The Works of Francis Balfour 3-20

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
Modern Notes:

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


Draft Version - Notice removed when completed.

Vol. III. A Treatise on Comparative Embryology 2 (1885)

CHAPTER XX. THE PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS

TJie Pectoral girdle.

Pisces. Amongst Fishes the pectoral girdle presents itself in its simplest form in Elasmobranchii, where it consists of a bent band of cartilage on each side of the body, of somewhat variable form, meeting and generally uniting with its fellow ventrally. Its anterior border is in close proximity with the last visceral arch, and a transverse ridge on its outer and posterior border, forming the articular surface for the skeleton of the limb, divides it into a dorsal part, which may be called the scapula, and a ventral part which may be called the coracoid.

In all the remaining groups of Fishes there is added to the cartilaginous band, which may wholly or partially ossify, an osseous support composed of a series of membrane bones.

In the types with such membrane bones the cartilaginous parts do not continue to meet ventrally, except in the Dipnoi where there is a ventral piece of cartilage, distinct from that bearing the articulation of the limb. The cartilage is moreover produced into two ventral processes, an anterior and a posterior, below the articulation of the limb ; which may be called, in accordance with Gegenbaur's nomenclature, the praecoracoid and coracoid. Of these the praecoracoid is far the most


600 THE PECTORAL GIRDLE.

prominent, and in the majority of cases the coracoid can hardly be recognised. The coracoid process is however well developed in the Selachioid Ganoids, and the Siluroid Teleostei. In Teleostei the scapular region often ossifies in two parts, the smaller of which is named by Parker praecoracoid, though it is quite distinct from Gegenbaur's praecoracoid. The membrane bones, as they present themselves in their most primitive state in Acipenser and the Siluroids, are dermal scutes embracing the anterior edge of the cartilaginous girdle. In Acipenser there are three scutes on each side. A dorsal scute known as the supra-clavicle, connected above with the skull by the posttemporal ; a middle piece or clavicle, and a ventral or infraclavicle (inter-clavicle), which meets its fellow below.

In most Fishes the primitive dermal scutes have become subdermal membrane bones, and the infra-clavicle is usually not distinct, but the two clavicles form the most important part of the membranous elements of the girdle. Additional membrane bones (post-clavicles) are often present behind the main row.

The development of these parts in Fishes has been but little studied.

In Scyllium, amongst the Elasmobranchii, I find that each half of the pectoral girdle develops as a vertical bar of cartilage at the front border of the rudimentary fin, and externally to the muscle-plates.

Before the tissue forming the pectoral girdle has acquired the character of true cartilage, the bars of the two sides meet ventrally by a differentiation in situ of the mesoblastic cells, so that, when the girdle is converted into cartilage, it forms an undivided arc, girthing the ventral side of the body. There is developed in continuity with the posterior border of this arc on the level of the fin a horizontal bar of cartilage, which is continued backwards along the insertion of the fin, and, as will be shewn in the sequel, becomes the metapterygium of the adult (figs. 344, bp and 348, mp). With this bar the remaining skeletal elements of the fin are also continuous.

The foramina of the pectoral girdle are not in the first instance formed by absorption, but by the non-development of the cartilage in the region of pre-existing nerves and vessels.


THE PECTORAL GIRDLE. 6oi

The development of these parts in Teleostei has been recently investigated by 'Swirski (No. 472) who finds in the Pike (Esox) that the cartilaginous pectoral girdle is at first continuous with the skeleton of the fin. It forms a rod with a dorsal scapular and ventral coracoid process. An independent mass of cartilage gives rise to a prascoracoid, which unites with the main mass, forming a triradiate bar like that of Acipenser or the Siluroids. The coracoid process becomes in the course of development gradually reduced.

'Swirski concludes that the so-called praecoracoid bar is to some extent a secondary element, and that the coracoid bar corresponds to the whole of the ventral part of the girdle of Elasmobranchii, but his investigations do not appear to me to be as complete as is desirable.

Amphibia and Amniota. The pectoral girdle contains a more or less constant series of elements throughout the Amphibia and Amniota ; and the differences in structure between the shoulder girdle of these groups and that of Fishes are so great that it is only possible to make certain general statements respecting the homologies of the parts in the two sets of types.

The generally accepted view, founded on the researches of Parker, Huxley, and Gegenbaur, is to the effect that there is a primitively cartilaginous coraco-scapular plate, homologous with that in Fishes, and that the membrane bones in Fishes are represented by the clavicle and inter-clavicle in the Sauropsida and Mammalia, which are however usually admitted to be absent in Amphibia. These views have recently been challenged by Gotte (No. 466) and Hoffmann (No. 467), on the ground of a series of careful embryological observations ; and until the whole subject has been worked over by other observers it does not seem possible to decide satisfactorily between the conflicting views. It is on all hands admitted that the scapulo-coracoid elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half of each plate becomes the scapula, which may subsequently become divided into a supra-scapula and scapula proper ; while the ventral half forms the coracoid, which is not always separated from the scapula, and is usually divided into a coracoid proper, a praecoracoid, and an epicoracoid. By the conversion of parts of the primitive cartilaginous plates into membranous tissue various fenestrae may be formed in the cartilage, and the bars


602 THE NATURE OF THE CLAVICLE.

bounding these fenestrae both in the scapula and coracoid regions have received special names ; the anterior bar of the coracoid region, forming the praecoracoid, being especially important. At the boundary between the scapula and the coracoid, on the hinder border of the plate, is placed the glenoid articular cavity to carry the head of the humerus.

The grounds of difference between Gotte and Hoffmann and other anatomists concern especially the clavicle and inter-clavicle. The clavicle is usually regarded as a membrane bone which may become to some extent cartilaginous. By. the above anatomists, and by Rathke also, it is held to be at first united with the coraco-scapular plate, of which it forms the anterior limb, free ventrally, but united dorsally with the main part of the plate ; and Gotte and Hoffmann hold that it is essentially a cartilage bone, which however in the majority of the Reptilia ossifies directly without passing through the condition of cartilage.

The interclavicle (episternum) is held by Gotte to be developed from a paired formation at the free ventral ends of the clavicles, but he holds views which are in many respects original as to its homologies in Mammalia and Amphibia. Even if Gotte's facts are admitted, it does not appear to me necessarily to follow that his deductions are correct. The most important of these is to the effect that the dermal clavicle of Pisces has no homologue in the higher types. Granting that the clavicle in these groups is in its first stage continuous with the coracoscapular plate, and that it may become in some forms cartilaginous before ossifying, yet it seems to me all the same quite possible that it is genetically derived from the clavicle of Pisces, but that it has to a great extent lost even in development its primitive characters, though these characters are still partially indicated in the fact that it usually ossifies very early and partially at least as a membrane bone 1 .

In treating the development of the pectoral girdle systematically it will be convenient to begin with the Amniota, which may be considered to fix the nomenclature of the elements of the shoulder girdle.

1 The fact of the clavicle going out of its way, so to speak, to become cartilaginous before being ossified, may perhaps be explained by supposing that its close connection with the other parts of the shoulder girdle has caused, by a kind of infection, a change in its histological characters.


II IK PECTORAL GIRDLE.


603


Lacertilia. The shoulder girdle is formed as two membranous plates, from the dorsal part of the anterior border of each of which a bar projects (Rathke, Gotte), which is free at its ventral end. This bar, which is usually (Gegenbaur, Parker) held to be independent of the remaining part of the shoulder girdle, gives rise to the clavicle and interclavicle. The scapulocoracoid plate soon becomes cartilaginous, while at the same time the clavicular bar ossifies directly from the membranous state. The ventral ends of the two clavicular bars enlarge to form two longitudinally placed plates, which unite together and ossify as the interclavicle.

Parker gives a very different account of the interclavicle in Anguis. He states that it is formed of two pairs of bones 'strapped on to the antero-inferior part of the prassternum,' which subsequently unite into one.

Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of a triradiate cartilage on each side, with one dorsal and two ventral limbs. It is admitted on all hands that the dorsal limb is the scapular element, and the posterior ventral limb the coracoid ; but, while the anterior ventral limb is usually held to be the praecoracoid, Gotte and Hoffmann maintain that, in spite of its being formed of cartilage, it is homologous with the anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the homologue of the clavicle.

Parker and Huxley (doubtfully) hold that the three anterior elements of the ventral plastron (entoplastron and epiplastra) are homologous with the interclavicle and clavicles, but considering that these plates appear to belong to a secondary system of dermal ossifications peculiar to the Chelonia, this homology does not appear to me probable.

Aves. There are very great differences of view as to the development of the pectoral arch of Aves.

About the presence in typical forms of the coraco-scapular plate and two independent clavicular bars all authors are agreed. With reference to the clavicle and interclavicle Parker (No. 468) finds that the scapular end of the clavicle attaches itself to and ossifies a mass of cartilage, which he regards as the mesoscapula, while the interclavicle is formed of a mass of tissue between the ends of the clavicles where they meet ventrally, which becomes the dilated plate at their junction.

Gegenbaur holds that the two primitive clavicular bars are simply clavicles, without any element of the scapula ; and states that the clavicles are not entirely ossified from membrane, but that a delicate band of cartilage precedes the osseous bars. He finds no interclavicle.

Gotte and Rathke both state that the clavicle is at first continuous with the coraco-scapular plate, but becomes early separated, and ossifies entirely as a membrane bone. Gotte further states that the interclavicles are formed as outgrowths of the median ends of the clavicles, which extend themselves at an early period of development along the inner edges of the two halves of the sternum. They soon separate from the clavicles, which subsequently meet to form the furculum ; while the interclavicular rudiments give rise, on the junction of the two halves of the sternum, to its keel, and to the ligament


604 THK PECTORAL GIRDLE.

connecting the furculum with the sternum. The observations of Gotte, which tend to shew the keel of the sternum is really an interclavicle, appear to me of great importance.

A prascoracoid, partially separated from the coracoid by a space, is present in Struthio. It is formed by a fenestration of a primitively continuous cartilaginous coracoid plate (Hoffmann). In Dromaeus and Casuarius clavicles are present (fused with the scapula in the adult Dromaeus), though absent in other Ratitae (Parker, etc.).

Mammalia. The coracoid element of the coraco-scapular plate is much reduced in Mammalia, forming at most a simple process (except in the Ornithodelphia) which ossifies however separately 1 .

With reference to the clavicles the same divergencies of opinion met with in other types are found here also.

The clavicle is stated by Rathke to be at first continuous with the coracoscapular plate. It is however soon separated, and ossifies very early, in the human embryo before any other bone. Gegenbaur however shewed that the human clavicle is provided with a central axis of cartilage, and this observation has been confirmed by Kolliker, and extended to other Mammalia by Gotte. The mode of ossification is nevertheless in many respects intermediate between that of a true cartilage bone and a membrane bone. The ends of the clavicles remain for some time, or even permanently, cartilaginous, and have been interpreted by Parker, it appears to me on hardly sufficient grounds, as parts of the mesoscapula and praecoracoid. Parker's so-called mesoscapula may ossify separately. The homologies of the episternum are much disputed. Gotte, who has worked out the development of the parts more fully than any other anatomist, finds that paired interclavicular elements grow out backwards from the ventral ends of the clavicles, and uniting together form a somewhat T-shaped interclavicle overlying the front end of the sternum. This condition is permanent in the Ornithodelphia, except that the anterior part of the sternum undergoes atrophy. But in the higher forms the interclavicle becomes almost at once divided into three parts, of which the two lateral remain distinct, while the median element fuses with the subjacent part of the sternum and constitutes with it the presternum (manubrium sterni). If Gotte' s facts are to be trusted, and they have been to a large extent confirmed by Hoffmann, his homologies appear to be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds that Gotte is mistaken as to the origin of the presternum.

Gegenbaur admits the lateral elements as parts of the interclavicle, while Parker holds that they are not parts of an interclavicle but are homologous with the omosternum of the Frog, which is however held by Gotte to be a true interclavicle.

1 This process, known as the coracoid process, is held by Sabatier to be the pnecoracoid ; while this author also holds that the upper third of the glenoid cavity, which ossifies by a special nucleus, is the true coracoid. The absence of a praecoracoid in the Ornithodelphia is to my mind a serious difficulty in the way of Sabatier's view.


THE PECTORAL GIRDLE. 605

Amphibia. In Amphibia the two halves of the shoulder girdle are each formed as a continuous plate, the ventral or coracoid part of which is forked, and is composed of a larger posterior and a smaller anterior bar-like process, united dorsally. In the Urodela the two remain permanently free at their ventral ends, but in the Anura they become united, and the space between them then forms a fenestra. The anterior process is usually (Gegenbaur, Parker) regarded as the praecoracoid, but Gotte has pointed out that in its mode of development it strongly resembles the clavicle of the higher forms, and behaves quite differently to the so-called praecoracoid of Lizards. It is however to be noticed that it differs from the clavicle in the fact that it is never segmented off from the coraco-scapular plate, a condition which has its only parallel in the equally doubtful case of the Chelonia. Parker holds that there is no clavicle present in the Amphibia, while Gegenbaur maintains that an ossification which appears in many of the Anura (though not in the Urodela) in the perichondrium on the anterior border of the cartilaginous bar above mentioned is the representative of the clavicle. Gotte's observations on the ossification of this bone throw doubt upon this view of Gegenbaur ; while the fact that the cartilaginous bar may be completely enclosed by the bone in question renders Gegenbaur's view, that there is present both a clavicle and prsecoracoid, highly improbable.

No interclavicle is present in Urodela, but in this group and in a number of the Anura, a process grows out from the end of each of the bars (praecoracoids) which Gotte holds to be the clavicles. The two processes unite in the median line, and give rise in front to the anterior unpaired element of the shoulder girdle (omosternum of Parker). They sometimes overlap the epicoracoids behind, and fusing with them bind them together in the median line. Parker who has described the paired origin of the so-called omosternum, holds that it is not homologous with the interclavicle, but compares it with his omosternum in Mammals.


BIBLIOGRAPHY.

(463) Bruch. " Ueber die Entwicklung der Clavicula und die Farbe des Blutes. " Zeit.f. wiss. Zool., \\. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a leurs differens ages." Memoires des savants etrang. Academic royale des sciences de Finstitut de France^ Vol. vi. 1835.

(465) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wirbelthiere, 2 Heft. Schultergiirtel der Wirbelthiere. Bmstflosse der Fische. Leipzig, 1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel." Archivf. mikr, Anat. Vol. xiv. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederlandisches Archivf. ZooL,Vol.v. 1879.

(468) W. K. Parker. "A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.


606 PELVIC GIRDLE.


(469) H. Rathke. Ueber die Entwicklung der Schildkrbten. Braunschweig, 1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der Saurier, 1853.

(471) A. Sabatier. Comparaison des ceinfures et des membres antMeurs et posttrtturs d. la Serie d. Vertttrh. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. iib. d. Entwick. d. Schultergiirtels n. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.


Pelvic girdle.

Pisces. The pelvic girdle of Fishes is formed of a cartilaginous band, to the outer and posterior side of which the basal element of the pelvic fin is usually articulated. This articulation divides it into a dorsal iliac, and ventral pubic section. The iliac section never articulates with the vertebral column.

In Elasmobranchii the two girdles unite ventrally, but the iliac section is only slightly developed. In Chimaera there is a well developed iliac process, but the pubic parts of the girdle are only united by connective tissue.

In the cartilaginous Ganoids the pelvic girdle is hardly to be separated from the skeleton of the fin. It is not united with its fellow, and is represented by a plate with slightly developed pubic and iliac processes.

In the Dipnoi there is a simple median cartilage, articulated with the limb, but not provided with an iliac process. In bony Ganoids and Teleostei there is on each side a bone meeting its fellow in the ventral line, which is usually held to be the rudiment of the pelvic girdle ; while Davidoff attempts to shew that it is the basal element of the fin, and that, except in Polypterus, a true pelvic girdle is absent in these types.

From my own observations I find that the mode of development of the pelvic girdle in Scyllium is very similar to that of the pectoral girdle. There is a bar on each side, continuous on its posterior border with the basal element of the fin (figs. 345 and 347). This bar meets and unites with its fellow ventrally before becoming converted into true cartilage, and though the iliac process (il) is never very considerable, yet it is better developed in the embryo than in the adult, and is at first directed nearly horizontally forwards.

Amphibia and Amniota. The primitive cartilaginous pelvic


PELVIC GIRDLE. 607


girdle of the higher types exhibits the same division as that of Pisces into a dorsal and a ventral section, which meet to form the articular cavity for the femur, known as the acetabulum. The dorsal section is always single, and is attached by means of rudimentary ribs to the sacral region of the vertebral column, and sometimes to vertebrae of the adjoining lumbar or caudal regions. It always ossifies as the ilium.

The ventral section is usually formed of two more or less separated parts, an anterior which ossifies as the pubis, and a posterior which ossifies as the ischium. The space between them is known as the obturator foramen. In the Amphibia the two parts are not separated, and resemble in this respect the pelvic girdle of Fishes. They generally meet the corresponding elements of the opposite side ventrally, and form a symphysis with them. The symphysis pubis, and symphysis ischii may be continuous (Mammalia, Amphibia).

The observations on the development of the pelvic girdle in the Amphibia and Amniota are nearly as scanty as on those of Fishes.

Amphibia. In the Amphibia (Bunge, No. 473) the two halves of the pelvic girdle are formed as independent masses of cartilage, which subsequently unite in the ventral line.

In the Urodelous Amphibia (Triton) each mass is a simple plate of cartilage divided into a dorsal and ventral section by the acetabulum. The ventral parts, which are not divided into two regions, unite in a symphysis comparatively late.

The dorsal section ossifies as the ilium. The ventral usually contains a single ossification in its posterior part which forms the ischium ; while the anterior part, which may be considered as representing the pubis, usually remains cartilaginous ; though Huxley (No. 475) states that it has a separate centre of ossification in Salamander, which however does not appear to be always present (Bunge). There is a small obturator foramen between the ischium and pubis, which gives passage to the obturator nerve. It is formed by the part of the tissue where the nerve is placed not becoming converted into cartilage.

There is a peculiar cartilage in the ventral median line in front of the pubis, which is developed independently of and much later than the true parts of the pelvic girdle. It may be called the praepubic cartilage.

Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two ventral (pubic and ischiad) processes. The acetabulum is placed on the outer side at the junction of the three processes, each of which may be


6o8 PECTORAL AND PELVIC GIRDLES.

considered to have a share in forming it. The distal ends of the pubis and ischium are close together when first formed, but subsequently separate. Each of them unites at a late stage with the corresponding process of the opposite side in a ventral symphysis. A centre of ossification appears in each of the three processes of the primitive cartilage.

Aves. In Birds the parts of the pelvic girdle no longer develop as a continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the Uuck, all the elements. The ilium early exhibits a short anterior process, but the pubis and ischium are at first placed with their long axes at right angles to that of the ilium, but gradually become rotated so as to lie parallel with it, their distal ends pointing backwards, and not uniting ventrally excepting in one or two Struthious forms.

Mammalia. In Mammalia the pelvic girdle is formed in cartilage as in the lower forms, but in Man at any rate the pubic part of the cartilage is formed independently of the remainder (Rosenberg). There are the usual three centres of ossification, which unite eventually into a single bone the innominate bone. The pubis and ischium of each side unite with each other ventrally, so as completely to enclose the obturator foramen.

Huxley holds that the so-called marsupial bones of Monotremes and Marsupials, which as shewn by Gegenbaur (No. 474) are performed in cartilage, are homologous with the praepubis of the Urodela ; but considering the great gap between the Urodela and Mammalia this homology can only be regarded as tentative. He further holds that the anterior prolongations of the cartilaginous ventral ends of the pubis of Crocodilia are also structures of the same nature.


BIBLIOGRAPHY.

(473) A. Bunge. Untersuch. z, Entwick. d. Beckengiirtels d. Amphibien, Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc." Proc. of Roy. Soc., Vol. xxvm. 1879.

(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et posterieurs dans la Serie d. Vertebrcs. Montpellier, 1880.

Comparison of Pectoral and Pelvic girdles.

Throughout the Vertebrata a more or less complete serial homology may be observed between the pectoral and pelvic girdles.

In the cartilaginous Fishes each girdle consists of a continuous band, a dorsal and ventral part being indicated by the articulation of the fin ; the former being relatively undeveloped in the pelvic


LIMBS. 609

girdle, while in the pectoral it may articulate with the vertebral column. In the case of the pectoral girdle secondary membrane bones become added to the primitive cartilage in most Fishes, which are not developed in the case of the pelvic girdle.

In the Amphibia and Amniota the ventral section of each girdle becomes divided into an anterior and a posterior part, the former constituting the praecoracoid and pubis, and the latter the coracoid and ischium ; these parts are however very imperfectly differentiated in the pelvic girdle of the Urodela. The ventral portions of the pelvic girdle usually unite below in a symphysis. They also meet each other ventrally in the case of the pectoral girdle in Amphibia, but in most other types are separated by the sternum, which has no homologue in the pelvic region, unless the praepubic cartilage is to be regarded as such. The dorsal or scapular section of the pectoral girdle remains free ; but that of the pelvic girdle acquires a firm articulation with the vertebral column.

If the clavicle of the higher types is derived from the membrane bones of the pectoral girdle of Fishes, it has no homologue in the pelvic girdle ; but if, as Gotte and Hoffmann suppose, it is a part of the primitive cartilaginous girdle, the ordinary view as to the serial homologies of the ventral sections of the two girdles in the higher types will need to be reconsidered.

Limbs.

It will be convenient to describe in this place not only the development of the skeleton of the limbs but also that of the limbs themselves. The limbs of Fishes are moreover so different from those of the Amphibia and Amniota that the development of the two types of limb may advantageously be treated separately.

In Fishes the first rudiments of the limbs appear as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.

These ridges are two in number on each side, an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected, but in some Elasmobranch embryos, more especially in Torpedo, they are connected together at their first development B. in. 39


6io


PAIRED FINS OF ELASMOBRANCHII.


by a line of columnar epiblast cells 1 . This connecting line of columnar epiblast is a very transitory structure, and after its disappearance the rudimentary fins become more prominent, consisting (fig. 343, &) of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold and the fin becomes a simple ridge of mesoblast, covered by epiblast. The pectoral fins are usually considerably ahead of the pelvic fins in development.

For the remaining history it is necessary to confine ourselves to Scylliurn as the only type which has been adequately studied.

The direction of the original ridge which connects the two fins of each side is nearly though not quite longitudinal, sloping somewhat obliquely downwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line a little way behind the anus.

The elongated ridge, forming the rudiment of each fin, gradually projects more and more, and so becomes broader in proportion to its length, but at the same time its actual attachment to the side of the body becomes shortened from behind forwards, so that what was originally the attached border becomes in part converted into the posterior border. This process is much more completely carried out in the case of the pectoral fins than in that of the pelvic, and the changes of form undergone by the pectoral fin in its development may be gathered from figs. 344 and 348.



FIG. 343. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ; cav. cardinal vein ; ua. vitelline artery ; u.v, vitelline vein ; al. duodenum ; /. liver ; sd. opening of segmented duct into the body cavity ; mp. muscle plate ; ;. umbilical canal.


1 I. M. I'alfour. Monograph on Elasmobranfh l-'hhes, pp. 1012.



LIMBS. 6ll

Before proceeding to the development of the skeleton of the fin it may be pointed out that the connection of the two rudimentary fins by a continuous epithelial line suggests the hypothesis that they are the remnants of two continuous lateral fins 1 .

Shortly after the view that the paired fins were remnants of continuous lateral fins had been put forward in my memoir on Elasmobranch Fishes, two very interesting papers were published by Thacker (No. 489) and Mivart (No. 484) advocating this view on the entirely independent grounds of the adult structure of the skeleton of the paired fins in comparison with that of the unpaired fins 2 .

The development of the skeleton has unfortunately not been as yet very fully studied. I have however made some investigations on this subject on Scyllium, and 'Swirski has also made some on the Pike.

In Scyllium the development of both the pectoral and pelvic fins is very similar.

In both fins the skeleton in its earliest stage consists of a bar springing from the posterior side of the pectoral or pelvic girdle, and running backwards parallel to the long axis of the body. The outer side of this bar is continued into a plate which

1 Both Maclise arid Humphry {Journal of Anat. and Pkys., Vol. v.) had previously suggested that the paired fins were related to the unpaired fins.

2 Davidoff in a Memoir (No. 477) which forms an important contribution to our knowledge of the structure of the pelvic fins has attempted from his observations to deduce certain arguments against the lateral fin theory of the limbs. His main argument is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. If this is the strongest argument which can be brought against the theory advocated in the text, there is I trust a considerable chance of its being generally accepted. For even granting that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral fin theory why the pelvic fins should not have shifted, and on the other hand the longitudinal cord connecting some of the spinal nerves in front of the pelvic fin may have another explanation. It might for instance be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their character such conclusions as those of Davidoff.

392


612


PAIRED FINS OF ELASMOBRANCHII.


extends into the fin, and which becomes very early segmented into a series of parallel rays at right angles to the longitudinal bar.

In other words, the primitive skeleton of both the fins consists of a longitudinal bar running along the base of the fin,



FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGITUDINAL AND HORIZONTAL SECTION.

The skeleton of the fin was still in the condition of embryonic cartilage. b.p. basipterygium (eventual metapterygium) ; fr. fin rays; p.g. pectoral girdle in transverse section; /. foramen in pectoral girdle; pc. wall of peritoneal cavity.

and giving off at right angles series of rays which pass into the fin. The longitudinal bar, which may be called the basipterygium, is moreover continuous in front with the pectoral or pelvic girdle as the case may be.

The primitive skeleton of the pectoral fin is shewn in longitudinal section in fig. 344, and that of the pelvic fin at a slightly later stage in fig. 345.

A transverse section shewing the basipterygium (inpi) of the pectoral fin, and the plate passing from it into the fin, is shewn in fig. 346.

Before proceeding to describe the later history of the two fins it may be well to point out that their embryonic structure completely supports the view which has been arrived at from the consideration of the soft parts of the fin.

My observations shew that the embryonic skeleton of the paired fin consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft part of the fin which has the form of a longitudinal ridge, and are continuous at their base with a longitudinal bar, which may very probably


LIMBS.


613


be due to secondary development. As pointed out by Mivart, a longitudinal bar is also occasionally formed to support the cartilaginous rays of unpaired fins. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of primitively independent rays, of which they believe the fin to have been originally composed. This view is probable enough in itself, but there is no trace



FIG. 345. PELVIC FIN OF A VERY YOUNG FEMALE EMBRYO OF SCYLLIUM STELLARE.

bb. basipterygium ; pu. pubic process of pelvic girdle ; il. iliac process of pelvic girdle.


in the embryo of the bar in question being formed by the coalesceace of rays, though the fact of its being perfectly continuous with the bases of the rays is somewhat in favour of this view 1 .

A point may be noticed here which may perhaps appear to be a difficulty, viz. that to a considerable extent in the pectoral, and to some extent in the pelvic fin the embryonic cartilage from which the fin-rays are developed is at first a continuous lamina, which subsequently segments into rays. I am however inclined to regard this merely as a result of the mode of conversion of the indifferent mesoblast into cartilage ; and in any case no conclusion adverse to the above view can be drawn from it, since I find that the rays of the unpaired fin are similarly segmented from a continuous lamina. In all cases the segmentation of the rays is to a large extent completed before the tissue in question is sufficiently differentiated to be called cartilage by an histologist.

Thacker and Mivart both hold that the pectoral and pelvic girdles have been evolved by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken to this view, and the fact of the pectoral and pelvic girdles originating continuously, and long remaining united with the

1 Thacker more especially founds his view on the adult form of the pelvic fins in the cartilaginous Ganoids ; Polyodon, in which the part which constitutes the basal plate in other forms is divided into separate segments, being mainly relied on. It is possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff, is secondary, but Thacker's view that the segmentation is a primitive character seems to me, in the absence of definite evidence to the reverse, the more natural one.


614


THE PELVIC FIN.


longitudinal bars of their respective fins is in favour of rather than against this view. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.

The later development of the skeleton of the two fins is more conveniently treated separately.

The pelvic fin. The changes in the pelvic fin are comparatively slight. The fin remains through life as a nearly horizontal lateral projection of the body, and the longitudinal bar the



FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG

EMBRYO OK SCYLLIUM STELLARE. mpt. basipterygial bar (metapterygium) ; fr. fin ray; m. muscles; hf. horny fibres.

basipterygium at its base always remains as such. It is for a considerable period attached to the pelvic girdle, but eventually becomes segmented from it. Of the fin rays the anterior remains directly articulated with the pelvic girdle on the separation of the basipterygium (fig. 347), and the remaining rays finally become segmented from the basipterygium, though they remain articulated with it. They also become to some extent transversely segmented. The posterior end of the basipterygial bar also becomes segmented off as the terminal ray.

The pelvic fin thus retains in all essential points its primitive arrangement.


LIMBS.


6l 5


The pectoral fin. The earliest stage of the pectoral fin



There


FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE.

bp. basipterygium ; m.o. process of basipterygium continued into clasper; il. iliac process of pectoral girdle ; pit. pubis.

differs from that of the pelvic fin only in minor points, is the same longitudinal or basipterygial bar to which the fin-rays are attached, whose position at the base of the fin is clearly seen in the transverse section (fig. 346, mpf). In front the bar is continuous with the pectoral girdle (figs. 344 and

348).

The changes which take place in the course of the further development are however very much more considerable in the case of the pectoral than in that of the pelvic fin. "' 3+8. F^OJJL ,,, v.

By the process spoken m p t me tapterygium (basipterygium of earlier

stage); me.p. rudiment of future pro- and mesopterygium ; sc. cut surface of scapular process ; cr. coracoid process;/;', foramen;/, horny fibres.



of above, by which the attachment of the pec


6l6 THE PECTORAL FIN.

toral fin to the body wall becomes shortened from behind forwards, the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (figs. 348 and 349, mp], constituting what Gegenbaur called the metapterygium, and eventually becomes segmented off from the pectoral girdle, simply articulating with its hinder edge.

The plate of cartilage, which is continued outwards from the basipterygium, or as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as in the case of the pelvic fin, and this is especially the case with the basal part of the plate. This basal part becomes in fact at first only divided into two parts (fig. 348) a small anterior part at the front end (me.p), and a larger posterior along the base of the remainder of the fin. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the rudiment of the mesopterygium and propterygium of Gegenbaur. It bears four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are borne by the edge of the plate continuous with the metapterygium.

The further changes in the cartilages of the limb are not important, and are easily understood by reference to fig. 349 representing the limb of a nearly full-grown embryo. The front end of the anterior basal cartilage becomes segmented off as a propterygium, bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium. The remainder of the now considerably segmented fin-rays are borne by the metapterygium.

The mode of development of the pectoral fin demonstrates that, as supposed by Mivart, the metapterygium is the homologue of the basal cartilage of the pelvic fin.

From the mode of development of the fins of Scyllium conclusions may be drawn adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom consider the primitive type of fin to be most nearly retained in Ceratodus, and to consist of a central multisegmented axis with numerous rays. Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodus, with a median axis and two


LIMBS.


6I 7


rows of rays ; but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (median or inner according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium ; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.

Gegenbaur's view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium ; not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays ; but the fact is that, as may easily be seen by an inspection of figs. 344 and 346, such a second set of lateral rays could not possibly have existed in a type . of fin like that found in the embryo 1 . With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill arches also falls ; in that his method of deriving the limbs from gill arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchs, could be derived from a visceral arch with its branchial rays 2 .

Gegenbaur's older view



FIG. 349. SKELETON OF THE PECTORAL FIN AND PART OF PECTORAL GIRDLE OF A NEARLY RIPE EMBRYO OF SCYLLIUM STELLARE.

m.p. metapterygium ; me.p. mesopterygium ; //. propterygium ; cr. coracoid process.


1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found in some Elasmobranch pectoral fins as rudiments of a second set of rays on the posterior side of the metapterygium, these rays will have to be regarded as structures in the act of being evolved, and not as persisting traces of a biserial fin.

2 Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as a result of his researches on Protopterus are interesting. The attachment which he describes between the external gills and the pectoral girdle is no doubt remarkable, but I would suggest that the observations we have on the vascular supply of these gills demonstrate that this attachment is secondary.


6l8 THE CHEIKOPTERYGIUM.

that the Elasmobranch fin retains a primitive uniserial type appears to me to be nearer the truth than his more recent view on this subject ; though I hold that the fundamental point established by the development of these parts in Scyllium is that the posterior border of the adult Elasmobranch fin is the primitive base line, i.e. the line of attachment of the fin to the side of the body.

Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The secondary character of the mesopterygium, and its total absence in the embryo Scyllium, appears to me as conclusive against Huxley's view, as the character of the embryonic fin is against that of Gegenbaur ; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchii by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.

With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski (No. 488) which unfortunately do not throw very much light upon the nature of the limb.

'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage, continuous with the pectoral girdle ; which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.

These investigations might be regarded as tending to shew that the basipterygium of Elasmobranchii is not represented in Teleostei, owing to the fin rays not having united into a continuous basal bar, but the observations are not sufficiently complete to admit of this conclusion being founded upon them with any certainty.

Tlie ckeiropterygium.

Observations on the early development of the pentadactyloid limbs of the higher Vertebrata are comparatively scanty.

The limbs arise as simple outgrowths of the sides of the body, formed both of epiblast and mesoblast. In the Amniota, at all events, they are processes of a special longitudinal ridge known as the Wolffian ridge. In the Amniota they also bear at their extremity a thickened cap of epiblast, which may be compared with the epiblastic fold at the apex of the Elasmobranch fin.

Both limbs have at first a precisely similar position, both being directed backwards and being parallel to the surface of the body.


I 111: CHEIROPTERYGIUM.


619


In the Urodela (Gotte) the ulnar and fibular sides are primitively dorsal, and the radial and tibial ventral : in Mammalia however Kolliker states that the radial and tibial edges are from the first anterior.

The exact changes of position undergone by the limbs in the course of development are not fully understood. To suit a terrestrial mode of life the flexures of the two limbs become gradually more and more opposite, till in Mammalia the corresponding joints of the two limbs are turned in completely opposite directions.

Within the mesoblast of the limbs a continuous blastema becomes formed, which constitutes the first trace of the skeleton of the limb. The corresponding elements of the two limbs, viz. the humerus and femur, radius and tibia, ulna and fibula, carpal and tarsal bones, metacarpals and metatarsals, and digits, become differentiated within this, by the conversion of definite regions into cartilage, which may either be completely distinct or be at first united. These cartilaginous elements subsequently ossify.

The later development of the parts, more especially of the carpus and tarsus, has been made the subject of considerable study ; and important results have been thereby obtained as to the homology of the various carpal and tarsal bones throughout the Vertebrata ; but this subject is too special to be treated of here. The early development, including the succession of the growth of the different parts, and the extent of continuity primitively obtaining between them, has on the other hand been but little investigated ; recently however the development of the limbs in the Urodela has been worked out in this way by two anatomists, Gotte (No. 482) and Strasser (No. 487), and their results, though not on all points in complete harmony, are of considerable interest, more especially in their bearing on the derivation of the pentadactyloid limb from the piscine fin. Till however further investigations of the same nature have been made upon other types, the conclusions to be drawn from Gotte and Strasser's observations must be regarded as somewhat provisional, the actual interpretation of various ontological processes being very uncertain.

The forms investigated are Triton and Salamandra. We may remind the reader that the hand of the Urodela has four digits, and the foot five, the fifth digit being absent in the hand 1 . In Triton the proximal row of carpal bones consists (using Gegenbaur's nomenclature) of (i) a radiale, and (2 and 3) an intermedium and ulnare, partially united. The distal row is formed of four carpals, of which the first often does not support the first 1 This seems to me clearly to follow from Gotte and Strasser's observations.


620 THE GHE1ROPTERYGIUM.

metacarpal ; while the second articulates with both the first and second metacarpals. In the foot the proximal row of tarsals consists of a tibiale, an intermedium and a fibulare. The distal row is formed of four tarsals, the first, like that in the hand, often not articulating with the first metatarsal, the second supporting the first and second metatarsals ; and the fourth the fourth and fifth metatarsals.

The mode of development of the hand and foot is almost the same. The most remarkable feature of development is the order of succession of the digits. The two anterior (radial or tibial) are formed in the first instance, and then the third, fourth and fifth in succession.

As to the actual development of the skeleton Strasser, whose observations were made by means of sections, has arrived at the following results.

The humerus with the radius and ulna, and the corresponding parts in the hind limb, are the first parts to be differentiated in the continuous plate of tissue from which the skeleton of the limb is formed. Somewhat later a cartilaginous centre appears at the base of the first and second fingers (which have already appeared as prominences at the end of the limb) in the situation of the permanent second carpal of the distal row of carpals ; and the process of chondrification spreads from this centre into the fingers and into the remainder of the carpus. In this way a continuous carpal plate of cartilage is established, which is on the one hand continuous with the cartilage of the two metacarpals, and on the other with the radius and ulna.

In the cartilage of the carpus two special columns may be noticed, the one on the radial side, most advanced in development, being continuous with the radius ; the other less developed column on the side of the ulna being continuous both with the ulna and with the radius. The ulna and radius are not united with the humerus.

In the further growth the third and fourth digits, and in the foot the fifth digit also, gradually sprout out in succession from the ulnar side of the continuous carpal plate. The carpal plate itself becomes segmented from the radius and ulna, and divided up into the carpal bones.

The original radial column is divided into three elements, a proximal the radiale, a middle element the first carpal, and a distal the second carpal already spoken of. The first carpal is thus situated between the basal cartilage of the second digit and the radiale, and would therefore appear to be the representative of a primitive middle row of carpal bones, of which the centrale is also another representative.

The centrale and intermedium are the middle and proximal products of the segmentation of the ulnar column of the primitive carpus, the distal second carpal being common both to this column and to the radial column.

The ulnar or fibular side of the carpus or tarsus becomes divided into a proximal element the ulnare or fibulare the ulnare remaining partially united with the intermedium. There are also formed from this plate two carpals to articulate with digits 3 and 4 ; while in the foot the corresponding elements articulate respectively with the third digit, and with the fourth and fifth digits.


THE CIIF.IROPTERYGIUM. 621

Gotte, whose observations were made in a somewhat different method to those of Strasser, is at variance with him on several points. He finds that the primitive skeleton of the limb consists of a basal portion, the humerus, continued into a radial and an ulnar ray, which are respectively prolonged into the two first digits. The two rays next coalesce at the base of the fingers to form the carpus, and thus the division of the limb into the brachium, antebrachium and manus is effected.

The ulna, which is primitively prolonged into the second digit, is subsequently separated from it and is prolonged into the third ; from the side of the part of the carpus connecting the ulna with the third digit the fourth digit is eventually budded out, and in the foot the fourth and fifth digits arise from the corresponding region. Each of the three columns connected respectively with the first, second, and third digits becomes divided into three successive carpal bones, so that Gotte holds the skeleton of the hand or foot to be formed of a proximal, a middle, and a distal row of carpal bones each containing potentially three elements. The proximal row is formed of the radiale, intermedium and ulnare ; the middle row of carpal i, the centrale and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two coalesced elements) and carpal 3.

The derivation of the cheiropterygium from the ichthyoptcrygium. All anatomists are agreed that the limbs of the higher Vertebrata are derived from those of Fishes, but the gulf between the two types of limbs is so great that there is room for a very great diversity of opinion as to the mode of evolution of the cheiropterygium. The most important speculations on the subject are those of Gegenbaur and Huxley.

Gegenbaur holds that the cheiropterygium is derived from a uniserial piscine limb, and that it consists of a primitive stem, to which a series of lateral rays are attached on one (the radial) side ; while Huxley holds that the cheiropterygium is derived from a biserial piscine limb by the "lengthening of the axial skeleton, accompanied by the removal of its distal elements further away from the shoulder-girdle and by a diminution in the number of the rays."

Neither of these theories is founded upon ontology, and the only ontological evidence we have which bears on this question is that above recorded with reference to the development of the Urodele limb.

Without holding that this evidence can be considered as in any way conclusive, its tendency would appear to me to be in favour of regarding the cheiropterygium as derived from a uniserial type of fin. The humerus or femur would appear to be the basipterygial bars (metapterygium), which have become directed outwards instead of retaining their original position parallel to the length of the body at the base of the fin. The anterior (proximal) fin-rays and the pro- and mesopterygium must be supposed to have become aborted, while the radius or ulna, and tibia or fibula are two posterior fin-rays (probably each representing several coalesced rays like the pro- and mesopterygium) which support at their distal extremities more numerous fin-rays consisting of the rows of carpal and tarsal bones.


622 THE CHEIROPTERYGIUM.

This view of the cheiropterygium corresponds in some respects with that put forward by Gotte as a result of his investigations on the development of the Urodele limbs, though in other respects it is very different. A difficulty of this view is the fact that it involves our supposing that the radial edge of the limb corresponds with the metapterygial edge of the piscine fin. The difficulties of this position have been clearly pointed out by Huxley, but the fact that in the primitive position of the Urodele limbs the radius is ventral and the ulna dorsal shews that this difficulty is not insuperable, in that it is easy to conceive the radial border of the fin to have become rotated from its primitive Elasmobranch position into the vertical position it occupies in the embryos of the Urodela, and then to have been further rotated from this position into that which it occupies in the adult Urodela and in all higher forms.

BIBLIOGRAPHY of the Limbs.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuckungen z. vergleich. Anat. d. Wirbelthiere. Leipzig, 1864 5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d. Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeitsckrift, Vol. V. 1870.

(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol. vii. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere." Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entivick. u. Regeneration d. Gliedmaassenskelets d. Molche. Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeil.f. iviss. Zool., Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi d. Menschen. " Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u. Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Untersitch. iib. d. Entwick. d. Schultergitrtels u. d. Skelcls d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history of the Vertebrate limbs." Trans, of the Connecticut Acad., Vol. ill. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut Acad., Vol. iv. 1877.


CHAPTER XXI.


THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE VASCULAR GLANDS.


The Body cavity.

IN the Ccelenterata no body cavity as distinct from the alimentary cavity is present ; but in the remaining Invertebrata the body cavity may (i) take the form of a wide space separating the wall of the gut from the body wall, or (2) may be present in a more or less reduced form as a number of serous spaces, or (3) only be represented by irregular channels between the muscular and connective-tissue cells filling up the interior of the body. The body cavity, in whatever form it presents itself, is probably filled with fluid, and the fluid in it may contain special cellular elements. A well developed body cavity may coexist with an independent system of serous spaces, as in the Vertebrata and the Echinodermata ; the perihaemal section of the body cavity of the latter probably representing the system of serous spaces.

In several of the types with a well developed body cavity it has been established that this cavity originates in the embryo from a pair of alimentary diverticula, and the cavities resulting from the formation of these diverticula may remain distinct, the adjacent walls of the two cavities fusing to form a dorsal and a ventral mesentery.

It is fairly certain that some groups, e.g. the Tracheata, with imperfectly developed body cavities are descended from ancestors which were provided with well developed body cavities, but how far this is universally the case cannot as yet be definitely decided, and for additional information on this subject the


624 CIIORDATA.


reader is referred to pp. 355 360 and to the literature there referred to.

In the Chaetopoda and the Tracheata the body cavity arises as a series of paired compartments in the somites of mesoblast (fig. 350) which have at first a very restricted extension on the ventral side of the body, but eventually extend dorsalwards and vcntralwards till each cavity is a half circle investing the alimentary tract ; on the dorsal side the walls separating the two



FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA LABYRINTHICA.

The section is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk segments and their nuclei.

i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument.

half cavities usually remain as the dorsal mesentery, while ventrally they are in most instances absorbed. The transverse walls, separating the successive compartments of the body cavity, generally become more or less perforated.

Chordata. In the Chordata the primitive body cavity is cither directly formed from a pair of alimentary diverticula (Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic plates of the two sides of the body (fig. 20).

As already explained (pp. 294 300) the walls of the dorsal sections of the primitive body cavity soon become separated from those of the ventral, and becoming segmented constitute the muscle plates, while the cavity within them becomes


I


THE BODY CAVITY.


625


the


obliterated : they are dealt with in a separate chapter. The ventral part of the primitive cavity alone constitutes the permanent body cavity.

The primitive body cavity in the lower Vertebrata is at first continued forwards into the region of the head, but on the formation of the visceral clefts the cephalic section of the body cavity becomes divided into a series of separate compartments. Subsequently these sections of the body cavity become obliterated ; and, since their walls give rise to muscles, they may probably be looked upon as equivalent to the dorsal sections of the body cavity in the trunk, and will be treated of in connection with the muscular system.

As a result of its mode of origin the body cavity in trunk is at first divided into two lateral halves ; and part of the mesoblast lining it soon becomes distinguished as a special layer of epithelium, known as the peritoneal epithelium, of which the part bounding the outer wall forms the somatic layer, and that bounding the inner wall the splanchnic layer. Between the two splanchnic layers is placed the gut. On the ventral side, in the region of the permanent gut, the two halves of the body cavity soon coalesce, the septum between them becoming absorbed, and the splanchnic layers of epithelium of the two sides uniting at the ventral side of the gut, and the somatic layers at the median ventral line of the body wall (fig.



In the lower Vertebrata the body cavity is originally present even in the post-anal region of the trunk, but usually atrophies early, frequently before the two halves coalesce.

On the dorsal side of the gut the B. III.


FIG. 351. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white matter of spinal cord ; pr. posterior nerve-roots ; cA. notochord ; x. sub-notochordal rod ; ao. aorta ; nip. muscle-plate ; nip 1 , inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body ; si. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.

40


626 ABDOMINAL PORES.


two halves of the body cavity never coalesce, but eventually the splanchnic layers of epithelium of the two sides, together with a thin layer of interposed mesoblast, form a delicate membrane, known as the mesentery, which suspends the gut from the dorsal wall of the body (figs. 119 and 351). On the dorsal side the epithelium lining of the body cavity is usually more columnar than elsewhere (fig. 351), and its cells partly form a covering for the generative organs, and partly give rise to the primitive germinal cells. This part of the epithelium is often known as the germinal epithelium.

Over the greater part of the body cavity the lining epithelium becomes in the adult intimately united with a layer of the subjacent connective tissue, and constitutes with it a special lining membrane for the body cavity, known as the peritoneal membrane.

Abdominal pores. In the Cyclostomata, the majority of the Elasmobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida (Chelonia and Crocodilia) the body cavity is in communication with the exterior by a pair of pores, known as abdominal pores, the external openings of which are usually situated in the cloaca 1 .

The ontogeny of these pores has as yet been but very slightly investigated. In the Lamprey they are formed as apertures leading from the body cavity into the excretory section of the primitive cloaca. This section would appear from Scott's (No. 87) observations to be derived from part of the hypoblastic cloacal section of the alimentary tract.

In all other cases they are formed in a region which appears to belong to the epiblastic region of the cloaca ; and from my observations on Elasmobranchs it may be certainly concluded that they are formed there in this group. They may appear as perforations (i) at the apices of papilliform prolongations of the body cavity, or (2) at the ends of cloacal pits directed from the exterior towards the body cavity, or (3) as simple slit-like openings.

