Book - An Introduction to the Study of Embryology: Difference between revisions

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{{Historic Disclaimer}}
{{Historic Disclaimer}}
= An Introduction to the Study of Embryology=
= An Introduction to the Study of Embryology=
 
[[File:Alfred Cort Haddon.jpg|thumb|alt=Alfred Cort Haddon|Alfred Cort Haddon (1855–1940)]]


By
By
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With gtumnwtf gltetration#.
Philadelphia : P. Blakiston, Son & Co., 1012 Walnut Street. 1887.
 
 
Philadelphia :


P. Blakiston, Son & Co.,
{|
| valign=middle|
To the memory of


1012 Walnut Street.
his beloved master and friend,


1887.
[[Embryology History - Francis Balfour|'''Francis Maitland Balfour''']]




 
This Book is dedicated by the Author.
TO
| [[File:Francis Balfour.jpg|alt=Francis Balfour (1851-1882)|thumb|200px|link=Embryology History - Francis Balfour|Francis Balfour (1851-1882)]]
 
|}
IT b c /!!> c m o v p
 
OF
 
HIS BELOVED MASTER AND FRIEND,
 
FRANCIS MAITLAND BALFOUR,
 
This Book
 
IS DEDICATED
 
BY
 
 
THE AUTHOR.




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Finally, I would here express my warmest thanks to my friend Professor G. B. Howes, of the Normal School of Science, South Kensington, for his kindness in reading the proofs and in making many valuable suggestions.
Finally, I would here express my warmest thanks to my friend Professor G. B. Howes, of the Normal School of Science, South Kensington, for his kindness in reading the proofs and in making many valuable suggestions.


==Chapter VI Organs Derived from the Hypoblast==
{{Historic Disclaimer}}
 
In a previous section the archenteron was left as a simple sac or tube, opening to the exterior anteriorly by the stomodseum, and posteriorly by the proctodaeum.
 
From what was said concerning the effects of the presence of a large amount of food-yolk, it will be obvious that there will be a discrepancy in the relative time of the development of various hypoblastic structures ; for example, in telolecithal ova the ventral wall of a considerable portion of the alimentary canal must of necessity be completed very late.
 
The primitive function of the hypoblast is undoubtedly alimentation, but in the course of evolution it has acquired several other functions. The digestive organs will now be first considered, and subsequently other hypoblastic derivatives will be described.
 
Digestive Organs. - The simple sac-like archenteron of the gastrula, as has already been described, is produced into pouches in a large number of animals.
 
When this occurs in Sponges the characteristic hypoblast cells (choano-flagellate cells) become restricted to the extremities (ciliated chambers) of the often complicated diverticula. All the exhalent canals are lined with flattened hypoblast cells.
 
The gastric diverticula of Coelenterates appear to be chiefly concerned with the circulation or distribution of the nutritive fluid, the actual process of digestion being probably confined to the stomach of the Hydromedusse, and the edges of the mesenteries in the Actinozoa (fig. 68).
 
In the Coelomata, or those animals provided with a true body cavity, these diverticula are cut off from the gastric cavity, and are henceforth spoken of as mesodermal structures.
 
The gastric diverticula of the Turbellarians, of certain Nemerteans, and of the Leeches, cannot be regarded as coelomic diverticula which have never severed their connection with the archenteron.
 
 
ORGANS DEBITED FROM THE HYPOBLAST.
 
 
169
 
 
It has been shown (p. 29) that in most centrolecithal ova (e.g., Crustacea) some of the hypoblast cells engulf the food-yolk which lies within the segmentation-cavity (fig. 22). In other ova the yolk is originally located within the primitive hypoblast. In both cases it is digested by those cells.
 
The actual conversion of the primitive hypoblast into special digestive cells has not been fully investigated, but it must be readily effected, as digestion and assimilation are primary properties of protoplasm.
 
The hypoblastic portion (mesenteron) of the alimentary canal is always divisible into definite regions, and, with the exception of most of the Arthropoda, it forms by far the largest section of the tract.
 
The various regions of the alimentary canal of different animals which appear to be similar had received corresponding names before their development was known, consequently many apparent morphological anomalies must be expected.
 
Usually among the , Invertebrates the stomodseum is prolonged as the oesophagus ; the mesenteron includes the stomach and intestine and their associated glands, while the proctodeum is small. The Arthropoda, as a whole, are an exception to this rule, for in Insects the mesenteron is that portion of the alimentary canal lying between the crop or proventriculus, when that is present, and the point of origin of the Malpighian tubes. The mesenteron may be a simple tube, or divided into regions, of which the anterior may possess numerous small caeca (some Beetles) or eight large ones (Cockroach). In low forms, such as the Myriapoda and Peripatus, the mesenteron is long and simple.
 
In the lower Crustacea the mesenteron is relatively long. There are in Amphipods, in addition to the two or four digestive caeca, which are so commonly present throughout the Crustacea, two long narrow tubes which open into the extreme hinder end of the mesenteron. These tubes are undoubtedly excretory, but, as Spencer has shown, they are hypoblastic and not epiblastic, they cannot be regarded as homologous with the Malpighian tubules of the Tracheata (p. 111).
 
The mesenteron of the Decapod Crustacea is restricted to the usually minute chamber between the so-called pyloric chamber (fig. 140) and the commencement of the intestine (proctodaeum) ; it is separated from the former by valves. It is to this that the term stomach should be restricted. The digestive gland or so
 
170
 
 
THE STUDY OF EMBRYOLOGY.
 
 
called “liver- opens by a wide aperture on each side into the mesenteron. The latter is the only portion of the alimentary canal of these animals which is not lined by cuticle.
 
In the Mollusca (figs. 1 8 and 84) only the buccal cavity is lined by epiblast, the stomach and intestine being archenteric derivatives. The stomodaeum gives rise to the buccal cavity and its organs (radula or odontophore, salivary glands), and to the oesophagus. The proctodaeum is very small. In the Cephalopoda the ink sac
 
 
Fig. 140. - Diagrammatic Sections of Embryos of the Cray-Fish (Astacus Fluviatilis). [From Huxley after Reichenbach.]
 