Considering the difference in development between the abdominal pores of most types, and those of the Cyclostomata, it is open to doubt whether these two types of pores are strictly homologous.

In the Cyclostomata they serve for the passage outwards of the generative products, and they also have this function in some of the few Teleostei in which they are found ; and Gegenbaur and Bridge hold that the primitive mode of exit of the generative products, prior to the development of the Miillerian ducts, was probably by means of these pores. I have elsewhere

1 For a full account of these structures the reader is referred to T. W. Bridge, "Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879.


THE BODY CAVITY.


627



suggested that the abdominal pores are perhaps remnants of the openings of segmental tubes ; there does not however appear to be any definite evidence in favour of this view, and it is more probable that they may have arisen as simple perforations of the body wall.

Pericardial cavity, pleural cavities, and diaphragm.

In all Vertebrata the heart is at first placed in the body cavity (fig. 353 A), but the part of the body cavity containing it afterwards becomes separated as a distinct cavity known as the pericardial cavity. In Elasmobranchii, Acipenser, etc. a passage is however left between the pericardial cavity and the body cavity ; and in the Lamprey a separation between the two cavities does not occur during the Ammoccete stage. In Elasmobranchii the pericardial cavity becomes established as a distinct space in front of the body cavity in the following way. When the two ductus Cuvieri, leading transversely from the sinus venosus to the cardinal veins, become developed, a horizontal septum, shewn on the right side in fig. 352, is formed to support them, stretching across from the splanchnic to the somatic side of the body cavity, and dividing the body cavity (fig. 352) in this part into (i) a dorsal section formed of a right and left division constituting the true body cavity (pp), and (2) a ventral part the pericardial cavity (pc). The septum is at first of a very small longitudinal extent, so that both in front and behind it (fig. 352 on the left side) the dorsal and ventral sections of the body cavity are in free communication. The septum soon however becomes prolonged, and ceasing to be quite horizontal, is directed obliquely upwards and forwards till it meets the dorsal wall of the body

40 2


-ht


FIG. 352. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F.

The figure shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the narrow passage which remains connecting the two cavities.

sp.c. spinal canal ; w. white matter of spinal cord ; pr. commissure connecting the posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; sv. sinus venosus ; cav. cardinal vein ; ht. heart ; pp. body cavity ; pc. pericardial cavity ; as. solid oesophagus ; /. liver ; nip. muscle-plate.


628 THE PERICARDIAL CAVITY.

Anteriorly all communication is thus early shut off between the body cavity and the pericardial cavity, but the two cavities still open freely into each other behind.

The front part of the body cavity, lying dorsal to the pericardial cavity, becomes gradually narrowed, and is wholly obliterated long before the close of embryonic life, so that in adult Elasmobranch Fishes there is no section of the body cavity dorsal to the pericardial cavity. The septum dividing the body cavity from the pericardial cavity is prolonged backwards, till it meets the ventral wall of the body at the point where the liver is attached by its ventral mesentery (falciform ligament). In this way the pericardial cavity becomes completely shut off from the body cavity, except, it would seem, for the narrow communications found in the adult. The origin of these communications has not however been satisfactorily worked out.

The septum between the pericardial cavity and the body cavity is attached on its dorsal aspect to the liver. It is at first nearly horizontal, but gradually assumes a more vertical position, and then, owing to the obliteration of the primitive anterior part of the body cavity, appears to mark the front boundary of the body cavity. The above description of the mode of formation of the pericardial cavity, and the explanation of its relations to the body cavity, probably holds true for Fishes generally.

In the higher types the earlier changes are precisely the same as those in Elasmobranch Fishes. The heart is at first placed within the body cavity attached to the ventral wall of the gut by a mesocardium (fig. 353 A). A horizontal septum is then formed, in which the ductus Cuvieri are placed, dividing the body cavity for a short distance into a dorsal (/./) and ventral (p.c) section (fig. 353 B). In Birds and Mammals, and probably also in Reptilia, the ventral and dorsal parts of the body cavity are at first in free communication both in front of and behind this septum. This is shewn for the Chick in fig- 353 A an d B, which are sections of the same chick, A being a little in front of B. The septum is soon continued forwards so as completely to separate the ventral pericardial and the dorsal body cavity in front, the pericardial cavity extending at this period considerably further forwards than the body cavity.

Since the horizontal septum, by its mode of origin, is


THE BODY CAVITY.


629


necessarily attached to the ventral side of the gut, the dorsal part of the primitive body space is divided into two halves by a median vertical septum formed of the gut and its mesentery (fig- 353 B). Posteriorly the horizontal septum grows in a slightly ventral direction along the under surface of the liver (fig- 354)j till it meets the abdominal wall of the body at the insertion of the falciform ligament, and thus completely shuts off the pericardial cavity from the body cavity. The horizontal septum forms, as is obvious from the above description, the dorsal wall of the pericardial cavity 1 .

A. B.



FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTYONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAI, CAVITY, A. BEING THE ANTERIOR SECTION.

p.p. body cavity; p.c. pericardial cavity; al. alimentary cavity ; au. auricle; v. ventricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; nip. muscle-plate; me. medullary cord.

With the complete separation of the pericardial cavity from the body cavity, the first period in the development of these parts is completed, and the relations of the body cavity to the

1 Kolliker's account of this septum, which he calls the mesocardium laterale (No. 298, p. 295), would seem to imply that in Mammals it is completed posteriorly even before the formation of the liver. I doubt whether this takes place quite so early as he implies, but have not yet determined its exact period by my own observations.


630


THE PERICARDIAL CAVITY.


pericardial cavity become precisely those found in the embryos of Elasmobranchii. The later changes are however very different. Whereas in Fishes the right and left sections of the body cavity dorsal to the pericardial cavity soon atrophy, in the higher types, in correlation with the relatively backward situation of the heart, they rapidly become larger, and receive the lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into the splanchnic mesoblast, in front of the body cavity ; but as they grow, they extend into the two anterior compartments of the body cavity, each attached by its mesentery to the mesentery of the gut (fig. 354, lg). They soon moreover extend beyond the region of the pericardium into the undivided body cavity behind. This holds not only for the embryos of the Amphibia and Sauropsida, but also for those of Mammalia.

To understand the further

rrianfrps in rhp nerirardial ravitv FlG> 354- SECTION THROUGH

pencaraiai cavity THECARDIACREGION OF AN EMBRYO

it is necessary to bear in mind its OF LACERTA MURALIS OF 9 MM. TO

, ,. ,, ,. . . SHEW THE MODE OF FORMATION OF

relations to the adjoining parts. THE PERICARDIAL CAVITY.



'-/it


It lies at this period completely ventral to the two anterior pro


ht. heart ; pc. pericardial cavity ; al. alimentary tract; lg. lung; /. liver ; pp. body cavity ; md. open longations of the body Cavity COn- end of Mullerian duct ; wd. Wolffian . . duct; vc. vena cava inferior; ao.

taming the lungs (fig. 354). Its aorta; ch. notochord; me. medullary

dorsal wall is attached to the gut, cord>

and is continuous with the mesentery of the gut passing to the dorsal abdominal wall, forming the posterior mediastinum of human anatomy.

The changes which next ensue consist essentially in the enlargement of the sections of the body cavity dorsal to the pericardial cavity. This enlargement takes place partly by the elongation of the posterior mediastinum, but still more by the two divisions of the body cavity which contain the lungs extending themselves ventrally round the outside of the peri


THE BODY CAVITY.


631


cardial cavity. This process is illustrated by fig. 355, taken from an embryo Rabbit. The two dorsal sections of the body cavity (pl.p] finally extend so as completely to envelope the pericardial cavity (pc\ remaining however separated from each other below by a lamina extending from the ventral wall of the pericardial cavity to the body wall, which forms the anterior mediastinum of human anatomy.

By these changes the pericardial cavity is converted into a closed bag, completely surrounded at its sides by the two lateral halves of the body cavity, which were primitively placed


SJ3. C.



FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL CAVITIES.

ht. heart; pc. pericardial cavity; //./ pleural cavity; Ig. lung; al. alimentary tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.

dorsally to it. These two sections of the body cavity, which in Amphibia and Sauropsida remain in free communication with the undivided peritoneal cavity behind, may, from the fact of their containing the lungs, be called the pleural cavities.

In Mammalia a further change takes place, in that, by the formation of a vertical partition across the body cavity, known as the diaphragm, the pleural cavities, containing the lungs,


632 THE VASCULAR SYSTEM.

become isolated from the remainder of the body or peritoneal cavity. As shewn by their development the so-called pleurae or pleural sacks are simply the peritoneal linings of the anterior divisions of the body cavity, shut off from the remainder of the body cavity by the diaphragm.

The exact mode of formation of the diaphragm is not fully made out ; the account of it recently given by Cadiat (No. 491) not being in my opinion completely satisfactory.

BIBLIOGRAPHY.

(491) M. Cadiat. "Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 1'cesophage." Journal de F Anatomic et de la Physiologic, Vol. xiv. 1878.


Vascular System.

The actual observations bearing on the origin of the vascular system, using the term to include the lymphatic system, are very scanty. It seems probable, mainly it must be admitted on d priori grounds, that vascular and lymphatic systems have originated from the conversion of indefinite spaces, primitively situated in the general connective tissue, into definite channels. It is quite certain that vascular systems have arisen independently in many types ; a very striking case of the kind being the development in certain parasitic Copepoda of a closed system of vessels with a red non-corpusculated blood (E. van Beneden, Heider), not found in any other Crustacea. Parts of vascular systems appear to have arisen in some cases by a canalization of cells.

The blood systems may either be closed or communicate with the body cavity. In cases where the primitive body cavity is atrophied or partially broken up into separate compartments (Insecta, Mollusca, Discophora, etc.) a free communication between the vascular system and the body cavity is usually present ; but in these cases the communication is no doubt secondary. On the whole it would seem probable that the vascular system has in most instances arisen independently of the body cavity, at least in types where the body cavity is


THE VASCULAR SYSTEM. 633

present in a well-developed condition. As pointed out by the Hertwigs, a vascular system is always absent where there is not a considerable development of connective tissue.

As to the ontogeny of the vascular channels there is still much to be made out both in Vertebrates and Invertebrates.

The smaller channels often rise by a canalization of cells. This process has been satisfactorily studied by Lankester in the Leech 1 , and may easily be observed in the blastoderm of the Chick or in the epiploon of a newlyborn Rabbit (Schafer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise to the walls, and the blood-corpuscles being derived either from nucleated masses set free within the vessels (the Chick) or from blood-corpuscles directly differentiated in the axes of the cells (Mammals).

Larger vessels would seem to be formed from solid cords of cells, the central cells becoming converted into the corpuscles, and the peripheral cells constituting the walls. This mode of formation has been observed by myself in the case of the Spider's heart, and by other observers in other Invertebrata. In the Vertebrata a more or less similar mode of formation appears to hold good for the larger vessels, but further investigations are still required on this subject. Gotte finds that in the Frog the larger vessels are formed as longitudinal spaces, and that the walls are derived from the indifferent cells bounding these spaces, which become flattened and united into a continuous layer.

The early formation of vessels in the Vertebrata takes place in the splanchnic mesoblast ; but this appears due to the fact that the circulation is at first mainly confined to the vitelline region, which is covered by splanchnic mesoblast.

The Heart.

The heart is essentially formed as a tubular cavity in the splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this cavity are formed of two layers, an outer thicker layer, which has at first only the form of a half tube, being incomplete on its dorsal side; and an inner lamina formed of delicate flattened cells. The latter is the epithelioid lining of the heart, and the cavity it contains the true cavity of the heart. The outer layer gives rise to the muscular wall and peritoneal covering of the heart. Though at first it has only the form of a half tube (fig.

1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science, Vol. XX. 1880.


634


THE HEART.


356), it soon becomes folded in on the dorsal side so as to form for the heart a complete muscular wall. Its two sides, after thus meeting to complete the tube of the heart, remain at first continuous with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery the mesocardium which attaches the heart to the ventral wall of the throat. The superficial stratum of the wall of the heart differentiates itself as the peritoneal covering. The inner epithelioid tube takes its origin at the time when the general cavity of the heart is being formed by the separation of the splanchnicmesoblastfrom the hypoblast. During this process (fig. 357) a layer of mesoblast remains close to the hypoblast, but connected with the main mass



FIG. 356. SECTION THROUGH THE DEVELOPING HEART OF AN EMBRYO OF AN ELASMOBRANCH (Pristiurus).

al. alimentary tract ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ht. heart.



FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.

hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochorcl ; x. thickening of hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ht. heart; //. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast.


THE VASCULAR SYSTEM.


635


of the mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form together the epithelioid lining of the heart ; between them is the cavity of the heart, which soon loses the protoplasmic trabeculae which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast, and resembles in its mode of origin that of other large vascular trunks.

The above description applies only to the development of the heart in those types in which it is formed at a period after the throat has become a closed tube (Elasmobranchii, Amphibia, Cyclostomata, Ganoids (?)). In a number of other cases, in which the heart is formed before the conversion of the throat into a closed tube, of which the most notable is that of Mammals (Hensen, Gotte, Kolliker), the heart arises as two independent

A.



B.


mes fir



FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE

SAME AGE AS FIG. 144 B. (From Kolliker.) B is a more highly magnified representation of part of A.

rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ; dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast; ^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of the pharynx.


636


THE HEART.


tubes (fig. 358), which eventually coalesce into an unpaired structure.

In Mammals the two tubes out of which the heart is formed appear at the sides of the cephalic plates, opposite the region of the mid- and hindbrain (fig. 358). They arise at a time when the lateral folds which form the ventral wall of the throat are only just becoming visible. Each half of the heart originates in the same way as the whole heart in Elasmobranchii, etc. ; and the layer of the splanchnic mesoblast, which forms the muscular wall for each part (ahh), has at first the form of a half tube open below to the hypoblast.

On the formation of the lateral folds of the splanchnic walls, the two halves of the heart become carried inwards and downwards, and eventually



FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE FORMATION OF THE HEART.

fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchnopleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins.


THE VASCULAR SYSTEM.


637


meet on the ventral side of the throat. For a short time they here remain distinct, but soon coalesce into a single tube.

In Birds, as in Mammals, the heart makes its appearance as two tubes, but arises at a period when the formation of the throat is very much more advanced than in the case of Mammals. The heart arises immediately behind the point up to which the ventral wall of the throat is established and thus has at first a A -shaped form. At the apex of the A , which forms the anterior end of the heart, the two halves are in contact (fig. 357), though they have not coalesced; while behind they diverge to be continued as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce from before backwards into a single structure. Fig. 359 A and B shews the heart during this process. The two halves have coalesced anteriorly (A) but are still widely separated behind (B). In Teleostei the heart is formed as in Birds and Mammals by the coalescence of two tubes, and it arises before the formation of the throat.

The fact that the heart arises in so many instances as a double tube might lead to the supposition that the ancestral Vertebrate had two tubes in the place of the present unpaired heart.

The following considerations appear to me to prove that this conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the formation of the heart, it is clear that a modification in the development of the heart would occur, in that the two halves of the heart would necessarily be formed widely apart, and only eventually united on the folding in of the wall of the throat. It is therefore possible to explain the double formation of the heart without having recourse to the above hypothesis of an ancestral Vertebrate with two hearts. If the explanation just suggested is the true one the heart should only be formed as two tubes when it arises prior to the formation of the throat, and as a single tube when formed after the formation of the throat. Since this is invariably found to be so, it may be safely concluded that the formation of the heart as two cavities is a secondary mode of development, which has been brought about by variations in the period of the closing in of the wall of the throat.

The heart arises continuously with the sinus venosus, which in the Amniotic Vertebrata is directly continued into the vitelline veins. Though at first it ends blindly in front, it is very soon connected with the foremost aortic arches.


638 THE HEART.


The simple tubular heart, connected as above described, grows more rapidly than the chamber in which it is contained, and is soon doubled upon itself, acquiring in this way an S-shaped curvature, the posterior portion being placed dorsally, and the anterior ventrally. A constriction soon appears between the dorsal and ventral portions.

The dorsal section becomes partially divided off behind from the sinus venosus, and constitutes the relatively thin-walled auricular section of the heart; while the ventral portion, after becoming distinct anteriorly from a portion continued forwards from it to the origin of the branchial arteries, which may be called the truncus arteriosus, acquires very thick spongy muscular walls, and becomes the ventricular division of the heart.

The further changes in the heart are but slight in the case of the Pisces. A pair of simple membranous valves becomes established at the auriculoventricular orifice, and further changes take place in the truncus arteriosus. This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a posterior section, called the conus arteriosus, provided with a series of transverse rows of valves, and an anterior section, called the bulb us arteriosus, not provided with valves, and leading into the branchial arteries. In most Teleostei (except Butirinus and a few other forms) the conus arteriosus is all but obliterated, and the anterior row of its valves alone preserved ; and the bulbus is very much enlarged 1 .

In the Dipnoi important changes in the heart are effected, as compared with other Fishes, by the development of true lungs. Both the auricular and ventricular chamber may be imperfectly divided into two, and in the conus a partial longitudinal septum is developed in connection with a longitudinal row of valves 2 .

In Amphibia the heart is in many respects similar to that of the Dipnoi. Its curvature is rather that of a screw than of a simple S. The truncus arteriosus lies to the left, and is continued into the ventricle which lies ventrally and more to the right, and this again into the dorsally placed auricular section.

After the heart has reached the piscine stage, the auricular section (Bombinator) becomes prolonged into a right and left auricular appendage^ A septum next grows from the roof of the auricular portion of the heart

1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit., Vol. n. 1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterienbogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.," Morphol. Jahrb., Vol. VI. 1880.

2 Boas holds that the longitudinal septum is formed by the coalescence of a row of longitudinal valves, but this is opposed to Lankester's statements, "On the hearts of Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. x. 1879.


THE VASCULAR SYSTEM. 639


obliquely backwards and towards the left, and divides it in two chambers ; the right one of which remains continuous with the sinus venosus, while the left one is completely shut off from the sinus, though it soon enters into communication with the newly established pulmonary veins. The truncus arteriosus 1 is divided into a posterior conus arteriosus (pylangium) and an anterior bulbus (synangium). The former is provided with a proximal row of valves at its ventricular end, and a distal row at its anterior end near the bulbus. It is also provided with a longitudinal septum, which is no doubt homologous with the septum in the conus arteriosus of the Dipnoi. The bulbus is well developed in many Urodela, but hardly exists in the Anura.

In the Amniota further changes take place in the heart, resulting in the abortion of the distal rows of valves of the conus arteriosus 2 , and in the splitting up of the whole truncus arteriosus into three vessels in Reptilia, and two in Birds and Mammals, each opening into the ventricular section of the heart, and provided with a special set of valves at its commencement. In Birds and Mammals the ventricle becomes moreover completely divided into two chambers, each communicating with one of the divisions of the primitive truncus, known in the higher types as the systemic and pulmonary aortae. The character of the development of the heart in the Amniota will be best understood from a description of what takes place in the Chick.

In Birds the originally straight heart (fig. 109) soon becomes doubled up upon itself. The ventricular portion becomes placed on the ventral and right side, while the auricular section is dorsal and to the left. The two parts are separated from each other by a slight constriction known as the canalis auricularis. Anteriorly the ventricular cavity is continued into the truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out from the auricle at a very early period. The general appearance of the heart, as seen from the ventral side on the fourth day, is shewn in fig. 360. Although the external divisions of the heart are well marked even before this stage, it is not till the end of the third day that the internal partitions become apparent ; and, contrary to what might have been anticipated from the evolution of these parts in the lower types, the ventricular septum is the first to be established.

1 For a good description of the adult heart vide Huxley, Article "Amphibia," in the Encyclopedia Britannic a.

2 It is just possible that the reverse may be true, vide note on p. 640. If however, as is most probable, the statement in the text is correct, the valves at the mouth of the ventricle in Teleostei are not homologous with those of the Amniota ; the former being the distal rov/ of the valves of the conus, the latter the proximal.



640 THE HEART OF AVES.

It commences on the third day as a crescentic ridge or fold springing from the convex or ventral side of the rounded ventricular portion of the heart, and on the fourth day grows rapidly across the ventricular cavity towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement of the truncus arteriosus, and dividing the twisted ventricular tube into two somewhat curved canals, one more to the left and above, the other to

the right and below. These commu- A ^) ) CA

nicate with each other, above the free edge of the partition, along its whole length.

Externally the ventricular portion as yet shews no division into two parts.

By the fifth day the venous end of the heart, though still lying somewhat to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON

forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION

VIEWED FROM THE VENTRAL SURFACE.

organ appearing to be drawn together.

The ventricular septum is complete. L ?.- lef t a , uricular appendage; C.A.

, e .. , . , , canahs auricularis ; v. ventricle ; b. trun The apex of the ventricles becomes cus arteriosus.

more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the auricular septum, while in the canalis auricularis, which is now at its greatest length, there is also to be seen a commencement of the valvular structures tending to separate the cavity of the auricles from those of the ventricles.

About the io6th hour, a septum begins to make its appearance in the truncus arteriosus in the form of a longitudinal fold, which according to Tonge (No. 495) starts at the end of the truncus furthest removed from the heart. It takes origin from the wall of the truncus between the fourth and fifth pairs of arches, and grows downwards in such a manner as to divide the truncus into two channels, one of which leads from the heart to the third and fourth pairs of arches, and the other to the fifth pair. Its course downwards is not straight but spiral, and thus the two channels into which it divides the truncus arteriosus wind spirally the one round the other.

At the time when the septum is first formed, the opening of the truncus arteriosus into the ventricles is narrow or slit-like, apparently in order to prevent the flow of the blood back into the heart. Soon after the appearance of the septum, however, semilunar valves (Tonge, No. 495) are developed from the wall of that portion of the truncus which lies between the free edge of the septum and the cavity of the ventricles 1 .

1 If Tonge is correct in his statement that the semilunar valves develop at some distance from the mouth of the ventricle, it would seem possible that the portion of the truncus between them and the ventricle ought to be regarded as the embryonic conus arteriosus, and that the distal row of valves of the conus (and not the proximal as suggested above, p. 639) has been preserved in the higher types.


THE VASCULAR SYSTEM.


641


The ventral and the dorsal pairs of valves are the first to appear : the former as two small solid prominences separated from each other by a narrow groove ; the latter as a single ridge, in the centre of which is a prominence indicating the point where the ridge will subsequently become divided into two. The outer valves appear opposite each other, at a considerably later period.

As the septum grows downwards towards the heart, it finally reaches the position of these valves. One of its edges then passes between the two ventral valves, and the other unites with the prominence on the dorsal valve-ridge. At the same time the growth of all the parts causes the valves to appear to approach the heart, and thus to be placed quite at the top of the ventricular cavities. The free edge of the septum of the truncus now

A. B.



FlG. 361. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY

OF INCUBATION.

A. from the ventral, B. from the dorsal side.

La. left auricular appendage; r.a. right auricular appendage ; r.v. right ventricle; l.v. left ventricle; b. truncus arteriosus.

fuses with the ventricular septum, and thus the division of the truncus into two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete ; the position of the valves not being very different from that in the adult heart.

That division of the truncus which opens into the fifth pair of arches is the one which communicates with the right ventricle, while that which opens into the third and fourth pairs communicates with the left ventricle. The former becomes the pulmonary artery, the latter the commencement of the systemic aorta.

The external constriction actually dividing the truncus into two vessels does not begin to appear till the septum has extended some way back towards the heart.

The semilunar valves become pocketed at a period considerably later than their first formation (from the H7th to the,i65th hour) in the order of their appearance.

At the end of the sixth day, and even on the fifth day (figs. 361 and 362), the appearance of the heart itself, without reference to the vessels which come from it, is not very dissimilar from that of the adult. The original


B. III.


4 1


642


THE HEART OF MAMMALIA.


r.a



l.v


FIG. 362. HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION, FROM THE VENTRAL SURFACE.

La. left auricular appendage ; r,a. right auricular appendage ; r.v. right ventricle ; l.v. left ventricle ; b. truncus arteriosus.


protuberance to the right now forms the apex of the ventricles, and the two auricular appendages are placed at the anterior extremity of the heart. The most noticeable difference (in the ventral view) is the still externally undivided condition of the truncus arteriosus.

The subsequent changes which the heart undergoes are concerned more with its internal structure than with its external shape. Indeed, during the next three days, viz. the eighth, ninth, and tenth, the external form of the heart remains nearly unaltered.

In the auricular portion, however, the septum which commenced on the fifth day becomes now more conspicuous. It is placed vertically, and arises from the ventral wall ; commencing at the canalis auricularis and proceeding towards the opening into the sinus venosus.

This latter structure gradually becomes reduced so as to become a special appendage of the right auricle. The inferior vena cava

enters the sinus obliquely from the right, so that its blood has a tendency to flow towards the left auricle of the heart, which is at this time the larger of the two.

The valves between the ventricles and auricles are now well developed, and it is about this time that the division of the truncus arteriosus into the aorta and pulmonary artery becomes visible from the exterior.

By the eleventh to the thirteenth day the right auricle has become as large as the left, and the auricular septum much more complete, though there is still a small opening, the foramen ovale, by which the two cavities communicate with each other.

The most important feature in which the development of the Reptilian heart differs from that of Birds is the division of the truncus into three vessels, instead of two. The three vessels remain bound up in a common sheath, and appear externally as a single trunk. The vessel not represented in Birds is that which is continued into the left aortic arch.

In Mammals the early stages in the development of the heart present no important points of difference from those of Aves. The septa in the truncus, in the ventricular, and in the auricular cavities are formed, so far as is known, in the same way and at the same relative periods in both groups. In the embryo Man, the Rabbit, and other Mammals the division of the ventricles is made apparent externally by a deep cleft, which, though evanescent in these forms, is permanent in the Dugong.

The attachment of the auriculo-ventricular valves to the wall of the ventricle, and the similar attachment of the left auriculo-ventricular valves in Birds, have been especially studied by Gegenbaur and Bernays (No. 492),


ARTERIAL SYSTEM. 643


and deserve to be noticed. In the primitive state the ventricular walls have throughout a spongy character ; and the auriculo-ventricular valves are simple membranous projections like the auriculo-ventricular valves of Fishes. Soon however the spongy muscular tissue of both the ventricular and auricular walls, which at first pass uninterruptedly the one into the other, grows into the bases of the valves, which thus become in the main muscular projections of the walls of the heart. As the wall of the ventricle thickens, the muscular trabeculas, connected at one end with the valves, remain at the other end united with the ventricular wall, and form special bands passing between the two. The valves on the other hand lose their muscular attachment to the auricular walls. This is the condition permanent in Ornithorhynchus. In higher Mammalia the ends of the muscular bands inserted into the valves become fibrous, from the development of intermuscular connective tissue, and the atrophy of the muscular elements. The fibrous parts now form the chordae tendinea?, and the muscular the musculi papillares.

The sinus venosus in Mammals becomes completely merged into the right auricle, and the systemic division of the truncus arteriosus is apparently not homologous with that in Birds.

In the embryos of all the Craniata the heart is situated very far forwards in the region of the head. This position is retained in Pisces. In Amphibia the heart is moved further back, while in all the Amniota it gradually shifts its position first of all into the region of the neck and finally passes completely within the thoracic cavity. The steps in the change of position may be gathered from figs. 109, in, and 118.

BIBLIOGRAPHY of the Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." Morphol. Jahrbuch,^o\. II. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f. mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX. 1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), and Balfour (292).

Arterial System.

In the embryos of Vertebrata the arterial system consists of a forward continuation of the truncus arteriosus, on the ventral

41 2


644


ARTERIES OF PISCES.


side of the throat (figs. 363, abr, and 364, a), which, with a few exceptions to be noticed below, divides into as many branches on each side as there are visceral arches. These branches, after traversing the visceral arches, unite on the dorsal side of the throat into a common trunk on each side. This trunk (figs. 363 and 364) after giving off one (or more) vessels to the head (c and c] turns backv/ards, and bends in towards the middle line, close to its fellow, immediately below the notochord (figs. 21 and 116) and runs backwards in this situation towards the end of the tail. The two parallel trunks below the notochord fuse very early into a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}.



ttbr v "a,

FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)

a. auricle ; v. ventricle ; abr. branchial artery ; c'. carotid ; ad. dorsal aorta ; s. branchial clefts; sv. sinus venosus; dc. ductus Cuvieri; n. nasal pit

There is given off from each collecting trunk from the visceral arches, or from the commencement of the dorsal aorta, a subclavian artery to each of the anterior limbs ; from near the anterior end of the dorsal aorta a vitelline artery (or before the dorsal aortae have united a pair of arteries fig. 125, R of A and L of A) to the yolk-sack, which subsequently becomes the main visceral artery 1 ; and from the dorsal aorta opposite the hind limbs one (or two) arteries on each side the iliac arteries to the hind limbs ; from these arteries the allantoic arteries are given off in the higher types, which remain as the hypogastric arteries after the disappearance of the allantois.

The primitive arrangement of the arterial trunks is with a few modifications retained in Fishes. With the development of the gills the vessels to the arches become divided into two parts connected by a capillary system in the gill folds, viz. into the

1 In Mammalia the superior inesenteric artery arises from the vitelline artery, which may probably be regarded as a primitive crclinco-mescnteric artery.


ARTERIAL SYSTEM.


branchial arteries bringing the blood to the gills from the truncus arteriosus, and the branchial veins transporting it to the dorsal aorta. The branchial vessels to those arches which do not bear gills, either wholly or partially atrophy; thus in Elasmobranchii the mandibular trunk, which is fully developed in the embryo (fig. 193, \av}, atrophies, except for a small remnant bringing blood to the rudimentary gill of the spiracle from the branchial vein of the hyoid arch. In Ganoids the mandibular artery atrophies, but the hyoid is usually preserved. In Teleostei both mandibular 1 and hyoid arteries are absent in the adult, except that there is usually left a rudiment of the hyoid, supplying the pseudobranch, which is similar to the rudiment of the mandibular artery in Elasmobranchii. In Dipnoi the mandibular artery atrophies, but the hyoid is sometimes preserved (Protopterus), and sometimes lost.

In Fishes provided with a well developed air-bladder this organ receives arteries, which arise sometimes from the dorsal aorta, sometimes from the caeliac arteries, and sometimes from the dorsal section of the last (fourth) branchial trunk. The latter origin is found in Polypterus and Amia, and seems to have been inherited by the Dipnoi where the air-bladder forms a true lung.

The pulmonary artery of all the air-breathing Vertebrata is derived from the pulmonary artery of the Dipnoi.

In all the types above Fishes considerable changes are effected in the primitive arrangement of the arteries in the visceral arches.

In Amphibia the piscine condition is most nearly retained 2 . The mandibular artery is never developed, and the hyoid artery is imperfect, being only connected with the cephalic vessels and never directly joining the dorsal aorta. It is moreover developed later than the arteries of the true branchial arches behind. The subclavian arteries spring from the common trunks which unite to form the dorsal aorta.

In the Urodela there are developed, in addition to the hyoid,

1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but is distinctly figured in Lereboullet (No. 71).

2 In my account of the Amphibia, Gotte (No. 296) has been followed.


646 ARTERIES OF THE AMNIOTA.

four branchial arteries. The three foremost of these at first supply gills, and in the Perennibranchiate forms continue to do so through life. The fourth does not supply a gill, and very early gives off, as in the Dipnoi, a pulmonary branch.

The hyoid artery soon sends forward a lingual artery from its ventral end, and is at first continued to the carotid which grows forward from the dorsal part of the first branchial vessel.

In the Caducibranchiata, where the gills atrophy, the following changes take place. The remnant of the hyoid is continued entirely into the lingual artery. The first branchial is mainly continued into the carotid and other cephalic branches, but a narrow remnant of the trunk, which originally connected it with the dorsal aorta, remains, forming what is known as a ductus Botalli. A rete mirabile on its course is the remnant of the original gill.

The second and third branchial arches are continued as simple trunks into the dorsal aorta, and the blood from the fourth arch mainly passes to the lungs, but a narrow ductus Botalli still connects this arch with the dorsal aorta.

In the Anura the same number of arches is present in the embryo as in the Urodela, all four branchial arteries supplying branchiae, but the arrangement of the two posterior trunks is different from that in the Urodela. The third arch becomes at a very early period continued into a pulmonary vessel, a relativelynarrow branch connecting it with the second arch. The fourth arch joins the pulmonary branch of the third. At the metamorphosis the hyoid artery loses its connection with the carotid, and the only part of it which persists is the root of the lingual artery. The first branchial artery ceases to join the dorsal aorta, and forms the root of the carotid : the so-called carotid gland placed on its course is the remnant of the gill supplied by it before the metamorphosis.

The second artery forms a root of the dorsal aorta. The third, as in all the Amniota, now supplies the lungs, and also sends off a cutaneous branch. The fourth disappears. The connection of the pulmonary artery with both the third and fourth branchial arches in the embryo appears to me clearly to indicate that this artery was primitively derived from the fonrtli arc/i as in the Urodela, and that its permanent connection


ARTERIAL SYSTEM.


647


with the third arch in the Anura and in all the Amniota is secondary.

In the Amniota the metamorphosis of the arteries is in all cases very similar. Five arches, viz. the mandibular, hyoid, and three branchial arches are always developed (fig. 364), but, owing to the absence of branchiae, never function as branchial arteries. Of these the main parts of the first two, connecting the truncus arteriosus with the collecting trunk into which the arterial arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.

The anterior part of the collecting trunk into which these vessels fall is not obliterated when they disappear, but is on the contrary continued forwards as a vessel supplying the brain, homologous with that found in Fishes. It constitutes the internal carotid. Similarly the anterior part of the trunk from which the mandibular and hyoid arteries sprang is continued forwards as a small vessel 1 , which at first passes to the oral region and constitutes in Reptiles the lingual artery, homologous with the lingual artery of the Amphibia ; but in Birds and Mammals becomes more important, and is then known as the external carotid (fig. 125). By these changes the roots of the external and internal carotids spring respectively from the ventral and dorsal ends of the primitive third artery, i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and thus this arterial arch persists in all types as the common carotid,



FIG. 364. DIAGRAM OF THE ARRANGEMENT OF THE ARTERIAL ARCHES IN AN EMBRYO OF ONE OF THE

AMNIOTA. (From Gegenbaur ; after RATHKE.)

a. ventral aorta; a", dorsal aorta; ' 2 > 3> 4> 5- arterial arches ; c. carotid artery.


1 His (No. 232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery, and one from the hyoid artery, forming the lingual artery. The vessel from which they spring is the external carotid. These observations of His will very probably be found to hold true for other types.


6 4 8


ARTERIAL ARCHES OF THE AMNIOTA.


and the basal part of the internal carotid. The trunk connecting the third arterial arch with the system of the dorsal aorta persists in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but is lost in the remaining Reptiles and in Birds and Mammals (fig. 366 B, C, D). It disappears earliest in Mammals (fig. 365 C), later in Birds (fig. 365 B), and still later in the majority of Reptiles.

The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta.

In all Reptiles it persists on both sides (fig. 366 A and B), but with the division of the truncus arteriosus into three vessels



ad


FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after Rathke.)

The first two arches have disappeared in all three. In A and B the last three are still complete, but in C the last two are alone complete.

/. pulmonary artery springing from the fifth arch, but still connected with the system of the dorsal aorta by a ductus Botalli; c. external carotid; <'. internal carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of fore-limb.

one of these, i.e. that opening furthest to the left side of the ventricle (e and d), is continuous with the right fourth arch, and also with the common carotid arteries (c) ; while a second springing from the right side of the ventricle is continuous with the left fourth arch (Ji and f). The right and left divisions of the fourth arch meet however on the dorsal side of the oesophagus to give origin to the dorsal aorta (g).

In Birds (fig. 366 C) the left fourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains as


ARTERIAL SYSTEM.


649


the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous with all the systemic arteries. Thus it comes about that in Birds the right fourth arch (e) alone gives rise to the dorsal aorta.

In Mammals (fig. 366 D) the truncus arteriosus is only divided into two, but the left fourth arch (>), instead of the right, is that continuous with the dorsal aorta, and the right fourth arch (/) is only continued into the right vertebral and right subclavian arteries.

The fifth arch always gives origin to the pulmonary artery (fig. 365, /) and is continuous with one of the divisions of the truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and Birds (fig. 366 C, i] and probably in Crocodilia, the right and left pulmonary arteries spring respectively from the right and left fifth arches, and during the greater part of embryonic life the parts of the fifth arches between the origins of the pulmonary arteries and the system of the dorsal aorta are preserved as ductus Botalli. These ductus Botalli persist for life in the Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig. 366 D, m) only one of the fifth arches gives origin to the two pulmonary arteries, viz. that on the right side in Ophidia, and the left in Mammalia.

The ductus Botalli of the fifth arch (known in Man as the ductus arteriosus) of the side on which the pulmonary arteries are formed, may remain (e.g. in Man) as a solid cord connecting the common stem of the pulmonary aorta with the systemic aorta.

The main history of the arterial arches in the Amniota has been sufficiently dealt with, and the diagram, fig. 366, copied from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely remains for me to say a few words about the subclavian and vertebral arteries.

The subclavian arteries in Fishes usually spring from the trunks connecting the branchial veins with the dorsal aorta. This origin, which is also found in Amphibia, is typically found in the embryos of the Amniota. In the Lizards this origin persists through life, but both subclavians spring from the right


650


ARTERIAL ARCHES OF THE AMNIOTA.


side. In most other types the origin of the subclavians is carried upwards, so that they usually spring from a trunk common to them and the carotids (arteria anonyma) (Birds and some Mammals); or the left one, as in Man and some other Mammals, arises from the systemic aorta just beyond the carotids. Various further modifications in the origin of the subclavians of the same general nature are found in Mammalia, A 13



FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL

ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From Mivart ; after Rathke.)

A. a. internal carotid; b. external carotid ; c. common carotid; d. ductus Botalli between the third and fourth arches ; e. right aortic trunk ; /. subclavian ; g. dorsal aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli between the pulmonary artery and the system of the dorsal aorta.

B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic trunk; e. vertebral artery;/, left aortic trunk of dorsal aorta; h. pulmonary artery ; i. ductus Botalli of pulmonary artery.

C. a. internal carotid ; b. external carotid ; c. common carotid ; d. systemic aorta; e. fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal aorta; h, left subclavian (fourth arch of left side); i. pulmonary artery; k. and /. right and left ductus Botalli of pulmonary arteries.

D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k. right vertebral; /. continuation of right subclavian; in. pulmonary artery; n. ductus Botalli of pulmonary artery.


THE VENOUS SYSTEM.


6 5 I


but they need not be specified in detail. The vertebral arteries usually arise in close connection with the subclavians, but in Birds they arise from the common carotids.

BIBLIOGRAPHY of the Arterial System.

(496) H. Rathke. " Ueb. d. Entwick. d. Arterien vv. bei d. Saugethiere von d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(-197) H. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier." Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

TJie Venous System,.

The venous system, as it is found in the embryos of Fishes, consists in its earliest condition of a single large trunk, which traverses the splanchnic mesoblast investing the part of the alimentary tract behind the heart. This trunk is directly continuous in front with the heart, and underlies the alimentary canal through both its praeanal and postanal sections. It is shewn in section in fig. 367, v, and may be called the subintestinal vein. This vein has been found in the embryos of Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs parallel to the dorsal aorta above, into which it is sometimes continued behind (Teleostei, Ganoidei, etc.).

In Elasmobranch embryos the subintestinal vein terminates, as may be gathered from sections (fig. 368, v.cau), shortly before the end of the tail. The same series of sections also shews that at the cloaca, where the gut enlarges and comes in contact with the skin, this vein bifurcates, the two branches uniting into a single vein both in front of and behind the cloaca.

In most Fishes the anterior part of this vein atrophies, the caudal section alone remaining, but the anterior section of it persists in the fold of the intestine in Petromyzon, and also remains in the spiral valve of some Elasmobranchii. In Amphioxus, moreover, it forms, as in the embryos of higher types, the main venous trunk, though even here it is usually broken up into two or three parallel vessels.

It no doubt represents one of the primitive longitudinal trunks of the vermiform ancestors of the Chordata. The heart and the branchial artery constitute a specially modified anterior continuation of this vein. The


652


THE SUBINTESTINAL VEIN.


-p.o


rp.r.


dilated portal sinus of Myxine is probably also part of it ; and if this is really rhythmically contractile 1 the fact would be interesting as shewing that this quality, which is now localised in the heart, was once probably common to the subintestinal vessel for its whole length.

On the development of the cardinal veins (to be described below) considerable changes are effected in the subintestinal vein. Its postanal section, which is known in the adult as the caudal vein, unites with the cardinal veins. On this junction being effected retrogressive changes take place in the praeanal section of the original subintestinal vessel. It breaks up in front into a number of smaller vessels, the most important of which is a special vein, which lies in the fold of the spiral valve, and which is more conspicuous in some Elasmobranchii than in Scyllium, in which the development of the vessel has been mainly studied. The lesser of the two branches connecting it round the cloaca with the caudal vein first vanishes, and then the larger ; and the two posterior cardinals are left as the sole forward continuations of the caudal vein. The latter then becomes prolonged forwards, so that the two cardinals open into it some little distance in front of the hind end of the kidneys. By these changes, and by the disappearance of the postanal section of the gut, the caudal vein is made to appear as a supraintestinal and not, as it really is, a subintestinal vessel.

From the subintestinal vein there is given off a branch which supplies the yolk-sack. This leaves the subintestinal vein close

1 J. Miiller holds that this sack is not rhythmically contractile.



FIG. 367. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle plate; ;;//'. inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body; st. segmental tube ; sd. segmental duct ; sp.v. spiral valve; v. subintestinal vein ; p.o. primitive generative cells.


THE VENOUS SYSTEM.


653


to the liver. The liver, on its development, embraces the subintestinal vein, which then breaks up into a capillary system in the liver, the main part of its blood coming at this period from the yolk-sack.

The portal system is thus established from the subintestinal vein ; but is eventually joined by the various visceral, and sometimes by the genital, veins as they become successively developed.

The blood from the liver is brought back to the sinus venosus by veins known as the hepatic veins, which, like the hepatic capillary system, are derivatives of the subintestinal vessel.

There join the portal system in Myxinoids and many Teleostei a number of veins from the anterior abdominal walls, representing a commencement of the anterior abdominal or epigastric vein of higher types 1 .

In the higher Vertebrates the original subintestinal vessel never attains a full development, even in the embryo. It is represented by (i) the ductus



FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x. subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta; v.cait. caudal vein.

1 Stannius, Vergleich. Anat., p. 251.


654


THE CARDINAL VEINS.


venosus, which, like the true subintestinal vein, gives origin (in the Amniota) to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether the partial atrophy of the subintestinal vessel was primitively caused by the development of the cardinal veins, or for some other reason, it is at any rate a fact that in all existing Fishes the cardinal veins form the main venous channels of the trunk.

Their later development than the subintestinal vessel as well as their absence in Amphioxus, probably indicate that they became evolved, at any rate in their present form, within the Vertebrate phylum.

The embryonic condition of the venous system, with a single large subintestinal vein is, as has been stated, always modified by the development of a paired system of vessels, known as the cardinal veins, which bring to the heart the greater part of the blood from the trunk.

The cardinal veins appear in Fishes as four paired longitudinal trunks (figs. 363 and 369), two anterior (/) and two posterior (c). They unite into two transverse trunks on either side, known as the ductus Cuvieri (dc), which fall into the sinus venosus, passing from the body wall to the sinus by a lateral mesentery of the heart already spoken of (p. 627, fig. 352). The anterior pair, known as the anterior cardinal or jugular veins, bring to the heart the blood from the head and neck. They are placed one on each side above the level of the branchial arches (fig. 299, a.cv). The posterior cardinal veins lie immediately dorsal to the mesonephros (Wolfifian body), and are mainly supplied by the blood from this organ and from the walls of the body (fig. 275, c.a.v). In many forms (Cyclostomata, Elasmobranchii and many Teleostei) they unite posteriorly with the caudal veins in the manner already described, and in a large number of instances the connecting branch between the two systems, in its passage through the mesonephros, breaks up into a capillary network, and so gives rise to a renal portal system.

The vein from the anterior pair of fins (subclavian) usually unites with the anterior jugular vein.



j


FIG. 369. DIAGRAM OF THE PAIRED VENOUS SYSTEM

OF A FISH. (From Gegenbaur. )

j. jugular vein (anterior cardinal vein) ; c. posterior cardinal vein; //. hepatic veins ; sv. sinus venosus ; dc. ductus Cuvieri.


THE VENOUS SYSTEM. 655

The venous system of the Amphibia and Amniota always differs from that of Fishes in the presence of a new vessel, the vena cava inferior, which replaces the posterior cardinal veins; the latter only being present, in their piscine form, during embryonic life. It further differs from that of all Fishes, except the Dipnoi, in the presence of pulmonary veins bringing back the blood directly from the lungs.

In the embryos of all the higher forms the general characters of the venous system are at first the same as in Fishes, but with the development of the vena cava inferior the front sections of the posterior cardinal veins atrophy, and the ductus Cuvieri, remaining solely connected with the anterior cardinals and their derivatives, constitute the superior venae cavae. The inferior cava receives the hepatic veins.

Apart from the non-development of the subintestinal vein the visceral section of the venous system is very similar to that in Fishes.

The further changes in the venous system must be dealt with separately for each group.

Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior cardinal veins arise as in Pisces. From the former the internal jugular vein arises as a branch ; the external jugular constituting the main stem. The subclavian with its large cutaneous branch also springs from the system of the anterior cardinal. The common trunk formed by the junction of these three veins falls into the ductus Cuvieri.

The posterior cardinal veins occupy the same position as in Pisces, and unite behind with the caudal veins, which Gotte has shewn to be originally situated below the post-anal gut. The iliac veins unite with the posterior cardinal veins, where the latter fall into the caudal vein. The original piscine condition of the veins is not long retained. It is first of all disturbed by the development of the anterior part of the important unpaired venous trunk which forms in the adult the vena cava inferior. This is developed independently, but unites behind with the right posterior cardinal. From this point backwards the two cardinal veins coalesce for some distance, to give rise to the posterior section of the vena cava inferior, situated between the kidneys 1 . The anterior sections of the cardinal veins subsequently atrophy. The posterior part of the cardinal veins, from their junction with the vena cava inferior to the caudal veins, forms a rhomboidal figure. The iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitu 1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and cannot be considered as completely established.


656 VEINS OF THE SNAKE.

dinal vessel on the outer border of the kidneys, which receives transverse vertebral veins and transmits their blood to the kidneys, thus forming a renal portal system. The anterior limbs of the rhomboid formed by the cardinal veins soon atrophy, so that the blood from the hind limbs can only pass to the inferior vena cava through the renal portal system. The posterior parts of the two cardinal veins (uniting in the Urodela directly with the unpaired caudal vein) still persist. The iliac veins also become directly connected with a new vein, the anterior abdominal vein, which has meanwhile become developed. Thus the iliac veins become united with the system of the vena cava inferior through the vena renalis advehens on the outer border of the kidney, and with the anterior abdominal veins by the epigastric veins.