C. Longitudinal section of an ovum in which the rudiments of the abdomen, of the hind-gut, and of the fore-gut have appeared. D. Later stage of similar embryo. E. Longitudinal section of newly-hatched embryo.
 
a. anus ; e. eye ; ep.b. epiblast : f.g. fore-gut (stomodaeum) ; f.g\ its oesophageal, and f.g 2 . its gastric portion ; h. heart; li.g. hind-gut (proctodaeum) ; m. mouth; m.g. midgut, mesenteron, or archenteron ; v. yolk. The dotted portions in D and E represent the nervous system.
 
early grows out as a simple diverticulum from the ventral wall of the hinder end of the intestine.,
 
Invertebrate Digestive Gland or “Liver.- - The large digestive gland associated with the mesenteron in the higher Invertebrates (Molluscs and Arthropods) is usually spoken of as a “liver.- As a matter of fact, it is now known to be a more universal digestive gland than its name would apply, and that it more closely corresponds in function with the Vertebrate pancreas, combining,
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
171
 
 
as it does, the function of liver and pancreas, it has been appropriately termed the hepato-pancreas. It is a complex gland which typically develops from the wall of the mesenteron (fig. 140) in the usual manner, but, in some forms, the liver appears to be formed by a metamorphosis of the remnant of the yolk-cells which remain after the formation of the mesenteron (fig. 84, B, y).
 
Mesenteron of Chordata. - The hypoblastic portion of the alimentary canal of the Chordata is divisible into the following regions: pharynx, oesophagus, stomach, and intestine (figs. 14 1,
 
143).
 
The egg being yolkless in Amphioxus, the archenteron (fig. 57) is directly converted into the alimentary canal of the adult.
 
The effect on the formation of the mesenteron by the presence
 
 
Fig. 141. - Isolated Alimentary Canal of Embryo Dog of Twenty-Five Days. Multiplied 5 diameters. [ From Kolliker after Bischoff .]
 
a. pharyngeal or branchial pouches ; 6. rudiment of laryngeal portion of the pharynx ; c. lungs ; d. stomach ;•/. liver; g. dorsal wall of the vitelline sac, with which the intestine still communicates by a large orifice (the umbilicus) ; h. rectum.
 
The inner white line indicates the hypoblast ; the surrounding dark border representing the splanchnic or visceral (mesoblastic) sheath of the alimentary tract. Compare with A, fig. 143.
 
 
A
 
at first of a small, and then of a gradually increasing amount of food-yolk, has already been described (p. 30). The constriction off of the digestive tract from the yolk-sac in telolecithal ova takes a comparatively long time, and not a few Fish are hatched with the yolk-sac still depending from their bodies. In fig. 141, which illustrates the isolated alimentary canal of an embryo Dog, viewed from the ventral surface, it will be seen that all the main organs have made their appearance while the umbilicus is still widely open (see also fig. 143). The neck of the yolk-sac gradually narrows to form the vitelline duct, and the first fold of the intestine (figs. 144, I; 143, c) occurs at the spot where the vitelline duct joins it. A diverticulum which occasionally occurs in Man in the lower part of the ileum is the persistent base of the vitelline duct ; and not unfrequently the proximal portion of the vitelline duct
 
 
172
 
 
THE STUDY OF EMBRYOLOGY.
 
 
may persist in Birds as a short tube connected with the small intestine.
 
Pharynx. - The pharynx probably extended along a considerable length of the body in the primitive Chordata, as is still the case in Amphioxus and Lampreys. The lateral walls were devoted to respiratory purposes, as will be described subsequently.
 
A deep ciliated groove, the endostyle, extends along the median ventral line of the pharynx (branchial sac) in Ascidians. The cilia work from before backwards and thus carry the mucus, which is secreted by the glandular cells of the endostyle, along with entangled food particles into the oesophagus.
 
The hypopharyngeal ridge of Amphioxus, with its glandular cells, has a similar function.
 
This region corresponds to the non-respiratory ventral portion of the pharynx of Balanoglossus.
 
Fig. 142. - Diagrammatic Longitudinal Section through the Head of a Larval Lamprey (Pefcromyzon. [From Claus after Balfour .]
 
Ab. optic vesicle; C. heart; cb. cerebellum ; c.h. cerebral hemisphere ; Chd. notochord; H. hypophysial (thyroid) involution ; inf. infundibulum ; ks. branchial pouches ; m.b. mid-brain ; md . medulla ; N. nervous system; O. stomodseum; 01. olfactory pit ; ot. auditory vesicle, represented as visible; pn. pineal gland (below which the optic thalamus is shown) ; v.cuo. ventral aorta; ve. velum. The oblique line between the velum and the first branchial pouch represents the left of a pair of ciliated grooves which converge on the median ventral line to meet the orifice of the thyroid.
 
 
A considerable groove is developed in the front portion of the floor of the pharynx in the larval Lamprey (fig. 142), and to a decreasing extent in higher forms.
 
We may therefore conclude that the ventral portion of the primitive pharynx was concerned in the transmission of food. The special mechanism by which this was effected afterwards degraded into the median element of the gland known as the thyroid body (see p. 183). It is possible that this change of function was correlated with the increase in size of the primitive Chordata and the consequent ability to eat larger prey. The latter, from their size, would not have the tendency to escape through the gill-slits, which minute organisms could easily do, and would further pass into the oesophagus without requiring the assistance of the ventral groove. The latter, owing to disuse, would naturally degenerate.
 
Throughout the Xchthyopsida the pharynx gradually becomes greatly shortened, as is also the case in Amphibia and Amniota.
 
(Esophagus. - The oesophagus calls for no special mention. It is a simple tube of variable length, which in some forms (Crocodilia and many Birds) has a ventral saccular dilatation or crop.
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
173
 
 
Stomach. - The oesophagus may pass imperceptibly or abruptly into the stomach. The stomach is usually a simple dilatation of the alimentary canal (figs. 141-144). Its exact form varies considerably, but it only becomes at all complicated in a few Mammals (e.g., Sloths, Cetacea, Buminants, some Marsupials and Bodents).
 
There is an instructive modification in the stomach of Buminants during growth. In the early foetus the relative size of the compartments and general form of the stomach are almost exactly those of the adult. After birth, owing to the milk-diet, the growth of the peptic stomach or abomasus is greatly in excess of that of the others; but as a herbivorous diet is acquired, the characteristic form of the adult stomach is re-acquired.
 
To secure increase of secreting surface without proportionate extent of superficies, crypts or pockets of digestive cells were developed forming simple glands. In time these became more complex, as was previously described for epiblastic glands (p. 106), the cells which actually secrete the digestive fluid being restricted to the blind extremities or alveoli of the gland.
 
Three types of such glands are found in Mammals ; the simple tubular crypts of Lieberkuhn in the small intestine. A gland with a non-glandular duct and a few simple tubules is illustrated by the peptic and pyloric glands of the stomach, and the glands of Brunner in the pylorus, while the liver and pancreas represent the most specialised form of gland.
 
Liver. - The “ liver - in Amphioxus, alone of all Chordata, retains its primitive tubular form. It is the earliest hypoblastic gland to be developed, and it is relatively very large in foetal life. It appears to be entirely absent in Balanoglossus.
 
In some of the lower Vertebrata (Elasmobranchs and Amphibia) (fig. 99) the liver arises from a single ventral diverticulum from the intestine, which soon becomes bilobed. In Birds and Mammals (fig. 1 41) the liver appears to be bilobed from the first.
 
The incipient liver buds out into a local thickening of the splanchnic mesoblast, which thus becomes penetrated by a number of rod-like prolongations (hepatic cylinders) of the primitive diverticula. As a rule the hepatic cylinders appear to be solid, but in Elasmobranchs Balfour found that they are hollow, as they are also stated to be in Amphibia. A system of ducts appears in due course. The hepatic cylinders have the peculiarity, which is unique among glands, of uniting with one another at numerous points, thus forming a network within the meshes of which the enveloping mesoblast develops into blood-vessels.
 
The gall-bladder is simply an enlargement of, or a diverticulum from, the main duct of the liver. Its presence is very variable ;
 
 
174
 
 
THE STUDY OF EMBKYOLOGY.
 
 
the number and position of the ducts of the liver opening into the intestine are also inconstant in various animals.
 
Pancreas. - The pancreas occurs very constantly among the Vertebrates. It is absent in the Cyclostomi and Perennibranchiate Amphibia, and rudimentary or absent in many Teleosts. The pancreas may be partially imbedded in the liver in Ganoids, and completely so in Siluroids. It first appears as a tubular outgrowth from the dorsal wall of the intestine, opposite to, but slightly behind, the diverticulum, which forms the rudiment of the liver. According to His, the pancreatic rudiment at first appears in front
 
 
Fig. 143. - Four Stages in the Development of the Human Alimentary Canal,
 
AS SEEN FROM THE LEFT SIDE AND ISOLATED. \After His .]
 
all. stalk of allantois; b.p. bursa pelvis; c. caecum; ep. epiglottis; g.e. genital eminence; k. kidney; l. liver; la. larynx ; l.d. duct of liver; Ig. lung; l.j. lower jaw; p. pancreas ; pr. proctodaeum ; R.p. Rathke -s pouch (hypophysial evagination), behind it in A and B is Seesfeel -s pouch ; st. stomach ; t. tongue ; tky. median rudiment of thymus- gland; tr. trachea; u. ureter; umb. umbilical vesicle; v.d. vitelline duct; W.d. Wolffian duct.
 
 
of the liver in the human embryo, and later shifts its position to behind that viscus (fig. 143, b-d). Hollow diverticula arise from the main duct, which continually subdivide. The surrounding mesoblast develops as usual into blood-vessels and connective tissue. In some cases two pancreatic diverticula have been observed.
 
Intestine. - The intestine is the post-gastric portion of the mesenteron. It is always a straight tube in epabryos, and persists as such in many of the lower Chordata. In other forms it becomes variously looped, owing to its length exceeding that of the body
 
cavity within which it lies.
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
175
 
 
The posterior portion of the intestine in the adult, but not in the embryo, is usually of markedly greater diameter than the anterior portion or small intestine ; it is known as the large intestine.
 
The secreting and absorbing surface of the alimentary canal is increased in the lowest Vertebrates by the development of a longitudinal fold projecting into the cavity of the intestine, which is known as the spiral valve.
 
The fold is slightly developed in the Cyclostomi, and reaches its highest state of development in some Elasmobranchs. It becomes less marked in the Ganoids, and traces of it may be found in the intestine of a few Teleosts. In no higher Vertebrate has it been definitely recognised. A similar fold is found in the intestine of some Ascidians ; such a fold may be compared with the typhlosole of certain Invertebrates (ex. Earthworm and Fresh-water Mussel).
 
 
Fig. 144. -Rough Diagrams Illustrating the Change in Relative Position undergone by the Digestive Tract in Mammals. [ From Landois and Stirling.]
 
6. colon ; o. vitelline duct ; r. rectum ; t. small intestine ; v. stomach.
 
 
Concomitantly, according to Wiedersheim, with the disappearance of the spiral valve in Fishes a number of hollow diverticula (pyloric caeca) make their appearance from the anterior region of the small intestine (duodenum). These are found in some Ganoids, in which group their development is not always inversely proportional to that of the spiral valve, and in most Teleosts, but in no other animals. Their function appears to be, in some forms, to increase the absorbing surface of the intestine, as a digestive function may be present or absent [Stirling, Macullum]. In a few Teleosts they occur side by side with the pancreas.
 
Those animals which possess a spiral valve have, in the main, an alimentary canal which pursues a straight course through the body cavity. In other forms (excepting Teleosts) the greater length of the intestine probably renders a spiral valve superfluous.
 
The relative length of the alimentary canal is largely dependent
 
 
176
 
 
THE STUDY OF EMBRYOLOGY.
 
 
upon the nature of the food of the animal. This is well illustrated in the case of the Frog -s tadpole. When still subsisting upon its stored-up food-yolk, the alimentary tract retains its primitive straight course (figs. 98, 99). After the tadpole is hatched it commences to feed upon decaying vegetable matter, and the intestine grows to a great length, and is coiled up like a watch-spring. Later on the young Frog takes to an animal diet, and the intestine is relatively very much shorter, and is only slightly looped.
 
The valvulae conniventes of Man, and similar folds in other animals, also serve to increase the absorbing surface of the small intestine. The development of all these structures is too obvious to require description.
 
In Mammals the end of the large intestine, where it passes into the small intestine, is usually enlarged to form the csecum. In Man there is at first no csecum (fig. 143, A-c), then a simple conical projection appears (fig. d) ; later the csecum lengthens, but the terminal portion does not keep pace with the growth of the base, and consequently becomes much narrower in calibre. The basal portion eventually grows so large that it is commonly called the csecum, while the true csecum is designated as the vermiform appendix. Several of the stages in the development of the human csecum are permanently retained in the adult stage in certain Mammals. It is not known whether the so-called vermiform appendix of the Wombat is, as in the higher Primates, a remnant of an originally elongated apex of the true csecum.
 
In some Armadillos the csecum is distinctly bilobed, and in Cyclothurus didactylus there are two distinct cseca. In addition to a capacious true csecum, Hyrax possesses a pair of simple conical Cseca in the large intestine.
 
In most Birds there are two cseca of variable length at the commencement of the large intestine.
 
A csecum is usually stated to first appear in Eeptiles, where it never attains a large size ; but Huxley has described and Howes has figured a representative of it in the Frog.
 
A simple rectal gland is found in Elasmobranchs.
 
Endodermal Muscles. - Muscular processes arising from the endodermal cells have been demonstrated by Jickeli in Hydra; these run transversely round the body, as opposed to the longitudinal direction of the similar fibres of the ectodermal cells. Endodermal muscular fibres have been demonstrated in the Actiniae by the brothers Hertwig.
 