The visceral venous system begins with the development of two vitelline veins, which at first join the sinus venosus directly. They soon become enveloped in the liver, where they break up into a capillary system, which is also joined by the other veins from the viscera. The hepatic system has in fact the same relations as in Fishes. Into this system the anterior abdominal vein also pours itself in the adult. This vein is originally formed of two vessels, which at first fall directly into the sinus venosus, uniting close to their opening into the sinus with a vein from the truncus arteriosus. They become prolonged backwards, and after receiving the epigastric veins above mentioned from the iliac veins, and also veins from the allantoic bladder, unite behind into a single vessel. Anteriorly the right vein atrophies and the left continues forward the unpaired posterior section.

A secondary connection becomes established between the anterior abdominal vein and the portal system ; so that the blood originally transported by the former vein to the heart becomes diverted so as to fall into the liver. A remnant of the primitive connection is still retained in the adult in the form of a small vein, the so-called vena bulbi posterior, which brings the blood from the walls of the truncus arteriosus directly into the anterior abdominal vein.

The pulmonary veins grow directly from the heart to the lungs.

For our knowledge of the development of the venous system of the Amniota we are mainly indebted to Rathke.

Reptilia. As an example of the Reptilia the Snake may be selected, its venous system having been fully worked out by Rathke in his important memoir on its development (No. 300).

The anterior (external jugular) and posterior cardinal veins are formed in the embryo as in all other types (fig. 370, vj and vc] ; and the anterior cardinal, after giving rise to the anterior vertebral and to the cephalic veins, persists with but slight modifications in the adult ; while the two ductus Cuvieri constitute the superior venos cavas.

The two posterior cardinals unite behind with the caudal veins. They are placed in the usual situation on the dorsal and outer border of the kidneys.


THE VENOUS SYSTEM.


657



U FIG. 370. ANTERIOR PORTION OF THE VENOUS SYSTEM OF AN EMBRYONIC SNAKE. (From Gegenbaur; after Rathke.)

vc. posterior cardinal vein; vj. jugular vein; DC. ductus Cuvieri ; vu. allantoic vein ; v. ventricle ; ba. truncus arteriosus ; a. visceral clefts ; /. auditory vesicle.


With the development of the vena cava inferior, to be described below, the blood from the kidneys becomes mainly transported by this vessel to the heart ; and the section of the posterior cardinals opening into the ductus Cuvieri gradually atrophies, their posterior parts remaining however on the outer border of the kidneys as the vena? renales advehentes 1 .

While the front part of the posterior cardinal veins is undergoing atrophy, the intercostal veins, which originally poured their blood into the posterior cardinal veins, become also connected with two longitudinal veins the posterior vertebral veins which are homologous with the azygos and hemiazygos veins of Man ; and bear the same relation to the anterior vertebral veins that the anterior and posterior cardinals do to each other.

These veins are at first connected by trans verse anastomoses with the posterior cardinals, but, on the disappearance of the front part of the latter, the whole of the blood from the intercostal veins falls into the posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into the jugular vein.

The posterior vertebral veins are at first symmetrical, but after becoming connected by transverse anastomoses, the right becomes the more important of the two.

The vena cava inferior, though considerably later in its development than the cardinals, arises fairly early. It constitutes in front an unpaired trunk, at first very small, opening into the right allantoic vein, close to the heart. Posteriorly it is continuous with two veins placed on the inner border of the kidneys 2 .

The vena cava inferior passes through the dorsal part of the liver, and in doing so receives the hepatic veins.

The portal system is at first constituted by the vitelline vein, which is directly continuous with the venous end of the heart, and at first receives the two ductus Cuvieri, but at a later period unites with the left ductus.

1 Rathke's account of the vena renalis advehens is thus entirely opposed to that which Gotte gives for the Frog, but my own observations on the Lizard incline me to accept Rathke's statements, for the Amniota at any rate.

2 The vena cava inferior does not according to Rathke's account unite behind with the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Gb'tte questions the accuracy of Rathke's statements on this head, but my own observations are entirely in favour of Rathke's observations, and lend no support whatever to Gotte's views.


B. III.


658 VEINS OF THE CHICK.

It soon receives a mesenteric vein bringing the blood from the viscera, which is small at first but rapidly increases in importance.

The common trunk of the vitelline and mesenteric veins, which may be called the portal vein, becomes early enveloped by the liver, and gives off branches to this organ, the blood from which passes by the hepatic veins to the vena cava inferior. As the branches in the liver become more important, less and less blood is directly transported to the heart, and finally the part of the original vitelline vein in front of the liver is absorbed, and the whole of the blood from the portal system passes from the liver into the vena cava inferior.

The last section of the venous system to be dealt with is that of the anterior abdominal vein. There are originally, as in the Anura, two veins belonging to this system, which owing to the precocious development of the bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}.

These veins, running along the anterior abdominal wall, are formed somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. They unite with two epigastric veins (homologous with those in the Anura), which connect them with the system of the posterior cardinal veins. The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein. This vein at first receives the vena cava inferior close to the heart, but eventually the junction of the two takes place in the region of the liver, and finally the anterior abdominal vein (as it comes to be after the atrophy of the allantois) joins the portal system and breaks up into capillaries in the liver 1 .

In Lizards the iliac veins join the posterior cardinals, and so pour part of their blood into the kidneys ; they also become connected by the epigastric veins with the system of the anterior abdominal or allantoic vein. The subclavian veins join the system of the superior venae cavas.

The venous system of Birds and Mammals differs in two important points from that of Reptilia and Amphibia. Firstly the anterior abdominal vein is only a foetal vessel, forming during foetal life the allantoic vein ; and secondly a direct connection is established between the vena cava inferior and the veins of the hind limbs and posterior parts of the cardinal veins, so that there is no renal portal system.

Aves. The Chick may be taken to illustrate the development of the venous system in Birds.

On the third day, nearly the whole of the venous blood from the body of the embryo is carried back to the heart by two main venous trunks, the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on each side to form the short transverse ductus Cuvieri (DC), both of which unite with the sinus venosus close to the heart. As the head and neck continue to enlarge, and the wings become developed, the single anterior

1 The junction between the portal system and the anterior abdominal vein is apparently denied by Rathke (No. 300, p. 173), hut this must he an error on his part.


THE VENOUS SYSTEM.


659



V.C.L


cardinal or jugular vein (fig. 371, /), of each side, is joined by two new veins : the vertebral vein, bringing back blood from the head and neck, and the subclavian vein from the wing (W\

On the third day the posterior cardinal veins are the only veins which return the blood from the hinder part of the body of the embryo.

About the fourth or fifth day, however, the vena cava inferior (fig. 371, V.C.L) makes its appearance. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running backward in the middle line below the aorta, and speedily losing itself in the tissues of the Wolffian bodies. When the true kidneys are formed it also receives blood from them, and thenceforward enlarging rapidly becomes the channel by which the greater part of the blood from the hinder part of the body finds its way to the heart. In proportion as the vena cava inferior increases in size, the posterior cardinal veins diminish.

The blood originally coming to them from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins, similar to those in Reptilia, which are however placed dorsally to the heads of the ribs, and join the anterior vertebral veins. With their appearance the anterior parts of the posterior cardinals disappear. The blood from the hind limbs becomes transported directly through the kidney into the vena cava inferior, without forming a renal portal system 1 .

On the third day the course of the vessels from the yolk-sack is very simple. The two vitelline veins, of which the right is already the smaller, form the ductus venosus, from which, as it passes through the liver on its way to the heart, are given off the two sets of vena advehentes and vena revehentes (fig. 371).

With the appearance of the allantois on the fourth day, a new feature is introduced. From the ductus venosus there is given off a vein which quickly divides into two branches. These, running along the ventral walls of the body from which they receive some amount of blood, pass to the allantois. They are the allantoic veins (fig. 371, U] homologous with the anterior abdominal vein of the lower types. They unite in front to form a single vein, which becomes, by reason of the rapid growth of the allantois, very long. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates ; and, its two


FIG. 371. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK AT THE COMMENCEMENT OF THE FIFTH

DAY.

H. heart ; d. c. ductus Cuvieri. Into the ductus Cuvieri of each side fall/, the jugular vein, W. the vein from the wing, and c. the inferior cardinal vein ; S. V. sinus venosus ; Of. vitelline vein ; U. allantoic vein, which at this stage gives off branches to the bodywalls ; V.C.l. inferior vena cava ; /. liver.


The mode in which this is effected requires further investigation.

42 2


66o


VEINS OF THE CHICK.



branches becoming large and conspicuous, there still appear to be two main allantoic veins. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.

On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more developed, it acquires a distinct venous system, and its blood is returned by veins which form a trunk, the mesenteric vein (fig. 372, M") falling into the vitelline vein at its junction with the allantoic vein.

These three great veins, in fact, form a large common trunk, which enters at once into the liver, and which we may now call the portal vein (fig. 372, P. V}. This, at its entrance into the liver, partly breaks up into the vena advehentes, and partly continues as the ductus venosus (D.V} straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the venas revehentes, carrying back the blood which circulates through the hepatic capillaries, join the ductus venosus close to its exit from the liver. By the time however that the vena cava has become a large and important vessel it is found that the venae revehentes, or as we may now call them the hepatic veins, have shifted their embouchment, and now fall directly into that vein, the ductus venosus making a separate junction rather higher up (fig. 372).

This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels ; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes obliterated ; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the hepatic veins into the vena cava.

Although the allantoic (anterior abdominal) vein is obliterated in the adult, there is nevertheless established an anastomosis between the portal system and the veins bringing the blood from the limbs to the vena cava


FIG. 372. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK DURING THE LATER DAYS OF INCUBATION.

H. heart ; V.S.R. right vena cava superior; V.S.L. left vena cava superior. The two venas cavrc superiores are the original 'ductus Cuvieri,' they open into the sinus venosus. J. jugular vein; Su.V. anterior vertebral vein ; In. V. inferior vertebral vein ; W. subclavian; V.C.I, vena cava inferior; D. V. ductus venosus ; P. V. portal vein ; M. mesenteric vein bringing blood from the intestines into the portal vein ; O.f. vitelline vein ; U. allantoic vein. The three last mentioned veins unite together to form the portal vein ; /. liver.


THE VENOUS SYSTEM.


66l


inferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as the coccygeo-mesenteric vein, which joins the portal vein ; while at the same time the posterior pelvic veins are connected with the common iliac veins by a vessel which unites with them close to their junction with the coccygeo-mesenteric vein.

Mammalia. In Mammals the same venous trunks are developed in the embryo as in other types (fig. 373 A). The anterior cardinals or external jugulars form the primitive veins of the anterior part of the body, and the internal jugulars and anterior vertebrals are subsequently formed. The subclavians (fig. 373 A, j), developed on the formation of the anterior limbs, also pour their blood into these primitive trunks. In the lower Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc., the two ductus Cuvieri remain as the two superior venae cavae, but more usually an anastomosis arises between the right and left innominate veins, and eventually the whole of the blood of the left superior cava is carried to the right side, and there is left only a single superior cava (fig. 373 B and C).



F IG - 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF

MAMMALS (MAN). (From Gegenbaur.)

j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein.

A. Stage in which the cardinal veins have already disappeared. Their position is indicated by dotted lines.

B. Later stage when the blood from the left jugular vein is carried into the right to form the single vena cava superior ; a remnant of the left superior cava being however still left.

C. Stage after the left vertebral vein has disappeared; the right vertebral remaining as the azygos vein. The coronary vein remains as the last remnant of the left superior vena cava.

A small rudiment of the left superior cava remains however as the sinus coronartus and receives the coronary vein from the heart (figs. 373 C, cor and 374, cs).

The posterior cardinal veins form at first the only veins receiving the


662


THE VEINS OF MAMMALIA.


blood from the posterior part of the trunk and kidneys ; and on the development of the hind limbs receive the blood from them also.

As in the types already described an unpaired vena cava inferior becomes eventually developed, and gradually carries off a larger and larger portion of the blood originally returned by the posterior cardinals. It unites with the common stem of the allantoic and vitelline veins in front of the liver.

At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior (fig. 374, il}. These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins (fig. 374, hy). Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia.

Posterior vertebral veins, similar to those of Reptilia and Birds, are established in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior



FIG. 374. DIAGRAM OF THE CHIEF

VENOUS TRUNKS OF MAN. (From Gegenbaur.)

cs. vena cava superior ; s. subclavian vein ; ji. internal jugular ; je. external jugular ; az. azygos vein ; ha. hemiazygos vein ; c. clotted line shewing previous position of cardinal veins ; ci. vena cava inferior ; r. renal veins ; il. iliac ; hy. hypogastric veins ; h. hepatic veins.

The dotted lines shew the position of embryonic vessels aborted in the adult.


cardinal veins (fig. 373 A) 1 .

On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374, f), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins (Ay).

Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein (fig. 374, az), the persisting part of the left forming the hemiazygos vein (ha}.

The remainder of the venous system is formed in the embryo of the vitelline and allantoic veins, the former being eventually joined by the mesenteric vein so as to constitute the portal vein.

1 Rathke, as mentioned above, holds that in the Snake the front part of the posterior cardinals completely aborts. Further investigations are required to shew whether there really is a difference between Mammalia and Reptilia in this matter.




THE VENOUS SYSTEM. 663

The vitelline vein is the first part of this system established, and divides near the heart into two veins bringing back the blood from the yolk-sack (umbilical vesicle). The right vein soon however aborts.

The allantoic (anterior abdominal) veins are originally paired. They are developed very early, and at first course along the still widely open somatic walls of the body, and fall into the single vitelline trunk in front. The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.

The succeeding changes have been somewhat differently described by Kolliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii.

With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver.

The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava.

At the close of foetal life the allantoic vein becomes obliterated up to its place of entrance into the liver ; the ductus venosus becomes a solid cord the so-called round ligament and the whole of the venous blood is brought to the liver by the portal vein 1 .

Owing to the allantoic (anterior abdominal) vein having merely a fcetal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established.


BIBLIOGRAPHY of the Venous System.

(498) J. Marshall. "On the development of the great anterior veins." Phil. Trans., 1859.

(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Saugethieren." MeckeVs Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere." Bericht. Jib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke (Nos. 299, 300, and 301).

1 According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a secondary connection established in the latter part of foetal life.


664 LYMPHATIC SYSTEM.


Lymphatic System.

The lymphatic system arises from spaces in the general parenchyma of the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.

In all the true Vertebrata certain parts of the system form definite trunks communicating with the venous system ; and in the higher types the walls of the main lymphatic trunks become quite distinct.

But little is known with reference to the ontogeny of the lymphatic vessels, but they originate late in larval life, and have at first the form of simple intercellular spaces.

The lymphatic glands appear to originate from lymphatic plexuses, the cells of which produce lymph corpuscles. It is only in Birds and Mammals, and especially in the latter, that the lymphatic glands form definite structures.

The Spleen. The spleen, from its structure, must be classed with the lymphatic glands, though it has definite relations to the vascular system. It is developed in the mesoblast of the mesogastrium, usually about the same time and in close connection with the pancreas.

According to Miiller and Peremeschko the mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which uniting with like processes from other cells form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these cells take place at a later period to form the so-called Malpighian corpuscles of the spleen.

BIBLIOGRAPHY of Spleen.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad. Wiss., Vol. LVI. 1867.

Suprarenal ^bodies.

In Elasmobranch Fishes two distinct sets of structures are found, both of which have been called suprarenal bodies. As shewn in the sequel both of these structures probably unite in the higher types to form the suprarenal bodies.

One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentally arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they are


SUPRARENAL BODIES. 665

closely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body.

The second body consists of an unpaired column of cells placed between the dorsal aorta and unpaired caudal vein, and bounded on each side by the posterior parts of the kidney. I propose to call it the interrenal body. In front it overlaps the paired suprarenal bodies, but does not unite with them. It is formed of a series of well-marked lobules, etc. In the fresh state Leydig (No. 506) finds that "fat molecules form the chief mass of the body, and one finds freely imbedded in them clear vesicular nuclei." As may easily be made out from hardened specimens it is invested by a tunica propria, which gives off septa dividing it into well-marked areas filled with polygonal cells. These cells constitute the true parenchyma of the body. By the ordinary methods of hardening, the oil globules, with which they are filled in the fresh state, completely disappear.

The paired suprarenal bodies (Balfour, No. 292, pp. 242 244) are developed from the sympathetic ganglia. These ganglia, shewn in an early stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a glandular part. The former constitutes the sympathetic ganglia of the adult ; the latter the true paired suprarenal bodies. The interrenal body is however developed (Balfour, No. 292, pp. 245 247) from indifferent mesoblast cells between the two kidneys, in the same situation as in the adult.

The development of the suprarenal bodies in the Amniota has been most fully studied by Braun (No. 503) in the Reptilia.

In Lacertilia they consist of a pair of elongated yellowish bodies, placed between the vena renalis revehens and the generative glands.

They are formed of two constituents, viz. (i) masses of brown cells placed on the dorsal side of the organ, which stain deeply with chromic acid, like certain of the cells of the suprarenals of Mammalia, and (2) irregular cords, in part provided with a lumen, filled with fat-like globules l , amongst which are nuclei. On treatment with chromic acid the fat globules disappear, and the cords break up into bodies resembling columnar cells.

The dorsal masses of brown cells are developed from the sympathetic ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent mesoblast cells as a thickening in the lateral walls of the inferior vena cava, and the cardinal veins continuous with it. The observations of Brunn (No. 504) on the Chick, and Kolliker (No. 298, pp. 953955) n the Mammal, add but little to those of Braun. They shew that the greater part of the gland (the cortical substance) in these two types is derived from the mesoblast, and that the glands are closely connected with sympathetic ganglia ; while Kolliker also states that the posterior part of the organ is unpaired in the embryo rabbit of 1 6 or 17 days.

The structure and development of what I have called the interrenal body

1 These globules are not formed of a true fatty substance, and this is also probably true for the similar globules of the interrenal bodies of Elasmobranchii.


666 SUPRARENAL BODIES.

in Elasmobranchii so closely correspond with that of the mesoblastic part of the suprarenal bodies of the Reptilia, that I have very little hesitation in regarding them as homologous 1 ; while the paired bodies in Elasmobranchii, derived from the sympathetic ganglia, clearly correspond with the part of the suprarenals of Reptilia having a similar origin ; although the anterior parts of the paired suprarenal bodies of Fishes have clearly become aborted in the higher types.

In Elasmobranch Fishes we thus have (i) a series of paired bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part.

BIBLIOGRAPHY of the Suprarenal bodies,

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien. " Arbeit, a. d. zool.-zoot. Institut Wurzlttrg, Vol. V. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." Archiv f. mikr. Anat., Vol. VIII. 1872.

(505) Fr. Leydig. Untersiich. iib. Fische u. fieptilten. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

1 The fact of the organ being unpaired in Elasmobranchii and paired in the Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired in the Rabbit.


CHAPTER XXII.


THE MUSCULAR SYSTEM.



IN all the Ccelenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of ectodermal or entodermal epithelial cells (figs. 375 and 376 B). The elements provided with these processes, which were first discovered by Kleinenberg, are known as myo-epithelial cells. Their contractile parts may either be striated (fig. 376) or non-striated (fig. 375). In some instances the epithelial part of the cell may nearly abort, its nucleus alone remaining (fig. 376 A) ; and in this way a layer of muscles lying completely below the surface may be established.

There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myo-epithelial cells of this kind. The more important of these groups are the Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and the Vertebrata 1 .

While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myo-epithelial cells, the mode of evolution of the

1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amoeboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vasoperitoneal vesicles are provided with muscular tails.


FIG. 375. MYO-EPITHELIAL CELLS OF HYDRA. (From Gegenbaur ; after Kleinenberg.)

m. contractile fibres.


668 THE MUSCULAR FIBRES.

muscular system of other types is still very obscure. The muscles may arise in the embryo from amoeboid or indifferent cells, and the Hertwigs 1 hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology.

The voluntary muscular system of the CJiordata.

The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myo-epithelial type. The embryonic muscle-cells are at first simple epithelial cells, but



FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after O. and R. Hertwig.)

A. Muscle-cell from the circular fibres of the subumbrella.

B. Myo-epithelial cells from the base of a tentacle.

soon become spindle-shaped : part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon in fig. 57.

The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which,

1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881.


THE MUSCULAR SYSTEM.


669


t>r



except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma.

The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites.

It has already been stated (pp. 292 ^296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elasmobranchii it will be convenient to commence with this group. Each somite is composed of two layers a somatic and a splanchnic both formed of a single row of columnar cells. Between these two layers is a cavity, which is at first directly continuous with the general body cavity, of which indeed it merely forms a specialised part (fig. 377). Before long the cavity becomes however completely constricted off from the permanent body cavity.

Very early (fig. 377) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove (% 378) that the cells in this situation (mp') have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and produce


FIG. 377. TRANSVERSE SECTION THROUGH THETRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

nc. neural canal ; pr. posterior root of spinal nerve ; x. subnotochordal rod ; ao. aorta ; sc. somatic mesoblast ; sf>. splanchnic mesoblast ; mp. muscle-plate ; mp', portion of muscle-plate converted into muscle ; Vr. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract.


THE MUSCLE-PLATES.


a mass of cells, placed between the muscles and the notochord ( Vr\ These cells form the commencing vertebral bodies, and have at first (fig. 378) the same segmentation as the somites from which they sprang.

After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates ; since they become directly converted into the whole voluntary muscular system of the trunk (fig. 379, mp}.

According to the statements of Bambeke and Go'tte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates (fig. 79). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle- plate of other Vertebrata, and exhibit a similar segmentation.

Osseous Fishes are stated to agree with Amphibians in the development of their somites and muscular system 1 , but further observations on this point are required.

In Birds the horizontal splitting of the mesoblast extends at first to the dorsal summit of the mesoblastic plates, but after the isolation of the somites the split between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part to persist. The somites on the second day, as seen in a transverse section (fig. 115, P.?'.), are somewhat quadrilateral in form but broader than they are deep.

Each at that time consists of a somewhat thick cortex of radi


FlG. 378. HORIZONTALSECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLL1UM CONSIDERABLY YOUNGER THAN 28 F.


The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.

ch. notochord ; ep. epiblast ; Vr, rudiment of vertebral body ; mp. muscle- plate ; mp' . portion of muscle-plate already differentiated into longitudinal muscles.


1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal., Vol. XI.


THE MUSCULAR SYSTEM. 671

ating rather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or lateral as they are at this period) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the upper and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16). The muscle-plate when formed (fig. 117) is found to consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity ; and no sooner is the muscle-plate formed than the middle portion of the inner layer becomes converted into longitudinal muscles. The avian muscle-plates have, in fact, precisely the same constitution as those of Elasmobranchii. The central space is clearly a remnant of the vertebral portion of the body cavity, which, though it wholly or partially disappears in a previous stage, reappears again on the formation of the muscle-plate.

The remainder of the somite, after the formation of the muscle-plate, is of very considerable bulk ; the cells of the cortex belonging to it lose their distinctive characters, and the major part of it becomes the vertebral rudiment.

In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to the dorsal summit of the mesoblastic plates, and the dorsal portions of the plates with their contained cavities become divided into somites, and are then separated off from the ventral. The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost. The further development resembles that in Birds.

The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements, take place on quite the same type throughout the Vertebrata, yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are important common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (i) the extension of the body cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscleplates ; (2) the primitive division of the vertebral plate into an outer (somatic) and an inner (splanchnic) layer, and the formation of a large part of the voluntary muscular system out of the inner


THE MUSCLE-PLATES.


sp.c


layer, which in all cases is converted into muscles earlier than the outer layer.

The conversion of the muscle-plates into muscles. It

will be convenient to commence this subject with a description of the changes which take place in such a simple type as that of the Elasmobranchii.

At the time when the muscleplates have become independent structures they form flat two-layered oblong bodies enclosing a slit-like central cavity (fig. 379, mp). The outer or somatic wall is formed of simple epithelial -like cells. The inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity of the plates the epithelium forming the remainder of the layer commences to insert itself; so that between the first-formed muscle and the cavity of the muscle-plate there appears a thin layer of cells, not however continuous throughout.

When first formed the muscleplates, as viewed from the exterior, have nearly straight edges ; soon however they become bent in the middle, so that the edges have an obtusely angular form, the apex of the angle being directed forwards. They are so arranged that the anterior edge of the one plate fits into the posterior edge of the one in front. In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa.

The growth of the plates is very rapid, and their upper ends



FIG. 379. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate; mp' . inner layer of muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; si. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; z/. subintestinal vein ; P.O. primitive generative cells.


THE MUSCULAR SYSTEM. 673

soon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, mp'). It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness by the addition of fresh spindle-shaped muscle-cells from the somatic layer as well as by the division of the already existing cells.

Thus both layers of the muscle-plate are concerned in forming the great longitudinal lateral muscles, though the splanchnic layer is converted into muscles very much sooner than the somatic 1 .

Each muscle-plate is at first a continuous structure, extending from the dorsal to the ventral surface, but after a time it becomes divided by a layer of connective tissue, which becomes developed nearly on a level with the lateral line, into a dorso-lateral and a ventro-lateral section. The ends of the muscle-plates continue for a long time to be formed of undifferentiated columnar cells. The complicated outlines of the inter-muscular septa become gradually established during the later stages of development, causing the well-known appearances of the muscles in transverse sections, which require no special notice here.

The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs (fig. 346). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs (fig. 380). Small portions of several muscle-plates (m.pl) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original dis 1 The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (vide fig. 57, mp) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles.

B. in. 43


674


THE MUSCLE-PLATES.


3,-n,



FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.

The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length ; whereas they join the spinal cord half-way between two posterior roots.

sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally directed nerve springing from posterior root; nip. muscle-plate; mp'. part of muscleplate already converted into muscles; vi.pl. part of muscle-plate which gives rise to the muscles of the limbs; /. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric) duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of hepatic duct with duodenum ; umc. umbilical canal.


THE MUSCULAR SYSTEM. 675

tinctness. There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscleplates themselves, after giving off buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds.

In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out.

It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish- like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out.

In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles.

The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive inter-muscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles.

In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kolliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.

432


676 THE HEAD-CAVITIES.


Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.

It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kolliker however believes that they have an independent origin.

The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscleplates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Gotte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs.

In the higher Vertebrata on the other hand the entrance of the muscleplates into the limbs has not been made out (Kolliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscleplates.

TJte Somites and muscular system of the head.

The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent to the somites of the trunk : they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii.

The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head ; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior extremity of the alimentary canal ; and are continuous behind with the general body-cavity (fig. 20 A, vp}. I propose calling them the head-cavities. The cavities of the two sides have no communication with each other.

Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft ; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections :


THE MUSCULAR SYSTEM.


677


vn~.



(i) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft ; (3) a section behind the first branchial cleft.

The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the mandibular arch.

As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole headcavity becomes on each side divided into (i) a premandibular section ; (2) a mandibular section (vide fig. 29 A, PP] > (3) a hyoid section ; (4) sections in each of the branchial arches.

The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells (fig. 381, ipp}. It is situated close to the eye, and presents a rounded or sometimes a triangular figure in section. The two halves of the cavity are prolonged ventralwards, and meet below the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them.

The next division of the head-cavity, which from its position


FIG. 381. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.

The section, owing to the cranial flexure, cuts both the foreand the hind-brain. It shews the premandibular and mandibular head-cavities ipp and ipp, etc. The section is moreover somewhat oblique from side to side.

fb. fore-brain ; /. lens of eye ; m. mouth ; pt. upper end of mouth, forming pituitary involution; lao. mandibular aortic arch; ipp. and ipp. first and second head-cavities; \vc. first visceral cleft; V. fifth nerve ; aim. auditory nerve ; VII. seventh nerve ; aa. dorsal aorta ; acv. anterior cardinal vein ; ch, notochord.


678 THE HEAD-CAVITIES.

may be called the mandibular cavity, presents a spatulate shape, being dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft (fig. 20, pp}. Like the previous space it is lined by a short columnar epithelium.

The mandibular aortic arch is situated close to its inner side (fig. 381, 2pp). After becoming separated from the lower part (Marshall), the upper part of the cavity atrophies about the time of the appearance of the external gills. Its lower part also becomes much narrowed, but its walls of columnar cells persist. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. In each of the remaining arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal dilated portion appears, however, to be present in the third or hyoid section alone (fig. 20), and even there disappears very soon, after being segmented off from the lower part (Marshall). The cavities in the posterior parts of the head become much reduced like those in its anterior part, though at rather a later period. FlG . 382 . HORIZONTAL

It has been shewn that the divi- SECTION THROUGH THE PENULTIMATE VISCERAL ARCH OF

sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS. with the exception of the anterior, e p. epiblast; vc. pouch of early become atrophied, not so how- hypoblast which will form the

walls of a visceral cleit ; //. CVer their walls. The cells forming segment of body-cavity in vis the walls both of the dorsal and ven- ceral arch ; aa ' aortic arch ' tral sections of these cavities become elongated, and finally become converted into muscles. Their exact history has not been followed in its details, but they almost unquestionably become the musculus contrictor superficialis and musculus interbranchialis 1 ; and probably also musculus levator mandibuli and other muscles of the front part of the head.

The anterior cavity close to the eye remains unaltered much longer than the remaining cavities.

1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zcltschrift, Vol. vn.



THE MUSCULAR SYSTEM.


679


Its further history is very interesting. In my original account of this cavity (No. 292, p. 208) I stated my belief that its walls gave rise to the eye-muscles, and the history of this process has been to some extent worked out by Marshall in his important memoir (No. 509).

Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus internus of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity.

Marshall brings evidence to shew that the rectus externus (as might be anticipated from its nerve supply) has no connection with the walls of the premandibular head-cavity, and finds that it arises close to the position originally occupied by the second and third cavities. Marshall has not satisfactorily made out the mode of development of the obliquus superior.

The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account of which has already been given at p. 461.

Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45, /ic\ the Newt (Osborn and Scott), and various Reptilia (Parker).

BIBLIOGRAPHY.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat. and Phys., Vol. vi. 1872.

(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.

(510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthierc." Silz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. . Entwick. d. Wirbelthiere. Berlin, 1879.

Vide 2^0 Gotte (No. 296), Kolliker (N o. 298), Balfour (No. 292), Huxley, etc.


CHAPTER XXIII.


EXCRETORY ORGANS.


EXCRETORY organs consist of coiled or branched and often ciliated tubes, with an excretory pore opening on the outer surface of the body, and as a rule an internal ciliated orifice placed in the body-cavity. In forms provided with a true vascular system, there is a special development of capillaries around the glandular part of the excretory organs. In many instances the glandular cells of the organs are filled with concretions of uric acid or some similar product of nitrogenous waste.

There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in the animal kingdom, but although there is not a little to be said for holding all these organs to be derived from some common prototype, the attempt to establish definite homologies between them is beset with very great difficulties.

Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and in the Rotifera excretory organs of a similar form to those of the Platyelminthes are also present.

These organs (Fraipont, No. 513) are more or less distinctly paired, and consist of a system of wide canals, often united into a network, which open on the one hand into a pair of large tubes leading to the exterior, and on the other into fine canals which terminate by ciliated openings, either in spaces between the connective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). The fine canals open directly into the larger ones, without first uniting into canals of an intermediate size.


EXCRETORY ORGANS.


68 1


The two large tubes open to the exterior, either by means of a median posteriorly placed contractile vesicle, or by a pair of vesicles, which have a ventral and anterior position. The former type is characteristic of the majority of the Trematoda, Cestoda. and Rotifera, and the latter of the Nemertea and some Trematoda. In the Turbellaria the position of the external openings of the system is variable, and in a few Cestoda (Wagner) there are lateral openings on each of the successive proglottides, in addition to the terminal openings. The mode of development of these organs is unfortunately not known.

Mollusca. In the Mollusca there are usually present two independent pairs of excretory organs one found in a certain number of forms during early larval life only 1 , and the other always present in the adult.

The larval excretory organ has been found in the pulmonate Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and possibly also in Paludina. It is placed in the anterior region of the body, and opens ventrally on each side, a short way behind the velum. It is purely a larval organ, disappearing before the close of the veliger stage. In the aquatic Pulmonata, where it is best developed, it consists on each side of a V-shaped tube, with a dorsally-placed apex, containing an enlargement of the lumen. There is a ciliated cephalic limb, lined by cells with concretions, and terminating by an internal opening near the eye, and a nonciliated pedal limb opening to the exterior 3 .

Two irreconcilable views are held as to the development of this system. Rabl (Vol. II. No. 268) and Hatschek hold that it is developed in the mesoblast ; and Rabl states that in Planorbis it is formed from the anterior mesoblast cells of the mesoblastic bands. A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while

1 I leave out of consideration an external renal organ found in many marine Gasteropod larvte, vide Vol. II. p. 280.

2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool. exfJr. et gener., Vol. VIII.

3 The careful observations of Fol seem to me nearly conclusive in favour of this limb having an external opening, and the statement to the reverse effect on p. 280 of Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be corrected.


682 POLYZOA.

the continuations of the two limbs are formed by perforated mesoblast cells.

According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventralwards and forwards. It thus acquires its V-shaped form.

In the terrestrial Pulmonata the provisional excretory organs are, according to Fol, formed as epiblastic invaginations, in the same way as those in the aquatic Pulmonata, but have the form of simple non-ciliated sacks, without internal openings.

The permanent renal organ of the Mollusca consists typically of a pair of tubes, although in the majority of the Gasteropoda one of the two tubes is not developed. It is placed considerably behind the provisional renal organ.

Each tube, in its most typical form, opens by a ciliated funnel into the pericardial cavity, and has its external opening at the side of the foot. The pericardial funnel leads into a glandular section of the organ, the lining cells of which are filled with concretions. This section is followed by a ciliated section, from which a narrow duct leads to the exterior.

As to the development of this organ the same divergence of opinion exists as in the case of the provisional renal organ.

Rabl's careful observations on Planorbis (Vol. II. No. 268) tend to shew that it is developed from a mass of mesoblast cells, near the end of the intestine. The mass becomes hollow, and, attaching itself to the epiblast on the left side of the anus, acquires an opening to the exterior. Its internal opening is not established till after the formation of the heart. Fol gives an equally precise account, but states that the first rudiment of the organ arises as a solid mass of epiblast cells. Lankester finds that this organ is developed as a paired invagination of the. epiblast in Pisidium, and Bobretzky also derives it from the epiblast in marine Prosobranchiata. In Cephalopoda on the other hand Bobretzky's observations (I conclude this from his figures) indicate that the excretory sacks of the renal organs are derived from the mesoblast.

Polyzoa. Simple excretory organs, consisting of a pair of ciliated canals, opening between the mouth and the anus, have


EXCRETORY ORGAN>.


68 3


been found by Hatschek and Joliet in the Entoproctous Polyzoa, and are developed, according to Hatschek, by whom they were first found in the larva, from the mesoblast

Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, with both peritoneal and external openings, are found in the Brachiopoda. They undoubtedly serve as genital ducts, but from their structure are clearly of the same nature as the excretory organs of the Chaetopoda described below. Their development has not been worked out.

Chaetopoda. Two forms of excretory organ have been met with in the Chaetopoda. The one form is universally or nearly universally present in the adult, and typically consists of a pair of coiled tubes repeated in every segment. Each tube has an internal opening, placed as a rule in the segment in front of that in which the greater part of the organ and the external opening are situated.

There are great variations in the structure of these organs, which cannot be dealt with here. It may be noted however that the internal opening may be absent, and that there may be several internal openings for each organ (Polynoe). In the Capitellidae moreover several pairs of excretory tubes have been shewn by Eisig (No. 512) to be present in each of the posterior segments.

The second form of excretory organ has as yet only been found in the larva of Polygordius, and will be more conveniently dealt with in connection with the development of the excretory system of this form.

There is still considerable doubt as to the mode of formation of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277), from his observations on the Oligochasta, holds that they develop as outgrowths of the epithelial layer covering the posterior side of the dissepiments, and secondarily become connected with the epidermis.

Hatschek finds that in Criodrilus they arise from a continuous linear thickening of the somatic mesoblast, immediately beneath the epidermis, and dorsal to the ventral band of longitudinal muscles. They break up into S-shaped cords, the anterior end of each of which is situated in front of a dissepiment, and is formed at first of a single large cell, while the posterior part is


684 CHvETOPODA.


continued into the segment behind. The cords are covered by a peritoneal lining, which still envelopes them, when in the succeeding stage they are carried into the body-cavity. They subsequently become hollow, and their hinder ends acquire openings to the exterior. The formation of their internal openings has not been followed.

Kleinenberg is inclined to believe that the excretory tubes take their origin from the epiblast, but states that he has not satisfactorily worked out their development.

The observations of Risig (No. 512) on the Capitellidae support Kowalevsky's view that the excretory tubes originate from the lining of the peritoneal cavity.

Hatschek (No. 514) has given a very interesting account of the development of the excretory system in Polygordius.

The excretory system begins to be formed, while the larva is still in the trochospere stage (fig. 383, npli), and consists of a provisional excretory organ, which is placed in front of the future segmented part of the body, and occupies a position very similar to that of the provisional excretory organ found in some Molluscan larvae (vide p. 68 1).

Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.

In its earliest stage the provisional excretory organ of Polygordius consists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS

, . , r 11-1 LARVA. (After Hatschek.)

each with an anterior funnel-like open- m _ moulh . ^ supraKBSO .

ing situated in the midst of the meSO- phageal ganglion ; nph. nephri11 11 . , dion ; ine.p. mesoblastic band;

blast cells, and a posterior external an _ anus 5 oL stomach . opening. The latter is placed immediately in front of what afterwards becomes the segmented region of the embryo. While the larva is still unsegmented, a second internal opening is formed for each tube (fig. 383, np/i) and the two openings so formed may eventually become divided into five (fig. 384 A), all communicating by a single pore with the exterior.

When the posterior region of the embryo becomes segmented,



EXCRETORY ORGANS.


685


paired excretory organs are formed in each of the posterior segments, but the account of their development, as given by Hatschek, is so remarkable that I do not think it can be definitely accepted without further confirmation.

From the point of junction of the two main branches of the larval kidney there grows backwards (fig. 384 B), to the hind end of the first segment, a very delicate tube, only indicated by its ciliated lumen, its walls not being differentiated. Near the front end of this tube a funnel, leading into the larval body cavity of the head, is formed, and subsequently the posterior end of the tube acquires an external opening, and the tube distinct walls. The communication with the provisional excretory organ is then lost, and thus the excretory tube of the first segment is established.

The excretory tubes in the second and succeeding segments are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from the preceding segment, and the subsequent separation of this part as a separate tube.

The tube may be continued with a sinuous course through



A A

A +

A.


Y

Y Y Y Y


J)


FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY SYSTEM OF POLYGORDIUS. (After Hatschek.)

several segments without a distinct wall. The external and internal openings of the permanent excretory tubes are thus secondarily acquired. The internal openings communicate with the permanent body-cavity. The development of the perma


686 GEPHYREA.


nent excretory tubes is diagrammatically represented in fig. 384 C and D.

The provisional excretory organ atrophies during larval life.

If Hatschek's account of the development of the excretory system of Polygordius is correct, it is clear that important secondary modifications must have taken place in it, because his description implies that there sprouts from the anterior excretory organ, while it has its own external opening, a posterior duct, which does not communicate either with the exterior or with the body-cavity! Such a duct could have no function. It is intelligible either (i) that the anterior excretory organ should lead into a longitudinal duct, opening posteriorly ; that then a series of secondary openings into the body-cavity should attach themselves to this, that for each internal opening an external should subsequently arise, and the whole break up into separate tubes ; or (2) that behind an anterior provisional excretory organ a series of secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.

Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one, similar in position and sometimes also in structure, to the provisional excretory organ of Polygordius, has so far only been found in the larvae of Echiurus and Bonellia.

In all the Gephyrea the so-called 'brown tubes' are apparently homologous with the segmented excretory tubes of Chaetopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a single highly modified tube in Bonellia, forming the oviduct and uterus ; a pair of tubes in the Gephyrea inermia, and two or three pairs in most Gephyrea armata, except Bonellia. Their development has not been studied.

In the Gephyrea armata there is always present a pair of posteriorly placed excretory organs, opening in the adult into the anal extremity of the alimentary tract, and provided with numerous ciliated peritoneal funnels. These organs were stated by Spengel to arise in Bonellia as outgrowths of the gut ; but in Echinrus Hatschek (No. 515) finds that they are developed from the somatic mesoblast of the terminal part of the trunk. They soon become hollow, and after attaching themselves to the epiblast on each side of the anus, acquire external openings. They are not at first provided with peritoneal funnels, but these parts of the organs become developed from a ring of cells at


EXCRETORY ORGANS.


687


their inner extremities ; and there is at first but a single funnel for each vesicle. The mode of increase of the funnels has not been observed, nor has it been made out how the organs themselves become attached to the hind-gut.

The provisional excretory organ of Echiurus is developed at an early larval stage, and is functional during the whole of larval life. It at first forms a ciliated tube on each side, placed in front of that part of the larva which becomes the trunk of the adult. It opens to the exterior by a fine pore on the ventral side, immediately in front of one of the mesoblastic bands, and appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal ciliated funnel. The primitively simple excretory organ becomes eventually highly complex by the formation of numerous branches, each ending in a slightly swollen extremity. These branches, in the later larval stages, actually form a network, and the inner end of each main branch divides into a bunch of fine tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.

In the larva of Bonellia Spengel has described a pair of provisional excretory tubes, opening near the anterior end of the body, which are probably homologous with the provisional excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).

Discophora. As in many of the types already spoken of, permanent and provisional excretory organs may be present in the Discophora. The former are usually segmentally arranged, and resemble in many respects the excretory tubes of the Chaetopoda. They may either be provided with a peritoneal funnel (Nephelis, Clepsine) or have no internal opening (Hirudo).

Bourne 1 has shewn that the cells surrounding the main duct in the medicinal Leech are perforated by a very remarkable network of ductules, and the structure of these organs in the Leech is so peculiar that it is permissible to state with due reserve their homology with the excretory organs of the Chaetopoda.

The excretory tubes of Clepsine are held by Whitman to be developed in the mesoblast.

1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of Micr. Science, Vol. XX. 1880.


688 ARTHROPODA.


There are found in the embryos of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which are not yet fully made out. In Nephelis they appear as one (according to Robin), or (according to Biitschli) as two successive pairs of convoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end of the U forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Fiirbringer is inclined from his own researches to believe that they open laterally. They contain a clear fluid.

In Hirudo, Leuckart has described three similar pairs of organs, the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement, from which a convoluted tube is continued for some distance backwards; the tube then turns forwards again, and after bending again upon itself opens to the exterior. The anterior part is broken up into a kind of labyrinthic network.

The provisional excretory organs of the Leeches cannot be identified with the anterior provisional organs of Polygordius and Echiurus.

Arthropoda. Amongst the Arthropoda Peripatus is the only form with excretory organs of the type of the segmental excretory organs of the Chsetopoda 1 .

These organs are placed at the bases of the feet, in the lateral divisions of the body-cavity, shut off from the main median division of the body-cavity by longitudinal septa of transverse muscles.

Each fully developed organ consists of three parts :

(i) A dilated vesicle opening externally at the base of a foot. (2) A coiled glandular tube connected with this, and subdivided again into several minor divisions. (3) A short terminal portion opening at one extremity into the coiled tube

1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." Quart. J, of Micr. Science, Vol. XIX. 1879.


EXCRETORY ORGANS. 689


and at the other, as I believe, into the body cavity. This section becomes very conspicuous, in stained preparations, by the intensity with which the nuclei of its walls absorb the colouring matter.

In the majority of the Tracheata the excretory organs have the form of the so-called Malpighian tubes, which always (vide Vol. II.) originate as a pair of outgrowths of the epiblastic proctodaeum. From their mode of development they admit of comparison with the anal vesicles of the Gephyrea, though in the present state of our knowledge this comparison must be regarded as somewhat hypothetical.

The antennary and shell-glands of the Crustacea, and possibly also the so-called dorsal organ of various Crustacean larvae appear to be excretory, and the two former have been regarded by Claus and Grobben as belonging to the same system as the segmental excretory tubes of the Chaetopoda.

Nematoda. Paired excretory tubes, running for the whole length of the body in the so-called lateral line, and opening in front by a common ventral pore, are present in the Nematoda. They do not appear to communicate with the body cavity, and their development has not been studied.

Very little is known with reference either to the structure or development of excretory organs in the Echinodermata and the other Invertebrate types of which no mention has been so far made in this Chapter.

Excretory organs and generative ducts of the Craniata.

Although it would be convenient to separate, if possible, the history of the excretory organs from that of the generative ducts, yet these parts are so closely related in the Vertebrata, in some cases the same duct having at once a generative and a urinary function, that it is not possible to do so.

The excretory organs of the Vertebrata consist of three distinct glandular bodies and of their ducts. These are (i) a small glandular body, usually with one or more ciliated funnels opening into the body cavity, near the opening of which there projects into the body cavity a vascular glomerulus. It is situated very far forwards, and is usually known as the head 44


690 ELASMOBRANCHII.


kidney, though it may perhaps be more suitably called, adopting Lankester's nomenclature, the pronepliros. Its duct, which forms the basis for the generative and urinary ducts, will be called the segmented duct.

(2) The Wolffian body, which may be also called the mesonepJiros. It consists of a series of, at first, segmentally (with a few exceptions) arranged glandular canals (segmental tubes) primitively opening at one extremity by funnel-shaped apertures into the body cavity, and at the other into the segmental duct. This duct becomes in many forms divided longitudinally into two parts, one of which then remains attached to the segmental tubes and forms the Wolffian or mesonepJiric duct, while the other is known as the Milllerian dnct.

(3) The kidney proper or metanephros. This organ is only found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolrfian duct.

The above parts do not coexist in full activity in any living adult member of the Vertebrata, though all of them are found together in certain embryos. They are so intimately connected that they cannot be satisfactorily dealt with separately.

Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the same time it forms a convenient starting point for studying the modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros. The development of the Elasmobranch excretory system has been mainly studied by Semper and myself.

The first trace of the system makes its appearance as a knob of mesoblast, springing from the intermediate cell-mass near the level of the hind end of the heart (fig. 385 K,pd). This knob is the rudiment of the abdominal opening of the segmental duct, and from it there grows backwards to the level of the anus a solid column of cells, which constitutes the rudiment of the segmental duct itself (fig. 385 B, pd). The knob projects towards the epiblast, and the column connected with it lies between the mesoblast and epiblast. The knob and column do not long remain solid, but the former acquires an opening into the body cavity (fig. 421, sd) continuous with a lumen, which


EXCRETORY ORGANS.


691


makes its appearance in the column (fig. 386, sd). The knob forms the only structure which can be regarded as a rudiment of the pronephros.


spn


spn



FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL

CLEFTS.

The sections illustrate the development of the segmental duct (pd) or primitive duct of the pronephros. In A (the anterior of the two sections) this appears as a solid knob (pd) projecting towards the epiblast. In B is seen a section of the column which has grown backwards from the knob in A.

spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. subnotochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate; ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //. body cavity; ep. epiblast; al. alimentary canal.