ORGANS DERIVED FROM THE HYPOBLAST. 177
 
Respiratory Organs of Invertebrates. - In but few Invertebrates does the alimentary tract function directly in respiration. The endoderm lining the general cavity of the body in Actinozoa is, however, probably largely concerned in respiration, especially in such forms as Edwardsia, Cerianthus, and Peachia, which live imbedded in the sand.
 
Respiration probably occurs all along the intestine in Proneomenia, and along the rectum in ETeomenia.
 
The anal respiration of many Crustacea is, as has already been stated (p. 109), really proctodseal.
 
The respiratory trees of most Holothuroidea are probably of hypoblastic origin. In other Echinoderms the ambulacral system is partially respiratory.
 
Chordata. - The anterior portion of the chordate mesenteron is mainly devoted to respiration ; this may appropriately be termed the branchial region, or, more shortly, the pharynx.
 
In most Chordata several pairs of wide lateral pouches arise from the sides of the pharynx and come into close contact with the external skin. There is apparently a slight invagination of the latter to meet the former ; an absorption of the applied membranes results in the formation of lateral slits (branchial or visceral clefts), by means of which the cavity of the pharynx is put into direct communication with the exterior.
 
Delicate processes of the hypoblastic epithelium covering the intermediate bars (branchial or visceral arches) constitute the gills or branchiae. These are richly supplied with blood by the branchial vessels (p. 226). True gills, however, are never developed in the Amniota at any period of life.
 
Almost invariably the anterior (hyomandibular) visceral cleft is the first to appear, the remainder appearing in order from before backwards.
 
 
The worm-like Balanoglossus has pharyngeal gill-slits which arise in the same manner as those of Vertebrates ; for a long time there is only one pair, hut subsequently they are repeated in pairs, increasing in number with the increase in the size of the body [Bateson]. The collar at the base of the proboscis grows backward as an opercular fold to a variable extent in different species of Balanoglossus, but it never extends beyond three gill-slits. The enclosed cavity is termed the atrial cavity by Bateson.
 
Van Beneden and Julin have shown that all Ascidians have but a single pair of visceral clefts, which arise as a pair of pharyngeal pouches met by corresponding epiblastic depressions. This condition is permanently retained by the interesting tailed form Appendicularia. In all other Ascidians the gill-clefts fuse together to form a single chamber (peribranchial cavity or atrium), which almost entirely sur M
 
 
178
 
 
THE STUDY OF EMBRYOLOGY.
 
 
rounds the pharynx (branchial sac). It is probable that the atrial pore is the persistent opening of the fused gill-slits. The atrium may be formed more especially from the hypoblastic or the epiblastic portion of these clefts. The numerous and usually irregular orifices (stigmata) in the pharynx clearly do not correspond with the gill-slits of higher forms, but are merely secondary perforations. We may say, with these authors, “ the Tunicata are Chordata with a single pair of branchial clefts, while the Yertebrata are furnished with several, and the Cephalochorda (Amphioxus) with a great number.-
 
In Amphioxus also a single pair of gill-slits first makes its appearance. This is subsequently followed by a large number (70-100), which slant obliquely from before backward. In the young form the gill-slits open directly to the exterior, but they are eventually covered by a pair of dorsal folds of skin which grow downwards, leaving a space between themselves and the gillslits (the branchial chamber or atrium). The two flaps of skin meet below the body and fuse throughout their whole extent except at one spot, the branchial or atrial pore. It will be readily apparent that the branchial chamber of Amphioxus is by no means homologous with that of Ascidians.
 
The number of gill-clefts never exceeds eight pairs in the Yertebrata. There are seven in the Cyclostomi and in Hexanchus, eight in Notidanus (Heptanchus), but six in all other Elasmobranchii ; amongst the Teleostei a further reduction in the number of clefts occurs, owing to the suppression of the hyoid pair.
 
The first cleft succeeding the mouth is termed the hyomandibular or hyoid cleft (spiracle), as it lies between the mandibular and hyoid arches. The second is correspondingly the hyobranchial or first branchial, and is bounded by the hyoid and the first branchial arches. The remaining slits are the branchial clefts.
 
Dohrn finds that the pair of ciliated grooves which lie in front of the gill-pouches in the Lamprey (fig. 142) is developed in the same manner as the branchial pouches, but an external opening is never acquired. This supposed lost pair of visceral clefts is termed by Dohrn spiracular or thyroidean.
 
Primitively all the visceral clefts were undoubtedly respiratory in function, and in many Eiasmobranchs the mandibular border of the spiracle bears a rudimentary gill. In Chimaera, some Ganoids, and many Teleosts, the hyoid border of the second cleft possesses only a rudimentary gill (opercular pseudobranch), which undergoes all stages of degeneration amongst the Teleosts, all the anterior gill-filaments having atrophied. The posterior gills have a tendency to disappear in Teleosts, the greatest reduction occurring in
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
179
 
 
Amphipnous cuchia, in which one branchial arch alone bears branchial filaments.
 
The gill-clefts in the Cyclostomes and Elasmobranchs are left quite exposed on the surface of the neck, but in Chimsera, Ganoids, Teleosts, and Dipnoi they are protected by a fold of skin (operculum), supported by skeletal elements ; the branchiostegal membrane and its supporting skeleton are derivatives of the hyoid arch. In some forms the border of the operculum fuses with the skin of the body, merely leaving a small orifice on each side leading from the branchial chamber.
 
In Amphibia the hyoid pharyngeal pouch never communicates with the exterior, but persists as the Eustachian recess. In larval life four, or rarely three (some Urodela), branchial clefts appear. The first, second, and third branchial arches develop external gills which may be covered by epiblast. These usually atrophy, and internal, probably hypoblastic, gills are developed on each side of the three branchial clefts. The internal gills are always lost, but in some Urodeles the external gills are retained throughout life. Cope has recently stated that the Siren loses and then re-acquires its external gills. Other Urodeles, which normally lose their gills when adult, may, however, become oexually mature while still retaining their gills (Axolotl).
 
An opercular fold grows back from each hyoid arch in Anura, and fusing above and below with the skin of the body, envelops the gills within a branchial chamber. At first the branchial chambers open widely to the exterior by an orifice on each side ; these persist in Dactylethra, according to Huxley. In Bombinator and certain other forms the openings of the branchial chambers unite to form a single ventral orifice. In the majority of Anura (Rana, Bufo), the two branchial chambers communicate by a ventral canal, and the opening of the right chamber is closed up, leaving a single asymmetrical pore on the left side.
 
External gills are present in some Ichthyopsida, but they have already been alluded to (p. 109).
 
The external gill filaments of Elasmobranch embryos arise as simple elongations of the posterior lamellae of each arch, the anterior not elongating at all. Dohrn finds that yolk is present in these filaments and in their veins, and also in the posterior branchial vein and the efferent arteries, but never in the heart or in the branchial artery. It would thus appear that these elongated filaments serve also to absorb the yolk.
 