While the lumen is gradually being formed, the segmental tubes of the mesonephros become established. They appear to arise as differentiations of the parts of the primitive lateral plates of mesoblast, placed between the dorsal end of the body cavity and the muscle-plate (fig. 386, st) 1 , which are usually known as the intermediate cell-masses.

The lumen of the segmental tubes, though at first very small, soon becomes of a considerable size. It appears to be established in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body cavity in the muscle-plates with the permanent body cavity. The lumen of each tube opens at its lower end into the dorsal part of the body cavity (fig. 386, st}, and each tube curls obliquely

1 In my original account of the development I held these tubes to be invaginations of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of my original statement from his investigations on the chick ; and from a re-examination of my specimens he arrived at the results stated above, and which I am now myself inclined to adopt.

442


692


ELASMOBRANCHII.


sp.c



backwards round the inner and dorsal side of the segmental duct, near which it at first ends blindly.

One segmental tube makes its appearance for each somite (fig. 265), commencing with that immediately behind the abdominal opening of the segmental duct, the last tube being situated a few segments behind the anus. Soon after their formation the blind ends of the segmental tubes come in contact with, and open into the segmental duct, and each of them becomes divided into four parts. These are (i) a section carrying the peritoneal opening, known as the peritoneal funnel, (2) a dilated vesicle into which this opens, (3) a coiled tubulus proceeding from (2), and terminating in (4) a wider portion opening into the segmental duct. At the same time, or shortly before this, each segmental duct unites with and opens into one of the horns of the cloaca, and also retires from its primitive position between the epiblast and mesoblast, and assumes a position close to the epithelium lining the body cavity (fig. 380, sd}. The general features of the excretory organs at this period are diagrammatically represented in the woodcut (fig. 387). In this fig. pd is the segmental duct and o its abdominal opening; s.t points to the segmental tubes, the finer details of whose structure are not represented in the diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one extremity of each of which opens into the body cavity, and the other into the segmental duct, which forms the only duct of the system, and communicates at its front end with the body cavity, and behind with the cloaca.


FIG. 386. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; nip, muscle-plate ; nip', inner layer of muscle-plate already converted into muscles ; Vr, rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; p.o. primitive generative cells.


EXCRETORY ORGANS. 693


The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line x, and in transverse section in figs. 388 and 389. The resulting ducts are (i) the Wolffian duct or mesonephric duct (wd\ dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Miillerian duct in the female, and the rudiments of this duct in the male. In the



FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN

ELASMOBRANCH EMBRYO.

pd. segmental duct. It opens at o into the body cavity and at its other extremity into the cloaca; x. line along which the division appears which separates the segmental duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.

female the formation of these ducts takes place (fig. 389) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Miillerian duct (pd}. A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen (fig. 389 A). The anterior part of the segmental duct is not divided, but remains continuous with the Mullerian duct, of which its anterior pore forms the permanent peritoneal opening 1 (fig. 387). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly

1 Five or six segmental tubes belong to the region of the undivided anterior part of the segmental duct, which forms the front end of the Mullerian duct ; but they appear to atrophy very early, without acquiring a definite attachment to the segmental duct.


694


ELASMOBRANCHIL


in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Miillerian duct, being shorter,



trd/



FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR I'ART OF THE SEGMENTAL DUCT OF A FEMALE EMBRYO OF SCYLLIUM CANICULA.

The figure shews how the segmental duct becomes split into the Wolffian or mesonephric duct above, and Miillerian duct or oviduct below.

wd. Wolffian or mesonephric duct; od. Miillerian duct or oviduct ; sd. segmental duct.


FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A

SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.

me. medullary canal; mp. muscle-plate; ch. notochord; ao. aorta; cav. cardinal vein; st. segmental tube. On the left side the section passes through the opening of a segmental tube into the body cavity. On the right this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; iv.d. Wolffian duct; m.d. Miillerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate; gr. the germinal ridge with the thickened germinal epithelium ; /. liver ; i. intestine with spiral valve.

and in the column of cells with which it is continuous being from the first incomplete.

The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening


EXCRETORY ORGANS.


695



into the Wolffian duct (fig. 390, px). The remainder of the vesicle becomes converted into a Malpighian body (mg}.

By the first of these changes 10^-4 M @W>f a tube is established connecting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the anterior part of the excretory organs in the adult, and most probably in the hinder part, yet it seems almost certain that the secondary and tertiary Malpighian bodies of the majority of segments are developed from its persisting blind end. Each of these


FIG. 390. LONGITUDINAL VERTICAL SECTION THROUGH PART OF THE MESONEPHROS OF AN EMBRYO OF SCYLLIUM.

The figure contains two examples of the budding of the vesicle of a segmental tube (which forms a Malpighian body in its own segment) to unite with the tubulus in the preceding segment close to its opening into the Wolffian (mesonephric) duct.

ge. epithelium of body-cavity; st. peritoneal funnel of segmental tube with its peritoneal opening; mg. Malpighian body; px. bud from Malphigian body uniting with preceding segment.


secondary and tertiary Malpighian bodies is connected with a convoluted tubulus (fig. 391, a.mg), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the


st.c



w.d


FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT. The figure shews a fibrous band passing from the primary to the secondary Malpighian bodies in two segments, which is the remains of the outgrowth from the primary Malpighian body.

sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Malpighian body; w.d. mesonephric (Wolffian) duct.


696 ELASMOBRANCI1II.


segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.

The third section of each tubulus becomes by continuous growth, especially in the hinder segments, very bulky and convoluted.

The general character of a slightly developed segment of the mesonephros at its full growth may be gathered from fig. 391. It commences with (i) a peritoneal opening, somewhat oval in form (st.d) and leading directly into (2) a narrow tube, the segmental tube, which takes a more or less oblique course backwards, and, passing superficially to the Wolffian duct (w.d}, opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather definite convolutions, and after uniting with tubuli from one, two, or more (according to the size of the segment) accessory Malpighian bodies (a.mg) smaller than the one into which the segmental tube falls, eventually opens by (4) a narrowish collecting tube into the Wolffian duct at the posterior end of the segment. Each segment is probably completely isolated from the adjoining segments, and never has more than one peritoneal funnel and one communication with the Wolffian duct.

Up to this time there has been no distinction between the anterior and posterior tubuli of the mesonephros, which alike open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in Scyllium canicula), either in some species elongate, overlap, while at the same time their openings travel backward so that they eventually open by apertures (not usually so numerous as the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian ducts, in the male; or in other species become modified, by a peculiar process of splitting from the Wolnian duct, so as to pour their secretion into a single duct on each side, which opens in a position corresponding with the numerous ducts of the other species (fig. 392). In both cases the modified posterior kidney-segments are probably equivalent to the per


EXCRETORY ORGANS. 697


manent kidney or metanephros of the amniotic Vertebrates, and for this reason the numerous collecting tubes or single collecting tube, as the case may be, will be spoken of as ureters. The anterior tubuli of the primitive excretory organ retain their early relation to the Wolffian duct, and form the permanent Wolffian body or mesonephros.

The originally separate terminal extremities of the Wolffian ducts always coalesce, and form a urinal cloaca, opening by a single aperture, situated at the extremity of the median papilla behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings, become obliterated.

In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes. Branches appear to grow from the first three or four or more of them (though probably not from their peritoneal openings), which pass to the base of the testis, and there uniting into a longitudinal canal, form a network, and receive the secretion of the testicular ampullae (fig. 393, nf). These ducts, the vasa efferent ia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as the longitudinal canal of the Wolffian body (l.c\ from which pass off ducts equal in number to the vasa efferentia, each of which normally ends in a Malpighian corpuscle. From the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian body, along which the semen is conveyed to the Wolffian duct (v.d). The Wolffian duct itself becomes much contorted and acts as vas deferens.

Figs. 392 and 393 are diagrammatic representations of the chief constituents of the adult urinogenital organs in the two sexes. In the adult female (fig. 392), there are present the following parts :

(1) The oviduct or Mullerian duct (m.d) split off from the segmental duct of the kidneys. Each oviduct opens at its anterior extremity into the body cavity, and behind the two oviducts have independent communications with the general cloaca.

(2) The mesonephric ducts (w.d), the other product of the


698


ELASMOBRANCHII.


segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior persisting segment of the mesonephros on each side, and unite behind to



FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT FEMALE ELASMOBRANCH.

m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are represented with openings into the body cavity, the posterior segmental tubes form the mesonephros ; ov. ovary.

open by a common papilla into the cloaca. The mesonephric duct receives the secretion of the anterior tubuli of the primitive mesonephros.

(3) The ureter which carries off the secretion of the kidney proper or metanephros. It is represented in my diagram in its most rare and differentiated condition as a single duct connected with the posterior segmental tubes.

(4) The segmental tubes (.$-./) some of which retain their


-S.t:



FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT MALE ELASMOBRANCH.

m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis; VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.


EXCRETORY ORGANS. 699


original openings into the body cavity, and others are without them. They are divided into two groups, an anterior forming the mesonephros or Wolffian body, which pours its secretion into the Wolffian duct ; and a posterior group forming a gland which is probably equivalent to the kidney proper of amniotic Craniata, and is connected with the ureter.

In the male the following parts are present (fig. 393):

(1) The Mlillerian duct (m.d], consisting of a small rudiment attached to the liver, representing the foremost end of the oviduct of the female.

(2) The mesonephric duct (w.d] which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (vd}. In the adult male its foremost part has a very tortuous course.

(3) The ureter (d\ which has the same fundamental constitution as in the female.

(4) The segmental tubes (s.t). The posterior tubes have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them to act as transporters of the semen.

Connected with the anterior tubes there are present (i) the vasa efferentia (VE], united on the one hand with (2) the central canal in the base of the testis (/), and on the other with the longitudinal canal of the Wolffian body (/<?). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in my diagram.

Apart from the absence of the pronephros the points which deserve notice in the Elasmobranch excretory system are (i) The splitting of the segmental duct into Wolffian (mesonephric) and Mullerian ducts. (2) The connection of the former with the mesonephros, and of the latter with the abdominal opening of the segmental duct which represents the pronephros of other types. (3) The fact that the Mullerian duct serves as oviduct, and the Wolffian duct as vas deferens. (4) The differentiation of a posterior section of the mesonephros into a special gland foreshadowing the metanephros of the Amniota.


/OO CYCLOSTOMATA.


Cyclostomata. The development of the excretory system amongst the Cyclostomata has only been studied in Petromyzon (Miiller, Furbringer, and Scott).

The first part of the system developed is the segmental duct. It appears in the embryo of about 14 days (Scott) as a solid cord of cells, differentiated from the somatic mesoblast near the dorsal end of the body cavity. This cord is at first placed immediately below the epiblast, and grows backwards by a continuous process of differentiation of fresh mesoblast cells. It soon acquires a lumen, and joins the cloacal section of the alimentary tract before the close of foetal life. Before this communication is established, the front end of the duct sends a process towards the body cavity, the blind end of which acquires a ciliated opening into the latter. A series of about four or five successively formed outgrowths from the duct, one behind the other, give rise to as many ciliated funnels opening into the body cavity, and each communicating by a more or less elongated tube with the segmental duct. These funnels, which have a metameric arrangement, constitute the pronephros, the whole of which is situated in the pericardial region of the body cavity.

On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into the body cavity, and covered by peritoneal epithelium. For a considerable period the pronephros constitutes the sole functional part of the excretory system.

A mesonephros is formed (Furbringer) relatively late in larval life, as a segmentally arranged series of solid cords, derived from the peritoneal epithelium. These cords constitute the rudiments of the segmental tubes. They are present for a considerable portion of the body cavity, extending backwards from a point shortly behind the pronephros. They soon separate from the peritoneal epithelium, become hollowed out into canals, and join the segmental duct. At their blind extremity (that originally connected with the peritoneal epithelium) a Malpighian body is formed.

The pronephros is only a provisional excretory organ, the atrophy of which commences during larval life, and is nearly completed when the Ammoccete has reached 180 mm. in length.


EXCRETORY ORGANS. 70 1

Further changes take place in connection with the excretory system on the conversion of the Ammoccete into the adult.

The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This cloaca also communicates by two apertures (abdominal pores) with the body cavity. The generative products are carried into the cloaca by these pores ; so that their transportation outwards is not performed by any part of the primitive urinary system. The urinogenital cloaca is formed by the separation of the portion of the primitive cloaca containing the openings of the segmental ducts from that connected with the alimentary tract.

The mesonephros of the Ammoccete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).

In Myxine the excretory system consists (i) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which the pronephros is composed open on the ventral side of the anterior portion of the segmental duct, which in old individuals is cut off from the posterior section of the duct. On the dorsal side of the portion of the segmental duct belonging to the pronephros there are present a small number of diverticula, terminating in glomeruli : they are probably to be regarded as anterior segmental tubes. (2) Of a mesonephros, which commences a considerable distance behind the pronephros, and is formed of straight extremely simple segmental tubes opening into the segmental duct (fig. 385).

The excretory system of Myxine clearly retains the characters of the system as it exists in the larva of Petromyzon.

Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by a duct, the nature of which is somewhat doubtful, but which is probably homologous with the mesonephric duct of other types.

The system commences in the embryo (Rosenberg, Oellacher, Gotte, Furbringer) with the formation of a groove-like fold of the somatic layer of peritoneal epithelium, which becomes gradually constricted into a canal; the process of constriction commencing in the middle and extending in both directions. The canal does not however close anteriorly, but remains open to the body cavity, thus giving rise to a funnel equivalent to the pronephric funnels of Petromyzon and Myxine. On the inner side of this


702


TELEOSTEI.


funnel there is formed a glomerulus, projecting into the body

cavity ; and at the same time that

this is being formed the anterior end

of the canal becomes elongated and

convoluted. The above structures

constitute a pronephros, while the

posterior part of the primitive canal

forms the segmental duct.

The portion of the body cavity with the glomerulus and peritoneal funnel of the pronephros (fig. 395, po) soon becomes completely isolated from the remainder, so as to form a closed cavity (gl). The development of the mesonephros does not take place till long after that of the pronephros. The segmental tubes which form it are stated by Fiirbringer to arise from solid ingrowths of peritoneal epithelium, developed successively from before backwards, but Sedgwick informs me that they arise as differentiations of the mesoblastic cells near the peritoneal epithelium. They soon become hollow, and unite with the segmental duct. Malpighian bodies are developed on their median portions. They grow very greatly in length, and become much convoluted, but the details of this process have not been followed out.

The foremost segmental tubes are situated close behind the pronephros, while the hindermost are in many cases developed in the post-anal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney of the adult, and the mesonephros the remainder ; the so-called caudal portion, where present, being derived (?) from the postanal segmental tubes.

In some cases the cephalic portion of the kidneys is absent



FIG. 394. PORTIONS OF THE MESONEPHROS OF MYXINE. (From Gegenbaur; after J. Miiller.)

a. segmental duct ; b. segmental tube; c. glomerulus ; d. afferent, e. efferent artery.

B represents a portion of A highly magnified.


EXCRETORY ORGANS. 703


in the adult, which probably implies the atrophy of the pronephros ; in other instances the cephalic portion of the kidneys is the only part developed. Its relation to the embryonic proncphros requires however further elucidation.

In the adult the ducts in the lower part of the kidneys lie as a rule on their outer borders, and almost invariably open into a



pr


FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT PARTS TEN DAYS BEFORE HATCHING.

pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x. subnotochordal rod ; al. alimentary tract.

urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few instances there is a common urinogenital pore.

In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It appears to me most probable, from the analogy of Lepidostcus, to be described in the next section, that these ducts are split off from the primitive segmental duct, and correspond with the Miillerian ducts of Elasmobranchii, etc. ; though on this point we have at present no positive embryological evidence (vide general considerations at the end of the Chapter). In the female Salmon and the male and female Eel the generative products are carried to the exterior by abdominal pores. It is possible that this may represent a primitive condition, though it


704


GANOIDEI.


is more probably a case of degeneration, as is indicated by the presence of ducts in the male Salmon and in forms nearly allied to the Salmonidae.

The coexistence of abdominal pores and generative ducts in Mormyrus appears to me to demonstrate that the generative ducts in Teleostei cannot be derived from the coalescence of the investment of the generative organs with the abdominal pores.

Ganoidei. The true excretory gland of the adult Ganoidei resembles on the whole that of Teleostei, consisting of an elongated band on each side the mesonephros an anterior dilatation of which probably represents the pronephros.

There is in both sexes a Mullerian duct, provided, except in Lepidosteus, with an abdominal funnel, which is however situated relatively very far back in the abdominal cavity. The Mullerian ducts appear to serve as generative canals in both sexes. In Lepidosteus they are continuous with the investment of the generative glands, and thus a relation between the generative ducts and glands, very similar to that in Teleostei, is brought about.

Posteriorly the Mullerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each side is clearly the primitive segmental duct. It receives the secretion of a certain number of the posterior mesonephric tubules, and usually unites with its fellow to form a kind of bladder, opening by a single pore into the cloaca, behind the anus. The duct which receives the secretion of the anterior mesonephric tubules is the true mesonephric or Wolffian duct.

The development of the excretory system, which has been partially worked out in Acipenscr and Lepidosteus 1 , is on the whole very similar to that in the Teleostei. The first portion of the system to



FIG. 396. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION.

me. medullary cord ; ms. mesoblast ; sg. segmental duct ; ch. notochord ; .r. subnotochordal rod; hy. hypoblast.


1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and myself.


EXCRETORY ORGANS.


705


be formed is the segmental duct. In Lepidosteus this duct is formed as a groove-like invagination of the somatic peritoneal epithelium, precisely as in Teleostei, and shortly afterwards forms a duct lying between the mesoblast and the epiblast (fig. 396, sg}. In Acipenser (Salensky) however it is formed as



FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER

EMBRYO. (After Salensky.)

Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. notochord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic plate.

a solid ridge of the somatic mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, Wg).

In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed. This gland is provided with but one 1 peritoneal opening, which together with the glomerulus belonging to it becomes encapsuled in a special section of the body cavity. The opening of the pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n. At this early stage of Acipenser (larva of 5 mm.) I could find no glomerulus.

The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the larvae of both these genera the pronephros is for a considerable period the only excretory organ. In Lepidosteus especially the development of the mesonephros occurs very late.

The development of the mesonephros has not been worked out in Lepidosteus, but in Acipenser the anterior segmental tubes become first established as (I believe) solid cords of cells, attached at one extremity to the peritoneal epithelium on each

1 I have not fully proved this point, but have never found more than one opening.


B. III.


45


GANOIDEI.


side of the insertion of the mesentery, and extending upwards and outwards round the segmental duct 1 . The posterior segmental tubes arise later than the anterior, and (as far as can be determined from the sections in my possession) they are formed independently of the peritoneal epithelium, on the dorsal side of the segmental duct.

In later stages (larvae of 7 10 mm.) the anterior segmental tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a Malpighian body, and the other end unites with the segmental duct. At a still later stage wide peritoneal funnels are es


sjy.c


mjo


pr.n



FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A

LARVA OF ACIPENSER 5 MM. IN LENGTH.

st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells, the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ; b.c. body cavity.

tablished, for at any rate a considerable number of the tubes, leading from the body cavity to the Malpighian bodies. These

1 Whether the segmental tubes are formed as ingrowths of the peritoneal epithelium, or in situ, could not be determined.


EXCRETORY ORGANS. 707

funnels have been noticed by Furbringer, Salensky and myself, but their mode of development has not, so far as I know, been made out. The funnels appear to be no longer present in the adult. The development of the Mullerian ducts has not been worked out.

Dipnoi. The excretory system of the Dipnoi is only known in the adult, but though in some respects intermediate in character between that of the Ganoidei and Amphibia, it resembles that of the Ganoidei in the important feature of the Mullerian ducts serving as genital ducts in both sexes.

Amphibia. In Amphibia (Gotte, Furbringer) the development of the excretory system commences, as in Teleostei, by the formation of the segmental duct from a groove formed by a fold of the somatic layer of the peritoneal epithelium, near the dorsal border of the body cavity (fig. 399, u). The anterior end of the groove is placed immediately behind the branchial region. Its posterior part soon becomes converted into a canal by a constriction which commences a short way from the front end of the groove, and thence extends backwards. This canal at first ends blindly close to the cloaca, into which however it soon opens.

The anterior open part of the groove in front of the constriction (fig. 399, n] becomes differentiated into a longitudinal duct, which remains in open communication with the body cavity by two (many Urodela) three (many Anura) or four (Cceciliidae) canals. This constitutes the dorsal part of the pronephros. The ventral part of the gland is formed from the section of the duct immediately behind the longitudinal canal. This part grows in length, and, assuming an S-shaped curvature, becomes placed on the ventral side of the first formed part of the pronephros. By continuous growth in a limited space the convolutions of the canal of the pronephros become more numerous, and the complexity of the gland is further increased by the outgrowth of blindly ending diverticula.

At the root of the mesentery, opposite the peritoneal openings of the pronephros, a longitudinal fold, lined by peritoneal epithelium, and attached by a narrow band of tissue, makes its appearance. It soon becomes highly vascular, and constitutes a glomerulus homologous with that in Petromyzon and Teleostei.

452


AMPHIBIA.


a*'


The section of the body cavity which contains the openings of the pronephros and the glomerulus, becomes dilated, and then temporarily shut off from the remainder. At a later period it forms a special though not completely isolated compartment. For a long time the pronephros and its duct form the only excretory organs of larval Amphibia. Eventually however the formation of the mesonephros commences, and is followed by the atrophy of the pronephros. The mesonephros is composed, as in other types, of a series of segmental tubes, but these, except in Cceciliidae, no longer correspond in number with the myotomes, but are in all instances more numerous. Moreover, in the posterior part of the mesonephros in the Urodeles, and through the whole length of the gland in other types, secondary and tertiary segmental tubes are formed in addition to the primary tubes.



FIG. 399. TRANSVERSE SECTION THROUGH A VERY YOUNG TADPOLE OF BOMBINATOR AT THE LEVEL OF THE ANTERIOR END OF THE YOLK-SACK. (After

Gotte.)

a. fold of epiblast continuous with the dorsal fin; is", neural cord; m. lateral muscle; as 1 . outer layer of muscle-plate; s. lateral plate of mesoblast ; b. mesentery ; u. open end of the segmental duct, which forms the pronephros ; f. alimentary tract ; f. ventral diverticulum which becomes the liver; e. junction of yolk cells and hypoblast cells ; d. yolk cells.


The development of the mesonephros commences in Salamandra (Fiirbringer) with the formation of a series of solid cords, which in the anterior myotomes spring from the peritoneal epithelium on the inner side of the segmental duct, but posteriorly arise independently of this epithelium in the adjoining mesoblast. Sedgwick informs me that in the

Frog the segmental tubes are throughout developed in the mesoblast, independently of the peritoneal epithelium. These cords next become detached from the peritoneal epithelium (in so far as they are primitively united to it), and after first assuming a vesicular form, grow out into coiled tubes, with a median limb the blind end of which assists in forming a Malpighian body, and a lateral limb which comes in contact with and opens into the segmental duct, and an intermediate portion connecting the two. At the junction of the median with the intermediate portion, and therefore at the neck of the Malpighian body, a canal grows out in a ventral direction, which meets the


EXCRETORY ORGANS. 709

peritoneal epithelium, and then develops a funnel-shaped opening into the body cavity, which subsequently becomes ciliated. In this way the peritoneal funnels which are present in the adult are established.

The median and lateral sections of the segmental tubes become highly convoluted, and the separate tubes soon come into such close proximity that their primitive distinctness is lost.

The first fully developed segmental tube is formed in Salamandra maculata in about the sixth myotome behind the pronephros. But in the region between the two structures rudimentary segmental tubes are developed.

The number of primary segmental tubes in the separate myotomes of Salamandra is as follows :

In the 6th myotome (i.e. the first with a true

segmental tube) 12 segmental tubes

yth roth myotome 23

IIth ... 34

I2th 3 4 or 4 5

I3th y> 45

1 3th i6th 56

It thus appears that the segmental tubes are not only more numerous than the myotomes, but that the number in each myotome increases from before backwards. In the case of Salamandra there are formed in the region of the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out of independent solid cords, which arise in the mesoblast dorsally to the tubes already established.

The secondary segmental tubes appear to develop out of these cords exactly in the same way as the primary ones, except that they do not join the segmental duct directly, but unite with the primary segmental tubes shortly before the junction of the latter with the segmental duct. In this way compound segmental tubes are established with a common collecting tube, but with numerous Malpighian bodies and ciliated peritoneal openings. The difference in the mode of origin of these compound tubes and of those in Elasmobranchii is very striking.

The later stages in the development of the segmental tubes have not been studied in the other Amphibian types.

In Cceciliidas the earliest stages are not known, but the tubes present in the adult (Spengel) a truly segmental arrangement, and in the young each of them is single, and provided with only a single peritoneal funnel. In the adult however many of the segmental organs become compound, and may have as many as twenty funnels, etc. Both simple and compound segmental tubes occur in all parts of the mesonephros, and are arranged in no definite order.

In the Anura (Spengel) all the segmental tubes are compound, and an enormous number of peritoneal funnels are present on the ventral surface, but it has not yet been definitely determined into what part of the segmental tubes they open.


710 AMPHIBIA.


Before dealing with the further changes of the Wolffian body it is necessary to return to the segmental duct, which, at the time when the pronephros is undergoing atrophy, becomes split into a dorsal Wolffian and ventral Mullerian duct. The process in Salamandra (Fiirbringer) has much the same character as in Elasmobranchii, the Mullerian duct being formed by the gradual separation, from before backwards, of a solid row of cells from the ventral side of the segmental duct, the remainder of the duct constituting the Wolffian duct. During the formation of the Mullerian duct its anterior part becomes hollow, and attaching itself in front to the peritoneal epithelium acquires an opening into the body cavity. The process of hollowing is continued backwards pari passu with the splitting of the segmental duct. In the female the process is continued till the Mullerian duct opens, close to the Wolffian duct, into the cloaca. In the male the duct usually ends blindly. It is important to notice that the abdominal opening of the Mullerian duct in the Amphibia (Salamandra) is a formation independent of the pronephros, and placed slightly behind it ; and that the undivided anterior part of the segmental duct (with the pronephros) is not, as in Elasmobranchii, united with the Mullerian duct, but remains connected with the Wolffian duct.

The development of the Mullerian duct has not been satisfactorily studied in other forms besides Salamandra. In Cceciliidae its abdominal opening is on a level with the anterior end of the Wolffian body. In other forms it is usually placed very far forwards, close to the root of the lungs (except in Proteus and Batrachoseps, where it is placed somewhat further back), and some distance in front of the Wolffian body.

The Mullerian duct is always well developed in the female, and serves as oviduct. In the male it does not (except possibly in Alytes) assist in the transportation of the genital products, and is always more or less rudimentary, and in Anura may be completely absent.

After the formation of the Mullerian duct, the Wolffian duct remains as the excretory channel for the Wolffian body, and, till the atrophy of the pronephros, for this gland also. Its anterior section, in front of the Wolffian body, undergoes a more or less complete atrophy.

The further changes of the excretory system concern (i) the junction in the male of the anterior part of the Wolffian body with the testis ; (2) certain changes in the collecting tubes of the


EXCRETORY ORGANS.


711


posterior part of the mesonephros. The first of these processes results in the division of the Wolffian body into a sexual and a non-sexual part, and in Salamandra and other Urodeles the division corresponds with the distribution of the simple and compound segmental tubes.

Since the development of the canals connecting the testes with the sexual part of the Wolffian body has not been in all points satisfactorily elucidated, it will be convenient to commence with a description of the adult arrangement of the parts (fig. 400 B). In most instances a non-segmental system of canals the vasa effcrentia (ve) coming from the testis, fall into a canal known as the longitudinal canal of the Wolffian body, from which there pass off transverse canals, which fall into, and are equal in number to, the primary Malpighian bodies of the sexual part of the gland. The spermatozoa, brought to the Malpighian bodies, are thence transported along the segmental tubes to the Wolffian duct, and so to the exterior. The system of canals connecting the testis with the Malpighian bodies is known as the testicular network. The number of segmental tubes connected with the testis varies very greatly. In Siredon there are as many as from 30 32 (Spengel).

The longitudinal canal of the Wolffian body is in rare instances (Spelerpes, etc.) absent, where the sexual part of the Wolffian body is slightly developed. In the Urodela the testes are united with the anterior part of the Wolffian body. In the Cceciliidas the junction takes place in an homologous part of the Wolffian body, but, owing to the development of the anterior segmental tubes, which are rudimentary in the Urodela, it is situated some way behind the front end. Amongst the Anura the connection of the testis with the tubules of the Wolffian body is subject to considerable variations. In Bufo cinereus the normal Urodele type is preserved, and in Bombinator the same arrangement is found in a rudimentary condition, in that there are transverse trunks from the longitudinal canal of the Wolffian body, which end blindly, while the semen is carried into the Wolffian duct by canals in front of the Wolffian body. In Alytes and Discoglossus the semen is carried away by a similar direct continuation of the longitudinal canal in front of the Wolffian body, but there are no rudimentary transverse canals passing into the Wolffian body, as in Bombinator. In Rana the transverse ducts which pass off from the longitudinal canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian bodies, enter directly into the collecting tubes near their opening into the Wolffian duct.


712 AMPHIBIA.


In most Urodeles the peritoneal openings connected with the primary generative Malpighian bodies atrophy, but in Spelerpes they persist. In the Cceciliidie they also remain in the adult state.

With reference to the development of these parts little is known except that the testicular network grows out from the primary Malpighian bodies, and becomes united with the testis. Embryological evidence, as well as the fact of the persistence of the peritoneal funnels of the generative region in the adults of some forms, proves that the testicular network is not developed from the peritoneal funnels.

Rudiments of the testicular network are found in the female Cceciliidae and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and of transverse canals passing from this to the Malpighian bodies, together with some branches passing into the mesovarium.

Amongst the Urodela the collecting tubes of the hinder non-sexual part of the Wolffian body, which probably represents a rudimentary metanephros, undergo in the male sex a change similar to that which they usually undergo in Elasmobranchii. Their points of junction with the Wolffian duct are carried back to the hindermost end of the duct (fig. 400 B), and the collecting tubes themselves unite together into one or more short ducts (ureters) before joining the Wolffian duct.

In Batrachoseps only the first collecting tube becomes split off in this way ; and it forms a single elongated ureter which receives all the collecting tubes of the posterior segmental tubes. In the female and in the male of Proteus, Menobranchus, and Siren the collecting tubes retain their primitive transverse course and open laterally into the Wolffian duct. In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the cloaca.

The urinary bladder of the Amphibia is an outgrowth of the ventral wall of the cloacal section of the alimentary tract, and is homologous with the allantois of the amniotic Vertebrata.

The subjoined diagram (fig. 400) of the urogenital system of Triton illustrates the more important points of the preceding description.

In the female (A) the following parts are present :

(1) The Mullerian duct or oviduct (od) derived from the splitting of the segmental duct.

(2) The Wolffian duct (sug) constituting the portion of the segmental duct left after the formation of the Mullerian duct.

(3) The mesonephros (r), divided into an anterior sexual part


EXCRETORY ORGANS.


7'3


connected with a rudimentary testicular network, and a posterior part. The collecting tubes from both parts fall transversely into the Wolffian duct.

(4) The ovary (ov).

(5) The rudimentary testicular network.

In the male (B) the following parts are present :

(1) The functionless though fairly developed Miillerian duct (;).

(2) The Wolffian duct (sug).

(3) The mesonephros (r) divided into a true sexual part, through the segmental tubes of which the semen passes, and a non-sexual part. The collecting tubes of the latter do not enter the Wolffian duct directly, but bend obliquely backwards and only fall into it close to its cloacal aperture, after uniting to form one or two primary tubes (ureters).

(4) The testicular network (ve) consisting of (i) transverse ducts from the testes, falling into (2) the longitudinal canal of the Wolffian body, from which (3) transverse canals are again given off to the Malpighian bodies.

Amniota. The amniotic Vertebrata agree, so far as is known, very closely amongst themselves in the formation of the urinogenital system.

The most characteristic feature of the system is the full development of a metanephros, which constitutes the functional kidney on the atrophy of the mesonephros or Wolffian body, which is a purely embryonic organ. The first part of the system to develop is a duct, which is usually spoken of as the Wolffian duct, but which is really the homologue of the seg


FIG. 400. DIAGRAM OF THE URINOGENITAL SYSTEM OF TRITON. (From Gegenbaur ; after Spengel.)

A. Female. B. Male. r. mesonephros, on the surface of which numerous peritoneal funnels are visible ; sug. mesonephric or Wolffian duct; od. oviduct (Miillerian duct); in. Miillerian duct of male ; ve. vasa efferentia of testis ; t. testis ; ov. ovary ; up. urinogenital pore.


714 AMNIOTA.


mental duct. It apparently develops in all the Amniota nearly on the Elasmobranch type, as a solid rod, primarily derived from the somatic mesoblast of the intermediate cell mass (fig. 401 W.d}\

The first trace of it is visible in an embryo Chick with eight somites, as a ridge projecting from the intermediate cell mass towards the epiblast in the region of the seventh somite. In the course of further development it continues to constitute such a ridge as far as the eleventh somite (Sedgwick), but from this point it grows backwards in the space between the epiblast and mesoblast In an embryo with fourteen somites a small lumen has appeared in its middle part and in front it is connected with rudimentary Wolffian tubules, which develop in continuity with it (Sedgwick). In the succeeding stages the lumen of the duct gradually extends backwards and forwards, and the duct itself also passes inwards relatively to the epiblast (fig. 402). Its hindend elongates till it comes into connection with, and opens into, the cloacal section of the hind-gut' 2 .

It might have been anticipated that, as in the lower types, the anterior end of the segmental duct would either open into the body cavity, or come into connection with a pronephros. Neither of these occurrences takes place, though in some types (the Fowl) a structure, which is probably the rudiment of a pronephros, is developed ; it does not however appear till a later stage, and is then unconnected with the segmental duct. The next part of the system to appear is the mesonephros or Wolffian body.

This is formed in all Amniota as a series of segmental tubes, which in Lacertilia (Braun) correspond with the myotomes, but in Birds and Mammalia are more numerous.

In Reptilia (Braun, No. 542), the mesonephric tubes develop as segmentally-arranged masses on the inner side of the Wolffian duct, and appear to be at first united with the peritoneal epithelium. Each mass soon becomes an oval vesicle, probably opening for a very short period into the

1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as developing from a groove opening to the peritoneal cavity, which subsequently becomes constricted into a duct. I have never met with specimens such as those figured by these authors.

2 The foremost extremity of the segmental duct presents, according to Gasser, curious irregularities and an anterior completely isolated portion is often present.


EXCRETORY ORGANS.


715


peritoneal cavity by a peritoneal funnel. The vesicles become very early detached from the peritoneal epithelium, and lateral outgrowths from them give rise to the main parts of the segmental tubes, which soon unite with the segmental duct.

In Birds the development of the segmental tubes is more complicated 1 .

The tubules of the Wolffian body are derived from the intermediate cell mass, shewn in fig. 401, between the upper end of the body cavity and the


g.o.



FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN

EMBRYO CHICK OF 45 HOURS.

M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ; p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. bloodvessel.

muscle-plate. In the Chick the mode of development of this mass into the segmental tubules is different in the regions in front of and behind about the sixteenth segment. In front of about the sixteenth segment the intermediate cell mass becomes detached from the peritoneal epithelium at certain points, remaining attached to it at other points, there being several such to each segment. The parts of the intermediate cell mass attached to the peritoneal epithelium become converted into S-shaped cords (fig. 402, st] which soon unite with the segmental duct (wd}. Into the commencement of each of these cords the lumen of the body cavity is for a short distance prolonged, so that this part constitutes a rudimentary peritoneal funnel.

1 Correct figures of the early stages of these structures were first given by Kolliker, but the correct interpretation of them and the first satisfactory account of the development of the excretory organs of Birds was given by Sedgwick (No. 549).


716


AMNIOTA.


In the Duck the attachment of the intermediate cell mass to the peritoneal epithelium is prolonged further back than in the Chick.

In the foremost segmental tubes, which never reach a very complete development, the peritoneal funnels widen considerably, while at the same time they acquire a distinct lumen. The section of the tube adjoining the wide peritoneal funnel becomes partially invaginated by the formation of a glomerulus, and this glomerulus soon grows to such an extent as to project through the peritoneal funnel, the neck of which it completely fills, into the body cavity (fig. 403, gl). There is thus formed a series of free peritoneal glomeruli belonging to the anterior Wolfnan tubuli 1 . These tubuli become however early aborted.

In the case of the remaining tubules developed from the S-shaped cords the attachment to the peritoneal epithelium is very soon lost. The cords acquire a lumen, and open into the segmental duct. Their blind extremities constitute the rudiments of Malpighian bodies.


am



FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH

ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.

am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c. spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.

1 These external glomeruli were originally mistaken by me (No. 539) for the glomeralus of the pronephros, from their resemblance to the glomerulus of the Amphibian pronephros. Their true meaning was made out by Sedgwick (No. 550).


EXCRETORY ORGANS.


717


In the posterior part of the Wolffian body of the Chick the intermediate cell mass becomes very early detached from the peritoneal epithelium, and at a considerably later period breaks up into oval vesicles similar to those of the Reptilia, which form the rudiments of the segmental tubes.

Secondary and tertiary segmental tubules are formed in the Chick, on the dorsal side of the primary tubules, as direct differentiations of the mesoblast. They open independently into the Wolffian duct.

In Mammalia the segmental tubules (Egli) are formed as solid masses in the same situation as in Birds and Reptiles. It is not known whether they are united with the peritoneal epithelium. They soon become oval vesicles, which develop into complete tubules in the manner already indicated.



After the establishment of the Wolffian body there is formed in both sexes in all the Amniota a duct, which in the female becomes the oviduct, but which is functionless and disappears more or less completely in the male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Mullerian duct of


FIG. 403. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK OF ABOUT IOO H.

gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao. aorta ; me. mesentery. The segmental tube, and the connection between the external and internal parts of the glomerulus are not shewn in this figure.



FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS). (After Balfour and Sedgwick. )

A is the nth section of the series. B i 5th

C i8th ,, ,,

gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.


7 i8


AMNIOTA.


the Ichthyopsida. In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick).

The pronephros, as I shall call it, consists of a slightly convoluted longitudinal canal with three or more peritoneal openings. In the earliest condition, it consists of three successive open involutions of the peritoneal epithelium, connected together by more or less well-defined ridge-like thickenings of the epithelium. It takes its origin from the layer of thickened peritoneal epithelium situated near the dorsal angle of the body cavity, and is situated some considerable distance behind the front end of the Wolfifian duct.

In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium,



FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times. m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes place; a. thickened portion of the germinal epithelium in which the primitive germinal cells C and o are lying ; E. modified mesoblast which will form the stroma of the ovary ; WK. Wolffian body ; y. Wolffian duct.


EXCRETORY ORGANS. 719

and develop a lumen. The condition of the structure at this stage is illustrated by fig. 404, representing three transverse sections through two grooves, and through the ridge connecting them.

The pronephros may in fact now be described as a slightly convoluted duct, opening into the body cavity by three groovelike apertures, and continuous behind with the rudiment of the true Miillerian duct.

The stage just described is that of the fullest development of the pronephros. In it, as in all the previous stages, there appear to be only three main openings into the body cavity ; but in some sections there are indications of the possible presence of one or two additional rudimentary grooves.

In an embryo not very much older than the one last described the pronephros atrophies as such, its two posterior openings vanishing, and its anterior opening remaining as the permanent opening of the Miillerian duct.

The pronephros is an extremely transitory structure, and its development and atrophy are completed between the QOth and i2Oth hours of incubation.

The position of the pronephros in relation to the Wolffian body is shewn in fig. 405, which probably passes through a region between two of the peritoneal openings. As long as the pronephros persists, the Mullerian duct consists merely of a very



FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour

and Sedgwick.)

In A the terminal portion of the duct is quite distinct ; in B it has united with the walls of the Wolffian duct.

md. Mullerian duct ; Wd. Wolffian duct.


72O AMNIOTA.


small rudiment, continuous with the hindermost of the three peritoneal openings, and its solid extremity appears to unite with the walls of the Wolffian duct.

After the atrophy of the pronephros, the Miillerian duct commences to grow rapidly, and for the first part of its course it appears to be split off as a solid rod from the outer or ventral wall of the Wolffian duct (fig. 406). Into this rod the lumen, present in its front part, subsequently extends. Its mode of development in front is thus precisely similar to that of the Miillerian duct in Elasmobranchii and Amphibia.

This mode of development only occurs however in the anterior part of the duct. In the posterior part of its course its growing point lies in a bay formed by the outer walls of the Wolffian duct, but does not become definitely attached to that duct. It seems however possible that, although not actually split off from the walls of the Wolrfian duct, it may grow backwards from cells derived from that duct.

The Miillerian duct finally reaches the cloaca though it does not in the female for a long time open into it, and in the male never does so.

The mode of growth of the Miillerian duct in the posterior part of its course will best be understood from the following description quoted from the paper by Sedgwick and myself.

"A few sections before its termination the Miillerian duct appears as a well-defined oval duct lying in contact with the wall of the Wolffian duct on the one hand and the germinal epithelium on the other. Gradually, however, as we pass backwards, the Miillerian duct dilates ; the external wall of the Wolffian duct adjoining it becomes greatly thickened and pushed in in its middle part, so as almost to touch the opposite wall of the duct, and so form a bay in which the Miillerian duct lies. As soon as the Miillerian duct has come to lie in this bay its walls lose their previous distinctness of outline, and the cells composing them assume a curious vacuolated appearance. No well-defined line of separation can any longer be traced between the walls of the Wolffian duct and those of the Miillerian, but between the two is a narrow clear space traversed by an irregular network of fibres, in some of the meshes of which nuclei are present.

The Miillerian duct may be traced in this condition for a considerable number of sections, the peculiar features above described becoming more and more marked as its termination is approached. It continues to dilate and attains a maximum size in the section or so before it disappears. A lumen may be observed in it up to its very end, but is usually irregular in outline and frequently traversed by strands of protoplasm. The Miillerian


EXCRETORY ORGANS. 721

duct finally terminates quite suddenly, and in the section immediately behind its termination the Wolffian duct assumes its normal appearance, and the part of its outer wall on the level of the Miillerian duct conies into contact with the germinal epithelium."

Before describing the development of the Mullerian duct in other Amniotic types it will be well to say a few words as to the identifications above adopted. The identification of the duct, usually called the Wolffian duct, with the segmental duct (exclusive of the pronephros) appears to be morphologically justified for the following reasons : (i) that it gives rise to part of the Mullerian duct as well as to the duct of the Wolffian body ; behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian body, before the Mullerian duct originates from it. (3) That it develops in a manner strikingly similar to that of the segmental duct of various lower forms.

With reference to the pronephros it is obvious that the organ identified as such is in many respects similar to the pronephros of the Amphibia. Both consist of a somewhat convoluted longitudinal canal, with a certain number of peritoneal openings ;

The main difficulties in the homology are :

(1) the fact that the pronephros in the Bird is not united with the segmental duct ;

(2) the fact that it is situated behind the front end of the Wolffian body. It is to be remembered in connection with the first of these difficulties

that in the formation of the Mullerian duct in Elasmobranchii the anterior undivided extremity of the primitive segmental duct, with the peritoneal opening, which probably represents the pronephros, is attached to the Mullerian duct, and not to the Wolffian duct ; though in Amphibia the reverse is the case. To explain the discontinuity of the pronephros with the segmental duct it is only necessary to suppose that the segmental duct and pronephros, which in the Ichthyopsida develop as a single formation, develop in the Bird as two independent structures a far from extravagant supposition, considering that the pronephros in the Bird is undoubtedly quite functionless.

With reference to the posterior position of the pronephros it is only necessary to remark that a change in position might easily take place after the acquirement of an independent development, and that the shifting is probably correlated with a shifting of the abdominal opening of the Mullerian duct.

The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota. The Mullerian duct is also usually stated to develop as a groove of the peritoneal epithelium, shewn in the Lizard in fig. 354, md., which is continued backward as a primitively solid rod in the space between B. ill. 46


722


AM N IOTA.


the Wolffian duct and peritoneal epithelium, without becoming attached to the Wolffian duct.

On the formation of the Miillerian duct, the duct of the mesonephros becomes the true mesonephric or Wolffian duct.

After these changes have taken place a new organ of great importance makes its appearance. This organ is the permanent kidney, or metanephros.

Metanephros. The mode of development of the metanephros has as yet only been satisfactorily elucidated in the Chick (Sedgwick, No. 549). The ureter and the collecting tubes of the kidney are developed from a dorsal outgrowth of the hinder part of the Wolffian duct. The outgrowth from the Wolffian duct grows forwards, and extends along the outer side of a mass of mesoblastic tissue which lies mainly behind, but somewhat overlaps the dorsal aspect of the Wolffian body.

This mass of mesoblastic cells may be called the metanephric blastema. Sedgwick, of the accuracy of whose account I have satisfied myself, has shewn that in the Chick it is derived from the intermediate cell mass of the region of about the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion of the intermediate cell mass of the region immediately in front of it, which breaks up into Wolffian tubules. The metanephric blastema remains however quite passive during the formation of the Wolffian tubules in the adjoining blastema ; and on the formation of the ureter breaks off from the Wolffian body in front, and, growing forwards and dorsalwards, places itself on the inner side of the ureter in the position just described.

In the subsequent development of the kidney collecting tubes grow out from the ureter, and become continuous with masses of cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.

The process just described appears to me to prove that the kidney of the A mniota is a specially differentiated posterior section of the primitive mesonephros.

According to the view of Remak and Kolliker the outgrowths from the ureter give rise to the whole of the tubuli uriniferi and the capsules of the Malpighian bodies, the mesoblast around them forming blood-vessels, etc. On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in


EXCRETORY ORGANS. 723


accordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formed in situ in the adjacent mesoblast.

Braun (No. 542) has arrived at the conclusion that in the Lacertilia the tissue, out of which the tubuli of the metanephros are formed, is derived from irregular solid ingrowths of the peritoneal epithelium, in a region behind the Wolffian body, but in a position corresponding to that in which the segmental tubes take their origin. These ingrowths, after separating from the peritoneal epithelium, unite together to form a cord into which the ureter sends the lateral outgrowths already described. These outgrowths unite with secreting tubuli and Malpighian bodies, formed in situ. In Lacertilia the blastema of the kidney extends into a postanal region. Braun's account of the origin of the metanephric blastema does not appear to me to be satisfactorily demonstrated.

The ureter does not long remain attached to the Wolffian duct, but its opening is gradually carried back, till (in the Chick between the 6th and 8th day) it opens independently into the cloaca.

Of the further changes in the excretory system the most important is the atrophy of the greater part of the Wolffian body, and the conversion of the Wolffian duct in the male sex into the vas deferens, as in Amphibia and the Elasmobranchii.

The mode of connection of the testis with the Wolffian duct is very remarkable, but may be derived from the primitive arrangement characteristic of Elasmobranchii and Amphibia.

In the structures connecting the testis with the Wolffian body two parts have to be distinguished, (i) that equivalent to the testicular network of the lower types, (2) that derived from the segmental tubes. The former is probably to be found in peculiar outgrowths from the Malpighian bodies at the base of the testes.

These were first discovered by Braun in Reptilia, and consist in this group of a series of outgrowths from the primary (?) Malpighian bodies along the base of the testis : they unite to form an interrupted cord in the substance of the testis, from which the testicular tubuli (with the exception of the seminiferous cells) are subsequently differentiated. These outgrowths, with the exception of the first two or three, become detached from the Malpighian bodies. Outgrowths similar to those in the male are found in the female, but subsequently atrophy.

Outgrowths homologous with those found by Braun have

46 2


724 AMNIOTA.


been detected by myself (No. 555) in Mammals. It is not certain to what parts of the testicular tubuli they give rise, but they probably form at any rate the vasa recta and rete vasculosum.

In Mammals they also occur in the female, and give rise to cords of tissue in the ovary, which may persist through life.

The comparison of the tubuli, formed out of these structures, with the Elasmobranch and Amphibian testicular network is justified in that both originate as outgrowths from the primary Malpighian bodies, and thence extend into the testis, and come into connection with the true seminiferous stroma.

As in the lower types the semen is transported from the testicular network to the Wolffian duct by parts of the glandular tubes of the Wolffian body. In the case of Reptilia the anterior two or three segmental tubes in the region of the testis probably have this function. In the case of Mammalia the vasa efferentia, i.e. the coni vasculosi, appear, according to the usually accepted view, to be of this nature, though Banks and other investigators believe that they are independently developed structures. Further investigations on this point are required. In Birds a connection between the Wolffian body and the testis appears to be established as in the other types. The Wolffian duct itself becomes, in the males of all Amniota, the vas deferens and the convoluted canal of the epididymis the latter structure (except the head) being entirely derived from the Wolffian duct.

In the female the Wolffian duct atrophies more or less completely.

In Snakes (Braun) the posterior part remains as a functionless canal, commencing at the ovary, and opening into the cloaca. In the Gecko (Braun) it remains as a small canal joining the ureter ; in Blindworms a considerable part of the canal is left, and in Lacerta (Braun) only interrupted portions.

In Mammalia the middle part of the duct, known as Gaertner's canal, persists in the females of some monkeys, of the pig and of many ruminants.

The Wolffian body atrophies nearly completely in both sexes ; though, as described above, part of it opposite the testis persists as the head of the epididymis. The posterior part of the gland from the level of the testis may be called the sexual part of the gland, the anterior part forming the non-sexual part.


EXCRETORY ORGANS. 725

The latter, i.e. the anterior part, is first absorbed ; and in some Reptilia the posterior part, extending from the region of the genital glands to the permanent kidney, persists till into the second year.

Various remnants of the Wolffian body are found in the adults of both sexes in different types. The most constant of them is perhaps the part in the female equivalent to the head of the epididymis and to parts also of the coiled tube of the epididymis, which may be called, with Waldeyer, the epoophoron 1 . This is found in Reptiles, Birds and Mammals ; though in a very rudimentary form in the first-named group. Remnants of the anterior non-sexual part of the Wolffian bodies have been called by Waldeyer parepididymis in the male, and paroophoron in the female. Such remnants are not (Braun) found in Reptilia, but are stated to be found in both male and female Birds, as a small organ consisting of blindly ending tubes with yellow pigment. In some male Mammals (including Man) a parepididymis is found on the upper side of the testis. It is usually known as the organ of Giraldes.

The Mlillerian duct forms, as has been stated, the oviduct in the female. The two ducts originally open independently into the cloaca, but in the Mammalia a subsequent modification of this arrangement occurs, which is dealt with in a separate section. In Birds the right oviduct atrophies, a vestige being sometimes left. In the male the Miillerian ducts atrophy more or less completely.

In most Reptiles and in Birds the atrophy of the Miillerian ducts is complete in the male, but in Lacerta and Anguis a rudiment of the anterior part has been detected by Leydig as a convoluted canal. In the Rabbit (Kolliker) 2 and probably other Mammals the whole of the ducts probably disappears, but in some Mammals, e.g. Man, the lower fused ends of the Miillerian ducts give rise to a pocket opening into the urethra, known as the uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the rudiments are more considerable, and may be continued into horns homologous with the horns of the uterus (Weber).

The hydatid of Morgani in the male is supposed (Waldeyer) to represent the abdominal opening of the Fallopian tube in the female, and therefore to be a remnant of the Miillerian duct.

Changes in the lower parts of the urinogenital ducts in the Amniota.

The genital cord. In the Monodelphia the lower part of the Wolffian ducts becomes enveloped in both sexes in a special

1 This is also called parovarium (His), and Rosenmiiller's organ.

2 Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but his account is by no means satisfactory, and its presence is distinctly denied by Kolliker.


726


AMNIOTA.


cord of tissue, known as -the genital cord (fig. 407, gc), within the lower part of which the MUllerian ducts are also enclosed. In the male the MUllerian ducts in this cord atrophy, except at their distal end where they unite to form the uterus masculinus. The Wolffian ducts, after becoming the vasa deferentia, remain for some time enclosed in the common cord, but afterwards separate from each other. The seminal vesicles are outgrowths of the vasa deferentia.

In the female the Wolffian ducts within the genital cord atrophy, though rudiments of them are for a long time visible or even permanently persistent. The lower parts of the MUllerian ducts unite to form the vagina and body of the uterus. The junction commences in the middle and extends forwards and backwards ; the stage with a median junction being retained permanently in Marsupials.

The urinogenital sinus and external generative organs. In all the Amniota, there open at first into the common cloaca the alimentary canal dorsally, the allantois ventrally, and the Wolffian and MUllerian ducts and ureters laterally. In Reptilia and Aves the embryonic condition is retained. In both groups the allantois serves as an embryonic urinary bladder, but while it atrophies in Aves, its stalk dilates to form a permanent urinary bladder in Reptilia. In Mammalia the dorsal part of the cloaca with the alimentary tract becomes first of all partially constricted off from the ventral, which then forms a urinogenital sinus (fig. 407, ug). In the course of development the urinogenital sinus becomes, in all Mammalia but the Ornithodelphia, completely separated from the intestinal cloaca, and the two parts obtain separate external openings. The ureters (fig. 407, 3) open higher up than the other ducts into the stalk of the allantois which dilates to form the bladder (4). The stalk connecting the bladder with the ventral wall of the body constitutes the urachus, and loses its lumen before the close of embryonic life. The part of the stalk of the allantois below the openings of the ureters narrows to form the urethra, which opens together with the Wolffian and MUllerian ducts into the urinogenital cloaca.

In front of the urinogenital cloaca there is formed a genital prominence (fig. 407, cp), with a groove continued from the


EXCRETORY ORGANS. 727

urinogenital opening ; and on each side a genital fold (&). In the male the sides of the groove on the prominence coalesce together, embracing between them the opening of the urinogenital cloaca ; and the prominence itself gives rise to the penis,



FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.)

The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are seen from the front.

3. ureter; 4. urinary bladder ; 5. urachus; of. genital ridge (ovary or testis) ; W. left Wolffian body ; x. part at apex from which coni vasculosi are afterwards developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of Wolffian and Mullerian ducts bound up in a common sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from which the labia majora or scrotum are developed.

along which the common urinogenital passage is continued. The two genital folds unite from behind forwards to form the scrotum.

In the female the groove on the genital prominence gradually disappears, and the prominence remains as the clitoris, which is therefore the homologue of the penis : the two genital folds form the labia majora. The urethra and vagina open independently into the common urinogenital sinus.


728 GENERAL CONCLUSIONS.

General conclusions and Summary.

Pronephros. Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg. Thus it is practically absent in the embryos of Elasmobranchii and the Amniota, but present in the larvae of all other forms.

This coincidence, on the principles already laid down in a previous chapter on larval forms, affords a strong presumption that the pronephros is an ancestral organ ; and, coupled with the fact that it is the first part of the excretory system to be developed, and often the sole excretory organ for a considerable period, points to the conclusion that the pronephros and its duct the segmental duct are the most primitive parts of the Vertebrate excretory system. This conclusion coincides with that arrived at by Gegenbaur and Fiirbringer.

The duct of the pronephros is always developed prior to the gland, and there are two types according to which its development may take place. It may either be formed by the closing in of a continuous groove of the somatic peritoneal epithelium (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of cells derived from the somatic mesoblast, which grows backwards between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).

It is quite certain that the second of these processes is not a true record of the evolution of 'the duct, and though it is more possible that the process observable in Amphibia and the Teleostei may afford some indications of the manner in which the duct was established, this cannot be regarded as by any means certain.

The mode of development of the pronephros itself is apparently partly dependent on that of its duct. In Petromyzon, where the duct does not at first communicate with the body cavity, the pronephros is formed as a series of outgrowths from the duct, which meet the peritoneal epithelium and open into the body cavity ; but in other instances it is derived from the anterior open end of the groove which gives rise to the segmental duct. The open end of this groove may either remain single


EXCRETORY ORGANS. 729

(Teleostci, Ganoidei) or be divided into two, three or more apertures (Amphibia). The main part of the gland in either case is formed by convolutions of the tube connected with the peritoneal funnel or funnels. The peritoneal funnels of the pronephros appear to be segmentally arranged.

The pronephros is distinguished from the mesonephros by developmental as well as structural features. The most important of the former is the fact that the glandular tubules of which it is formed are always outgrowths of the segmental duct ; while in the mesonephros they are always or almost always 1 formed independently of the duct.

The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros; unless the structures found in Myxine are to be regarded as such. Functionally the place of such Malpighian bodies is taken by the vascular peritoneal ridge spoken of in the previous pages as the glomerulus.

That this body is really related functionally to the pronephros appears to be indicated (i) by its constant occurrence with the pronephros and its position opposite the peritoneal openings of this body ; (2) by its atrophy at the same time as the pronephros ; (3) by its enclosure together with the pronephridian stoma in a special compartment of the body-cavity in Teleostei and Ganoids, and its partial enclosure in such a compartment in Amphibia.

The pronephros atrophies more or less completely in most types, though it probably persists for life in the Teleostei and Ganoids, and in some members of the former group it perhaps forms the sole adult organ of excretion.

The cause of its atrophy may perhaps be related to the fact that it is situated in the pericardial region of the body-cavity, the dorsal part of which is aborted on the formation of a closed pericardium ; and its preservation in Teleostei and Ganoids may on this view be due to the fact that in these types its peritoneal funnel and its glomerulus are early isolated in a special cavity.

Mesonephros. The mesonephros is in all instances composed of a series of tubules (segmental tubes) which are developed independently of the segmental duct. Each tubule is

1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an exception to the general rule that there is no outgrowth from the segmental or metanephric duct to meet the segmental tubes.


730 GENERAL CONCLUSIONS.

typically formed of (i) a peritoneal funnel opening into (2) a Malpighian body, from which there proceeds (3) a coiled glandular tube, finally opening by (4) a collecting tube into the segmental duct, which constitutes the primitive duct for the mesonephros as well as for the pronephros.

The development of the mesonephridian tubules is subject to considerable variations.

(1) They may be formed as differentiations of the intermediate cell mass, and be from the first provided with a lumen, opening into the body-cavity, and directly derived from the section of the body-cavity present in the intermediate cell mass; the peritoneal funnels often persisting for life (Elasmobranchii).

(2) They may be formed as solid cords either attached to or independent of the peritoneal epithelium, which after first becoming independent of the peritoneal epithelium subsequently send downwards a process, which unites with it and forms a peritoneal funnel, which may or may not persist (Acipenser, Amphibia).

(3) They may be formed as in the last case, but acquire no secondary connection with the peritoneal epithelium (Teleostei, Amniota). In connection with the original attachment to the peritoneal epithelium, a true peritoneal funnel may however be developed (Aves, Lacertilia).

Physiological considerations appear to shew that of these three methods of development the first is the most primitive. The development of the tubes as solid cords can hardly be primary.

A question which has to be answered in reference to the segmental tubes is that of the homology of the secondarily developed peritoneal openings of Amphibia, with the primary openings of the Elasmobranchii. It is on the one hand difficult to understand why, if the openings are homologous in the two types, the original peritoneal attachment should be obliterated in Amphibia, only to be shortly afterwards reacquired. On the other hand it is still more difficult to understand what physiological gain there could be, on the assumption of the non-homology of the openings, in the replacement of the primary opening by a secondary opening exactly similar to it. Considering the great variations in development which occur in undoubtedly homologous parts I incline to the view that the openings in the two types are homologous.


EXCRETORY ORGANS.


731


In the majority of the lower Vertebrata the mesonephric tubes have at first a segmental arrangement, and this is no doubt the primitive condition. The coexistence of two, three, or more of them in a single segment in Amphibia, Aves and Mammalia has recently been shewn, by an interesting discovery of Eisig, to have a parallel amongst Chaetopods, in the coexistence of several segmental organs in a single segment in some of the Capitellidae.

In connection with the segmental features of the mesonephros it is perhaps worth recalling the fact that in Elasmobranchii as well as other types there are traces of segmental tubes in some of the postanal segments. In the case of all the segmental tubes a Malpighian body becomes established close to the extremity of the tube adjoining the peritoneal opening, or in an homologous position in tubes without such an opening. The opposite extremity of the tube always becomes attached to the segmental duct.

In many of the segments of the mesonephros, especially in the hinder ones, secondary and tertiary tubes become developed in certain types, which join the collecting canals of the primary tubes, and are provided, like the primary tubes, with Malpighian bodies at their blind extremities.

There can it appears to me be little or no doubt that the secondary tubes in the different types are homodynamous if not homologous. Under these circumstances it is surprising to find in what different ways they take their origin. In Elasmobranchii a bud sprouts out from the Malpighian body of one segment, and joins the collecting tube of the preceding segment, and subsequently, becoming detached from the Malpighian body from which it sprouted, forms a fresh secondary Malpighian body at its blind extremity. Thus the secondary tubes of one segment are formed as buds from the segment behind. In Amphibia (Salamandra) and Aves the secondary tubes develop independently in the mesoblast. These great differences in development are important in reference to the homology of the metanephros or permanent kidney, which is discussed below.

Before leaving the mesonephros it may be worth while putting forward some hypothetical suggestions as to its origin and relation to the pro


732 GENERAL CONCLUSIONS.

nephros, leaving however the difficult questions as to the homology of the segmental tubes with the segmental organs of Chastopods for subsequent discussion.

It is a peculiarity in the development of the segmental tubes that they at first end blindly, though they subsequently grow till they meet the segmental duct with which they unite directly, without the latter sending out any offshoot to meet them 1 . It is difficult to believe that peritoneal infundibula ending blindly and unprovided with some external orifice can have had an excretory function, and we are therefore rather driven to suppose that the peritoneal infundibula which become the segmental tubes were either from the first provided each with an orifice opening to the exterior, or were united with the segmental duct. If they were from the first provided with external openings we may suppose that they became secondarily attached to the duct of the pronephros (segmental duct), and then lost their external openings, no trace of these structures being left, even in the ontogeny of the system. It would appear to me more probable that the pronephros, with its duct opening into the cloaca, was the only excretory organ of the unsegmented ancestors of the Chordata, and that, on the elongation of the trunk and its subsequent segmentation, a series of metameric segmental tubes became evolved opening into the segmental duct, each tube being in a sort of way serially homologous with the primitive pronephros. With the segmentation of the trunk the latter structure itself may have acquired the more or less definite metameric arrangement of its parts.

Another possible view is that the segmental tubes may be modified derivatives of posterior lateral branches of the pronephros, which may at first have extended for the whole length of the body-cavity. If there is any truth in this hypothesis it is necessary to suppose that, when the unsegmented ancestor of the Chordata became segmented, the posterior branches of the primitive excretory organ became segmentally arranged, and that, in accordance with the change thus gradually introduced in them, the time of their development became deferred, so as to accord to a certain extent with the time of formation of the segments to which they belonged. The change in their mode of development which would be thereby introduced is certainly not greater than that which has taken place in the case of segmental tubes, which, having originally developed on the Elasmobranch type, have come to develop as they do in the posterior part of the mesonephros of Salamandra, Birds, etc.

Genital ducts. So far the origin and development of the excretory organs have been considered without reference to the modifications introduced by the excretory passages coming to serve as generative ducts. Such an unmodified state of the

1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior segmental tubes of the Chick form an exception to this general statement.


EXCRETORY ORGANS. 733


excretory organs is perhaps found permanently in Cyclostomata 1 and transitorily in the embryos of most forms.

At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (vide p. 626).

The secondary relations of the excretory ducts to the generative organs seem to have been introduced by an opening connected with the pronephridian extremity of the segmental duct having acquired the function of admitting the generative products into it, and of carrying them outwards ; so that primitively the segmental duct must have served as efferent duct both for the generative products and the pronepJiric secretion (just as the Wolffian duct still does for the testicular products and secretion of the Wolffian body in Elasmobranchii and Amphibia).

The opening by which the generative products entered the segmental duct can hardly have been specially developed for this purpose, but must almost certainly have been one of the peritoneal openings of the pronephros. As a consequence (by a process of natural selection) of the segmental duct having both a generative and a urinary function, a further differentiation took place, by which that duct became split into two a ventral Mullerian duct and a dorsal Wolffian duct.

The Mullerian duct was probably continuous with one or more of the abdominal openings of the pronephros which served as generative pores. At first the segmental duct was probably split longitudinally into two equal portions, and this mode of splitting is exceptionally retained in some Elasmobranchii ; but the generative function of the Mullerian duct gradually impressed itself more and more upon the embryonic development, so that, in the course of time, the Mullerian duct developed less and less at the expense of the Wolffian duct. This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in this respect a less primitive condition than the Elasmobranchii ; while in Aves it has been carried even further, and it seems possible that in some Amniota the Mullerian and segmental

1 It is by no means certain that the transportation outwards of the genital products by the abdominal pores in the Cyclostomata may not be the result of degeneration.


734 GENERAL CONCLUSIONS.

ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fiirbringer). The abdominal opening no doubt also became specialised. At first it is quite possible that more than one pronephric abdominal funnel may have served for the entrance of the generative products ; this function being, no doubt, eventually restricted to one of them.

Three different types of development of the abdominal opening of the Mullerian duct have been observed.

In Amphibia (Salamandra) the permanent opening of the Mullerian duct is formed independently, some way behind the pronephros.

In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Mullerian duct, and no true pronephros appears to be formed.

In Birds the anterior of the three openings of the rudimentary pronephros remains as the permanent opening of the Mullerian duct.

These three modes of development very probably represent specialisations of the primitive state along three different lines. In Amphibia the specialisation of the opening appears to have gone so far that it no longer has any relation to the pronephros. It was probably originally one of the posterior openings of this gland.

In Elasmobranchii, on the other hand, the functional opening is formed at a period when we should expect the pronephros to develop. This state is very possibly the result of a differentiation by which the pronephros gradually ceased to become developed, but one of its peritoneal openings remained as the abdominal aperture of the Mullerian duct. Aves, finally, appear to have become differentiated along a third line ; since in their ancestors the anterior (?) pore of the head-kidney appears to have become specialised as the permanent opening of the Mullerian duct.

The Mullerian duct is usually formed in a more or less complete manner in both sexes. In Ganoids, where the separation between it and the Wolffian duct is not completed to the cloaca, and in the Dipnoi, it probably serves to carry off the generative products of both sexes. In other cases however only the female


EXCRETORY ORGANS.


735


products pass out by it, and the partial or complete formation of the Mullerian duct in the male in these cases needs to be explained. This may be done either by supposing the Ganoid arrangement to have been the primitive one in the ancestors of the other forms, or, by supposing characters acquired primitively by the female to have become inherited by both sexes.

It is a question whether the nature of the generative ducts of Teleostei can be explained by comparison with those of Ganoids. The fact that the Mullerian ducts of the Teleostean Ganoid Lepidosteus attach themselves to the generative organs, and thus acquire a resemblance to the generative ducts of Teleostei, affords a powerful argument in favour of the view that the generative ducts of both sexes in the Teleostei are modified Mullerian ducts. Embryology can however alone definitely settle this question.

In the Elasmobranchii, Amphibia, and Amniota the male products are carried off by the Wolffian duct, and they are transported to this duct, not by open peritoneal funnels of the mesonephros, but by a network of ducts which sprout either from a certain number of the Malpighian bodies opposite the testis (Amphibia, Amniota), or from the stalks connecting the Malpighian bodies with the open funnels (Elasmobranchii). After traversing this network the semen passes (except in certain Anura) through a variable number of the segmental tubes directly to the Wolffian duct. The extent of the connection of the testis with the Wolffian body is subject to great variations, but it is usually more or less in the anterior region. Rudiments of the testicular network have in many cases become inherited by the female.

The origin of the connection between the testis and Wolffian body is still very obscure. It would be easy to understand how the testicular products, after falling into the body-cavity, might be taken up by the open extremities of some of the peritoneal funnels, and how such open funnels might have groove-like prolongations along the mesorchium, which might eventually be converted into ducts. Ontogeny does not however altogether favour this view of the origin of the testicular network. It seems to me nevertheless the most probable view which has yet been put forward.

The mode of transportation of the semen by means of the mesonephric tubules is so peculiar as to render it highly improbable that it was twice acquired, it becomes therefore necessary to suppose that the Amphibia and


736 GENERAL CONCLUSIONS.

Amniota inherited this mode of transportation of the semen from the same ancestors as the Elasmobranchii. It is remarkable therefore that in the Ganoidei and Dipnoi this arrangement is not found.

Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the Miillerian duct serving for both sexes is the primitive arrangement, and the Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary condition, which has originated at a stage in the evolution of the Vertebrata when some of the segmental tubes had begun to serve as the efferent ducts of the testis, and has resulted in consequence of a degeneration of the latter structures. Although the second alternative is the more easy to reconcile with the affinities of the Ganoid and Elasmobranch types, as indicated by the other features of their organization, I am still inclined to accept the former ; and consider that the incomplete splitting of the segmental duct in Ganoidei is a strong argument in favour of this view.

Metanephros. With the employment of the Wolffian duct to transport the semen there seems to be correlated (i) a tendency of the posterior segmental tubes to have a duct of their own, in which the seminal and urinary fluids cannot become mixed, and (2) a tendency on the part of the anterior segmental tubes to lose their excretory function. The posterior segmental tubes, when connected in this way with a more or less specialised duct, have been regarded in the preceding pages as constituting a metanephros.

This differentiation is hardly marked in the Anura, but is well developed in the Urodela and in the Elasmobranchii ; and in the latter group has become inherited by both sexes. In the Amniota it culminates, according to the view independently arrived at by Semper and myself, (i) in the formation of a completely distinct metanephros in both sexes, formed however, as shewn by Sedgwick, from the same blastema as the Wolffian body, and (2) in the atrophy in the adult of the whole Wolffian body, except the part uniting the testis and the Wolffian duct.

The homology between the posterior metanephridian section of the Wolffian body, in Elasmobranchii and Urodela, and the kidney of the Amniota, is only in my opinion a general one, i.e. in both cases a common cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more or less similar differentiation of parts.

Fiirbringer has urged against Semper's and my view that no satisfactory proof of it has yet been offered. This proof has however, since Fiirbringer wrote his paper, been supplied by Sedgwick's observations. The development of the kidney in the Amniota is no doubt a direct as opposed to a phylogenetic development ; and the substitution of a direct for


EXCRETORY ORGANS. 737


a phylogenetic development has most probably been rendered possible by the fact that the anterior part of the mesonephros continued all the while to be unaffected and to remain as the main excretory organ during foetal life.

The most serious difficulty urged by Fiirbringer against the homology is the fact that the ureter of the metanephros develops on a type of its own, which is quite distinct from the mode of development of the ureters of the metanephros of the Ichthyopsidan forms. It is however quite possible, though far from certain, that the ureter of Amniota may be a special formation confined to that group, and this fact would in no wise militate against the homology I have been attempting to establish.

Comparison of the Excretory organs of the Chordata and Invertebrata.

The structural characters and development of the various forms of excretory organs described in the preceding pages do not appear to me to be sufficiently distinctive to render it possible to establish homologies between these organs on a satisfactory basis, except in closely related groups.

The excretory organs of the Platyelminthes are in many respects similar to the provisional excretory organ of the trochosphere of Polygordius and the Gephyrea on the one hand, and to the Vertebrate pronephros on the other ; and the Platyelminth excretory organ with an anterior opening might be regarded as having given origin to the trochosphere organ, while that with a posterior opening may have done so for the Vertebrate pronephros 1 .

Hatschek has compared the provisional trochosphere excretory organ of Polygordius to the Vertebrate pronephros, and the posterior Chastopod segmental tubes to the mesonephric tubes ; the latter homology having been already suggested independently by both Semper and myself. With reference to the comparison of the pronephros with the provisional excretory organ of Polygordius there are two serious difficulties :

(1) The pronephric (segmental) duct opens directly into the cloaca, while the duct of the provisional trochosphere excretory organ opens anteriorly, and directly to the exterior.

(2) The pronephros is situated within the segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts ; while the provisional trochosphere organ is placed in front of the segmented region of the trunk, and is in no way segmented.

The comparison of the mesonephric tubules with the segmental excretory organs of the Chaetopoda, though not impossible, cannot be satisfactorily admitted till some light has been thrown upon the loss of the supposed external openings of the tubes, and the origin of their secondary connection with the segmental duct.

1 This suggestion has I believe been made by Fiirbringer. B. III. 47


738 BIBLIOGRAPHY.


Confining our attention to the Invertebrata it appears to me fairly clear that Hatschek is justified in holding the provisional trochosphere excretory organs of Polygordius, Echiurus and the Mollusca to be homologous. The atrophy of all these larval organs may perhaps be due to the presence of a well-developed trunk region in the adult (absent in the larva), in which excretory organs, probably serially homologous with those present in the anterior part of the larva, became developed. The excretory organs in the trunk were probably more conveniently situated than those in the head, and the atrophy of the latter in the adult state was therefore brought about, while the trunk organs became sufficiently enlarged to serve as the sole excretory organs.

BIBLIOGRAPHY OF THE EXCRETORY ORGANS. Invertebrata.

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Stat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d. Cesto'ides." Archives de Biologic, Vol. I. 1880.

(514) B. Hatschek. "Studien lib. Entwick. d. Anneliden." Arbeit, a. d. zool. Instit. Wien, Vol. I. 1878.

(515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d. zool. Instit. Wien, Vol. in. 1880.

EXCRETORY ORGANS OF VERTEBRATA. General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." yournal of Anat. and Phys., Vol. X. 1876.

(517) Max. Furbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meek el. Zur Morphol. d. Hani- u. Geschlechtnverkz.d. Wirbelthiere, etc. Halle, 1848.

(519) Joh. Miiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Rathke. " Beobachtungen u. Betrachtungen u. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825.

(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875 (522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.


1 The papers of Furbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.


BIBLIOGRAPHY. 739


ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat., Vol. XI. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k. Ak. Wiss. Berlin, 1845.

(525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschri/t, Vol. IX. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. n. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Entivicklung d. Teleostierniere. Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersitchungen ii. die mdnnlichen Geschleehts- und Harnwerkzeuge d. nackten Amphibien. Dorpat, 1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Hisiologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wiirzburg. Vol. III. 1876.

(538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit. f. wiss. Zool., Vol. IV.

Vide also Gotte (No. 296).

Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of a head -kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878.

(540 ) Banks. On the Wolffian bodies of the fatus and their remains in the adult. Edinburgh, 1864.

472


74O BIBLIOGRAPHY.


(541) Th. Bornhaupt. Untersuchungen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. iv. 1877.

(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d. WolfFschen Ganges im Hiihnerei." Mini. Acad. Imp. Petersbourg, vn. Series, Vol. xxvil. 1880.

(544) Th. Egli. Beitrage zur Anat. und Entwick. d. Geschlechtsorgane. Inaug. Diss. Zurich, 1876.

(545) E. Gasser. Beitrage zur Entwicklungsgeschichte d. Allantois, der Milllcr'schen Gange u. des Afters. Frankfurt, 1874.

(546) E. Gasser. "Beob. iib. d. Entstehung d. Wolff schen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Hiihnerembryonen." Sitz. d. GeseU. zur Befdrderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchting iiber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.

(550) A. Sedgwick. "On the development of the structure known as the glomerulus of the head-kidney in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. xiv. 1879.

(553) E. H. Weber. Zusdtze z. Lehre von Baue u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298).


CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS.

GENERATIVE ORGANS.

THE structure and growth of the ovum and spermatozoon were given in the first chapter of this work, but their derivation from the germinal layers was not touched on, and it is this subject with which we are here concerned. If there are any structures whose identity throughout the Metazoa is not open to doubt these structures are the ovum and spermatozoon ; and the constancy of their relations to the germinal layers would seem to be a crucial test as to whether the latter have the morphological importance usually attributed to them.

The very fragmentary state of our knowledge of the origin of the generative cells has however prevented this test being so far very generally applied.

Porifera. In the Porifera the researches of Schulze have clearly demonstrated that both the ova and the spermatozoa take their origin from indifferent cells of the general parenchyma, which may be called mesoblastic. The primitive germinal cells of the two sexes are not distinguishable ; but a germinal cell by enlarging and becoming spherical gives rise to an ovum ; and by subdivision forms a sperm-morula, from the constituent cells of which the spermatozoa are directly developed.

Ccelenterata. The greatest confusion prevails as to the germinal layer from which the male and female products are derived in the Ccelenterata 1 .

1 E. van Beneden (No. 556) was the first to discover a different origin for the generative products of the two sexes in Hydractinia, and his observations have led to numerous subsequent researches on the subject. For a summary of the observations on the Hydroids vide Weismann (No. 560).


742 CCELENTERATA.


The following apparent modes of origin of these products have been observed.

(1) The generative products of both sexes originate in the ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free Gonophores of Hydromedusae, the Siphonophora, and probably the Ctenophora.

(2) The generative products of both sexes originate in the entoderm (hypoblast) : Plumularia and Sertularella, amongst the Hydroids, and the. whole of the Acraspeda and Actinozoa.

(3) The male cells are formed in the ectoderm, and the female in the entoderm : Gonothyraea, Campanularia, Hydractinia, Clava.

In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Ccelenterata have led, it would seem to be necessary either to hold that there is no definite homology between the germinal layers in the different forms of Ccelenterata, or to offer some satisfactory explanation of the behaviour of the genital products, which would not involve the acceptance of the first alternative.

Though it can hardly be said that such an explanation has yet been offered, some observations of Kleinenberg (No. 557) undoubtedly point to such an explanation being possible.

Kleinenberg has shewn that in Eudendrium the ova migrate freely from the ectoderm into the endoderm, and vice versa ; but he has given strong grounds for thinking that they originate in the ectoderm. He has further shewn that the migration in this type is by no means an isolated phenomenon.

Since it is usually only possible to recognise generative elements after they have advanced considerably in development, the mere position of a generative cell, when first observed, can afford, after what Kleinenberg has shewn, no absolute proof of its origin. Thus it is quite possible that there is really only one type of origin for the generative cells in the Ccelenterata.

Kleinenberg has given reasons for thinking that the migration of the ova into the entoderm may have a nutritive object. If this be so, and there are numerous facts which shew that the position of generative cells is often largely influenced by their nutritive requirements, it seems not impossible


GENERATIVE ORGANS. 743

that the endodermal position of the generative organs in the Actinozoa and acraspedote Medusre may have arisen by a continuously earlier migration of the generative cells from the ectoderm into the endoderm ; and that the migration may now take place at so early a period of the development, that we should be justified in formally holding the generative products to be endodermal in origin.

\Ve might perhaps, on this view, formulate the origin of the generative products in the Ccelenterata in the following way :

Both ova and spermatozoa primitively originated in the ectoderm, but in order to secure a more complete nutrition the cells which give rise to them exhibit in certain groups a tendency to migrate into the endoderm. This migration, which may concern the generative cells of one or of both the sexes, takes place in some cases after the generative cells have become recognisable as such, and very probably in other cases at so early a period that it is impossible to distinguish the generative cells from indifferent embryonic cells.

Very little is known with reference to the origin of the generative cells in the triploblastic Invertebrata.

Chaetopoda and Gephyrea. In the Chaetopoda and Gephyrea, the germinal cells are always developed in the adult from the epithelial lining of the body cavity ; so that their origin from the mesoblast seems fairly established.

If we are justified in holding the body cavity of these forms to be a derivative of the primitive archenteron (vide pp. 356 and 357) the generative cells may fairly be held to originate from a layer which corresponds to the endoderm of the Ccelenterata 1 .

Chaetognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Biitschli, has been recently treated with great detail by O. Hertwig 2 .

The generative cells appear during the gastrula stage, as two large cells with conspicuous nuclei, which are placed in the hypoblast lining the archenteron, at the pole opposite the blastopore. These cells soon divide, and at the same time pass out of the hypoblast, and enter the archenteric cavity (fig. 408 - A, ge). The division into four cells, which is not satisfactorily represented ifl my diagram, takes place in such a way that two

1 The Hertwigs (No. 271) state that in their opinion the generative cells arise from the lining of the body cavity in all the forms whose body cavity is a product of the archenteron. We do not know anything of the embryonic development of the generative organs in the Echinodermata, but the adult position of the generative organs in this group is very unfavourable to the Hertwigs' view.

2 O. Hertwig, Die Chcetognathen. Jena, 1880


744


CH^ETOGNATHA.


cells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the



FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after

Biitschli, and B after Kowalevsky.) The three embryos are represented in the same positions.

A. Represents the gastrula stage.

B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.

C. Represents a later stage, in which the mouth involution (in) has become continuous with the alimentary tract, and the blastopore is closed.

///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs.

ovaries, one half of each primitive cell thus forming an ovary, and the other a testis.



FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal

surface. B, from the side. (After Biitschli.)

m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; <.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchnopleure ; ge. generative organs.


GENERATIVE ORGANS.


745


When the archenteric cavity is divided into a median alimentary tract, and two lateral sections forming the body cavity, the generative organs are placed in the common vestibule into which both the body cavity and alimentary cavity at first open (fig. 408).

The generative organs long retain their character as simple cells. Eventually (fig. 409) the two ovaries travel forwards, and apply themselves to the body walls, while the two testes also become separated by a backward prolongation of the median alimentary tract.

On the formation of the transverse septum dividing the tail from the body, the ovarian cells lie immediately in front of this septum, and the testicular cells in the region behind it.

Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek finds that the generative organs originate from a pair of specially large mesoblast cells, situated in the space between the stomach and the floor of the vestibule. The two cells undergo changes, which have an obvious resemblance to those of the generative cells of the Chsetognatha. They become surrounded by an investment of mesoblast cells, and divide so as to form two masses. Each of these masses at a later period separates into an anterior and a posterior part. The former becomes the ovary, the latter the testis.

Nematoda. In the Nematoda the generative organs are derived from the division of a single cell which would appear to be mesoblastic 1 .

Insecta. The generative cells have been observed at a very early embryonic stage in several insect forms (Vol. II. p. 404), but the observations so far recorded with reference to them do not enable us to determine with certainty from which of the germinal layers they are derived.

Crustacea. In Moina, one of the Cladocera, Grobben 2 has shewn that the generative organs are derived from a single cell, which becomes differentiated during the segmentation. This cell, which is in close contiguity with the cells from which both the mesoblast and hypoblast originate, subsequently divides ;

1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.

2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil. Wien. Vol. II. 1879.


746


CHORDATA.


sp.c


but at the gastrula stage, and after the mesoblast has become formed, the cells it gives rise to are enclosed in the epiblast, and do not migrate inwards till a later stage. The products of the division of the generative cell subsequently divide into two masses. It is not possible to assign the generative cell of Moina to a definite germinal layer. Grobben, however, thinks that it originates from the division of a cell, the remainder of which gives rise to the hypoblast.

Chordata. In the Vertebrata, the primitive generative cells (often known as primitive ova) are early distinguishable, being imbedded amongst the cells of two linear streaks of peritoneal epithelium, placed on the dorsal side of the body cavity, one on each side of the mesentery (figs. 405 C and 4io,/0). They appear to be derived from the epithelial cells amongst which they lie ; and are characterized by containing a large granular nucleus, surrounded by a considerable body of protoplasm. The peritoneal epithelium in which they are placed is known as the germinal epithelium.

It is at first impossible to distinguish the germinal cells which will become ova from those which will become spermatozoa.

The former however remain within the peritoneal epithelium (fig. 41 1), and become converted into ova in a manner more particularly described in Vol. II. pp. 54 59.

The history of the primitive germinal cells in the male has not been so adequately worked out as in the female.

The fullest history of them is that given by Semper (No. 559) for the Elasmobranchii, the general accuracy of which I can fully support ;



FIG. 410. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal cord ; W. white matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; mp. muscle-plate ; mp'. inner layer of- muscle-plate already converted into muscles ; Vr. rudiment of vertebral body ; st. segmental tube; sd. segmental duct; sp.v. spiral valve ; v. subintestinal vein ; i>.o. primitive generative cells.


GENERATIVE ORGANS.


747


though with reference to certain stages in the history further researches are still required 1 .

In Elasmobranchii the male germinal cells, instead of remaining in the germinal epithelium, migrate into the adjacent stroma, accompanied I believe by some of the indifferent epithelial cells. Here they increase in number, and give rise to masses of variable form, composed partly of true germinal cells, and partly of smaller cells with deeply staining nuclei, which are, I believe, derived from the germinal epithelium.



FIG. 411. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OK SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.

These masses next break up into ampullae, mainly formed of germinal cells, and each provided with a central lumen ; and these ampullae attach themselves to tubes derived from the smaller cells, which are in their turn continuous with the testicular network. The spermatozoa are developed from the cells forming the walls of the primitive ampulla;; but the process of their formation does not concern us in this chapter.

In the Reptilia Braun has traced the passage of the primitive germinal cells into the testicular tubes, and I am able to confirm his observations on this point : he has not however traced their further history.

1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring my own observations into accord with his as to the structure of the Elasmobranch testis.


MODE OF EXIT OF GENITAL PRODUCTS.


In Mammalia the evidence of the origin of the spermatospores from the germinal epithelium is not quite complete, but there can be but little doubt of its occurrence 1 .

In Amphioxus Langerhans has shewn that the ova and spermatozoa are derived from similar germinal cells, which may be compared to the germinal epithelium of the Vertebrata. These cells are however segmentally arranged as separate masses (vide Vol. II. p. 54).

BIBLIOGRAPHY.

(554) G. Balbiani. Lemons s. la generation des Vcrlebrcs. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart, J. of Micr. Science, Vol. xvm.

(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit. f. -wiss. Zool., Vol. xxxv. 1881.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.zoot. Inslit. Wilrzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a. d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Fitffcalso O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

GENITAL DUCTS.

The development and evolution of the generative ducts is as yet very incompletely worked out, but even in the light of our present knowledge a comparative review of this subject brings to light features of considerable interest, and displays a fruitful field for future research.

In the Ccelenterata there are no generative ducts.

In the Hydromedusae and Siphonophora the generative products are liberated by being dehisced directly into the surrounding medium ; while in the Acraspeda, the Actinozoa and the Ctenophora, they are dehisced into parts of the gastrovascular system, and carried to the exterior through the mouth.

The arrangement in the latter forms indicates the origin of

1 An entirely different view of the origin of the sperm cells has been adopted by Balbiani, for which the reader is referred to his Memoir (No. 554).


GENITAL DUCTS.


749


the methods of transportation of the genital products to the exterior in many of the higher types.

It has been already pointed out that the body cavity in a very large number of forms is probably derived from parts of a gastrovascular system like that of the Actinozoa.

When the part of the gastrovascular system into which the generative products were dehisced became, on giving rise to the body cavity, shut off from the exterior, it would be essential that some mode of transportation outwards of the generative products should be constituted.

In some instances simple pores (probably already existing at the time of the establishment of a closed body cavity) become the generative ducts. Such seems probably to have been the case in the Chaetognatha (Sagitta) and in the primitive Chordata.

In the latter forms the generative products are sometimes dehisced into the peritoneal cavity, and thence transported by the abdominal pores to the exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus they pass by dehiscence into the atrial cavity, and thence through the gill slits and by the mouth, or by the abdominal pore (?) to the exterior. The arrangement in Amphioxus and the Teleostei is probably secondary, as possibly also is that in the Cyclostomata ; so that the primitive mode of exit of the generative products in the Chordata is still uncertain. It is highly improbable that the generative ducts of the Tunicata are primitive structures.

A better established and more frequent mode of exit of the generative products when dehisced into the body cavity is by means of the excretory organs. The generative products pass from the body cavity into the open peritoneal funnels of such organs, and thence through their ducts to the exterior. This mode of exit of the generative products is characteristic of the Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata, and probably also of the Mollusca. It is moreover quite possible that it occurs in the Polyzoa, some of the Arthropoda, the Platyelminthes and some other types.

The simple segmental excretory organs of the Polychaeta, the Gephyrea and the Brachiopoda serve as generative canals, and in many instances they exhibit no modification, or but a very slight one, in connection with their secondary generative


750 DERIVATION FROM EXCRETORY ORGANS.

function ; while in other instances, e.g. Bonellia, such modification is very considerable.

The generative ducts of the Oligochaeta are probably derived from excretory organs. In the Terricola ordinary excretory organs are present in the generative segments in addition to the generative ducts, while in the Limicola generative ducts alone are present in the adult, but before their development excretory organs of the usual type are found, which undergo atrophy on the appearance of the generative ducts (Vedjovsky).

From the analogy of the splitting of the segmental duct of the Vertebrata into the Miillerian and Wolffian ducts, as a result of a combined generative and excretory function (vide p. 728), it seems probable that in the generative segments of the Oligochasta the excretory organs had at first both an excretory and a generative function, and that, as a secondary result of this double function, each of them has become split into two parts, a generative and an excretory. The generative part has undergone in all forms great modifications. The excretory parts remain unmodified in the Earthworms (Terricola), but completely abort on the development of the generative ducts in the Limicola. An explanation may probably be given of the peculiar arrangements of the generative ducts in Saccocirrus amongst the Polychaeta (vide Marion and Bobretzky), analogous to that just offered for the Oligochaeta.

The very interesting modifications produced in the excretory organs of the Vertebrata by their serving as generative ducts were fully described in the last chapter ; and with reference to this part of our subject it is only necessary to call attention to the case of Lepidosteus and the Teleostei.

In Lepidosteus the Mullerian duct appears to have become attached to the generative organs, so that the generative products, instead of falling directly into the body cavity and thence entering the open end of a peritoneal funnel of the excretory organs, pass directly into the Mullerian duct without entering the body cavity. In most Teleostei the modification is more complete, in that the generative ducts in the adult have no obvious connection with the excretory organs.

The transportation of the male products to the exterior in all the higher Vertebrata, without passing into the body cavity, is in principle similar to the arrangement in Lepidosteus.

The above instances of the peritoneal funnels of an excretory organ becoming continuous with the generative glands, render it highly probable that there may be similar instances amongst the In vertebrata.


GENITAL DUCTS.


751


As has been already pointed out by Gegenbaur there are many features in the structure of the genital ducts in the more primitive Mollusca, which point to their having been derived from the excretory organs. In several Lamellibranchiata 1 (Spondylus, Lima, Pecten) the generative ducts open into the excretory organs (organ of Bojanus), so that the generative products have to pass through the excretory organ on their way to the exterior. In other Lamellibranchiata the genital and excretory organs open on a common papilla, and in the remaining types they are placed close together.

In the Cephalopoda again the peculiar relations of the generative organs to their ducts point to the latter having primitively had a different, probably an excretory, function. The glands are not continuous with the ducts, but are placed in special capsules from which the ducts proceed. The genital products are dehisced into these capsules and thence pass into the ducts.

In the Gasteropoda the genital gland is directly continuous with its duct, and the latter, especially in the Pulmonata and Opisthobranchiata, assumes such a complicated form that its origin from the excretory organ would hardly have been suspected. The fact however that its opening is placed near that of the excretory organ points to its being homologous with the generative ducts of the more primitive types.

In the Discophora, where the generative ducts are continuous with the glands, the structure both of the generative glands and ducts points to the latter having originated from excretory organs.

It seems, as already mentioned, very possible that there are other types in which the generative ducts are derived from the excretory organs. In the Arthropoda for instance the generative ducts, where provided with anteriorly placed openings, as in the Crustacea, Arachnida and the Chilognathous Myriapoda, the Pcecilopoda, etc., may possibly be of this nature, but the data for deciding this point are so scanty that it is not at present possible to do more than frame conjectures.

The ontogeny of the generative ducts of the Nematoda and

1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d. Thierreichs, Vol. in. p. 404.


752 DERIVATION FROM EXCRETORY ORGANS.

the Insecta appears to point to their having originated independently of the excretory organs.

In the Nematoda the generative organs of both sexes originate from a single cell (Schneider, Vol. I. No. 390).

This cell elongates and its nuclei multiply. After assuming a somewhat columnar form, it divides into (i) a superficial investing layer, and (2) an axial portion.

In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the further development the two ends of the column become the blind ends of the ovary, and the axial tissue they contain forms the germinal tissue of nucleated protoplasm. The superficial layer gives rise to the epithelium of the uterus and oviduct. The germinal tissue, which is originally continuous, is interrupted in the middle part (where the superficial layer gives rise to the uterus and oviduct), and is confined to the two blind extremities of the tube.

In the male the superficial layer, which gives rise tc the epithelium of the vas deferens, is only formed at the hinder ond of the original column. In other respects the development takes place as in the female.

In the Insecta again the evidence, though somewhat conflicting, indicates that the generative ducts arise very much as in Nematodes, from the same primitive mass as the generative organs. In both of these types it would seem probable that the generative organs were primitively placed in the body cavity, and attached to the epidermis, through a pore in which their products passed out ; and that, acquiring a tubular form, the peripheral part of the gland gave rise to a duct, the remainder constituting the true generative gland. It is quite possible that the generative ducts of such forms as the Platyelminthes may have had a similar origin to those in Insecta and Nematoda, but from the analogy of the Mollusca there is nearly as much to be said for regarding them as modified excretory organs.

In the Echinodermata nothing is unfortunately known as to the ontogeny of the generative organs and ducts. The structure of these organs in the adult would however seem to indicate that the most primitive type of echinoderm generative organ consists of a blind sack, projecting into the body cavity, and opening by


GENITAL DUCTS. 753


a pore to the exterior. The sack is lined by an epithelium, continuous with the epidermis, the cells of which give rise to the ova or spermatozoa. The duct of these organs is obviously hardly differentiated from the gland ; and the whole structure might easily be derived from the type of generative organ characteristic of the Hydromedusae, where the generative cells are developed from special areas of the ectoderm, and, when ripe, pass directly into the surrounding medium.

If this suggestion is correct we may suppose that the generative ducts of the Echinodermata have a different origin to those of the majority of 1 the remaining triploblastica.

Their ducts have been evolved in forms in which the generative products continued to be liberated directly to the exterior, as in the Hydromedusae ; while those of other types have been evolved in forms in which the generative products were first transported, as in the Actinozoa, into the gastrovascular canals 2 .

1 It would be interesting to have further information about Balanoglossus.

2 These views fit in very well with those already put forward in Chapter xm. on the affinities of the Echinodermata.


B. III.


48


CHAPTER XXV.