 
180
 
 
THE STUDY OF EMBKYOLOGY.
 
 
In none of the Amniota do the visceral clefts bear gills at any period of life. In all forms there are four pairs of clefts, the last two being very small in Mammals. The visceral arches between the clefts are well marked (fig. 145, k", k"'), each possessing a central artery ; hut in Mammals the last cleft is not bounded by a posterior arch. In Man, at least, none of the visceral clefts are actually perforated [His], and the fourth and fifth external visceral furrows are withdrawn into a fold or sinus of the neck (sinus prsecervicalis), (figs. 146, 147).
 
The visceral clefts close up and entirely disappear, with the exception of the first (hyoid or hyomandibular), which, as has already been described (151), persists as the Eustachian tube and tympanic cavity.
 
 
Fig. 145. - Head of Embryo Rabbit of Ten Days.
 
Magnified 12 diameters. [ From Kolliker.]
 
a. eye ; at. atrium or primitive auricle of the heart ; b. aortic bulb ; k\ k", k" -. first (mandibular), second (hyoid), third (xst branchial) visceral arch ; m. mouth ; 0. superior maxillary process, and u. inferior maxillary (mandibular) process of the right side ; s. mid-brain, which forms the interior extremity of the body ; v. anterior portion of head and fore-brain ; v. ventricle of the heart.
 
 
Intestinal Respiration. - Many Teleosts swallow atmospheric air, which passes along the alimentary canal and is ejected by the anus. There can be no doubt that this is a method of supplementary respiration. In these forms the hypoblast of the intestine is a respiratory tissue. Gage finds that the papillate mucous membrane of the pharynx of the American fresh-water Turtle, Aspidonectes spinifer, is distinctly respiratory in function, but this does not appear to hold good for other forms [Haswell].
 
Air-Bladder. - A tubular diverticulum grows out from the dorsal side of the oesophagus or stomach in most Ganoids (fig. 152, A, a.b) and Teleosts. In the Salmon and Carp [Yon Baer] it arises just in front of the liver, and slightly to the right side. It grows backwards, and in some cases forwards as well. Excepting in some Teleosts this structure persists as the air-bladder.
 
It is possible that the primitive diverticulum from the mesenteron, which afterwards developed into the air-bladder, was originally connected with secretion. A
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
181
 
 
small sac of unknown function occurs on the dorsal wall of the gullet in some Elasmobranchs.
 
The gases contained in the alimentary canal, and also, perhaps, air swallowed for respiratory purposes, would naturally tend to collect in a dorsal diverticulum. A hydrostatic apparatus would thus be formed, the muscular walls of the sac (airbladder) enabling the quantity of the contained gases to be regulated.
 
In some Fish (Physoclysti) the duct (pneumatic duct) by means of which the airbladder communicates with the alimentary canal becomes closed ; and in others, as in the Pleuronectidse, the air-bladder may entirely disappear.
 
In the Physoclysti the amount of gas in the air-bladder is regulated by diffusion through a network of blood-vessels. Under some conditions the fish may respire with the air secreted in the air-bladder by its own blood-vessels ; but this is a purely accessory and temporary mode of respiration.
 
The air-bladder in some Ganoids and Teleosts, and notably in the Dipnoids, is cellular and very vascular, and atmospheric air is in some of them known to be sucked in through the mouth, so that the air-bladder functions like a true lung.
 
In Gurnards and other Teleosts the air-bladder is used in making grunting sounds. In many Teleosts the air-bladder functions as an accessory auditory organ, either by impinging directly on the vestibulum of the internal ear, or by being indirectly connected with it by means of a chain of ossicles. The auditory function is most highly developed in the Siluroids, in which group the air-bladder becomes strangely modified, and may come anteriorly into close contact with the body-wall immediately behind the shoulder-girdle. The body- wall may become extremely thin at this spot, so as to form a regular tympanum. It is interesting to note that this tympanic membrane, like the tympanum of the ear, is lined externally by epiblast and internally by hypoblast.
 
In no organ of Vertebrates is there so varied a change of function as there is in this enteric diverticulum of Fishes.
 
Lungs. - The lungs are developed from the ventral wall of the oesophagus immediately behind the pharynx as an elongated groove, which abruptly terminates posteriorly (fig. 143, A, Ig). This ventral groove becomes constricted off from the oesophagus, except at its anterior end (glottis), where it still retains its connection with the pharynx (fig. 143, la). The blind slightly swollen extremity of the newly formed tube is the rudiment of the lung, and the duct is the trachea.
 
The lung very early exhibits a bilobed character (figs. 14 1, c; 146, c, T). Some observers state that it is from the first distinctly paired.
 
In most Amniota the surrounding splanchnic mesoblast becomes greatly thickened, and the hypoblastic sac-like lungs burrow into the stroma, dividing and subdividing as they advance. Eventually an extremely ramified system of tubes is formed in Mammals, each ultimate branch of which being terminally distended into a sacculated ampulla (infundibulum).
 
The primitive sac-like character of the lungs (fig. 143, b) is retained in the Amphibia and most Reptilia, the walls being merely infolded to give increased respiratory surface.
 
 
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THE STUDY OF EMBRYOLOGY.
 
 
In the Chameleons variable branched prolongations of the lungs project freely into the body cavity. Analogous diverticula appear in the embryos of Birds, and ultimately form the air-sacs. Prolongations from the latter pass into many of the bones in most Birds, the penetration of these delicate sacs into the bones being due to bone-absorption consequent on pressure.
 
The cartilaginous rings of the bronchi and trachea and the cartilages of the larynx are of mesoblastic origin.
 
The air-bladder of the Dipnoids is clearly homologous with the same organ of other Fishes, but in this remarkable group of animals the air-bladder is distinctly double ; its walls are greatly infolded (“spongy - or “ cellular -) and very vascular ; the blood supply is taken directly from the last aortic arch, and not from the caeliac artery, the blood being returned directly to the heart, and not to the liver, as in other Fishes ; lastly, the wide pneumatic duct opens on the ventral wall of the throat (the same also occurs in the Ganoid Polypterus). In all these points the air-bladder of the Dipnoi resembles the lungs of Amphibia. From these facts it is usually concluded that lungs are directly derived from the air-bladder of Fishes.
 
Minot, however, has suggested that the lungs have been evolved by the modification of a pair of gill-pouches, which do not break through in the neck, but grow down into the thorax (figs. 14 1, c, 146, c, l).
 
Albrecht considers it erroneous to homologise dorsal with ventral organs, and points out the difficulty of the migration of the dorsal air-bladder to a sub -oesophageal position. In the Gymnodont Teleosts, in addition to the dorsal air-bladder, there are ventral air-sacs proceeding from the oesophagus, by means of which these fishes can inflate themselves. These sacs are considered by him as homologous with lungs, and heterologous to the dorsal air-bladder. The air-bladder of Polypterus would therefore be the homologue of the lungs. Dorsal diverticula from the oesophagus opposite the larynx may normally (Pig) or abnormally (Man) be present.
 
 
Tongue. - Born finds that in the Pig the tongue is developed from the anterior portion of the ventral floor of the pharynx. The space between the ventral ends of the first and second visceral arches is at first depressed; but later a longitudinal ridge grows up, separated from the arches on each side by a groove. The anterior portion of this ridge grows forward and becomes the free part of the tongue. The tongue does not extend back beyond the second arch, but the posterior portion of the ridge projects between the third and fourth arches and develops the epiglottis. As Minot points out, the epithelium covering of the tongue is thus hypoblastic in origin.
 
If the above statement is correct, the taste-buds on the papillae are hypoblastic sense organs. The gustatory goblet-cells on the tongue of Amphibia possibly have a similar origin. The goblet-shaped organs in the mouth and pharynx of Fishes may have a similar function, but those of the mouth appear to be homologous with similar organs situated in the skin. Macullum has very recently recorded the occurrence of taste-buds in the oesophagus of the Sturgeon ; he also states that he has found them in Ampliioxus as far back as the opening of the hepatic caecum.
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
183
 
 
According to His, the tongue in Man has a double origin. From the anterior region of the ventral space (mesobranchial area), between the visceral arches of an early embryo, a small round projection (tuberculum impar) is formed ; behind this are a pair of folds (furcula), which eventually will form the epiglottis. The ventral portions of the second and third arches grow towards their fellows of the opposite side between the tuberculum and the furcula. The basal growths of the arches form the roots of the future tongue, and unite together behind the tuberculum impar;
 
 
Fig. 146. - Diagrams Illustrating the Visceral Arches and Development op the Tongue in the Human Embryo. [After His.]
 
Seen from above, the dorsal (posterior) wall of the pharynx being supposed to be cut away. In B. the branchial blood-vessels are indicated.
 
F. furcula ; l. lungs ; R. roots of the tongue ; s.pc. sinus praecervicalis ; T. tuberculum impar, body of the tongue in D ; 1-5. visceral arches.
 
the median pit between these structures is the rudiment of the median lobe of the thyroid body. The tongue is formed by the fusion of the two roots with the tuberculum.
 
Thyroid Body. - The generally received view of the significance of the thyroid body has already been mentioned (p. 172).
 
In the Lamprey (fig. 142) the thyroid body arises as a wide diverticulum from the floor of the anterior portion of the pharynx. The orifice becomes restricted to a pore and eventually disappears. During larval life it consists of a median ciliated portion communicating with a pair of complicated lateral glandular sacs.
 
In some higher forms the thyroid is stated to develop as a tubular diverticulum or solid down-growth from the anterior region of the pharynx, which later becomes bilobed. Subsequently it is quite detached from the pharynx, and is produced into
 
 
184
 
 
THE STUDY OF EMBRYOLOGY.
 
 
a number of hollow or solid processes, between which connective tissue septa and blood-vessels enter.
 
Born reconciles various conflicting observations regarding the origin of the thyroid body in Mammals by finding that, according to his investigations, the organ has a double origin. An unpaired portion arises as an invagination from the floor of the pharynx opposite the front edge of the second visceral cleft. It separates from the pharyngeal epithelium, expands laterally, and migrates backwards. The other portion of the thyroid is derived from the paired remnants of the fourth visceral clefts. These are at first somewhat pear-shaped hollow sacs, but on becoming connected with the central portion they acquire a spongy interior. Pischelis confirms Born -s statements from his researches on the Pig, Rabbit, and Birds. His finds, in the human embryos, that the median thyroid rudiment arises as a hollow diverticulum between the third visceral arches, and that the lateral portions are evaginated from the posterior end of the pharynx near the glottis (fig. 147, l.thyr). The several parts become separated from their parent tissues and sink into the deeper portion of the neck. The duct of invagination of the median portion persists for some time as the ductus thyreoglossus (fig. 147, d.thyr). The foramen csecum, cornu medium, and the various bursse which may be present iq the adult are rudiments of this duct.
 
The last investigation on the thyroid body is that of De Meuron, who finds that the median element is always (Elasmobranch, Frog, Lizard, Fowl, Sheep, and Man) developed from a median pit in the pharynx at the level of the second visceral arch. He homologises the supra-pericardial bodies of Elasmobranchs [Van Bemmelen] and Amphibia [De Meuron] with the accessory or lateral thyroid bodies of the Amniota (the left alone occurs in Acanthias and Lacerta). The structure of both resembles that of the median element. These bodies arise as a pair of diverticula behind the sixth branchial cleft (seventh visceral), which is imperfectly developed in many Elasmobranchs. It may be concluded that these represent a degenerate pair of gills as in Heptanchus, in which there are seven branchial clefts, the supra-pericardial bodies are absent. In the higher Fishes and larval Amphibia the lateral rudiments of the thyroid develop directly from the pharynx behind the last (fifth branchial cleft. Owing to a further reduction of the clefts, which also disappear without leaving a trace and a consequent shortening of the pharynx, the lateral thyroids appear to develop from the fourth branchial cleft ; this is most marked in Mammals. The similarity in structure of the fully-developed lateral thyroids with the median element and their close connection in adult Mammals rather tend to support Dohrn -s hypothesis concerning the primitive condition of the median thyroid, i.e., that it represents a pair of degraded hyoid clefts.
 
Thymus Gland. - Maurer finds that in the embryo Trout the thymus takes its origin from four thickenings of the epithelium of the visceral clefts on each side of the body, a rudiment being situated in the dorsal angle of each of the four clefts. A proliferation of the epithelium takes place, and the four rudiments on each side become fused together. Each lateral thymus gland sinks into the underlying mesoblast and takes on the character of a lymphatic gland.
 
Dohrn states that there is primitively one thymus rudiment for each branchial cleft in Elasmobranchs, but the fifth disappears in the Sharks. The separation of these rudiments from the epithelium is due to the shortening of the clefts and the bending of the visceral arches. The parts thus isolated gave rise to a new organ, the thymus, which was afterwards transmitted by heredity to higher Vertebrates.
 
In the Pig, according to Born, the thymus arises as a pair of ventral evaginations from near the inner openings of the third pair of visceral clefts, the outer portions of which atrophy. The end of the thyroid rests against the pericardium at the spot where the aorta leaves it. The central cavity disappears, and many branches grow out from the solid cord, mainly in the direction of the heart.
 