THE ALIMENTARY CANAL AND ITS APPENDAGES, IN THE CHORDATA.

THE alimentary canal in the Chordata is always formed of three sections, analogous to those so universally present in the Invertebrata. These sections are (i) the mesenteron lined by hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and (3) the proctodaeum or anal section lined like the stomodaeum by epiblast.

Mesenteron.

The early development of the epithelial wall of the mesenteron has already been described (Chapter XI.). It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig. 420, ne). It often remains for a long time widely open in the middle towards the yolk-sack.

It has already been shewn that from the dorsal wall of the mesenteron the notochord is separated off nearly at the same time as the lateral plates of mesoblast (pp. 292 300).

The subnotochordal rod. At a period slightly subsequent to the formation of the notochord, and before any important differentiations in the mesenteron have become apparent, a remarkable rod-like body, which was first discovered by Gotte, becomes split off from the dorsal wall of the alimentary tract in all the Ichthyopsida. This body, which has a purely provisional existence, is known as the subnotochordal rod.


MESENTERON.


755


It develops in Elasmobranch embryos in two sections, one situated in the head, and the other in the trunk.

The section in the trunk is the first to appear. The wall of the alimentary canal becomes thickened along the median dorsal line (fig. 412, r), or else produced into a ridge into which there penetrates a narrow prolongation of the lumen of the alimentary canal. In either case the cells at the extreme summit become gradually constricted off as a rod, which lies immediately dorsal to the alimentary tract, and ventral to the notochord (fig. 413, *).



FIG. 412. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.

df. dorsal fin ; sp.c. spinal cord ; //. body cavity ; sp. splanchnic layer of mesoblast ; so. somatic layer of mesoblast; mp'. portion of splanchnic mesoblast commencing to be differentiated into muscles ; ch. notochord ; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract ; al. alimentary tract.


FIG. 413. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

nc. neural canal ; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp'. portion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract.


In the hindermost part of the body its mode of formation differs somewhat from that above described. In this part the alimentary wall is' very thick, and undergoes no special growth prior to the formation of the subnotochordal rod ; on the contrary, a small linear portion of the wall becomes scooped out along the median dorsal line, and eventually separates from the remainder as the rod in question. In the trunk the splitting off of the rod takes place from before backwards, so that the anterior part of it is formed before the posterior.

The section of the subnotochordal rod in the head would appear to develop in the same way as that in the trunk, and the splitting off from the throat proceeds from before backwards.

482


756 MESENTERY.


On the formation of the dorsal aorta, the subnotochordal rod becomes separated from the wall of the gut and the aorta interposed between the two (fig. 367, *).

When the subnotochordal rod attains its fullest development it terminates anteriorly some way in front of the auditory vesicle, though a little behind the end of the notochord ; posteriorly it extends very nearly to the extremity of the tail and is almost co-extensive with the postanal section of the alimentary tract, though it does not reach quite so far back as the caudal vesicle (fig. 424, b x). Very shortly after it has attained its maximum size it begins to atrophy in front. We may therefore conclude that its atrophy, like its development, takes place from before backwards. During the later embryonic stages not a trace of it is to be seen. It has also been met with in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in all of which it appears to develop in fundamentally the same way as in Elasmobranchii. In Acipenser it appears to persist in the adult as the subvertebral ligament (Bridge, Salensky). It has not yet been found in a fully developed form in any amniotic Vertebrate, though a thickening of the hypoblast, which may perhaps be a rudiment of it, has been found by Marshall and myself in the Chick (fig. 1 10, x).

Eisig has instituted an interesting comparison between it and an organ which he has found in a family of Chaetopods, the Capitellidas. In these forms there is a tube underlying the alimentary tract for nearly its whole length, and opening into it in front, and probably behind. A remnant of such a tube might easily form a rudiment like the subnotochordal rod of the Ichthyopsida, and as Eisig points out the prolongation into the latter during its formation of the lumen of the alimentary tract distinctly favours such a view of its original nature. We can however hardly suppose that there is any direct genetic connection between Eisig's organ in the Capitellidas and the subnotochordal rod of the Chordata.


Splanchnic mesoblast and mesentery- The mesentcron consists at first of a simple hypoblastic tube, which however becomes enveloped by a layer of splanchnic mesoblast. This layer, which is not at first continued over the dorsal side of the mesenteron, gradually grows in, and interposes itself between the hypoblast of the mesenteron, and the organs above. At the same time it becomes differentiated into two layers, viz. an outer cpithelioid layer which gives rise to part of the peritoneal epithelium, and an inner layer of undifferentiated cells which in time becomes converted into the connective tissue and muscular walls of the mesenteron. The connective tissue layers become first formed, while of the muscular layers the circular is the first to make its appearance.


ALIMENTARY CANAL. 757

Coincidently with their differentiation the connective tissuestratum of the peritoneum becomes established.

The Mesentery. Prior to the splanchnic mesoblast growing round the alimentary tube above, the attachment of the latter structure to the dorsal wall of the body is very wide. On the completion of this investment the layer of mesoblast suspending the alimentary tract becomes thinner, and at the same time the alimentary canal appears to be drawn downwards and away from the vertebral column.

In what may be regarded as the thoracic division of the general pleuroperitoneal space, along that part of the alimentary canal which will form the oesophagus, this withdrawal is very slight, but it is very marked in the abdominal region. In the latter the at first straight digestive canal comes to be suspended from the body above by a narrow flattened band of mesoblastic tissue. This flattened band is the mesentery, shewn commencing in fig. 117, and much more advanced in fig. 1 19, M. It is covered on either side by a layer of flat cells, which form part of the general peritoneal epithelioid lining, while its interior is composed of indifferent tissue.

The primitive simplicity in the arrangement of the mesentery is usually afterwards replaced by a more complicated disposition, owing to the subsequent elongation and consequent convolution of the intestine and stomach.

The layer of peritoneal epithelium on the ventral side of the stomach is continued over the liver, and after embracing the liver, becomes attached to the ventral abdominal wall (fig. 380). Thus in the region of the liver the body cavity is divided into two halves by a membrane, the two sides of which are covered by the peritoneal epithelium, and which encloses the stomach dorsally and the liver ventrally. The part of the membrane between the stomach and liver is narrow, and constitutes a kind of mesentery suspending the liver from the stomach : it is known to human anatomists as the lesser omentum.

The part of the membrane connecting the liver with the anterior abdominal wall constitutes the fa lei form or suspensory ligament of the liver. It arises by a secondary fusion, and is not a remnant of a primitive ventral mesentery (vide pp. 624 and 625).


758 MESENTERY.


The mesentery of the stomach, or mesogastrium, enlarges in Mammalia to form a peculiar sack known as the greater omentum.

The mesenteron exhibits very early a trifold division. An anterior portion, extending as far as the stomach, becomes separated off as the respiratory division. On the formation of the anal invagination the portion of the mesenteron behind the anus becomes marked off as the postanal division, and between the postanal section and the respiratory division is a middle portion forming an intestinal and cloacal division.

The respiratory division of the mesenteron.

This section of the alimentary canal is distinguished by the fact that its walls send out a series of paired diverticula, which meet the skin, and after a perforation has been effected at the regions of contact, form the branchial or visceral clefts.

In Amphioxus the respiratory region extends close up to the opening of the hepatic diverticulum, and therefore to a position corresponding with the commencement of the intestine in higher types. In the craniate Vertebrata the number of visceral clefts has become reduced, but from the extension of the visceral clefts in Amphioxus, combined with the fact that in the higher Vertebrata the vagus nerve, which is essentially the nerve of the branchial pouches, supplies in addition the walls of the oesophagus and stomach, it may reasonably be concluded, as has been pointed out by Gegenbaur, that the true respiratory region primitively included the region which in the higher types forms the oesophagus and stomach.

In Ascidians the respiratory sack is homologous with the respiratory tract of Amphioxus.

The details of the development of the branchial clefts in the different groups of Vertebrata have already been described in the systematic part of this work.

In all the Ichthyopsida the walls of a certain number of clefts become folded ; and in the mesoblast within these folds a rich capillary network, receiving its blood from the branchial arteries, becomes established. These folds constitute the true internal gills.


ALIMENTARY CANAL.


759


In addition to internal gills external branchial processes covered by epiblast are placed on certain of the visceral arches in the larva of Polypterus, Protopterus and many Amphibia. The external gills have probably no genetic connection with the internal gills.

The so-called external gills of the embryos of Elasmobranchii are merely internal gills prolonged outwards through the gill clefts.

The posterior part of the primitive respiratory division of the mesenteron becomes, in all the higher Vertebrata, the oesophagus and stomach. With reference to the development of these parts the only point worth especially noting is the fact that in Elasmobranchii and Teleostei their lumen, though present in very young embryos, becomes at a later stage completely filled up, and thus the alimentary tract in the regions of the oesophagus and stomach becomes a solid cord of cells (fig. 23 A, ces)\ as already suggested (p. 61) it seems not impossible that this feature may be connected with the fact that the cesophageal region of the throat was at one time perforated by gill clefts.

In addition to the gills two important organs, viz. the thyroid body and the lungs, take their origin from the respiratory region of the alimentary tract.

Thyroid body. In the Ascidians the origin of a groovelike diverticulum of the ventral wall of the branchial sack, bounded by two lateral folds, and known as the endostyle or hypopharyngeal groove, has already been described (p. 18). This groove remains permanently open to the pharyngeal sack,



FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA

OF PETROMYZON. (From Gegenbaur ; after Calberla.)

o. mouth ; 6. olfactory pit ; v. septum between stomodteum and mesenteron ; h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle.


760


THE THYROID BODY.



and would seem to serve as a glandular organ secreting mucus. As was first pointed out by W. Miiller there is present in Amphioxus a very similar and probably homologous organ, known as the hypopharyngeal groove.

In the higher Vertebrata this organ never retains its primitive condition in the adult state. In the larva of Petromyzon there is, however, present a ventral groove-like diverticulum of the throat, extending from about the second to the fourth visceral cleft. This organ is shewn in longitudinal section in fig. 414, h, and in transverse section in fig. 415, and has been identified by W. Muller (Nos. 565 and 566) with the hypopharyngeal groove of Amphioxus and Ascidians. It does not, however, long retain its primitive condition, but its opening becomes gradually reduced to a pore, placed between the third and fourth of the permanent clefts (fig. 416, tli). This opening is retained throughout the Ammoccete condition, but the organ becomes highly complicated, with paired anterior and posterior horns and a median spiral portion. In the adult the connection with the pharynx is obliterated, and the organ is partly absorbed and partly divided up into a series of glandular follicles, and eventually forms the thyroid body.

From the consideration of the above facts W. Muller was led to the conclusion tJiat the tJiyroid body of the Craniata was derived from the endostyle or Jiypopharyngeal groove. In all the higher Vertebrata the thyroid body arises as a diverticulum of the ventral wall of the throat in the region either of the mandibular or hyoid arches (fig. 417, Tk}, which after being segmented off becomes divided up into follicles.

In Elasmobranch embryos it appears fairly early as a diverticulum from the ventral surface of the throat in the region of the niandibular arc/i, extending from the border of the mouth to the point where the ventral aorta divides into the two aortic branches of the mandibular arch (fig. 417, Th}.


FIG. 415. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH THE BRANCHIAL REGION OF YOUNG LARV.K OF PETROMYZON. (From Gegenbaur ; after Calberla.)

d. branchial region of throat.


ALIMENTARY CANAL.


761


Somewhat later it becomes in Scyllium and Torpedo solid, though still retaining its attachment to the wall of the oesophagus. It continues to grow in length, and becomes divided up into a number of solid branched lobules separated by connective tissue septa. Eventually its connection with the throat becomes lost, and the lobules develop a lumen. In Acanthias the lumen of the gland is retained (W. Miiller) till after its detachment from the


-- "


Pti



FIG. 416. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4 '8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. The letter tv pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.

c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ; mb. mid-brain ; cb, cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.

throat. It preserves its embryonic position through life. In Amphibia it originates, as in Elasmobranchii, from the region of the mandibular arch ; but when first visible it forms a double epithelial wall connecting the throat with the nervous layer of the epidermis. It subsequently becomes detached from the epidermis, and then has the usual form of a diverticulum from the throat. In most Amphibians it becomes divided into two lobes, and so forms a paired body. The peculiar connection between the thyroid diverticulum and the epidermis in Amphibia has been noted by Gotte in Bombinator, and by Scott and Osborn in Triton. It is not very easy to see what meaning this connection can have.

In the Fowl (W. Miiller) the thyroid body arises at the end of the second or beginning of the third day as an outgrowth from the hypoblast of the throat, opposite the point of origin of the anterior arterial arch. This outgrowth becomes by the fourth day a solid mass of cells, and by the fifth ceases to be connected with the epithelium of the throat, becoming at the same time bilobed. By the seventh day it has travelled somewhat backwards, and the two lobes have completely separated from each other. By


762


THE THYROID BODY.


the ninth day the whole is invested by a capsule of connective tissue, which sends in septa dividing it into a number of lobes or solid masses of cells, and by the sixteenth day it is a paired body composed of a number of hollow branched follicles, each with a ' membrana propria,' and separated from each other by septa of connective tissue. It finally travels back to the point of origin of the carotids.

Amongst Mammalia the thyroid arises in the Rabbit (Kolliker) and Man (His) as a hollow diverticulum of the throat at the bifurcation of the foremost pair of aortic arches. It soon however becomes solid, and is eventually detached from the throat and comes to lie on the ventral side of the larynx or windpipe. The changes it undergoes are in the main similar to those in the lower Vertebrata. It becomes partially constricted into two lobes, which remain however united by an isthmus 1 . The fact that the thyroid sometimes arises in the region of the first and sometimes in that of the second cleft is probably to be explained



Tli


FIG. 417. SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO, AT THE LEVEL OF THE AUDITORY INVOLUTION.

Th. rudiment of thyroid body ; aup. auditory pit ; aim. ganglion of auditory nerve ; iv. v. roof of fourth ventricle ; a.c.v. anterior cardinal vein ; aa. aorta ; f.aa aortic trunk of mandibular arch ; //. head cavity of mandibular arch ; Ivc. alimentary pouch which will form the first visceral cleft.


by its rudimentary character.

The Thymus gland. The thymus gland may conveniently be dealt with here, although its origin is nearly as obscure as its function. It has usually been held to be connected with the lymphatic system. Kolliker was the first to shew that this view was probably erroneous, and he attempted to prove that it was derived in the Rabbit from the walls of one of the visceral clefts, mainly on the ground of its presenting in the embryo an epithelial character.

1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a pair of epithelial vesicles, which are developed as outgrowths of the walls of the first pair of visceral clefts. He attempts to explain the contradictory observations of other embryologists by supposing that they have mistaken the ventral ends of visceral pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair of visceral clefts, at a much later period than would appear from the observations of His and Kolliker. In view of the comparative development of this organ it is difficult to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain the supposed errors of his predecessors is certainly not capable of being applied in the case of Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that the method of investigation by transverse sections, which has been usually employed, is less liable to error than that by longitudinal sections which he has adopted.


ALIMENTARY CANAL. 763


Stieda (No. 569) has recently verified Kolliker's statements. He finds that in the Pig and the Sheep the thymus arises as a paired outgrowth from the epithelial remnants of a pair of visceral clefts. Its two lobes may at first be either hollow (Sheep) or solid (Pig), but eventually become solid, and unite in the median line. Stieda and His hold that in the adult gland, the so-called corpuscles of Hassall are the remnants of the embryonic epithelial part of the gland, and that the lymphatic part of it is of mesoblastic origin ; but Kolliker believes the lymphatic cells to be direct products of the embryonic epithelial cells.

The posterior visceral clefts in the course of their atrophy give rise to various more or less conspicuous bodies of a pseudo-glandular nature, which have been chiefly studied by Remak 1 .

Swimming bladder and lungs. A swimming bladder is present in all Ganoids and in the vast majority of Teleostei. Its development however is only imperfectly known.

In the Salmon and Carp it arises, as was first shewn by Von Baer, as an outgrowth of the alimentary tract, shortly in front of the liver. In these forms it is at first placed on the dorsal side and slightly to the right, and grows backwards on the dorsal side of the gut, between the two folds of the mesentery.

The absence of a pneumatic duct in the Physoclisti would appear to be due to a post-larval atrophy.

In Lepidosteus the air-bladder appears to arise, as in the Teleostei, as an invagination of the dorsal wall of the oesophagus.

In advanced embryos of Galeus, Mustelus and Acanthias, MikluchoMaclay detected a small diverticulum opening on the dorsal side of the oesophagus, which he regards as a rudiment of a swimming bladder. This interpretation must however be regarded as somewhat doubtful.

The lungs. The lungs originate in a nearly identical way in all the Vertebrate forms in which their development has been observed. They are essentially buds or processes of the ventral wall of the primitive oesophagus.

At a point immediately behind the region of the visceral clefts the cavity of the alimentary canal becomes compressed laterally, and at the same time constricted in the middle, so that its transverse section (fig. 418 i) is somewhat hourglass-shaped, and shews an upper or dorsal chamber d, joining on to a lower or ventral chamber / by a short narrow neck.

1 For details on these organs vide Kolliker, Entwicklungsgeschichte, p. 88 1.


764


THE LUNGS.



The hinder end of the lower tube enlarges (fig. 418 2), and then becomes partially divided into two lobes (fig. 418 3). All these parts at first freely communicate, but the two lobes, partly by their own growth, and partly by a process of constriction, soon become isolated posteriorly; while in front they open into the lower chamber of the oesophagus (fig. 422).

By a continuation forwards of the process of constriction the lower chamber of the oesophagus, carrying with it the two lobes above mentioned, becomes gradually transformed into an independent tube, opening in front by a narrow slit-like aperture into the oesophagus. The single tube in front is the rudiment of the trachea and larynx, while the two diverticula behind become (fig. 419, Ig) the bronchial tubes and lungs.

While the above changes are taking place in the hypoblastic walls of the alimentary tract, the splanchnic mesoblast surrounding these structures becomes very much thickened ; but otherwise bears no marks of the internal changes which are going on, so that the above formation of the lungs and trachea cannot be seen from the surface. As the paired diverticula of the lungs grow backwards, the mesoblast around them takes however the form of two lobes, into which they gradually bore their way.

There do not seem to be any essential differences in the mode of formation of the above structures in the types so far observed, viz. Amphibia, Aves and Mammalia. Writers differ as to whether the lungs first arise as


FlG. 418. FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE LUNGS.

(After Gotte.)

a. mesoblast; b. hypoblast; d. cavity of digestive canal ; /. cavity of the pulmonary diverticulum.

In (i) the digestive canal has commenced to be constricted into an upper and lower canal ; the former the true alimentary canal, the latter the pulmonary tube; the two tubes communicate with each other in the centre.

In (2) the lower (pulmonary) tube has become expanded.

In (3) the expanded portion of the tube has become constricted into two tubes, still communicating with each other and with the digestive canal.

In (4) these are completely separated from each other and from the digestive canal, and the mesoblast has also begun to exhibit externally changes corresponding to the internal changes which have been going on.


ALIMENTARY CANAL.


765


re


paired diverticula, or as a single diverticulum ; and as to whether the rudiments of the lungs are established before those of the trachea. If the above account is correct it would appear that any of these positions might be maintained. Phylogenetically interpreted the ontogeny of the lungs appears however to imply that this organ was first an unpaired structure and has become secondarily paired, and that the trachea was relatively late in appearing.

The further development of the lungs is at first, in the higher types at any rate, essentially similar to that of a racemose gland. From each primitive diverticulum numerous branches are given off In Aves and Mammalia (fig. 355) they are mainly confined to the dorsal and lateral parts. These branches penetrate into the surrounding mesoblast and continue to give rise to secondary and tertiary branches. In the meso


At


FIG. 419. SECTION THROUGH THE CARDIAC REGION OF AN EMBRYO OF LACERTA MURALIS OF 9 MM. TO SHEW THE MODE OF FORMATION OF THE PERICARDIAL CAVITY.

ht. heart ; pc . pericardial cavity ; al. alimentary tract; Ig. lung; /. liver; pp. body cavity; md. open end of Mullerian duct; wd. Wolffian duct ; vc. vena cava inferior ; ao. aorta; ch. notochord; me, medullary cord.


blast around them numerous capillaries make their appearance, and the further growth of the bronchial tubes is supposed by Boll to be due to the mutual interaction of the hitherto passive mesoblast and of the hypoblast.

The further changes in the lungs vary somewhat in the different forms.

The air sacks are the most characteristic structures of the avian lung. They are essentially the dilated ends of the primitive diverticula or of their main branches.

In Mammalia (Kolliker, No. 298) the ends of the bronchial tubes become dilated into vesicles, which may be called the primary air-cells. At first, owing to their development at the ends of the bronchial branches, these are confined to the surface of the lungs. At a later period the primary air-cells divide each into two or three parts, and give rise to secondary air-cells, while at the same time the smallest bronchial tubes, which continue all the while to divide, give rise at all points to fresh air-cells. Finally the bronchial tubes cease to become more branched, and the air-cells belonging to each minute lobe come in their further growth to open into a common chamber.


766 THE CLOACA.


Before the lungs assume their function the embryonic air-cells undergo a considerable dilatation.

The trachea and larynx. The development of the trachea and larynx does not require any detailed description. The larynx is formed as a simple dilatation of the trachea. The cartilaginous structures of the larynx are of the same nature as those of the trachea.

It follows from the above account that the whole pulmonary structure is the result of the growth by budding of a system of branched hypoblastic tubes in the midst of a mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium of the tubes, and the mesoblast providing the elastic, muscular, cartilaginous, vascular, and other connective tissues of the tracheal and bronchial walls.

There can be no doubt that the lungs and air-bladder are homologous structures, and the very interesting memoir of Eisig on the air-bladder of the Chaetopoda 1 shews it to be highly probable that they are the divergent modifications of a primitive organ, which served as a reservoir for gas secreted in the alimentary tract, the gas in question being probably employed for respiration when, for any reason, ordinary respiration by the gills was insufficient.

Such an organ might easily become either purely respiratory, receiving its air from the exterior, and so form a true lung ; or mainly hydrostatic, forming an air-bladder, as in Ganoidei and Teleostei.

It is probable that in the Elasmobranchii the air-bladder has become aborted, and the organ discovered by Micklucho-Maclay may perhaps be a last remnant of it.

The middle division of the mesenteron. The middle division of the mesenteron, forming the intestinal and cloacal region, is primitively a straight tube, the intestinal region of which in most Vertebrate embryos is open below to the yolksack.

Cloaca. In the Elasmobranchii, the embryos of which probably retain a very primitive condition of the mesenteron, this region is not at first sharply separated from the postanal section behind. Opposite the point where the anus will even 1 H. Eisig, " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden." Mittheil. a. d. zool. Station z. Neafel, Vol. II. 1881.


ALIMENTARY CANAL.


767


tually appear a dilatation of the mesenteron arises, which comes in contact with the external skin (fig. 28 E, an}. This dilatation becomes the hypoblastic section of the cloaca. It communicates behind with the postanal gut (fig. 424 D), and in front with the intestine ; and may be defined as the dilated portion of the alimentary tract which receives the genital and urinary ducts and opens externally by the proctodczum.

In Acipenser and Amphibia the cloacal region is indicated as a ventral diverticulum of the mesenteron even before the closure of the blastopore. It is shewn in the Amphibia at an early stage in fig. 73, and at a later period, when in contact with the skin at the point where the anal invagination is about to appear, in fig. 420.



FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF

BOMBINATOR. (After Gotte.)

m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; ch. notochord ; pn. pineal gland.

In the Sauropsida and Mammalia the cloaca appears as a dilatation of the mesenteron, which receives the opening of the allantois almost as soon as the posterior part of the mesenteron is established.

The eventual changes which it undergoes have been already dealt with in connection with the urinogenital organs.

Intestine. The region in front of the cloaca forms the intestine. In certain Vertebrata it nearly retains its primitive character as a straight tube ; and in these types its anterior part is characterised by the presence of a peculiar fold, which in a highly specialised condition is known as the spiral valve. This structure appears in its simplest form in Ammocoetes. It


768 THE INTESTINE.


there consists of a fold in the wall of the intestine, giving to the lumen of this canal a semilunar form in section, and taking a half spiral.

In Elasmobranchii a similar fold to that in Ammoccetes first makes its appearance in the embryo. This fold is from the first not quite straight, but winds in a long spiral round the intestine. In the course of development it becomes converted into a strong ridge projecting into the lumen of the intestine (fig. 388, /). The spiral it makes becomes much closer, and it thus acquires the form of the adult spiral valve. A spiral valve is also found in Chimaera and Ganoids. No rudiment of such an organ is found in the Teleostei, the Amphibia, or the higher Vertebrata.

The presence of this peculiar organ appears to be a very primitive Vertebrate character. The intestine of Ascidians exhibits exactly the same peculiarity as that of Ammoccetes, and we may probably conclude from embryology that the ancestral Chordata were provided with a straight intestine having a fold projecting into its lumen, to increase the area of the intestinal epithelium.

In all forms in which there is not a spiral valve, with the exception of a few Teleostei, the intestine becomes considerably longer than the cavity which contains it, and therefore necessarily more or less convoluted.

The posterior part usually becomes considerably enlarged to form the rectum or in Mammalia the large intestine.

In Elasmobranchii there is a peculiar gland opening into the dorsal side of the rectum, and in many other forms there is a caecum at the commencement of the rectum or of the large intestine.

In Teleostei, the Sturgeon and Lepidosteus there opens into the front end of the intestine a number of caecal pouches known as the pancreatic caeca. In the adult Sturgeon these pouches unite to form a compact gland, but in the embryo they arise as a series of isolated outgrowths of the duodenum.

Connected with the anterior portion of the middle region of the alimentary canal, which may be called the duodenum, are two very important and constant glandular organs, the liver and the pancreas.


ALIMENTARY CANAL.


769


ITlf



The liver. The liver is the earliest formed and largest glandular organ in the embryo.

It appears in its simplest form in Amphioxus as a single unbranched diverticulum of the alimentary tract, immediately behind the respiratory region, which is directed forwards and placed on the left side of the body.

In all true Vertebrata the gland has a much more complicated structure. It arises as a ventral outgrowth of the duodenum (fig. 420, /). This outgrowth may be at first single, and then grow out into two lobes, as in Elasmobranchii (fig. 421) and Amphibia, or have from the first the form of two somewhat unequal diverticula, as in Birds (fig. 422), or again as in the Rabbit (Kolliker) one diverticulum may be first formed, and a second one appear somewhat later. The hepatic diverticula, whatever may be their primitive form, grow into a special thickening of the splanchnic mesoblast.

From the primitive diverticula there are soon given off a number of hollow buds (fig. 421) which rapidly increase in length and number, and form the so-called hepatic cylinders. They soon anastomose and unite together, and so constitute an irregular network. Coincidently with the formation of the hepatic network the united vitelline and visceral vein or veins (u.v\ in their passage through the liver, give off numerous branches, and gradually break up into a plexus of channels which form a secondary network amongst the hepatic cylinders. In Amphibia these channels are stated by Gotte to be lacunar, but in Elasmobranchii, and probably Vertebrata generally, they arc from the first provided with distinct though delicate walls. B. in. 49


FIG. 421. SECTION THROUGH THE VENTRAL PART OF THE TRUNK OF A YOUNG EMBRYO OF SCYLLIUM AT THE LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ; cav. cardinal vein; ua. vitelline artery ; nv. vitelline vein united with subintestinal vein ; al. duodenum ; /. liver ; sd. opening of segmental duct into the body-cavity ; mp. muscle-plate ; urn. umbilical canal.


770


THE LIVER.


It is still doubtful whether the hepatic cylinders are as a rule hollow or solid. In Elasmobranchii they are at first provided with a large lumen, which though it becomes gradually smaller never entirely vanishes. The same seems to hold good for Amphibia and some Mammalia. In Aves the lumen of the cylinders is even from the first much more difficult to see, and the cylinders are stated by Remak to be solid, and he has been followed in this matter by Kolliker. In the Rabbit also Kolliker finds the cylinders to be solid.

The embryonic hepatic network gives rise to the parenchyma of the adult liver, with which in its general arrangement it closely agrees. The blood-channels are at first very large, and have a very irregular arrangement ; and it is not till comparatively late that the hepatic lobules with their characteristic vascular structures become established.

The biliary ducts are formed either from some of the primitive hepatic cylinders, or, as would seem to be the case in Elasmobranchii and Birds (fig. 422), from the larger diverticula of the two primitive outgrowths.

The gall-bladder is so inconstant, and the arrangement of the ducts opening into the intestine so variable, that no general statements can be made about them. In Elasmobranchii the primitive median diverticulum (fig. 421) gives rise to the ductus choledochus. Its anterior end dilates to form a gall-bladder.

In the Rabbit a ductus choledochus is formed by a diverticulum from the intestine at the point of insertion of the two primitive lobes. The gall-bladder arises as a diverticulum of the right primitive lobe.

The liver is relatively very large during embryonic life and has, no doubt, important functions in connection with the circulation.



r


FIG. 422. DIAGRAM OF THE DIGESTIVE TRACT OF A CHICK UPON THE FOURTH DAY. (After Gotte.)

The black line indicates the hypoblast. The shaded part around it is the splanchnic mesoblast.

Ig. lung ; st. stomach ; p. pancreas ; /. liver.


ALIMENTARY CANAL.


771


The pancreas. So far as is known the development of the pancreas takes place on a very constant type throughout the series of craniate Vertebrata, though absent in some of the Teleostean fishes and Cyclostomata, and very much reduced in most Teleostei and in Petromyzon.

It arises nearly at the same time as the liver in the form of a hollow outgrowth from the dorsal side of the intestine nearly opposite but slightly behind the hepatic outgrowth (fig. 422, /). It soon assumes, in Elasmobranchii and Mammalia, somewhat the form of an inverted funnel, and from the expanded dorsal part of the funnel there grow out numerous hollow diverticula into the passive splanchnic mesoblast.

As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The funnel-shaped receptacle loses its origi nal form, and elongating, assumes the character of a duct.

From the above mode of development it is clear that the glandular cells of the pancreas are derived from the hypoblast.

Into the origin of the varying arrangements of the pancreatic ducts it is not possible to enter in detail. In some cases, e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a division of the primitive gland and duct. In other cases, e.g. the Bird, a second diverticulum springs from the alimentary tract. In a large number of instances the primitive condition with a single duct is retained.

Postanal section of the mesenteron. In the embryos of all the Chordata there is a section of the mesenteron placed behind the anus. This section invariably atrophies at a comparatively early period of embryonic life ; but it is much better developed in the lower forms than in the higher. At its posterior extremity it is primitively continuous with the neural tube (fig. 420), as was first shewn by Kowalevsky.

The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore.

In the Tunicata the section of the mesenteron, which in all probability corresponds to the postanal gut of the Vertebrata, is that immediately

492



772 POSTANAL SECTION OF THE MESENTERON.

following the dilated portion which gives rise to the branchial cavity

and permanent intestine. It has already

been shewn that from the dorsal and

lateral portions of this section of the

primitive alimentary tract the notochord

and muscles of the Ascidian tadpole are

derived. The remaining part of its walls

forms a solid cord of cells (fig. 423, al'},

which either atrophies, or, according to

Kowalevsky, gives rise to blood-vessels.

In Amphioxus the postanal gut, FIG. 423. TRANSVERSE OPTICAL

.hough distinctly developed, is no, very % long, and atrophies at a comparatively (After Kowalevsky.) early period. The sect i on ; s f rom an embryo of

In Elasmobranchii this section of the the same age as fig. 8 iv.

alimentary tract is very well developed, ch - notochord ; nc neural 1 canal ;

. , , me. mesoblast ; of. hypoblast of and persists for a considerable period of ta ji <

embryonic life. The following is a history of its development in the genus Scyllium.

Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.

The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast.

In the succeeding stages, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about -J of that of the whole alimentary tract.

Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.

Posteriorly (A) there is present a terminal vesicle (alv) '25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (nc) ; to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B al}. Its average diameter is about '084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (x), but this does not extend as far back as the terminal vesicle.

The thick-walled stalk of the vesicle is connected with the cloacal section


ALIMENTARY CANAL.


773


of the alimentary tract by a very narrow thin-walled tube (C of). This for the most part has a fairly uniform calibre, and a diameter of not more than 035 mm. Its walls are formed of flattened epithelial cells. At a point not far from the cloaca it becomes smaller, and its diameter falls to -03 mm. In



cl.al


FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc . neural canal ; al. postanal gut ; alv. caudal vesicle of postanal gut ; x. subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta; v.cau, caudal vein.

front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (D al}.

Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.

The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.

In Teleostei the vesicle at the end of the tail, discovered by Kupffer,


774 THE STOMOD/EUM.


(fig- 34> hyv) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii.

In Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewh in the embryo of Bombinator in fig. 420.

Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period



FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypoblast ; p.a.g, postanal gut ; pr. remains of primitive streak folded in on the ventral side ; al. allantois ; me. splanchnic mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kolliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425, p.a.g.} and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms.

The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter xii., p. 323.


The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract.

In Ascidians this pit is placed on the dorsal surface (fig. 9, o), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically


THE STOMOD/EUM.


775



(vide p. 5), but further observations on its development are required.

In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the forebrain (fig. 426), and is deeper in Petromyzon (fig. 416, ;) than in any other known form.

From the primary buccal cavity or stomodaeum there grows out the pituitary pit (fig. 426, pt\ the development of which has already been described (p. 435).

The wall separating the stomodaeum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original limits of the primitive buccal cavity become obliterated ; while a secondary buccal cavity, partly lined by hypoblast and partly by epiblast, becomes established.

This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the posterior nares.

In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity.

In the Amniota further important changes take place.

In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427, /), and the two plates, meeting in the middle line, form a horizontal septum dividing the front part of the primitive buccal cavity into a dorsal respiratory section (), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. The


FIG. 426. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG PRISTIURUS EMBRYO.

r.unpaired rudimentofthecerebral hemispheres \pn. pineal gland ; /w.infundibulum ; //.ingrowth from mouth to form the pituitary body ; mb. mid-brain ; cb. cerebellum ; ch. notochord; al. alimentary tract; Zaa. artery of mandibular arch.


THE TEETH.



two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.

An internasal septum (fig. 427, e) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.

In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate.

The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the posterior part of the buccal cavity.

The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth ; but the latter alone will engage our attention here.

The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth.

As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis. The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter.


FIG. 427. DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE TRUE MOUTH BELOW. (From Gegenbaur.)

p. palatine plate of superior maxillary process; m. permanent mouth ; n. posterior part of nasal passage; e. internasal septum.


Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoid


THE STOMOD/KUM.


777


and branchial arches ; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region.

The teeth are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla.

The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.

Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior ; so that it becomes constituted of (i) an external layer of columnar cells, and (2) a central core of rounded cells ; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a papilla of the subepithelial connective tissue (p] which has in the meantime been developed.

From the above epithelial structure, which may be called the enamel organ, and from the papilla it covers, which maybe spoken of as the dental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule ; known as the dental capsule.

Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar transformation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a somewhat different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papilla? which project into the highly vascular tissue of the dental sheath. Between



FIG. 428. DIAGRAM SHEWING THE DEVELOPMENT OF THE TEETH. (From Gegenbaur.)

p. dental papilla ; e. enamel organ.


778 THE PROCTOD/EUM.

the epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana praeformativa.

The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium.

The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine while the inner portion remains as the pulp.

The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.

By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.

The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.

The general development of the enamel organs and dental papillae is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.

In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured (fig. 428).

The arrangement represented in fig. 428, in which successive enamel organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth.

The Proctodceuni.

In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communication


THE PROCTOD/EUM.


779


with the exterior by means of an epiblastic invagination, constituting a proctodseum.

This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodseum ; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication.

The mode of formation of the proctodaeum, which is in general extremely simple, is illustrated by fig. 420 an.

In most forms the original boundary between the cpiblast of the proctodaeum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication.



FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy, hypoblast ; p.a.g. postanal gut ; pr. remains of primitive streak folded in on the ventral side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

In Birds the formation of the proctodseum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa Fabricii.

The proctodseum first appears when the folding off of the tail end of the embryo commences (fig. 429, an} and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut.

The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodaeum. The actual perforation of the septum between the proctodeeum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of fcetal life, and the approxi


780 BIBLIOGRAPHY.


mation of the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces.

The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctodaeum, with which the bursa Fabricii communicates.

BIBLIOGRAPHY. Alimentary Canal and its appendages.

(561) B. Afanassiew. "Ueber Bau u. Entwicklung d. Thymus d. Saugeth." Archivf. mikr. Anat. Bd. xiv. 1877.

(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Hiihnerembryonen." Archivf. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beilrdge zur Entivicklungsgeschichle d. Darmkanah im Hiihnchen. 1867.

(565) W. Millie r. "Ueber die Entwickelung der Schilddriise." Jenaische Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaische Zeitschrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische Untcrsuchungen. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. iib. d. Entwick. d. Glandula Thymus, Glandula thyroidea,u. Glandula car otica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad. Petrop. 1766.

(571) H. Wolfler. Ueb. d. Entwick. u. d. Bau d. Schilddriise. Berlin, 1880. Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of Micros. Science, Vol. in. 1855.

(573) R. Owen. Odontography . London, 1840 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative. London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros. Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker's Histology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.


INDEX TO VOLUME III.


Abdominal muscles, 675

Abdominal pore, 626, 749

Acipenser, development of, 102; affinities of, 1 1 8 ; comparison of gastrula of, 279 ; pericardial cavity of, 627

Actinotrocha, 373

Air-bladder of Teleostei, 77; Lepidosteus, 117; blood supply of, 645 ; general account of, 763 ; homologies of, 766

Alciope, eye of, 480

Alisphenoid region of skull, 569

Alimentary canal and appendages, development of, 754

Alimentary tract ofAscidia, 18; Molgula, 22; Pyrosoma, 24; Salpa, 31 ; Elasmobranchii, 52; Teleostei, 75; Petromyzon, 93, 97; Acipenser, no; Amphibia, 129, 136; Chick, 167; respiratory region of, 754; temporary closure of oesophageal region of, 759

Allantois, development of in Chick, 191, 198; blood-vessels of in Chick, 193; Lacerta, 205, 209; early development of in Rabbit, 229, of Guinea-pig, 264; origin of, 309. See also ' Placenta ' and 'Bladder

Alternation of generations in Ascidians, origin of, 35 ; in Botryllus, 35 ; Pyrosoma, 36; Salpa, 36; Doliolum, 36

Alytes, branchial chamber of, 136; yolksack of, 139; branchiae, 141 ; Miillerian duct of, 710

Amblystoma, ovum of, 120; larva of, 142,

H3

Amia, ribs of, 561

Ammocoetes, 95; metamorphosis of, 97;

eye of, 498 Amnion, early development of in Chick,

185; later history of in Chick, 196;

Lacerta, 204, 210; Rabbit, 229; origin

of, 3.07. 39

Amphibia, development of, 120; viviparous, 121; gastrula of, 277; suctorial mouth of, 317; cerebellum of, 426; infundibulum of, 431; pineal gland of, 433; cerebrum of, 439; olfactory lobes of, 444; nares of, 553; notochord and its sheath, 548; vertebral column of, 554; ribs of, 561 ; branchial arches of, 574; mandibular and hyoid arches of, 582 ; columella of, 582 ; pectoral girdle of, 605; pelvic girdle of, 607; limbs of, 619; heart of, 638; arterial system of, f>45 ; venous system of, 655 ; excretory


system of, 707 ; vasa efierentia of, 711; liver of, 769; postanal gut of, 774; stomodaeum of, 778

Amphiblastula larva of Porifera, 344

Amphioxus, development of, i ; gastrula of, 275 ; formation of mesoblast of, 292 ; development of notochord of, 293; head of, 314; spinal nerves of, 461; olfactory organ of, 462 ; venous system of, 651; transverse abdominal muscle f> 673; generative cells of, 748; liver of, 769; postanal gut of, 772; stomodaeum of, 777

Amphistylic skulls, 578

Angular bone, 594

Anterior abdominal vein, 653

Anura, development of, 121; epiblast of, 125; mesoblast of, 128; notochord of, 128; hypoblast of, 129; general growth of embryo of, 131; larva of, 134; vertebral column of, 556 ; mandibular arch of, 584

Anus of Amphioxus, 7 ; Ascidia, 18; Pyrosoma, 28 ; Salpa, 31 ; Elasmobranchii, 57; Amphibia, 130, 132; Chick, 167; primitive, 324

Appendicularia, development of, 34

Aqueductus vestibuli, 519

Aqueous humour, 497

Arachnida, nervous system of, 409; eye of, 481

Area, embryonic, of Rabbit, 218; epiblast

of, 219; origin of embryo from, 228

area opaca of Chick, 150; epiblast,

hypoblast, and mesoblast of, 159 area pellucida of Chick, 150; of Lacerta, 202

area vasculosa of Chick, 194; mesoblast of, 1 60; of Lizard, 209; Rabbit, 228, 229

Arteria centralis retinas, 503

Arterial system of Petromyzon, 97; constitution of in embryo, 643 ; of Fishes, 644; of Amphibia, 645; of Amniota, 647

Arthropoda, head of, 313 ; nervous system of, 409 ; eye of, 480 ; excretory organs of, 688

Articular bone of Teleostei, 581 ; of Sauropsida, 588

Ascidia, development of, 9

Ascidians. See 'Tunicata'

Ascidiozooids, 25

Atrial cavity of Amphioxus, 7; Ascidia, 18; Pyrosoma, 24


7 82


INDEX.


Atrial pore of Amphioxus, 7; Ascidia, 20; Pyrosoma, 28 ; Salpa, 32

Auditory capsules, ossifications in, 595, 59.6

Auditory involution of Elasmobranchii, 57; Teleostei, 73; Petromyzon, 89, 92; Acipenser, 106; Lepidosteus, 114; Amphibia, 127; Chick, 170

Auditory nerve, development of, 459

Auditory organs, of Ascidia, 15; of Salpa, 31; of Ammocoetes, 98; Ganoidei, 108, 114; of Amphibia, 127; of Aves, 170; general development of, 512; of aquatic forms, 512; of land forms, 513; of Ccelenterata, 513; of Mollusca, 515; of Crustacea, 516; of Vertebrata, 517; of Cyclostomata, 89, 92, 518; of Teleostei, Lepidosteus and Amphibia, 518; of Mammalia, 519; accessory structures of, 527; ofTunicata, 528

Auriculo-ventricular valves, 642

Autostylic skulls, 579

Aves, development of, 145; cerebellum of, 426; midbrain of, 427; infundibulum of, 431; pineal gland of, 434; pituitary body of, 436; cerebrum of, 439 ; olfactory lobes of, 444 ; spinal nerves of, 449 ; cranial nerves of, 455 ; vagus of, 458; glossopharyngeal of, 458; vertebral column of, 557; ossification of vertebral column of, 558; branchial arches of, 572, 573; pectoral girdle of, 603; pelvic girdle of, 608; heart of, 637 ; arterial system of, 647 ; venous system of, 658; muscle-plates of, 670; excretory organs of, 714; mesonephros of, 715; pronephros of, 718; Miillerian duct of, 718, 720; nature of pronephros of, 721 ; connection of Miillerian duct with Wolffian in, 720 ; kidney of, 722; lungs of, 764; liver of, 769; postanal gut of, 774

Axolotl, 142, 143; ovum of, 120; midbrain of, 427; mandibular arch of, 583

Basilar membrane, 524

Basilar plate, 565

Basipterygium, 612

Basisphenoid region of skull, 569

Bilateral symmetry, origin of, 373-376

Bile duct, 770

Bladder, Amphibia, 131 ; of Amniota, 726

Blastodermic vesicle, of Rabbit, first development of, 217; of 7th day, 222; Guinea-pig, 263; meaning of, 291

Blastoderm of Pyrosoma, 24; Elasmobranchii, 41; Chick, 150; Lacerta 202

Blastopore, of Amphioxus, 2; of Ascidia, II ; Elasmobranchii, 42, 54, 62 ; Petromyzon, 87; Acipenser, 104 ; Amphibia, 125, 130; Chick, 153; Rabbit, 216; true Mammalian, 226; comparative history of closure of, 284, 288; summary of fate of, 340; relation of to primitive anus, 324


Blood-vessels, development of, 633

Body cavity, of Ascidia, 2 1 ; Molgula, 2 1 ; Salpa, 31; Elasmobranchii, 47 ; of Teleostei, 75 ; Petromyzon, 94 ; Chick, 169; development of in Chordata, 325; views on origin of, 356 360, 377; of Invertebrata, 623; of Chordata, 624; of head, 676

Bombinator, branchial chamber of, 136; vertebral column of, 556

Bonellia, excretory organs of, 687

Bones, origin of cartilage bones, 542 ; origin of membrane bones, 543; development of, 543; homologies of membrane bones, 542 ; homologies of cartilage bones, 545

Brachiopoda, excretory organs of, 683 ; generative ducts of, 749

Brain, of Ascidia, IT, 15; Elasmobranchii, 56, 59, 60; Teleostei, 77; Petromyzon, 89, 92 ; Acipenser, 105 ; Lepidosteus, 113; early development of in Chick, 170; flexure of in Chick, 175; later development of in Chick, 176; Rabbit, 229, general account of development of, 419; flexureof, 420; histogeny of, 422

Branchial arches, prseoral, 570; disappearance of posterior, 573; dental plates of in Teleostei, 574; relation of to head cavities, 571 ; see ' Visceral arches'

Branchial chamber of Amphibia, 136

Branchial clefts, of Amphioxus, 7 ; of Ascidia, 18, 20; Molgula, 23; Salpa, 32; of Elasmobranchii, 57, 59 01; Teleostei, 77; Petromyzon, 91, 96; Acipenser, 105; Lepidosteus, 114, 116; Amphibia, 132, 133; Chick, 178; Rabbit, 231; praeoral, 312, 318; of Invertebrata, 326; origin of, 326

Branchial rays, 574

Branchial skeleton, development of, 572, 592; of Petromyzon, 96, 312, 571; of Ichthyopsida, 572; dental plates of in Teleostei, 574; relation of to head cavities, 572

Branchiae, external of Elasmobranchii, 6r, 62; of Teleostei, 77; Acipenser, 107; Amphibia, 127, 133, 135

Brood-pouch, of Salpa, 29 ; Teleostei, 68, Amphibia, 12 1

Brown tubes of Gephyrea, 686

Bulbus arteriosus, of Pishes, 638 ; Amphibia, 639

Bursa Fabricii, 167, 779

Canalis auricularis, 639 Canalis reuniens, 521 Capitellidre, excretory organs of, 683 Carcharias, placenta of, 66 Cardinal vein, 652 Carnivora, placenta of, 250 Carpus, development of, 620 Cartilage bones of skull, 595 ; homologies of, 595


INDEX.


783


Cat, placenta of, 250

Caudal swellings of Elasmobranchii, 46,

55; Teleostei, 72; Chick, 162, 170 Cephalic plate of Elasmobranchii, 55 Cephalochorda, development of, i Cephalopoda, eyes of, 473 477 Cerebellum, Petromyzon, 93; Chick, 176;

general account of development of, 424,

425

Cerebrum of Petromyzon, 93, 97; Chick, 175 ; general development of, 429, 438; transverse fissure of, 443 Cestoda, excretory organs of, 68 1 Cetacea, placenta, 255 Chtetognatha, nervous system of, 349; eye of, 479 ; generative organs of, 743 ; generative ducts of, 749 Chcetopoda, head of, 313; eyes of, 479; excretory organs of, 683; generative organs of, 743 ; generative ducts of, 749 Charybdnea, eye of, 472 Cheiroptera, placenta of, 244 Cheiropterygium, 618; relation of to ich thyopterygium, 621

Chelonia, development of, 210; pectoral girdle of, 603 ; arterial system of, 649 Chick, development of, 145 ; general growth of embryo of, 1 70 ; rotation of embryo of, 173; fcetal membranes of, 185; epiblast of, 150, 166; optic nerve and choroid fissure of, 500

Chilognatha, eye of, 481

Chilopoda, eye of, 481

Chimasra, lateral line of, 539 ; vertebral column of, 548; nares of, 533

Chiromantis, oviposition of, 121

Chorda tympani, development of, 460

Chordata, ancestor of, 311; branchial system of, 312; evidence from Ammocuetes, 312; head of, 312; mouth of, 318; table of phylogeny of, 327

Chorion, 237; villi of, 237, 257

Choroid coat, Ammoccetes, 99; general account of, 487

Choroid fissure, of Vertebrate eye, 486, 493 ; of Ammocoetes, 498 ; comparative development of, 500; of Chick, 501; of Lizards, 501 ; of Elasmobranchii, 502 ; of Teleostei, 503 ; Amphibia, 503 ; Mammals, 503, 504

Choroid gland, 320

Choroid pigment, 489

Choroid plexus, of fourth ventricle, 425 ; of third ventricle, 432 ; of lateral ventricle, 442

Ciliated sack of Ascidia, 18; Pyrosoma, 26; Salpa, 31

Ciliary ganglion, 461

Ciliary muscle, 490

Ciliary processes, 488; comparative development of, 506

Clavicle, 600

Clitoris, development of, 727

Clinoid ridge, 569

Cloaca, 766


Coccygeo-mesenteric vein, 66 1

Cochlear canal, 519

Coecilia, development of, 143; pronephros of, 707; mesonephros of, 709; Mill lerian duct of, 710

Coelenterata, larvae of, 367 ; eyes of, 47 1 ; auditory organs of, 513; generative organs of, 741

Columella auris, 529; of Amphibia, 582 ; of Sauropsida, 588

Commissures, of spinal cord, 417; of brain, 431, 432, 439, 443

Coni vasculosi, 724

Conus arteriosus, of Fishes, 638; of Amphibia, 638

Coracoid bone, 599

Cornea, of Ammocretes, 99 ; general development of, 495 ; corpuscles of, 496 ; comparative development of, 499; of Mammals, 499

Coronoid bone, 595

Corpora geniculata interna, 428

Corpora quadrigemina, 428

Corpora striata, development of, 437

Corpus callosum, development of, 443

Corti, organ of, 522; structure of, 525; fibres of, 525 ; development of, 526

Cranial flexure, of Elasmobranchii, 58, 60; of Teleostei, 77; Petromyzon, 93, 94; of Amphibia, 131, 132; Chick, 174; Rabbit, 231; characters of, 321; significance of, 322

Cranial nerves, development of, 455; relation of to head cavities, 461 ; anterior roots of, 462 464; view on position of roots of, 466

Crocodilia, arterial system of, 649

Crura cerebri, 429

Crustacea, nervous system of, 41 1 ; eye of, 481; auditory organs of, 515; generative cells of, 745 ; generative ducts of,

75

Cupola, 524

Cutaneous muscles, 676

Cyathozooid, 25

Cyclostomata, auditory organs of, 517; olfactory organ of, 532; notochord and vertebral column of, 546, 549; abdominal pores of, 626 ; segmental duct of, 700 ; pronephros of, 700 ; mesonephros of, 700 ; generative ducts of, 733, 749 ; venous system of, 651 ; excretory organs of, 700

Cystignathus, oviposition of, 122

Dactylethra, branchial chamber of, 136;

branchise of, 136; tadpole of, 140 Decidua reflexa, of Rat, 242 ; of Insecti vora, 243; of Man, 245 Deiter's cells, 526 Dental papilla, 777 Dental capsule, 777 Dentary bone, 595 Dentine, 780 Descemet's membrane, 496


784


INDEX.


Diaphragm, 631 ; muscle of, 676

Dipnoi, nares of, 534; vertebral column of, 548; membrane bones of skull of, 592 ; heart of, 638 ; arterial system of, 645 ; excretory system of, 707 ; stomodseum of, 777

Diptera, eye of, 481

Discophora, excretory organs of, 687

Dog, placenta of, 248

Dohni, on relations of Cyclostomata, 84 ; on ancestor of Chordata, 311, 319

Doliolum, development of, 28

Ductus arteriosus, 649

Ductus Botalli, 648

Ductus Cuvieri, 654

Ductus venosus Arantii, 663

Dugong, heart of, 642

Dysticus, eye of, 481

Ear, see ' Auditory organ '

Echinodermata, secondary symmetry of larva of, 380; excretory organs of, 689 ; generative ducts of, 752

Echinorhinus, lateral line of, 539; vertebral column of, 548

Echiurus, excretory organs of, 686

Ectostosis, 543

Edentata, placenta of, 248, 250, 256

Eel, generative ducts of, 703

Egg-shell of Elasmobranchii, 40 ; Chick, 146

Elasmobranchii, development of, 40; viviparous, 40; general features of development of, 55 ; gastrulaof, 281 ; development of mesoblast of, 294 ; notochord of, 294 ; meaning of formation of mesoblast of, 295; restiform tracts of, 425 ; optic lobes of, 427 ; cerebellum of, 425 ; pineal gland of, 432 ; pituitary body of, 435 ; cerebrum of, 438 ; olfactory lobes of, 444 ; spinal nerves, 449 ; cranial nerves of, 457; sympathetic nervous system of, 466; nares of, 533; lateral line of, 539; vertebral column of, 549 ; ribs of, 560 ; parachordals of, 567 ; mandibular and hyoid arches of, 576 ; pectoral girdle of, 600 ; pelvic girdle of, 607; limbs of, 609; pericardial cavity of, 627; arterial system of, 644 ; venous system of, 65 1 ; muscle-plates of, 668 ; excretory organs of, 690 ; constitution of excretory organs in adult of, 697; spermatozoa of, 747 ; swimming-bladder of, 763 ; intestines of, 767 ; liver of, 769; postanal gut of, 772

Elrcoblast of Pyrosoma, 28; Salpa, 30

Elephant, placenta of, 249

Embolic formation of gastrula, 333

Enamel organ, 777

Endolymph of ear, 522

Endostosis, 543

Endostyle of Ascidia, 18, 759; Pyrosoma, 25; Salpa, 32

Epiblast, of Elasmobranchii, 47 ; Teleostei, 71, 75; Petromyzon, 86; Lcpid


osteus, 112; Amphibia, 122, 125; Chick, 149, 166; Lacerta, 203; Rabbit, 216, 219; origin of in Rabbit, 221 ; comparative account of development of, 300

Epibolic formation of gastrula, 334

Epichordal formation of vertebral column, 556

Epicrium glutinosum, 143

Epidermis, in Ccelenterata, 393; protective structures of, 394

Epididymis, 724

Epigastric vein, 653

Episkeletal muscles, 676

Episternum, 602

Epoophoron, 725

Ethmoid bone, 597

Ethmoid region of skull, 570

Ethmopalatine ligament of Elasmobranchs, 576

Euphausia, eye of, 483

Eustachian tube, of Amphibia, 135; Chick, 1 80; Rabbit, 232; general development of, 528

Excretory organs, general constitution of, 680; of Platyelminthes, 680; of Mollusca, 681; of Polyzoa, 682; of Brachiopoda, 683 ; of Choetopoda, 683 ; of Gephyrea, 686 ; of Discophora, 687 ; of Arthropoda, 688; of Nematoda, 689; of Echinodermata, 689 ; constitution of in Craniata, 689; of Elasmobranchii, 690; constitution of in adult Elasmobranch, 697; of Petromyzon, 700; of Myxine, 701 ; of Teleostei, 701 ; of Ganoidei, 704; of Dipnoi, 707; of Amphibia, 707; of Amniota, 713; comparison of Vertebrate and Invertebrate, 737

Excretory system, of Elasmobranchii, 49 ; Teleostei, 78; Petromyzon, 95, 98; Acipenser, 99; Amphibia, 133

Exoccipital bone, 595

Exoskeleton, dermal, 393 395 ; epidermal, 393396

External generative organs, 726

Extra-branchial skeleton, 572

Eye, of Ascidia, 16; Salpa, 31; Elasmobranchii, 56, 57, 58; Teleostei, 73; Petromyzon, 92, 98; Aves, i/o; Rabbit, 229; general development of, 470; evolution of, 470, 471; simple, 480; compound, 481 ; aconous, 482; pseudoconous, 482 ; of Invertebrata, 471; of Vertebrata, 483 ; comparative development of Vertebrate, 497 ; of Ammoccetes, 497 ; of Tunicata, 507 ; of Chordata, general views on, 508 ; accessory eyes of Fishes, 509; muscles of, 677

Eyelids, development of, 506

Falciform ligament, 757

Falx cerebri, 439

Fasciculi terctes, of Elasmobranchii. 426

Feathers, development of, 396


INDEX.


785


Fenestra rotunda and ovalis, 529

Fertilization, of Amphioxus, 2 ; of Urochorda, 9; Salpa, 29; Elasmobranchii, 46; of Teleostei, 68; Petromyzon, 84 ; Amphibia, 120; Chick, 145 ; Reptilia, 202 ; meaning of, 331

Fifth nerve, development of, 460

Fifth ventricle, 443

Fins, of Elasmobranchii, 62 ; Teleostei, 78; Petromyzon, 94, 95; Acipenser, 109; Lepidosteus, 118; relation of paired to unpaired, 611, 612 ; development of pelvic, 614; development of pectoral, 615; views on nature of paired fins, 616

Fissures of spinal cord, 417

Foetal development, 360 ; secondary variations in, 361

Foot, 618

Foramen of Munro, 430, 438

Foramen ovale, 642

Forebrain, of Elasmobranchii, 55, 59, 60; Petromyzon, 93 ; general development of, 428

Formative cells, of Chick, 154

Fornix, development of, 443

Fornix of Gottsche, 428

Fourth nerve, 464

Frontals, 592

Fronto-nasal process of Chick, 179

Gaertner's canals, 724

Gall-bladder, 770

Ganoidei, development of, 102; relations of, 118; nares of, 534; notochord of, 546 ; vertebral column of, 546, 553 ; ribs of, 561 ; pelvic girdle of, 606; arterial system of, 645 ; excretory organs of, 704; generative ducts of, 734

Gastropoda, eye of, 472

Gastrula, of Amphioxus, 2; of Ascidia, lo; Elasmobranchii, 43, 44 ; Petromyzon, 86; Acipenser, 103; Amphibia, 123; comparative development of, in Invertebrata, 275 ; comparison of Mammalian, 291 ; phylogenetic meaning of, 333 ; ontogeny of (general), 333 ; phylogeny of, 338 343 ; secondary types of, 34!

Geckos, vertebral column of, 557

Generative cells, development of, 74! ; origin of in Ccelenterata, 741 ; of Invertebrata, 743 ; of Vertebrata, 746

Generative ducts, of Teleostei, 704, 735 ; of Ganoids, 704; of Cyclostomata, 733; origin of, 733 ; of Lepidosteus, 735, 750 ; development and evolution of, 748 ; of Ccelenterata, 748 ; of Sagitta, 749 ; of Tunicata, 749 ; Cheetopoda, Gephyrea, etc., 749; of Mollusca, 751; of Discophora, 751 ; of Echinodermata,

75*

Generative system of Elasmobranchii, 51 Gephyrea, nervous system of, 412; excretory organs of, 686 ; generative cells of, 743 ; generative ducts of, 749

B. III.


Germinal disc, of Elasmobranchii, 40; Teleostei, 68 ; Chick, 147

Germinal epithelium, 746

Germinal layers, summary of organs <lrrived from, in Vertebrata, 304 ; historical account of views of, 332 ; homologies of in the Metazoa, 345

Germinal wall of Chick, 152, 159; structure and changes of, 160

Geryonia, auditory organ of, 5 r 5

Gill of Salpa, 31

Giraldes, organ of, 725

Glands, epidermic, development of, 397

Glomerulus, external, of Chick, 716

Glossopharyngeal nerve, development of,

45 6 > 457 Grey matter of spinal cord, 417; of brain,

423 Growth in length of Vertebrate embryo,

306 Guinea-pig, primitive streak of, 223;

notochord of, 226 ; placenta of, 242 ;

development of, 262 Gymnophiona, see ' Ccecilia '

Habenula perforata, 525

Hairs, development of, 396

Halichrerus, placenta of, 250

Hand, 619

Head, comparative account of, 313; segmentation of, 314

Head cavities, of Elasmobranchii, 50 ; Petromyzon, 90, 96; Amphibia, 127; general development of, 676

Head-fold of Chick, 157, 167

Head kidney, see ' Pronephros '

Heart, of Pyrosoma, 25; Elasmobranchii, 50, 58 ; Petromyzon, 94, 97 ; Acipenser, 106; Chick, 170 ; first appearance of in Rabbit, 230; general development of, 633 ; of Fishes, 635, 637 ; of Mammalia, 638; of Birds, 637, 639; meaning of development of, 637 ; of Amphibia, 638 ; of Amniota, 639 ; change of position of, 643

Hind-brain, Elasmobranchii, 55, 59, 60 ; Petromyzon, 93 ; general account of, 424

Hippocampus major, development of, 442

Hirudo, development of blood-vessels of, 633 ; excretory organs of, 688

Horse, placenta of, 253

Hyaloid membrane, 492

Hylodes, oviposition of, 1 21 ; metamorphosis of, -1 37

Hyobranchial cleft, 572

Hyoid arch, of Chick, 179; general account of, 572, 575 ; modifications of, e !73> 577 > f Elasmobranchii, 576; of Teleostei, 577 ; of Amphibia, 582 ; of Sauropsida, 588; of Mammalia,

589

Hyomandibular bar of Elasmobranchii, 576, 577 ; of Teleostei, 579 ; of Amphibia, 582

50


86


INDEX.


Hyomandibular cleft, of Fetromyzon, 91 ; Chick, 179 ; general account of, 572

Hyostylic skulls, 582

Hypoblast of Elasmobranchii, 5! ; Teleostei, 71, 75; Petromyzon, 86; Acipenser, 104; Lepidosteus, 113; Amphibia, 122, 129; Chick, 151, 167 ; Lacerta, 203; Rabbit, 215, 216, 219 ; origin of in Rabbit, 220

Hyposkeletal muscles, 675

Ilyrax, placenta of, 249

Incus, 529, 590

Infraclavicle, 600

Infundibulum of Petromyzon, 92 ; Chick, 175 ; general development of, 430

Insectivora, placenta of, 243

Insects, nervous system of, 410 ; eye of, 481; generative organs of, 745; generative ducts of, 751

Intercalated pieces of vertebral column,

55 1

Interclavicle, homologies of, 602

Intermediate cell-mass of Chick, 183

Intermuscular septa, 672

Interorbital septum, 570

Interrenal bodies, 665

Iris, 489 ; comparative development of,

506

Iris of Ammoccetes, 98 Island of Reil, 444

Jacobson's organ, 537 Jugal bone, 594

Kidney, see ' Metanephros '

Labia majora, development of, 727

Labial cartilages, 597

Labium tympanicum, 525 ; vestibulare,

5 2 5

Lacertilia, general development of, 202 ; nares of, 537 ; pectoral girdle of, 603 ; pelvic girdle of, 607 ; arterial system of, 649

Lacrymal bone, 593

Lacrymal duct, 506

Lacrymal glands, 506

Lremargus, vertebral column of, 548

Lagena, 524

Lamina spiralis, 524

Lamina terminalis, 438

Larva of Amphioxus, 2 ; of Ascidia, 1 5 it ; Teleostei, 81 ; Petromyzon, 89, 95; Lepidosteus, 117, 318; Amphibia, 134, 142; types of, in the Invertebrata, 363

Larvre, nature, origin, and affinities of, 360 386; secondary variations of less likely to be retained, 362 ; ancestral history more fully recorded in, 362 ; secondary variations in development of, 363 ; ontogenetic record of secondary variations in, 361; of freshwater and land animals, 362; types of, 36.2; phosphorescence of, 364; of Coelenterata,


367 ; table of, 365 ; of Invertebrata, 367 et seq.

Larynx, 766

Lateral line sense organs, 538 ; comparison of, with invertebrate, 538 ; development of, in Teleostei, 538 ; development of, in Elasmobranchii, 539

Lateral ventricle, 438 ; anterior cornu of, 440 ; descending cornu of, 440 ; choroicl plexus of, 443

Layers, formation of, in Elasmobrancliii, 41, 56 ; Teleostei, 71 ; Petromyzon, 85 ; Acipenser, 103 ; Lepidosteus, 1 1 1 ; Amphibia, 121; Chick, 150, 152; Lacerta, 202; Rabbit, 215 227; comparison of Mammalia with lower forms, 226, 289; comparison of formation of in Vertebrata, 275; origin and homologies of, in the Metazoa, 331

Leech, see ' Hirudo '

Lemuridre, placenta, 256

Lens, of Elasmobranchii, 57, 58 ; Petromyzon, 94, 99; Acipenser, 106 ; Lepidosteus, 115 ; Amphibia, 127 ; Chick, 177 ; of Vertebrate eyes, 485 ; general account of, 493 ; capsule of, 493 ; comparative development of, 499 ; of Amphibia, Teleostei, Lepidosteus, 499

Lepidosteus, development of, 1 1 1 ; larva of, 117; relations of, 119; spinal nerves of, 455; ribs of, 561 ; generative ducts of, 704, 735 ; swimming-bladder of,

763

Ligamentum pectinatum, 490

Ligamentum suspensorium, 557, 558

Ligamentum vesicse medium, 239

Limbs, of Elasmobranchii, 59 ; Teleostei, 80 ; first appearance of in Chick, 184 ; Rabbit, 232 ; muscles of, 673 ; of Fishes, 609; relation of, to unpaired fins of Fishes, 611, 612; of Amphibia, 61 8

Liver of Teleostei, 78 ; Petromyzon, 95, 96; Acipenser, no; Amphibia 130; general account of, 769

Lizard, development of, 202; general growth of embryo of, 208 ; Mullerian duct of, 721

Lizzia, eye of, 471

Lobi inferiores, 431

Lungs of Amphibia, 137 ; development of, 763 ; homology of, 766

Lymphatic system, 664

Malleus, 529, 591 ; views on, 591 Malpighian bodies, development of accessory in Elasmobranchs, 695 Mammalia, development of, 214; comparison of gastrula of, 291 ; cerebellum of, 427 ; infundibulum of, 431 ; pineal gland of, 434; pituitary body of, 436; cerebrum of, 439 ; spinal nerves of, 449 ; sympathetic of, 466; vertebral column of, 558; branchial arches of, 573, 574; mandibular and hyoid arches of, 589 ; pectoral girdle of, 604; pelvic girdle of,


INDEX.


787


608 ; heart of, 636 ; arterial system of, 647; venous system of, 661 ; muscleplates of, 671 ; mesonephros of, 714; testicular network of, 724 ; urinogenital sinus of, 727 ; spermatozoa of, 747 ; lungs of, 765 ; intestines of, 768 ; liver of> 769; postanal gut of, 774; stomodseum of, 775

Mammary gland, development of, 398 Man, placenta of, 244 ; general account of development of, 265 ; characters of embryo of, 270

Mandibular arch of Elasmobranchii, 62, 576; Petromyzon, 91 ; Acipenser, 106, 116; Chick, 179; general account of,

572, 575; modification of to form jaws,

573, 575; of Teleostei, 580; of Amphibia, 582; Sauropsida, 588; Mammalia, 589

Mandibular bar, evolution of, 311, 321

Manis, placenta of, 256

Marsupial bones, 608

Marsupialia, foetal membranes of, 240 ; cerebellum of, 426 ; corpus callosum of, ' 443 ; uterus of, 726

Maxilla, 594

Meatus auditorius externus, of Chick, 181; development of, 527

Meckelian cartilage, of Elasmobranchii, 576; of Teleostei, 581 ; of Amphibia, 584, 585; of Sauropsida, 588 ; of Mammalia, 590

Mediastinum anterior and posterior, 630

Medulla oblongata, of Chick, 176 ; general development of, 425

Medullary plate of Amphioxus, 4, 5 ; of Ascidia, n; Elasmobranchii, 44, 47, 55; Teleostei, 72; Petromyzon, 88; Acipenser, 104; Lepidosteus, 1 1 1 ; Amphibia, 126, 127, 131; Chick, 159; Lacerta, 204; Rabbit, 223, 227, 228; primitive bilobed character of, 303, 317

Medusae, auditory organs of, 513

Membrana capsulo-pupillaris, 494, 504,

507

Membrana elastica externa, 546

Membrana limitans of retina, 491

Membrana tectoria, 522, 525

Membrane bones, of Amphibia, 582 ; of Sauropsida, 588; of Mammalia, 590; of mandibular arch, 593 ; of pectoral girdle, 599, 602 ; origin of, 592 ; homologies of, 593

Membranous labyrinth, development of in Man, 519

Menobranchus, branchial arches of, 142

Mesenteron of Elasmobranchii, 43 ; Teleostei, 75 ; Petromyzon, 85 ; Acipenser, 104; Amphibia, 123, 124, 129; Chick, 167; general account of, 754

Mesentery, 626, 756

Mesoblast, of Amphioxus, 6 ; Ascidia, 17, 20; Pyrosoma, 24; Salpa, 30; Elasmobranchii, 44, 47; Teleostei, 75; Petromyzon, 86; Acipenser, 105; Lepi


dosteus, 113; Amphibia, 125, 128, 129; of Chick, 154, 167; double origin of in Chick, 154, 158, 159; origin of from lips of blastopore in Chick, 158; of area vasculosa of Chick, iOo; Lacerta, 203; origin of in Rabbit, 218, 223; of area vasculosa in Rabbit, 227; comparative account of formation of, 292 ; discussion of development of in Vertebrata, 297 ; meaning of development of in Amniota, 298; phylogenetic origin of, 346 ; summary of ontogeny of, 349 352 ; views on ontogeny of, 352 360

Mesoblastic somites, of Amphioxus, 6 ; Elasmobranchii, 48, 55 ; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 114; Amphibia, 129, 131; Chick, 161, 1 80; Rabbit, 228; development of in Chordata, 325; meaning of development of, 331; of head, 676

Mesogastrium, 758

Mesonephros, of Teleostei, 78, 702; Petromyzon, 95, 98, 700; Acipenser, 1 10, 705; Amphibia, 134, 708; Chick, 184, 714; general account of, 690 ; development of in Elasmobranchs, 691 ; of Cyclostomata, 700 ; Ganoidei, 705 ; sexual and non-sexual part of in Amphibia, 710; of Amniota, 713, 724; summary and general conclusions as to, 729; relation of to pronephros, 731

Mesopterygium, 616

Metagenesis of Ascidians, 34

Metamorphosis of Amphibia, 137, 140

Metanephros, 690; development of in Elasmobranchii, 697; of Amphibia, 712; of Amniota, 713; of Chick, 722; of Lacertilia, 723; phylogeny of, 736

Metapterygium, 616

Metapterygoid, of Elasmobranchii, 576; of Teleostei, 581

Metazoa, evolution of, 339, 342 ; ancestral form of, 333, 345

Mid-brain, of Elasmobranchii, 55, 58, 59; Petromyzon, 92; general account of development of, 427

Moina, generative organs of, 745

Molgula, development of, 22

Mollusca, nervous system of, 414 ; eyes of, 472; auditory organs of, 515; excretory organs of, 68 1

Monotremata, foetal membranes of, 240 ; cerebellum of, 426; corpus callosum of, 443 ; cerebrum of, 443 ; urinogenital sinus of, 726

Mormyrus, generative ducts of, 704

Mouth, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 27; Salpa, 31; Elasmobranchii, 57, 60, 61, 62; Petromyzon, 92, 94, 95, 99; Acipenser, 107; Lepidosteus, 118; Amphibia, 129, 132, "134; Rabbit, 231 ; origin of, 317

Mouth, suctorial, of Petromyzon, 99; Acipenser, 107; Lepidosteus, 116, 317; Amphibia, 133, 141, 317


88


INDEX.


Mullerian duct, 690; of Elasmobranchs, 693 ; of Ganoids, 704 ; of Amphibia, 710; of Aves, 717,720; opening of into cloaca, 727; origin of, 733; summary of development of, 733; relation of to pronephros, 733

Muscle-plates, of Amphioxus, 6; Elasmobranchii, 49, 668 ; Teleostei, 670 ; Petromyzon, 94; Chick, 183, 670; general development of, 669 ; of Amphibia, 670; Aves, 670; of Mammalia, 671; origin of muscles from, 672

Muscles, of Ascidia, II, 17; development of from muscle-plates, 672; of limbs, 673 ; of head, 676 ; of branchial arches, 678; of eye, 678

Muscular fibres, epithelial origin of, 667

Muscular system, development of, 667; of Chordata, 668

Mustelus, placenta of, 66

Myoepithelial cells, 667

Mysis, auditory organ of, 517

Myxine, ovum of, loo; olfactory organ of, 533 ; portal sinus of, 652 ; excretory system of, 701

Nails, development of, 397

Nares, of Acipenser, 108; of Ichthyopsida, 534; development of in Chick, 535; development of in Lacertilia, 537; development of in Amphibia, 537

Nasal bones, 592

Nasal pits, Acipenser, 108; Chick, 176; general development of, 531

Nematoda, excretory organs of, 689 ; generative organs of, 745 ; generative ducts of, 752

Nemertines, nervous system of, 311 ; excretory organs of, 68 1

Nerve cord, origin of ventral, 378

Nerves, spinal, 449 ; cranial, 455 466

Nervous system, central, general account of development of in Vertebrata, 415 ; conclusions as to, 445; sympathetic, 466

Nervous system, of Amphioxus, 4; Ascidia, 15, 16; Molgula, 22; Pyrosoma, 24, 25; Salpa, 30, 31; Elasmobranchii, 44; Teleostei, 77 ; Petromyzon, 89, 93; Acipenser, 105; Amphibia, 126; comparative account of formation of central, 301; of Sagitta, 349; origin of in Ccelenterata, 349; of pneoral lobe, 377, 380; evolution of, 400405; development of in Invertebrates, 406; of Arthropoda, 408; of Gephyrea, 412; Mollusca, 414

Neural canal, of Ascidia, 10; Teleostei, 72; Petromyzon, 88; Acipenser, 105; Lepidosteus, 114; Amphibia, 126, 131 ; Chick, 1 66, 171 ; Lacerta, 208; closure of in Frog and Amphioxus, 279; closure of in Elasmobranchii, 284; phylogcuctic origin of, 316

Neural crest, 449, 456, 457


Neurenteric canal, of Amphioxus, 4, 5 ; Ascidia, lo; Elasmobranchii, 54; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 113; Aves, 162; Lacerta, 203, 206; general account of, 323; meaning of, 3 2 3

Newt, ovum of, 120; development of, I2 55 general growth of, 141

Notidanus, vertebral column of, 548; branchial arches of, 572

Notochord of Amphioxus, 6; Ascidia, II, 17; Elasmobranchii, 51; Teleostei, 74; Petromyzon, 86, 94; Acipenser, 104; Lepidosteus, 113; Amphibia, 128, 129; Chick, 157; canal of, in Chick, 163; Lacerta, 204, 205; Guinea-pig, 226; comparative account of formation of, 292, 325; sheath of, 545; later histological changes in, 546; cartilaginous sheath of, 547; in head, 566; absence of in region of trabeculas, 567

Notodelphys, brood-pouch of, 121 ; branchiae of, 140

Nototrema, brood-pouch of, 121

Nucleus pulposus, 559

Oceania, eye of, 471

Occipital bone, 595

CEsophagus, solid, of Elasmobranchii, 61, 759; of Teleostei, 78

Olfactory capsules, 571

Olfactory lobes, development of, 444

Olfactory nerves, Ammoccetes, 99; general development of, 464

Olfactory organ, of aquatic forms, 531; Insects and Crustacea, 531; of Tunicata, 532 ; of Amphioxus, 532 ; of Vertebrata, 533; Petromyzon, 533; of Myxine, 533

Olfactory sacks, of Elasmobranchii, 60; Teleostei, 73; Petromyzon, 92, 97; Acipenser, 106, 108; Lepidosteus, 116; Chick, 176

Oligochreta, excretory organs of, 683

Olivary bodies, 426

Omentum, lesser and greater, 757

Onchidium, eye of, 473

Opercular bones, 593

Operculum, of Teleostei, 77; Acipenser, 107; Lepidosteus, 117, 118; Amphibia,

r 3.5.

Ophidia, development of, 210; arterial system of, 649 ; venous system of, 656

Optic chiasma, 430, 493

Optic cup, retinal part of, 488 ; ciliary portion of, 489

Optic lobes, 428

Optic nerve, development of, 492 ; comparative development of, 500

Optic thalami, development of, 431

Optic vesicle, of Elasmobranchii, 57 59; Teleostei, 74, 499 ; Petromyzon, 89, 92 ; Acipenser, 106; Lepidosteus, 115; Chick, 170; Rabbit, 229; general development of, 429 ; formation of secon


INDKX.


7*9


dary, 487 ; obliteration of cavity of, 488 ; comparative development of, 499; of Lepidosteus and Teleostei, 499. See also ' Eye '

Ora serrata, 488

Orbitosphenoid region of skull, 570

Organs, classification of, 391 ; derivation of from germinal layers, 392

Orycteropus, placenta of, 249

Otic process of Axolotl, 583; of Frog, 585 et seq.

Otoliths, 512

Oviposition, of Amphioxus, i ; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 84; Amphibia, 121; Reptilia, 202

Ovum, of Amphioxus, i; Pyrosoma, 23; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 83 ; Myxine, loo; Acipenser, 102; Lepidosteus, in; Amphibia, 120; Chick, 146; Reptilia, 202 ; Mammalia, 214; of Porifera, 741; migration of in Ccelenterata, 742; Vertebrata, 746

Palatine bone, of Teleostei, 580; origin of, 594

Pancreas, Acipenser, no; general development of, 770

Pancreatic caeca, of Teleostei, etc. 768

Papillae, oral, of Acipenser, 108; Lepidosteus, n6

Parachordals, 565, 566

Parasphenoid bone, 594

Parepididymis, 725

Parietal bones, 592

Paroophorori, 725

Parovarium, 725

Pectoral girdle, 599 ; of Elasmobranchs, 600; of Teleostei, 600; of Amphibia and Amniota, 60 1 ; comparison of with pelvic, 608

Pecten, eye of, 479

Pecten, of Ammoccetes, 498; of Chick, 501 ; Lizard, 501 ; Elasmobranchs, 501

Pedicle, of Axolotl, 484 ; of Frog, 485

Pelobates, branchial apertures of, 136; vertebral column of, 556

Pelodytes, branchial chamber of, 135

Pelvic girdle, 606; of Fishes, 606; Amphibia and Amniota, 607 ; of Lacertilia, 607 ; of Mammalia, 608 ; comparison with pectoral, 608

Penis, development of, 727

Peribranchial cavity, of Amphioxus, 7; of Ascidia, 18; Pyrosoma, 24

Pericardial cavity, of Pyrosoma, 26 ; Elasmobranchii, 49 ; Petromyzon, 94; general account of, 626; of Fishes, 627 ; of Amphibia, Sauropsida and Mammalia, 628

Perichordal formation of vertebral column, 5^6

Perilymph of ear, 523 Periotic capsules, ossifications in, 595, 596


Peripatus, nervous system of, 409 ; eye of 480 ; excretory organs of, 688

Peritoneal membrane, 626

Petromyzon, development of, 83; affinities of, 83, 84; general development of, 87; hatching of, 89; comparison of gastrula of, 280; branchial skeleton of, 312, 572; cerebellum of, 425; pineal gland of, 434 ; pituitary body of, 436 ; cerebrum of, 439; auditory organ of, 517; olfactory organ of, 533; comparison of oral skeleton of with Tadpole, 586; pericardial cavity of, 627; abdominal pores of, 626 ; venous system of, 651 ; excretory organs of, 700; segmental duct of, 700; pronephros of, 700; mesonephros of, 700 ; thyroid body of, 760; postanalgut of, 774; stomodx-um

of, 775

Phosphorescence of larvae, 364

Phylogeny, of the Chordata, 327; of the Metazoa, 384

Pig, placenta of, 251; mandibular and hyoid arches of, 589

Pineal gland, of Petromyzon, 93 ; Chick, 175; general development of, 432; nature of, 432, 434

Pipa, brood-pouch of, 121 ; metamorphosis of, 139; yolk-sack of, 140; vertebral column of, 556

Pituitary body, of Rabbit, 231 ; general development of, 435 ; meaning of, 436 ; Placenta, of Salpa, 29; Elasmobranchii, 66; of Mammalia, 232; villi of, 235 ; deciduate and non-deciduate, 239; comparative account of, 239 259 ; characters of primitive type of, 240; zonary, 248; non-deciduate, 250; histology of, 257; evolution of, 259

Placoid scales, 395

Planorbis, excretory organs of, 68 1

Planula, structure of, 367

Pleural cavities, 631

Pleuronectidae, development of, 80

Pneumatoccela, characters of, 327

Polygordius, excretory organs of, 684

Polyophthalmus, eye of, 479

Polypedates, brood-pouch of, 121

Polyzoa, excretory organs of, 682 ; generative cells of, 745 ; generative ducts

of, 751

Pons Varolii, 426, 427

Pori abdominales, Ammoccetes, 99

Porifera, ancestral form of, 345 ; development of generative cells of, 74!

Portal vein, 653

Postanal gut of Elasmobranchii, 58, 59, 60; Teleostei, 75; Chick, 169; general account of, 323, 772

Prsemaxilla, 594

Praeopercular bone, 593

Prrcoral lobe, ganglion of, 377, 380

Prefrontals, 597

Presphenoid region of skull, 570

Primitive groove of Chick, 1 55


790


INDEX.


Primitive streak, of Chick, 152, 161; meaning of, 153; origin of mesoblast form in Chick, 154; continuity of hypoblast with epiblast at anterior end of, in Chick, 156; comparison of with blastopore, 165 ; fate of, in Chick, 165 ; of Lacerta, 203; of Rabbit, 221; of Guinea-pig, 223 ; fusion of layers at, in Rabbit, 224; comparison of with blastopore of lower forms, 226, 287 ; of Mammalia, 290

Processus falciformis of Ammoccetes, 498 ; of Elasmobranch, 502 ; of Teleostei , 503 Proctodseum, 778

Pronephros, of Teleostei, 78, 701 ; Petromyzon, 95, 99, 700; Acipenser, 106, no; Amphibia, 134, 707; general account of, 689 ; of Cyclostomata, 700 ; of Myxine, 701 ; Ganoidei, 705 ; of Amniota, 714; of Chick, 718; summary of and general conclusions as to, 728; relation of, to mesonephros, 731 ; cause of atrophy of, 729 Prootic, 596, 597 Propterygium, 616 Proteus, branchial arches of, 142 Protochordata, characters of, 327 Protoganoidei, characters of, 328 Protognathostomata, characters of, 328 Protopentadactyloidei, characters of, 329 Protovertebrata, characters of, 328 Pseudis, Tadpole of, 139; vertebral

column of, 556

Pseud ophryne, yolk-sack of, 140; Tadpole of, 140 Pterygoid bone, of Teleostei, 581; origin

of, 597

Pterygoquadrate bar, of Elasmobranchii, 576; of Teleostei, 581; Axolotl, 584; F r g, 584; ofSauropsida, 588; of Mammalia, 589

Pulmonary artery, origin of, 645 ; of Amphibia, 645 ; of Amniota, 649

Pulmonary vein, 655

Pupil, 489

Pyrosoma, development of, 23

Quadrate bone of Teleostei, 581 ; of Axolotl, 584; Frog, 585; Sauropsida, 588

Quadratojugal bone, 594

Rabbit, development of, 214; general growth of embryo of, 227 ; placenta of, 248

Radiate symmetry, passage from to bilateral symmetry, 373 376

Raja, caudal vertebras of, 553

Rat, placenta of, 242

Recessus labyrinthi, 519

Reissner's membrane, 524

Reptilia, development of, 202; viviparous, 202; cerebellum of, 426; infundibulum of, 431; pituitary body of, 436; cerebrum of, 439; vertebral column of,


556; arterial system of, 648; venous system of, 656; mesonephros of, 713; testicular network of, 723; spermatozoa of, 747

Restiform tracts of Elasmobranchii and Teleostei, 425

Retina, histogenesis of, 490

Retinulse, 482

Rhabdom, 482

Rhinoderma, brood-pouch of, 121; metamorphosis of, 1 39

Ribs, development of, 560

Roseniniiller's organ, 725

Rotifera, excretory organs of, 680

Round ligament of liver, 663

Ruminantia, placenta of, 253

Sacci vasculosi, 437

Sacculus hemisphericus, 519; of Mammals, 519, 520

Sagitta. See ' Chaetognatha'

Salpa, sexual development of, 29; asexual development of, 33

Salamandra, larva of, 142; vertebral column of, 553; limbs of, 619; mesonephros of, 708; Miillerian duct of, 710

Salmonidse, hypoblast of, 71; generative ducts of, 704

Sauropsida, gastrula of, 286; meaning of primitive streak of, 288; blastopore of, 289 ; mandibular and hyoid arches of, 588 ; pectoral girdle of, 60 1

Scala, vestibuli, 522; tympani, 523; media, 522

Scales, general development of, 396 ; development of placoid scales, 395

Scapula, 599

Sclerotic, 488

Scrotum, development of, 727

Scyllium, caudal vertebrse of, 553; mandibular and hyoid arches of, 578; pectoral girdle of, 600; limbs of, 610; pelvic fin of, 614; pectoral fin of, 615

Segmental duct, 690 ; development of in Elasmobranchs, 690; of Cyclostomata, 700; of Teleostei, 701; of Ganoidei, 704, 705 ; of Amphibia, 707 ; of Amniota, 713

Segmental organs, 682

Segmental tubes, 690 ; development of in Elasmobranchs, 691 ; rudimentary anterior in Elasmobranchs, 693 ; development of secondary, 731

Segmentation cavity, of Elasmobranchii, 42 44; Teleostei, 69, 85, 86; Amphibia, 122, 125

Segmentation, meaning of, 331

Segmentation of ovum, in Amphioxus, 2 ; Ascidia, 9 ; Molgula, 22 ; Pyrosoma, 23; Salpa, 30; Elasmobranchii, 40; Telostei, 69; Petromyzon, 84; Acipenser, IOT, Lcpidosteus, in; Amphibia, 122, 124; Newt, 125; Chick, 146; Lizard, 202: Rabbit, 214


INDEX.


791


Semicircular canals, 519

Sense organs, comparative account of development of, 304

Septum lucidum, 443

Serous membrane, Lacerta, 209; of Rabbit, 237

Seventh nerve, development of, 459

Shell-gland of Crustacea, 689

Shield, embryonic, of Chick, 151 ; of Lacerta, 202

SimiadiK, placenta of, 247

Sinus rhomboidalis, of Chick, 162

Sinus venosus, 637

Sirenia, placenta of, 255

Sixth nerve, 463

Skate, mandibular and hyoid arches of,

577

Skeleton, elements of found in Vertebrata, 542

Skull, general development of, 564 ; historical account of, 564 ; development of cartilaginous, 566; cartilaginous walls of, 570; composition of primitive cartilaginous cranium, 565

Somatopleure, of Chick, 170

Spelerpes, branchial arches of, 142

Spermatozoa, of Porifera, 741; of Vertebrata, 746

Sphenoid bone, 595

Sphenodon, hyoid arch of, 588

Spinal cord, general account of, 415; white matter of, 415; central canal of, 417, 418; commissures of, 417; grey matter of, 417; fissures of, 418

Spinal nerves, posterior roots of, 449; anterior roots of, 453

Spiracle, of Elasmobranchii, 62 ; Acipenser, 105; Amphibia, 136

Spiral valve. See 'Valve'

Spleen, 664

Splenial bone, 595

Squamosal bone, 593

Stapes, 529; of Mammal, 590

Sternum, development of, 562

Stolon of Doliolum, 29 ; Salpa, 33

Stomodaeum, 774

Stria vascularis, 524

Styloid process, 591

Sub-intestinal vein, 65 1 ; meaning of,

651

Syngnathus, brood-pouch of, 68 Subnotochordal rod, of Elasmobranchii,

54; Petromyzon, 94; Acipenser, no;

Lepidosteus, 115; general account of,

754; comparison of with siphon of

Chsetopods, 756

Subzonal membrane, 237; villi of, 236 Sulcus of Munro, 432 Supraclavicle, 600 Suprarenal bodies, 664 Supra-temporal bone, 593 Swimming bladder, see Air bladder Sylvian aqueduct, 428 Sylvian fissure, 444 Sympathetic ganglia, development of, 467


Tadpole, 134, 139, 140; phylogenetic meaning of, 137; metamorphosis of, 137; m can ing of suctorial mouth of, 585

Tail of Teleostei, 80; Acipenser, 109; Lepidosteus, 109; Amphibia, 132

Tarsus, development of, 620

Teeth, horny provisional, of Amphibia, 136; general development of, 776; origin of, 777

Teleostei, development of, 68; viviparous, 68; comparison of formation of layers in, 286; restiform tracts of, 425 ; mid-brain of, 425 ; infundibulum of, 431 ; cerebrum of, 439; nares of, 534; lateral line of, 538; notochord and membrana elastica of, 549 ; vertebral column of, 553; ribs of, 561; hyoid and mandibular arches of, 579; pectoral girdle of, 601 : pelvic girdle of, 606; limbs of, 618; heart of, 637; arterial system of, 645; muscle-plates of, 670; excretory organs of, 701 ; generative ducts of, 704, 735, 749; swimming bladder of, 763 ; postanal gut of,

Teredo, nervous system of, 414

Test of Ascidia, 14; Salpa, 31

Testicular network, of Elasmobranchs, 697 ; of Amphibia, 712 ; Reptilia, 723 ; of Mammals, 724

Testis of Vertebrata, 746

Testis, connection of with Wolffian body, in Elasmobranchii, 697; in Amphibia, 710; in Amniota, 723; origin of, 735

Thalamencephalon of Chick, 175; general development of, 430

Third nerve, development of, 461

Thymus gland, 762

Thyroid gland, Petromyzon, 92 ; general account of, 759; nature of, 760; development of in Vertebrata, 761

Tooth. See 1 Teeth'

Tori semicirculares, 428

Tornaria, 372

Trabeculas, 565, 567; nature of, 568

Trachea, 766

Trematoda, excretory organs of, 68 1

Triton alpestris, sexual larva of, 143

Triton, development of limbs of, 619} urinogenital organs of, 7 12

Truncus arteriosus, 638; of Amphibia, 638; of Birds, 639

Turiicata, development of mesoblast of, 293; test of, 394; eye of, 507; auditory organ of, 530; olfactory organ of, 532; generative duct of, 749 ; intestine of, 767; postanal gut of, 771; stomodseum of, 775

Turbellaria, excretory organs of, 68 1

Tympanic annulus of *'rog, 587

Tympanic cavity, of Amphibia, 135; Chick, 1 80; Rabbit, 232; general development of, 528; of Mammals, 591

Tympanic membrane, of Chick, 180; general development of, 528


792


INDEX.


Tympanohyal, 591

Umbilical canal of Elasmobranchii, 54,

57, 58, 59

Umbilical cord, 238; vessels of, 239

Ungulata, placenta of, 250

Urachus, 239, 726

Ureters, of Elasmobranchii, 696; development of, 723

Urethra, 727

Urinary bladder of Amphibia, "Jii; of Amniota, 726

Urinogenital organs, see Excretory organs

Urinogenital sinus of Petromyzon, 700; of Sauropsida, 726; of Mammalia, 727

Urochorda, development of, 9

Uterus, development of, 726; of Marsupials, 726

Uterus masculinus, 726

Utriculus, 519

Uvea of iris, 489

Vagus nerve, development of, 456, 457; intestinal branch of, 458; branch of to lateral line, 459

Valve, spiral, of Petromyzon, 97; Acipenser, no; general account of, 767

Valves, semilunar, 641; auriculo-ventricular, 642

Vasa efferentia, of Elasmobranchs, 697 ; of Amphibia, 711; general origin of, 724

Vascular system, of Amphioxus, 8; Petromyzon, 97; Lepidosteus, 116; general development of, 632

Vas deferens, of Elasmobranchii, 697 ; of Amniota, 723

Vein, sub-intestinal of Petromyzon, 97 ; Acipenser, no; Lepidosteus, 116

Velum of Petromyzon, 9 1

Vena cava inferior, development of, 655

Venous system of Petromyzon, 97; general development of, 651; of Fishes, 651 ; of Amphibia and Amniota, 655 ; of Reptilia, 656; of Ophidia, 656; of Aves, 658; of Mammalia, 661

Ventricle, fourth, of Chick, 176; history of, 424

Ventricle, lateral, 438, 440; fifth, 443

Ventricle, third, of Chick, 175

Vertebral bodies, of Chick, 183

Vertebral column, development of, 545, 549; epichordal and perichordal development of in Amphibia, 556

Vespertilionidse, early development of, 217

Vieussens, valve of, 426

Villi, placental, of zona radiata, 235 ; subzonal membrane, 235; chorion, 237;


Man, 246; comparative account of, 2 575 of young human ovum, 265, 269

Visceral arches, Amphioxus, 7 ; Elasmobranchii, 57 60; Teleostei, 77; Acipenser, 1 06; Lepidosteus, 116; Amphibia, 133; Chick, 177; Rabbit, 231; prseoral, 570; relation of to head cavities, 572; disappearance of posterior, 573; dental plates of in Teleostei, 574

Visual organs, evolution of, 470

Vitelline arteries of Chick, 195

Vitelline veins of Chick, 195

Vitreous humour, of Ammoccetes, 98 ; general development of, 494; blood* vessels of in Mammals, 503 ; mesoblastic ingrowth in Mammals, 503

Vomer, 594

White matter, of spinal cord, 415; of brain, 423

Wolffian body, see ' Mesonephros '

Wolffian duct, first appearance of in Chick, 183; general account of, 690; of Elasmobranchs, 693 ; of Ganoids, 704; of Amphibia, 710; of Amniota, 713; atrophy of in Amniota, 724

Wolffian ridge, 185

Yolk blastopore, of Elasmobranchii, 64

Yolk, folding off of embryo from, in Elasmobranchii, 55; in Teleostei, 76; Acipenser, 106; Chick, 168, 170

Yolk nuclei, of Elasmobranchii, 41, 53; Teleostei, 69, 75

Yolk, of Elasmobranchii, 40; Teleostei, 68; Petromyzon, 96; Acipenser, 109; Amphibia, 122, 129; Chick, 146; influence of on formation of layers, 278; influence of on early development,

341, 342

Yolk-sack, Amphibia, 131, 140, 141; enclosure of, 123

.Yolk-sack, development of in Rabbit, 227; of Mammalia reduced, 227; circulation of in Rabbit, 233 ; enclosure of in Sauropsida, 289

Yolk-sack, enclosure of, Petromyzon, 86

Yolk-sack, Lepidosteus, 118

Yolk-sack of Chick, enclosure of, 160; stalk of, 174; general account of, 193; circulation of, 195 ; later history of, 198

Yolk-sack of Elasmobranchii, enclosure of, 62, 283; circulation of, 64

Yolk-sack of Lacerta, 209 ; circulation of, 209

Yolk-sack, Teleostei, 75, 81; enclosure of, 75 ; circulation of, 81

Zona radiata, villi of, 237 Zonula of Zinn, 495


BIBLIOGRAPHY.