His finds in the human embryo that the primary (epithelial) rudiment of the
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
185
 
 
thymus arises from the epithelium of the inner portion of the fourth, third, and partly also of the second visceral clefts. These parts become massed together and separated from the outer skin (fig. 147, thm). He asserts that it is developed from the epidermis and cannot be regarded as a hypoblastic structure, since in Man none of the visceral clefts become perforated.
 
It is evident that the function of the gland in the Amniota is secondarily acquired, and that it is a degraded epithelial organ, "which, from its relation to the gill-clefts in Fishes, may possibly have been some form of sense organ.
 
De Meuron has also studied the development of the thymus. In Ichthyopsida and Sauropsida it arises as solid thickenings of the epithelium of the dorsal side of the branchial clefts. In Fishes it arises from the first four branchial clefts, in the Lizard from the second, third, and fourth, and in Birds from the third and fourth visceral
 
 
A
 
 
Fig. 147. - Development of the Thymus Gland and Thyroid Body in the Human Embryo. [After His.]
 
 
A. Transverse section through the hinder portion of the head. B. Transverse section through the larynx of an older embryo. C. Profile reconstruction of the thyroid and thymus glands, seen from below. D. The same seen from the side.
 
ao.a, ao.d. ascending and descending aorta ; c. carotid artery ; d.thyr. ductus thyreoglossus ; ep. epiglottis ;/.c. foramen caecum ;j.v. jugular vein ; l. larynx; l.thyr. lateral thyroid rudiment; m.thyr. median thyroid rudiment; ce. oesophagus; p.a. plicae aryepiglotticse ; ph. pharynx ; r.t. roots of the tongue ; t. tongue ; thm. rudiment of the thymus gland ; tr. trachea ; ix. glossopharyngeal ; x. vagus ; xi. hypoglossal ; xn. spinal accessory nerves ; 1-5. visceral arches.
 
 
clefts. In the last three groups the thickening of the third cleft is the largest. In Anura the second visceral cleft alone develops a thymus. The history of the thymus is very different in Mammals ; dorsal rudiments are developed, as in Birds, from the third and fourth visceral clefts, but nearly the whole of the adult organ is derived from a ventral caecum from the third branchial cleft.
 
Gustatory Organ of Amphioxus. - The organ usually known as the olfactory organ of Amphioxus consists of an outer ciliated sac opening to the exterior and also into an inner sense-organ, which again communicates with the mouth. Hatschek finds that the whole organ is developed from the left of a pair of archenteric diverticula in front of the mouth, and that it is therefore of purely hypoblastic origin. It probably is an organ of taste. Hatschek and Dohrn are inclined to homologise it with the hypophysis.
 
 
186
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Excretory Organs. - The only excretory organs which appear to be of hypoblastic origin are the paired urinary tubes which occur in Amphipods. They arise from the extreme hind-end of the mesenteron, there being a distinct break of continuity when the latter ceases and the hind-gut (rectum) begins [Spencer]. Their development is unknown.
 
Skeletal Structures. - Notochord. - The primitive axial supporting rod or skeleton (notochord), or chorda dorsalis, which is peculiar to the Chordata, and from which they derive their name, is of hypoblastic origin.
 
Hemichordata. - Bateson has shown that in the larval Balanoglossus (B. kowalevskii) the median dorsal wall of the pharynx is constricted off and grows forward as a short hollow diverticulum of hypoblast, which afterwards becomes solid except posteriorly, where its lumen opens throughout life into the pharynx. The cells scon become vacuolated as in the notochord of higher forms.
 
Urochordata. - In Ascidians the notochord is developed solely in the tail, it being derived from the dorsal wall of the caudal arclienteron.
 
Cephalochordata. - In Amphioxus the notochord is, as it were, pinched off from the median dorsal wall of the archenteron (fig. 56 nch). Ultimately its folded appearance and its connection with the archenteron are lost. It is constricted off from before backwards.
 
Vertebrata. - In the lower Vertebrates the notochord is distinctly derived from the dorsal wall of the mesenteron (archenteron). Hertwig -s researches on the development of the Newt (Triton) show that the dorsal hypoblast (usually referred to in this book as invaginated hypoblast and the chorda entoblast of Hertwig and others) not only is distinctly columnar, as opposed to the rounded ordinary hypoblast cells, but it also lines a distinct groove (fig. 148). The two sides of the notochordal groove, as it may be termed, come together and form a solid rod of cells, the arrangement of which gives no indication of their origin. The notochordal groove is scarcely apparent in the Frog.
 
Mitsukuri and Ishikawa have demonstrated that the notochord in the Snapping Turtle (Trionvx japonicus) (fig. 149) is developed in a manner perfectly comparable with that of the Newt. Indications of a similar origin of the notochord are found in Lizards, and notably in Mammals.
 
In Birds the axial hypoblast very early becomes converted into
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
187
 
 
the rudiment of the notochord, and this may occur almost before the permanent hypoblast can be recognised as such; hence the supposition of some authors that the notochord was derived from the mesoblast.
 
 
Fig. 148. - Transverse Section of the Dorsal Portion oe an Embryo Newt (Triton). [ 4 /f«‘ O. Hertivig.]
 
a. mesenteron ; ax.hy. axial hypoblast in process of forming the notochord; b.c. coelom (body-cavity) ; ep. epiblast ; hy. digestive hypoblast ; n.p. neural plate ; so.m. somatic mesoblast; sp.m. splanchnic mesoblast.
 
 
The rudiment of the notochord consists of a solid rod of cells lying between the neural tube and the mesenteron. Posteriorly it is connected with the fusion of the layers which occurs at the
 
 
(Fig. 149. - Formation of Notochord in Trionyx. [ After Mitsukuri and Ishikawa.]
 
A. Transverse section through the head region before the closure of the neural groove B. D. Portions of successive sections of the same embryo. The shading of the epiblast is purely diagrammatic.
 
am. amnion ; ax.hy. axial hypoblast ; ep.a. epiblastic, and hy.a. hypoblastic layer of amnion; hy. hypoblast; to. mesoblast; n.c. neural canal: nch. notochord.
 
dorsal lip of the blastopore (fig. 62), or, when there is no distinct blastoporic passage, as in the Fowl, it passes into the primitive streak. At a later stage the notochord terminates anteriorly
 
 
188
 
 
THE STUDY OF EMBRYOLOGY.
 
 
behind the infundibulum, its extremity being often recurved. Posteriorly the notochord terminates at the end of the tail.
 
A definite sheath (elastica limitans interna) is soon formed as a secretion from the peripheral cells of the notochord. The cells of the notochord become vacuolated, so that the notochord has a spongy appearance ; a few nuclei surrounded by a little protoplasm remain attached to some of the meshes of the network (figs. 150, 152, 173, 175, ch).
 
The notochord and its sheath are replaced in most Vertebrates, leaving only a small rudiment, as will be mentioned in the description of the development of the vertebral column (pp. 196-199).
 