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UROCHORDA.

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B. Hi. a


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(48) Joh. M tiller. Clatter Haie des Aristoteles und iiber die Verschiedenheitcn unler den Haifachen und Rochen in der Entivicklung des Eies. Berlin, 1840.

(49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter." Sitz. der k. Akad. Wien, Vol. LXXIII. 1873.

(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv fiir micro. Anat., Vol. XI. 1875.

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Ill


(52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirlwllosen. Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. n. 1875.

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TELEOSTEI.

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(58) Ch. van Bambeke. "Premiers effets de la fecondation sur les cufs de Poissons: sur 1'origine et la signification du feuillet muqueux on glandulaire chez les Poissons Osseux." Comptes Rendus des Seances de f Academic des Sciences, Tome

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(62) A. Gotte. "Beitrage zur Entwicklungsgeschichte der Wirbelthiere." Archiv f. mikr. Anat., Vol. IX. 1873.

(63) A. Gotte. " Ueber d. Entwicklung d. Central-Nervensystems der Teleostier." Archiv f. mikr. Anat., Vol. xv. 1878.

(64) A. Gotte. " Entwick. d. Teleostierkeime." Zoologischer Anzeiger, No. 3. 1878.

(65) W. His. " Untersuchungen Uber die Entwicklung von Knochenfischen, etc." Zeit.f. Anat. it. Entwicklungsgeschichte, Vol. I. 1876.

(66) W. His. "Untersuchungen Uber die Bildung des Knochenfischembryo (Salmen)." Archiv f. Anat. u. Physiol., 1878.

(67) E. Klein. "Observations on the early Development of the Common 'Trout." Quart. J. of Micr. Science, Vol. XVI. 1876.

~^* (68) C. Kupffer. " Beobachtungen Uber die Entwicklung der Knochenfische." Archiv f. mikr. Anat., Bd. iv. 1868.

(69) C. Kupffer. Ueber Laichen u. Entwicklung des Ostsee-Herings. Berlin, 1878.

(70) M. Lereboullet. "Recherches sur le developpement du brochet de la perche et de 1'ecrevisse." Annales des Sciences Nat., Vol. I., Series iv. 1854.

(71) M. Lereboullet. " Recherches d'Embryologie comparee sur le developpement de la Truite." An. Sci. Nat., quatrieme serie, Vol. XVI. 1861.

(72) T. Oellacher. " Beitrage zur Entwicklungsgeschichte der Knochenfische nach Beobachtungen am Bachforellenei." Zeit. f. wiss. Zool., Vol. xxn., 1872, and' Vol. xxni., 1873.

(72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschen u. Thiere. Leipzig, 1832-3. Part n. Blennius.

(73) Reineck. " Ueber die Schichtung des Forellenkeims." Archiv f. mikr. Anat., Bd. V. 1869.

(74) S. Strieker. "Untersuchungen Uber die Entwicklung der Bachforelle." Sitzungsberichte der Wiener k. Akad. d. Wiss., 1865. Vol. LI. Abth. 2.

(75) Carl Vogt. " Embryologie des Salmones." Histoire Naturelle des Poissons de F Europe Centrale. L. Agassiz. 1842.

(76) C.Weil. " Beitrage zur Kenntniss der Knochenfische." Silzungsbcr. doWiener kais. Akad. der Wiss., Bd. i.xvi. 1872.

a 2


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CYCLOSTOMATA.

(77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri." Zeit.f. iviss. Zool., Vol. xxx. 1877.

(78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda clorsalis d. Teleostier u. d. Petromyzonten." Morpholog. Jahrbuch, Vol. in. 1877.

(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtimg am Ei d. Neunaugen. Konigsberg, 1878.

(80) Aug. Muller. " Ueber die Entwicklung d. Neunaugen." Miiller s Archiv, 1856.

(81) Aug. Muller. Beobachtungen iib. d. Befruchtungserscheinungen im Ei d. Neunaugen. Konigsberg, 1864.

(82) W. Muller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen. ' Jcnaische Zeitschrift, Vol. IX. 1875.

(83) Ph. Owsjannikoff. "Die Entwick. von d. Flussneunaugen. " ^ Vorlauf. Mittheilung. Melanges Biologiqttcs tires du Bulletin de VAcad. Imp. St Pttersbourg, Vol. vn. 1870.

(84) Ph. Owsjannikoff. On the development of Petromyzon fiuviatihs (Russian).

(85) Anton Schneider. Beitrdge z. vergleich. Anat. a. Entwick. d. Wirbelthiere. Quarto. Berlin, 1879.

(86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte Preisschrift. Haarlem, 1856.

(87) W. B. Scott. " Vorlaufige Mittheilung iib. d. Entwicklungsgeschichte d. Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. ill. Jahrg. 1880.

GANOIDEI. A cipenseridce.

(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung. " Bull. Soc. Nat. Moscow, 1871.

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick. d. Store." Vorlauf. Mittheilung. Melanges Biologizes tires du Bulletin d. VAcad. Imp. St Petersbowg, Vol. VII. 1870.

(90) W. Salensky. "Development of the Sterlet (Acipenser ruthenus)." 2 Parts. Proceedings of the Society of Naturalists in the imperial University of Kasan. 1878 and 9 (Russian). Part I., abstracted in Hoffmann and Schwalbe's Jahresbcricht for 1878.

(91) W. Salensky. " Zur Embryologie d. Ganoiden (Acipenser)." Zoologischer Anzeiger, Vol. I., Nos. n, 12, 13.

Lepidosteidce.

(92) Al. Agassiz. "The development of Lepidosteus." Proc. Amer. Acad. of Arts and Sciences, Vol. xm. 1878.

AMPHIBIA.

(93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate brun." Mc/noires coitronncs, etc. de I 1 Acad. roy. de Belgique, 1868.

(94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens." /!iill,-tin dc V Acad. roy. de Belgique, 1875.

(95) Ch. van Bambeke. " Nouvelles recherches sur 1'embryologie des Batraciens." Archives de Biologic, Vol. I. 1880.

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(97) B. Benecke. "Ueber die Entwicklung des Erdsalamanders." Zoolo. isch er An zeiger, 1880.


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(100) A. Ecker. Icones Physiolog. 1851 1859.

(101) A. Gotte. Die Entivicklungsgeschichte der Unkc. Leipzig, 1875.

(102) C. K. Hoffmann. "Amphibia." Klassen u. Ordnungen d. T/iierrdchs, 18731879.

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(107) W. K. Parker. " On the structure and development of the Skull of the Batrachia." Phil. Trans., Vol. CXLVI., Part 2. 1876.

(108) W. C. H. Peters. " Ueber die Entwicklung der Coecilien und besonders von Coecilia compressicauda." Berlin. Monatsbericht, p. 40, 1874.

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AVES.

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(134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entwick. d. ers. Blutgefiisse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. Akad., Vol. LXIII. 1871.

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(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug. Diss. Berlin, 1875.

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(152) C. F. Wolff. Theoria generationis. Halse, 1759.

(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriitcten Hiinchen. Halle, 1812.

REPTILIA.

(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Keptilien. Konigsberg, 1878.


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(155) C. Kupffer. "Die Entstehung d. Allantois u. <1. Gastrula d. Wirbclthiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.

Lacertilia.

(156) F. M. Balfour. " On the early Development of the Lacertilia, together with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(157) Emmert u. Hochstetter. " Untersuchung lib. d. Entwick. d. Eidechsen in ihren Eiern." Reil's Archiv, Vol. X. 1811.

(158) M. Lereboullet. "Developpement de la Truite, du Lc/ard et du Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn. 1862.

(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia. Phil. Trans., Vol. 170, p. 2. 1879.

(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d. Gesell. z. Be/or, d. gesam. Naturwiss. Marburg. July 23, 1880.

Ophidia.

(161) H. Dutrochet. " Recherches s. 1. en veloppes du foetus." Mem. d. Soc. Mcd. if Emulation, Paris, Vol. vm. 1.816.

(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans. , Vol. 169, Part II. 1878.

(163) H. Rathke. EntTvick. d. Natter. Konigsberg, 1839.

Chelonia.

(164) L. Agassiz. Contributions to the Natural History of the United Slates, Vol. u. 1857. Embryology of the Turtle.

(165) W. K. Parker. "On the development of the skull and nerves in the green Turtle." Proc. of the Roy. Soc., Vol. xxvin. 1879. Vide also Nature, April 14, 1879, and Challenger Reports, Vol. I. 1880.

(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.

Crocodilia.

(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.

MAMMALIA.

(168) K. E. von Baer. Ueb. Entwicklungsgcschichte d. Jhiere. Konigsberg,

(169) Barry. "Researches on Embryology." First Series. Philosophical Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid. 1840.

(170) Ed. van Beneden. La maturation de Foeuf, la fecondation et les premieres phases du developpement embryonaire d. Mammiferes. Bruxelles, 1875.

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(174) Th. L. W. Bischoff. Entivicklungsgeschichte des Kanmcheneies. Braunschweig, 1842.

(175) Th. L. W. Bischoff. Entwicklungsgeschuhte des Hundeeies.

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(176) Th. L. W. Bischoff. Entivicklungsgesclnchte des Meerschivcinchens.

Giessen. 1852.


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(179) Th. L. W. Bischoff. Historisch-kritische B enter kungen z. d. naicstcn Alittheilungen iil>. d. erste Entwick. d. Siitigethiereier. Miinchen, 1877.

(180) M. Coste. Embryogenie comparee. Paris, 1837.

(181) E. Haeckel. Anthropogenie, Entwicklungsgeschichte des Menschen. Lci])zig, 1874.

(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens u. Meerschweinchens." Zeit.f. Anat. u. Entwick., Vol. I. 1876.

(183) A. Kolliker. Entivicklungsgeschichte d. Menschen u. d. hb'hcren Thiere. Leipzig, 1879.

(184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoologist her Anseiger, Nos. 61, 62, Vol. in. 1880.

(185) N. Lieberkiihn. Ueber d. Keimblatter d. Siiugethiere. Doctor- Jubelfeier d. Herrn H. Nasse. Marburg, 1879.

(186) N. Lieberkiihn. "Z. Lehre von d. Keimblattern d. Saugethiere." Sitz. d. Gesell. z. Beford. d. gesam. Natunviss. Marburg, No. 3. 1880.

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(189) E. A. S chafer. " Description of a Mammalian ovum in an early condition of development." Proc. Roy. Soc., No. 168. 1876.

(190) E. A. Schafer. "A contribution to the history of development of the guinea-pig." Journal of Anal, and Phys. , Vol. x. and xi. 1876 and 1877.

Fcetal Membranes and Placenta of Mammalia.

(191) John Anderson. Anatomical and Zoological Researches in Western Yunnan. London, 1878.

(192) K. E. von Baer. Untersuchungen iiber die Gef&ssverbindung zwischen Mutter und Fruc/tf, 1828.

(193) C. G. Cams. Tabulae anatomiam comparali-vam illustrantes. 1831, 1840.

(194) H. C. Chapman. "The placenta and generative apparatus of the Elephant." Journ. Acad. Nat. Sc., Philadelphia. Vol. viii. 1880.

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(196) Ecker. Icones Physiologicae. 1852-1859.

(197) G. B. Ercolani. 7'he utricular glands of the uterus, etc., translated from the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations of memoirs published in the Mem. deW Accad. d. Scienze d. Bologna, and additional matter written specially for the translation.

(198) G. B. Ercolani. Nuove ricerche sulla placenta nei pesci cartilaginosi e nei mammiferi. Bologna, 1 880.

(199) Eschricht. De organis quae respirationi et mttritioni fcetus Mammaliutn inservinnt. Hafniae, 1837.

(200) A. H. Gar rod and W. Turner. "The gravid uterus and placenta of Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878.

(201) P. Hart ing. Het ei en de placenta van Halicore Dugong. Inaug. diss. Utrecht. " On the ovum and placenta of the Dugong." Abstract by Prof. Turner. Journal of Anat. and Phys., Vol. xin.

(202) Th. H. Huxley. The Elements of Comparative Anatomy. London, 1864.

(203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Verh. der Wiirzb. phys.-med. Gesellschaft, Bd. x.

(204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Virginiana)." Amer. Phil. Soc. Trans., Vol. x. 1853.

(205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences Nat., Ser. 3, Vol. I. 1844.


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IX


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(207) Alf. Milne-Edwards. " Observations sur quelqucs points <le I'Kmbryologie des Lemuriens, etc." Ann. Sci. Nat., Ser. V., Vol. xv. 1872.

(208) Alf. Milne-Edwards. " Sur la conformation du placenta chcz le Tainandua." Ann. des Sci. Nat., xv. 1872.

(209) Alf. Milne-Edwards. " Kecherches s. 1. enveloppes fcetales du Tatou a neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vill. 1878.

(210) R. Owen. "On the generation of Marsupial animals, with a description of the impregnated uterus of the Kangaroo." Phil. Trans., 1834.

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(213) R. Owen. "Description of the foetal membranes and placenta of the Elephant." Phil. Trans., 1857.

(214) R.Owen. On the Anatomy of Vertebrates, Vol. III. London, 1868.

(215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions of the Zoological Society, Vol. V. 1866.

(216) W. Turner. "Observations on the structure of the human placenta." Journal of Anat. and Phys., Vol. vn. 1868.

(217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc. Edinb,, Vol. xxvi. 1872.

(218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffrnanni)." Trans, of R. Society of Edinburgh, Vol. xxvn. 1875.

(219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)." Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.

(220) W.Turner. "On the placentation of the Cape Ant-eater (Orycteropus capensis)." Journal of Anat. and Phys., Vol. X. 1876.

(221) W. Turner. Lectures on the Anatomy of the Placenta. First Series. Edinburgh, 1876.

(222) W. Turner. "Some general observations on the placenta, with special reference to the theory of Evolution." Journal of Anat. and Phys., Vol. XI. 1877.

(223) W.Turner. " On the placentation of the Lemurs." Phil. Trans., Vol. 166, p. 2. 1877.

(224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878.

(225) W. Turner. "The cotyledonary and diffused placenta of the Mexican deer (Cervus Americanus). " Journal of Anat. and Phys., Vol. xm. 1879.


Human Embryo.

(226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies." Archiv f. Gynaekologie, Bd. xm. 1878.

(227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaekologie, Bd. xn. 1877.

(228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der Graviditat." Wiener medicinische Wochenschrift, 1877.

(229) M. Coste. Histoire generale et particuliere du developpement des corps organises, 1847-59.

(230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859.

(231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d. menschlichen Embryos." Archiv f. Anat. u. Phys., 1877.

(232) W. His. Anatomie menschlicher Etnbryonen, Part I. Embryonen d. ersten Monats. Leipzig, 1880.

(233) J. Kollmann. " Die menschlichen Eier von 6 MM. Grosse." Archiv f.


Anat. und Phys., 1879.

(234) W. Krause. Phys., 1875.

(235) W. Krause. /. wiss. Zool., Vol. xxxv.


Ueber d. Allantois d. Menschen." Archiv f. Anat. und


' Ueber zwei friihzeitige menschliche Embryonen." 1880.


Zeit.


X BIBLIOGRAPHY.


(236) L. Loewe. "Im Sachen cler Eihaute jiingster menschlicher Eicr. " Archiv fiir Gynaekologie, Bd. xiv. 1879.

(237) C. B. Reichert. " Beschreibung einer friihzeitigen menschlichcn Frucht im blaschenformigen Bildungszustande (sackformiger Keim von Baer) nebst vergleichenden Untersuchungen iiber die blaschenformigen Friichte der Saugethiere und des Menschen. " Abhandlungcn der konigl. Akad. d, Wiss, zu Berlin, 1873.

(238) Allen Thomson. "Contributions to the history of the structure of the human ovum and embryo before the third week after conception ; with a description of some early ova." Edinburgh Med. Siirg.Journal, Vol. LI I. 1839.

COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS IN THE VERTEBRATA.

(239) F. M. Balfour. "A comparison of the early stages in the development of Vertebrates." Quart. J. of Micr. Science, Vol. xv. 1875.

(240) F. M. Balfour. "A monograph on the development of Elasmobranch Fishes." London, 1878.

(241) F. M. Balfour. " On the early development of the Lacertilia together with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anal. it. Entwick., Vol. II. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.

(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad. des Sciences St Petersbourg, Ser. vii. Tom. XI. 1867.

(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lanceolatus." Archiv f. mikr. Anal., Vol. XIII. 1877.

(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." Zool. Anzeiger, Vol. II. 1879, PP- 5 2 ' 593' 61?.

(247) R. Remak. Untersuchungen iib. d. Entiuicklung d. Wirbelthiere, 1850 1858.

(248) A. Rauber. Primitivstreifen ti. Neurula d. Wirbelthiere, Leipzig, 1877.

PHYLOGENY OF THE CHORDATA.

(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, London, 1878.

(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.

(251) E. Haeckel. Schb'pfungsgeschichte. Leipzig. Vide also Translation. The History of Creation. King and Co. , London. 1876.

(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Antliropogeny. Kegan Paul and Co., London, 1878.

(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus." Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. xi. 1867, and Archiv f. ?nikr. Anat., Vol. XIII. 1877.

(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.

(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. u. 1875.

(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876 1877.

GENERAL WORKS ON EMBRYOLOGY.

(257) Allen Thomson. British Association Address, 1877.

(258) A. Agassiz. "Embryology of the Ctenophoroe." Mem. Amcr. Acad. of Arts and Sciences, Vol. X. 1874.

(259) K. E. von Baer. Ueb. Entivicklnngsgeschichle d. Thiere. Konigsberg, 18281837.


BIBLIOGRAPHY.


XI


(260) F. M. Balfour. "A Comparison of the Early Stages in the Development of Vertebrates." Qttart. Journ. of Micr. Set., Vol. XV. 1875.


(261) 1874.


C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastnca-theorie. Wieii,


(262) C. Claus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.

(263) A. Dohrn. Der Ursprung d. Wirbdlhiere u. d. Princip des Functionswechsds. Leipzig, 1875.

(264) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also Translation. Elements of Comparative Anatomy. Macmillan Co. 1878.

(265) A. Gotte. Ent^vicklungsgeschichte d. Unke. Leipzig, 1874.

(266) E. Haeckel. Studien z. Gastrcca-theorie, Jena, 1877; anc ' a ' so Jenaische Zeitschrift, Vols. vm. and IX. 1874-5.

(267) E. Haeckel. Schdpfungsgeschichte. Leipzig. Vide also Translation, The History of Creation. King & Co., London, 1878.

(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Atithropogeny. Kegan Paul & Co., London, 1878.

(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden." Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.

(270) O. and R. Hertwig. " Die Actinien." Jenaische Zeitschrift, Vols. xiil. and XIV. 1879.

(271) O. and R. Hertwig. Die Cctlomtheorie. Jena, 1881.

(272) O. Hertwig. Die Chatognathen. Jena, 1880.

(273) R. Hertwig. Ueb. d. Ban d. Ctenophoren. Jena, 1880.

(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill, 1877.

(274*) T. H. Huxley. "On the Classification of the Animal Kingdom." Quart. J. of Micr. Science, Vol. XV. 1875.

(275) N. Kleinenberg. Hydra, eine anatomisch-entivicklungsgeschichte Untersnchung. Leipzig, 1872.

(276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hbh. Thiere. Leipzig, 1879.

(277) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Acad. Petersbourg, Series vii. Vol. xvi. 1871.

(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist.

1873 (279) E. R. Lankester. " Notes on Embryology and Classification." Quart.

Jotirn. of Alter. Set., Vol. xvn. 1877.

(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme." Zeit. f. wiss. Zool., Vol. xxiv. 1874.

(281) E. Metschnikoff. " Spongiologische Studien." Zeit. f. wiss. Zool., Vol. xxxn. 1879.

(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines of Comparative Embryology. Holt and Co., New York, 1876.

(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol. x. 1876.

(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschneke (Planorbis)." Morph. Jahrbuch, Vol. v. 1879.

(285) H. Rathke. Abhandhmgen z. Bildung und Enlwicklungsgesch.d. Menschen u. d. Thiere. Leipzig, 1833.

(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig, 1829.

(287) R. Remak. Untersuch. ilb. d. Entwick. d. Wirbelthiere. Berlin, 1855.

(288) Salensky. " Bemerkungen lib. Haeckels Gastrsea-theorie." Archiv /. Naturgeschichte, 1874.

(289) E. Schafer. "Some Teachings of Development." Quart. Jotint. of Micr. Science, Vol. xx. 1880.

(290) C. Semper. " Die Verwandtschaftbeziehungen d. gegliederten Thiere." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876-7.


Xll BIBLIOGRAPHY.


GENERAL WORKS DEALING WITH THE DEVELOPMENT OF THE ORGANS OF THE CHORDATA.

(291) K. E. von Baer. Ueber Enlwicklungsgeschichte d. Thiere. Konigsberg, ! 828 1837.

(292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878.

(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Siiugdhiere u. d. Menschen. Leipzig, 1842.

(294) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878. Vide also English translation, Elements of Comp. Anatomy. London, 1878.

(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I. London, 1874.

(296) Alex. Gotte. Entwickhmgsgeschichte d. Unke. Leipzig, 1875.

(297) W. His. Untersuch. ilb. d. erste Anlage d. Wirbelthierleibes. Leipzig, 1868.

(298) A. K 6 Hiker. Entwickhmgsgeschichte d. Menschen u. der hoheren Thiere. Leipzig, 1879.

(299) H. Rathke. Abhandlungen u. Bildung und Entwickhingsgeschichle d. Menschen u. d. Thiere. Leipzig, 1838.

(300) H. Rathke. Entwicklungs. d. Natter. Konigsberg, 1839.

(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.

(302) R. Remak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere. Berlin, 18501855.

(303) S. L. Schenk. Lehrbuch d. vergleich. Embryologie d. Wirbelthiere. Wien, 1874.

. EPIDERMIS AND ITS DERIVATIVES. General.

(304) T. H. Huxley. " Tegumentary organs." Todd's Cyclopedia of Anat. and Physiol.

(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv /. mikr. Anat. Vol. XV. 1876. Pft&also Kolliker (No. 298).

Scales of the Pisces.

(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zahne d. Selachier." Jenaische Zeitschrift, Vol. vill. 1874.

(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrbuch, Vol. u. 1876. (Siluroiden u. Acipenseridae.)

(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbuch, Vol. v. 1879.

Feathers.

(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.

(310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit.f. wiss. Zool., Vol. xxx. 1878.

Sweat-glands.

(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes Rendus, Dec. 29, 1879.


BIBLIOGRAPHY. xiii


Mammary glands.

(312) C. Creighton. "On the development of the Mamma and the Mammary function." Jour, of Anat. and Phys. , Vol. xi. 1877.

(313) C. Gegenbaur. " Bemerkungen lib. d. Milchdriisen-Papillen d. Saugethiere." Jenaische Zeit.. Vol. VII. 1873.

(314) M. Huss. " Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wiederkauern." Jenaische Zeit., Vol. vil. 1873.

(315) C. Langer. " Ueber d. Bau u. d. Entwicklung d. Milchdriisen." Denk. d. k. Akad. Wiss. Wien, Vol. in. 1851.

THE NERVOUS SYSTEM. Evolution of the Nervous System.

(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the British Association." 1880.

(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen, Discomedusen." Denk. d. math.-natiirwiss. Classe d. k. Akad. Wiss. Wien, Vol. xxxvin. 1877.

(318) Th. Eimer. Zoologische Studien a. Capri. I. Ueber Beroe ovatus. Ein Beitrag z. Anat. d. Rippenquallen. Leipzig, 1873.

(319) V. Hensen. " Zur Entwicklung d. Nervensystems. " Virchow's Archiv, Vol. xxx. 1864.

(320) O. and R. Hertwig. Das Nerven system u. d. Sinnesorgane d. Medusen. Leipzig, 1878.

(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriicksichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xiii. 1879.

(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift, Vol. xiv. 1880.

(323) A. W. Hubrecht. "The Peripheral Nervous System in Palseo- and Schizonemertini, one of the layers of the body-wall." Quart, y. of Micr. Science, Vol. xx. 1880.

(324) N. Kleinenberg. Hydra, eine anatomisch-entwickhmgsgeschichthche Untersuchung. Leipzig, 1872.

(325) A. Kowalevsky. " Embryologische Studien an Wtirmern u. Arthropoden." Mem. Acad. Petersboiirg, Series vil., Vol. XVI. 1871.

(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita." Phil. Trans. 1878.

Nervous System of the Invertebrata.

(327) F. M. Balfour. "Notes on the development of the Araneina." Quart. J. of Micr. Science, Vol. xx. 1880.

(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren.' Jenaische Zeitschrift, Vol. XI. 1877.

(329) N. Kleinenberg. "The development of the Earthworm, Lumbncus Trapezoides." Quart. J. of Micr. Science, Vol. xix. 1879.

(330) A. Kowalevsky. "Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, Series vin., Vol. xvi. 1871.

(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Flusskrebses." Zeit.f. wiss. Zool, Vol. xxix. 1877.

Central Nervous System of the Vertebrata.

(332) C. J. Carus. Versuch einer Darstellung d. Nervensystems, etc. Leipzig,

(333) J. L. Clark. " Researches on the development of the spinal cord in Man, Mammalia and Birds." Phil. Trans., 1862.


xiv BIBLIOGRAPHY.


(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. " Centralblatt f. d. med. \Vissenschaften, 1 868. Nr. 8.

(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen und der hb'heren Wirbelthiere. Tiibingen, 1869.

(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v. Ecker und Lindenschmidt. Vol. ill. 1868.

(337) E. Ehlers. " Die Epiphyse am Gehirn d. Plagiostomen." Zeit.f.wiss. Zool. Vol. xxx., suppl. 1878.

(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des Menschen. Auf Grtind entwicklungsgeschichtlicher Untersuchungen. Leipzig, 1876.

(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoisfs Archiv, Bd. xxx. 1864.

(340) L. Lowe. " Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Saugethiere u. d. Menschen." Berlin, 1880.

(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nervensystems d. Wirbelthiere." Mitthcil. a. d. embryol. Instit. Wien, Vol. u. 1880.

(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ." Quart. J. of Micr. Science, Vol. xix. 1879.

(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.

(344) W. Miiller. " Ueber Entwicklung und Bau der Hypophysis und des Processus infundibuli cerebri. " Jenaische Zeitschrift. Bd. vi. 1871.

(345) H. Rahl- Ruck hard. "Die gegenseitigen Verhaltnisse d. Chorda, Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d. einzelnen Theile d. Fischgehirns." Morphol. Jahrbuch, Vol. vi. 1880.

(346) H. Rathke. " Ueber die Entstehung der glandula pituitaria. " Mullens Archiv f. Anat. und Physiol. , Bd. V. 1838.

(347) C. B. Reich ert. Der Bau des menschlichen Gehirns. Leipzig, 1859 u 1861.

(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns." Zcitschrift f. wiss. Zoologie, 1862. Bd. xi.

(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Sitz. d. Jenaischen Gesell.f. Med. u. Natttnviss. Jan. 23, 1880.

(350) Fried. Tiedemann. Anatomie und Bildungsgeschichte des Gehirns im Foetus des Menschen. Niirnberg, 1816.

Peripheral Nervous System of the Vertebrata.

(351) F. M. Balfour. "On the development of the spinal nerves in Elasmobranch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A monograph on the development of Elasmobranch Fishes. London, 1878, pp. 191216.

(352) W. His. " Ueb. d. Anfiinge d. peripherischen Nervensystems." Archiv f. Anat. u. Physiol., 1879.

(353) A. M. Marshall. " On the early stages of development of the nerves in Birds." Jottrnal of Anat. and Fkys.,Vo\. XI. 1877.

(354) A. M. Marshall. "The development of the cranial nerves in the Chick." Quart, y. of Micr. Science, Vol. xvni. 1878.

(355) A. M. Marshall. "The morphology of the vertebrate olfactory organ." Quart, y. of Micr. Science, Vol. xix. 1879.

(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmobranchs." Quart, y. of Micr. Science, Vol. xxi. 1881.

(357) C. Schwalbe. "Das Ganglion oculomotorii. " Jenaische Zeitschrift, Vol. xni. 1879.

Sympathetic Nervous System.

(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes. London, 1878, p. 173.

(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung d. Ganglien d. Sympatheticus." Mittheil. a. d. cmbryologischen Instit. Wien. Heft III. 1879.


BIBLIOGRAPHY. XV


THE EYE.

Eye of the Mollusca.

(362) N. Bobretzky. " Observations on the development of the Cephalopoda " (Russian). Nachrichtcn d. kaiserlichen Gesell. d. Frennde der Natuna iss. Anthropolog. Ethnogr. bei d. Universitdt Moskau.

(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit. f. wiss. Zool., Bd. xxiv. 1874.

(364) V. Hensen. "Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss. Zool., Vol. xv. 1865.

(365) E. R. Lankester. " Observations on the development of the Cephalopoda." Quart. J. of Micr. Science, Vol. xv. 1875.

(366) C. Semper. Ueber Sehorganevon Typus d. Wirbelthicraugen. Wiesbaden, 1877.

Eye of the Arthropoda.

(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.

(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden. Palinurus und Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.

(369) E. Claparede. "Morphologic d. zusammengesetzten Auges bei den Arthropoden." Zeit. f. wiss. Zool., Bd. X. 1860.

(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden. Gottingen, 1879.

The Vertebrate Eye.

(371) J.Arnold. Beitrage zur Entwicklungsgeschichle des A uges. Heidelberg, 1874.

(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wiirzliurger naturwissenschaftliche Zeitschrift, Bd. 8.

(373) L. Kessler. Zur Ent^vicklung d. Auges d. Wirbclthiere. Leipzig, 1877.

(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.

(375) N. Lieberkiihn. " Beitrage z. Anat. d. embryonalen Auges." Archiv f. Anat. und Phys., 1879.

(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.

(377) V. Mihalkowics. "Untersuchungen iiber den Kamm des Vogelauges." Archiv f. mikr. Anat., Vol. IX. 1873.

(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wirbelthiere." Festgabe Carl Ludwig. Leipzig, 1874.

(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische." Wiener Sitzungsberichte, Bd. LV. 1867.

Accessory organs of the Vertebrate Eye.

(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien." Morphologisches Jahrbuch, Bd. II. 1876.

(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. V. 1879.

Eye of the T2tnicata,

(382) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.

(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1872.


xvi BIBLIOGRAPHY.


AUDITORY ORGANS. Auditory organs of tlie Invertebrata.

(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeil.f. wiss. Zool., Vol. xui. 1863.

(385) O. and R. Her twig. Das Nervensystem u. d. Sinnesorgane d. Medusen. Leipzig, 1878.

Auditory organs of the Vertebrata.

(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d. kaiserl. Leap. Carol. Akad. d. Wissenschaft., Vol. xxxv.

(387) C. Hasse. Dievergleich. Morphologieu. Histologied. hciutigen Gehororgane d. Wirbelthiere. Leipzig, 1873.

(388) V. Hensen. "Zur Morphologie d. Schnecke." Zeit. f, wiss. ZooI.,Vo\.

XIII. 1863.

(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim bebrliteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. VI.

(390) Reissner. De Auris internee formatione. Inaug. Diss. Dorpat, 1851.

Accessory parts of Vertebrate Ear.

(391) David Hunt. "A comparative sketch of the development of the ear and eye in the Pig. " Transactions of the International Otological Congress, 1 876.

(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres." Morphol. Jahrbiich, Vol. ill. 1877.

(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trommelfelles." Mittheil. a. d. embryol. Instit. Wien, Heft I. 1877.

OLFACTORY ORGAN.

(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.

(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen." Festschrift f. Rienecker, 1877.

(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." Quart. Journ. of Micr. Science, Vol. xix., 1879.

SENSE-ORGANS OF THE LATERAL LINE.

(397) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, pp. 141 146. London, 1878.

(398) H. Eisig. "Die Segmentalorgane d. Capitelliden." Mitlhcil. a. d. zool. Station zu Neapel, Vol. I. 1879.

(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(400) Fr. Ley dig. Lehrbuch d. Histologie des Menschen u. d. Thiere. Hamm.

T857 (401) Fr. Ley dig. Nene Beitrdge z. anat. Kenntniss d. Haiitdecke u. IJautsinnesorgane d. Fische. Halle, 1879.

(402) F. E. Schulze. "Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.

(403) C. Semper. "Das Urogenitalsystem d. Selachier." Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. II.

(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische." Archiv f. mikr. Anat., Vol. xvil. and xvni. 1879 and 1880.

ORIGIN OF THE SKELETON.

(405) C. Gegenbaur. "Ueb. primare u. secundare Knochenliildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium." Jciiaischc Zeitschrifl, Vol. in. 1867.


BIBLIOGRAPHY. xvii


(406) O. Hertwig. "Ueber Bau u. Entwicklung cl. Placoidschuppcn u. d. Ziihne d. Selachicr." Jetiaische Zeitschrift, Vol. vm. 1874.

(407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bcdeutung f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supplementheft, 1874.

(408) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrlmch, Vol. u. 1876. (Siluroiden u. Acipenseriden.)

(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. I'olypterus)." Morph. Jahrbnch, Vol. v. 1879.

(410) A. Kolliker. "AllgemeineBetrachtungenub. die Entstehungd. knocliernen Schadels d. Wirbelthiere. " Berichle v. d. konigl. zoot. Anstalt z. \Viirzlwrg, 1849.

(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bichir." Zeit.f. wiss. Zool., Vol. V. 1858.

(412) H. Muller. "Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc." Zeit. f. wiss. Zool., Vol. IX. 1859.

(413) Williamson. "On the structure and development of the Scales and Bones of Fishes." Phil. Trans., 1851.

(414) Vrolik. " Studien iib. d. Verknocherung u. die Knochen d. Schadels d. Teleostier." Niederldndisches Archiv f. Zoologie, Vol. i.


NOTOCHORD AND VERTEBRAL COLUMN.

(415) Cartier. " Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeitschrift fur wiss. Zool., Bd. xxv. Suppl. 1875.

(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wirbelsaule der Amphibien und Reptilien. Leipzig, 1862.

(417) C. Gegenbaur. "Ueber die Entwickelung der Wirbelsaule des Lepidosteus mit vergleichend anatomischen Bemerkungen." Jenaisckc Zeitschrift, Bd. ill. 1863.

(418) C. Gegenbaur. "Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische Zeitschrift, Vol. v. 1870.

(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d. Wirbelthiere." II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol. xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879 (Teleostier).

(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der Wirbelsaule u. s. w." Hasse, Anatomische Studiett, 1872.

(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.

(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical, medicin. Gesellschaft in Wiirzburg, Bd. X.

(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier insbesondere iiber die Wirbel der Lamnoidei." Abhandhmgen der senkenbergischen naturforschenden Gesellschaft in Frankfurt, Bd. V.

(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.

(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische und Reptilien. Berlin, 1853.

(426) Aug. Muller. "Beobachtungen zur vergleichenden Anatomic der Wirbelsaule." Miiller's Archiv. 1853.

(427) J. Muller. " Vergleichende Anatomic der Myxinoiden u. der Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungcn der koniglichen Akademie der Wissenschaften zu Berlin. 1834.

(428) W. Muller. "Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.

(429) A. Schneider. Beitrage z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

B. III. *


xviii BIBLIOGRAPHY.


RIBS AND STERNUM.

(430) C. Claus. " Beitrage z. vergleich. Osteol. d. Vertcbraten. I. Rippen u. unteres Bogensystem." Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(431) A. E. Fick. "Zur Entwicklungsgeschichte d. Rippen und Querfortsritze." Archiv f. Anat. und Physiol. 1879.

(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.

(433) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergiirtel." Archiv f. mikr. Anat., Vol. xiv. 1877.

(434) C. Hasse u. G. Born. " Bcmerkungen lib. d. Morphologic d. Rippen." Zoologischer Anzeiger, 1879.

(435) C.K.Hoffmann. " Beitrage z. vergl. Anat. d. Wirbelthiere." Niederliind. Archiv Zool., Vol. iv. 1878.

(436) W. K. Parker. " A monograph on the structure and development of the shoulder-girdle and sternum." Ray Soc. 1867.

(437) H. Rathke. Ueb. d. Ban u. d. Enlivicklung d. Brustbeins d. Sanricr.

1853 (438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen." Morphol. Jahrlmch., Vol. VI. 1880.

THE SKULL.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a leur differents ages." Paris, Mem. savans tirang. 1835, and An. Sci. Nat. Vol. I. 1834.

(440) C. Gegenbaur. UntersucJmngen z. vergleich. Anat. d. Wirbelthiere, III. Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehbrorgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr, Anat., Vol. xi. 1874, suppl.

(443) T. H. Huxley. "On the theory of the vertebrate skull." Proc. Royal Soc., Vol. ix. 1858.

f444) T.H.Huxley. The Elements of Comparative Anatomy . London, 1869.


(445 (446 (447


T. H. Huxley. "On the Malleus and Incus." Proc. Zool. Soc.,

T. H. Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."


Journ. of Anat. and Phys., Vol. X. 1876.

(448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the Common Fowl (Gallus Domesticus). " Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the Pig (Susscrofa)." Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the Batrachia." Part II. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the Common Snake (Tropidonotus natrix)." Phil. Trans. , 1878.

(456) W. K. Parker. "On the structure and development of the skull in Sharks and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(1.17) W. K. Parker. "On the structure and development of the skull in the Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans., 1879.


BIBLIOGRAPHY,


(458) W. K. Parker. "The development of the Green Turtle." The Zoolo-v of the Voyage of H.M.S. Challenger. Vol. I. pt. v.

(459) W. K. Parker. "The structure and development of the skull in the Batrachia." 1't. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Bettany. The Morphology of the Skull. London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1830.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Mailer's Archiv, 1837.

(462) W. Salensky. " Beitrage z. Entwick. d. knorpeligen Gehorknochelchen." Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull; Gotte (No. 296).

THE PECTORAL GIRDLE.

(463) Bruch. "Ueber die Entwicklung der Clavicula und die Farbe des Blutes." Zeit.f. wiss. Zool., IV. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a leurs differents ages." Memoires des savants etrang. Academic royale des sciences de Finstitut de France, Vol. VI. 1835.

(465) C. Gegenbaur. Unterstichungen zur vergleichenden Anatomic der Wirbelthiere, i Heft. Schultergilrtel der Wirbelthiere. Brustflosse der Fische. Leipzig, 1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel. " Archiv f. mikr. Anat. Vol. XIV. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederldndisches Archiv f. Zool. , Vol. V. 1879.

(468) W. K. Parker. " A Monograph on the Structure and Development of the Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.

(469) H. Rathke. Ueber die Entwicklung der Schildkroten. Braunschweig, 1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der Satirier, 1853.

(471) A. Sab a tier. Comparaison des ceintures et des menibres anteneurs et posterieurs d. la Serie d. Vertebrcs. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. lib. d. Entwick. d. Schultergiirtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

THE PELVIC GIRDLE.

(473) A. Bunge. Untersuch. z. Entwick. d. Beckengilrtels d. Amphibien, Reptilien u. Vdgel. Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc." Proc. of Roy. Soc., Vol. xxvin. 1879.

(476) A. S aba tier. Comparaison des ceintures et des membres anterieurs ct postb-ieurs dans la Serie d. Vertebres. Montpellier, 1880.

SKELETON OF THE LIMBS.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuchungen z. vergleich. Anat. d. Wirbelthiere. Leipzig, 18645. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d. Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeilschrift, Vol. V. 1870.


XX BIBLIOGRAPHY.


(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol. vn. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere." Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entwick. u. Regeneration d. Gliedmaassenskelets d. Molche. Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthiere." Zeit.f. wiss. Zool., Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi d. Menschen." Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u. Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Unterstich. iib. d. Entwick. d. Schnltergiirtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history of the Vertebrate limbs." Trans, oftke Connecticut Acad., Vol. III. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut Acad., Vol. IV. 1877.

PLEURAL AND PERICARDIAL CAVITIES.

(491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de 1'cesophage." Journal de FAnatomie et de la Physiologic, Vol. xiv. 1878.

VASCULAR SYSTEM. The Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen." Morphol. Jahrbuch, Vol. 11. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f. mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX. 1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296), and Balfour (292).

The Arterial System.

(496) H. Rathke. "Ueb. d. Entwick. d. Arterien w. bei d. Saugethiere von d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(41)7) PI. Rathke. " Untersuchungen iib. d. Aortenwurzeln d. Saurier." Denkschriften d. k. Akad. Wien, Vol. xiil. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

The Venous System.

(498) J.Marshall. "On the development of the great anterior veins." Phil. Trans., 1859.


BIHLIOGRAI'IIY. XXJ


(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge thieren." Meckel 's Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbclthiere." Bericht. iib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke (Nos. 299, 300, and 301).

THE SPLEEN.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. "Ueb. d. Entwick. d. Milz." Silz. d. Wien. Akad. Wiss., Vol. LVI. 1867.

THE SUPRARENAL BODIES.

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilian." Arbeit, a. d. zool.-zoot. Institut Wilrzburg, Vol. v. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren." Archiv f. mikr. Anat., Vol. vni. 1872.

(505) Fr. Leydig. Untersuch. ilb. Fische u. Reptilien. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

THE MUSCULAR SYSTEM OF THE VERTEBRATA.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." J our n. of Anat. and Phys., Vol. vi. 1872.

(508) J. Miiller. "Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." Quart. J. of Micr. Science, Vol. XXI. 1881.

(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." Sitz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc.

EXCRETORY ORGANS.

INVER TEBRA TA .

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool. Slat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Irematc Cestoides." Archives de Biologie, Vol. I. 1880.

(514) B. Hatschek. "Studien iib. Entwick. d. Annehden. Arbeit, a. d. zool. Instil. Wien, Vol. I. 1878. .

(515) B. Hatschek. "Ueber Entwick. von Echmrus, etc. Arbeit, a.

zool. Instit. Wien, Vol. ill. 1880.

VERTEBRATA.

General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of Vertebrates." Journal of Anat. and Phys., Vol. X. 1876.


XXJi BIBLIOGRAPHY.


(517) Max. Fiirbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meckel. Zur Morphol. d. Harn- u. Geschlechtswerkz.d. Wirbelthiere, etc. Halle, 1848.

(519) Job. Mtiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Ratbke. "Beobachtungen u. Betrachtungen ii. d. Entwicklung d. Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in Dantzig, Bd. I. 1825.

(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. ubrigen Wirbelthiere." Arb. a. d. zool.-zoot. Insiit. Wiirzburg, Vol. u.

1875 (522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.

ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anal., Vol. xi. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. M uller. " Untersuchungen ii. d. Eingeweide d. Fische. " Abh. d. k. Ak. Wiss. Berlin, 1845.

(525) W. Muller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa." Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Muller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschrift, Vol. ix. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol. Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d. k. k. Akad. Wiss. Wien, Vol. II. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Enlwicklung d. Teleostierniere. Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologisclie Untcrsiiclniii^cn ii. die mdnnlichcn Geschlec/its- tmd Harmverkzeuge d. nackten Amphibien. Dorpat, 1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Ley dig. Analomie d. Amphibien u. Keptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Histologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien." Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d. zool.- zoot. Instil. Wiirzburg. Vol. in. 1876.

(538) Von Wittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit. f. wiss. Zool., Vol. iv.

Vide also Gotte (No. 296).

1 The papers of Fiirbringer, Semper and Waldeyer contain full references to the literature of the Vertebrate excretory organs.


BIBLIOGRAPHY. xxiii


Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of ahead-kidney in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. XIX. 1878.

(540) Banks. On the Wolffian bodies of the foetus and their remains in the adult. Edinburgh, 1864.

(541) Th. Bornhaupt. UntersucJnmgen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. IV. 1877.

(543) J. Dansky u. J. Kostenitsch. " Ueb. d. Entwick. d. Keimblatter u. d. Wolffschen Ganges im Htihnerei." Me"m. Acad. Imp. Petersbourg, vn. Series, Vol. xxvn. 1880.

(544) Th. Egli. Beitrdge zur Anat. tmd Entiuick. d. Geschlechtsorgane. Inaug. Diss. Zurich, 1876.

(545) E. Gasser. Beitrdge zur Entwickhmgsgeschichte d. Allantois, der MiUler' schen Giinge u. des Afters. Frankfurt, 1874.

(546) E. Gasser. " Beob. iib. d. Entstehung d. WolfFschen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Htihnerembryonen." Sitz. d. Cesell. zur Beforderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchung liber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.

(550) A. Sedgwick. "On the development of the structure known as the glomerulus of the head -kidney in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. XIV. 1879.

(553) E. H. Weber. Zusdtze z, Lehre von Bane u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297), Kolliker (No. 298).

GENERATIVE ORGANS.

(554) G. Balbiani. Lemons s. la generation des Vertebres. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart. J. of Micr. Science, Vol. XVIII.

(556) E. van Beneden. "De la distinction originelledutecticuleet del'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrhim." Zeit. f. wiss. Zool., Vol. xxxv. 1 88 r.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche. " Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach clem Ursprung d. Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

ALIMENTARY CANAL AND ITS APPENDAGES.

(561) B. Afanassiew. " Ueber Bau u. Entwicklung d. Thymus d. Saugeth." Archiv f. mikr. Anat. Bd. XIV. 1877.


XXIV BIBLIOGRAPHY.


(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffhung hei Hiihneremhryonen." Archiv f. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beitrage zur Entwicklungsgeschichte 'd. Darmkanah im Hithnchcn. 1867.

(565) W. Miiller. " Ueber die Entwickelung der Schilddriise." ycnaische Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaischc Zeitschrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische UntersucJnmgcn. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d. Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. lib. d. Entivick. d. Glandula Thymus, Glandula thyroidea, u. Glandula carotica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad. Petrop. 1766.

(571) A. Wblfler. Ueb. d. Entwick. it. d. Ban d. Schilddriise. Berlin, 1880. Vide also Kolliker (298), Qotte (296), His (232 and 297), Foster and Balfour (2!)5),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of Micros. Science, Vol. III. 1855.

(573) R. Owen. Odontography. London, 1840 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative. London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros. Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker 's Histology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.




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