Sub-Notochordal Rod. - A solid rod of cells is developed from the dorsal wall of the alimentary canal in Ichthyopsida after the formation of the notochord (figs. 150, 173, 175, x).
 
This sub-notochordal rod, as it is termed, has about the same extension as the notochord. Its function or homology is unknown, but it appears to persist as the sub-vertebral ligament in the Sturgeon.
 
Significance of the Notochord. - Few embryological problems are more obscure than the probable phylogenetic significance of the notochord. The embryological evidence points to its hvpoblastic origin. We are justified in assuming the primitive, or at all events the archaic, nature of its development in the Amphioxus (fig. 56) and the Newt (fig. 148). The variations which are met with in other Vertebrates can be reduced to the type of the Newt, as is proved by the Chelonia (fig. 149).
 
The development of the urochord in the Ascidians is manifestly a degraded process.
 
The restricted notochord of Balanoglossus develops in an essentially similar manner to that of Amphioxus, but the central lumen is retained for a much longer period. It is interesting to note that in some Amniota a transient canal occurs at the posterior end of the notochord.
 
Upon an examination of the figures given by authors illustrating the development of the notochord in Balanoglossus, Amphioxus, the Newt, Chelonia, Lizards, and Mammals, the conclusion seems to be almost inevitable that we must regard the notochord as a secondary structure. It may be that the ancestor of the Chordata possessed a longitudinal groove along the neural aspect of its alimentary tract, which may have had some special secretory (? mucous) function. The extremely early acquisition of distinctive histological characters may be recalled in this connection.
 
The closure of the notochordal groove in ontology at the time of the constriction off of the archenteric diverticula from the mesenteron is suggestive of phylogenetic synchrony.
 
It is not difficult to imagine that a rod of cells, even though containing at first a small lumen, might form a mechanical support to the body which would prove of considerable value, and, being internal, it would grow with the growth and requirements of the animal.
 
Urinary Bladder. - The urinary bladder is properly speaking a hypoblastic organ, but it is more convenient to deal with it at the same time as the uro-genital ducts (P- 2 59)
 
==Chapter VII. Organs Derived from the Mesoblast==
 
However it arises, the mesoblast gives rise to the deeper layer of the skin, i.e., the derma or cutis.; to the whole of the muscular system in animals higher than the Ccelenterata ; to nearly all the internal supporting structures of the body ; to the lining membrane of the body-cavity, peritoneum, in the broadest sense of the term : to the whole of the vascular system ; to the excretory organs ; and to the generative glands.
 
Indifferent Mesoblast. - Under the term indifferent mesoblast may be classed the general parenchyma of the body of the lowest Metazoa.
 
In the Porifera, between the two primitive epithelia of the body irregular amoeboid cells occur in greater or less abundance, imbedded in a jelly-like matrix. Sollas suggests the appropriate term of archseocytes for such cells. The origin of these mesamoeboids has been described ; they function in various ways, probably mainly in nutrition, by carrying food-products to various parts of the organism, and in the transportation of waste matter, in this respect resembling the leucocytes of higher animals. Many of the mesamoeboids secrete spicules; some develop into m uscle- cells ; others constitute germ-cells, and some are stated to act as nervecells.
 
The oval or anastomosing stellate cells in the gelatinous tissue of Scyphomedusse arise mostly from the hypoblast, and the muscular stellate cells of Ctenophora from the epiblast, though some are stated by Chun to be of hypoblastic origin. There may be connective-tissue cells in the fibrillar lamina of Actinozoa.
 
The mesamoeboids enclosed within the spacious segmentationcavity of larval Echinoderms have many functions to perform ; as Metschnikoff has shown, they devour degenerate tissues (see p. 274), and they also secrete the larval skeleton (fig. 16, m.s.).
 
The spongy parenchyma which fills up the space between the
 
 
190
 
 
THE STUDY OF EMBRYOLOGY.
 
 
epiblast of the skin and the hypoblast of the meseateron in Platyhelminths appears to be of mesenchymatous origin. These cells are essentially “ indifferent - in character, and Lankester has shown how that in the Leech this tissue, which he terms skeletotrophic, may insensibly pass into blood-vessels and blood-cells on the one hand, or into connective tissues generally on the other. A good deal of the intermediate parenchymatous tissue of Molluscs might be placed in this category.
 
In higher forms the wandering cells of the body (colourless blood corpuscles, leucocytes), retain their amoeboid nature, and probably have diverse functions. The generative or germ-cells may be considered as the least specialised cells in the body.
 
Dermal Mesoblast. - That mesoblastic tissue which immediately underlies the embryonic epiblast, and which constitutes the derma or cutis of the adult, may be termed dermal or peripheral mesoblast.
 
Such, for instance, are those mesamceboids which in Echinoderms are enclosed between the lining membrane of the body-cavity and the epiblast. They constitute the main thickness of the body-wall, and are productive of muscles, ligaments, and the calcareous spicules, plates, and spines.
 
It would be superfluous to enumerate the various aspects which the dermal mesoblast assumes.
 
The derma of Vertebrates typically consists of - (i.) Connective tissue fibres and elastic fibres. The fibres of the derma in Ichthyopsida are usually arranged in more or less regular vertical and horizontal bundles, whereas those of the Amniota are irregularly felted together. (2.) Pigment cells and wandering leucocytes. (3.) Often a deeper layer of fat cells. (4.) Non-striated muscular fibres; and, lastly, it is penetrated by blood-vessels and nerves from the one side, and by glands and hair-bulbs on the other.
 
Muscular System. - There is considerable uncertainty with regard to the exact origin of the muscular system of many Invertebrates. In some cases it is wholly or partially mesenchymatous (Echinodermata, Platyhelminths). In the Echinoderms the epithelial cells of the archenteric diverticula are stated by Metschnikoff to possess muscular processes, but it is not known whether these furnish all the muscular elements of the body-wall. The external muscle fibres, which cause the movements of the spines of the Echinoids, are almost certainly not so derived. The muscles are known to be mesothelial in origin in the Earthworm ; but even
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
191
 
 
in the Chaetopoda and Arthropoda (?) mesenchymatous elements are stated by some observers to be present, and these may possibly form muscle-cells.
 
In the Chordata the muscular system is entirely of mesothelial origin, being derived from the somatic and splanchnic layers.
 
The first muscles to make their appearance in Amphioxus
 
 
Fig. 150. - Transverse Section trough the Trunk oe an Elasmobranch Embryo (Pristiurus). [From Balfour.]
 
al. mesenteron ; ao. aorta ; mp. muscle-plate ; uip'. portion of muscle-plate converted into muscle ; nc. neural canal ; pr. dorsal root of spinal nerve arising from the neural crest ; sc. somatic mesoblast ; sp. splanchnic mesoblast; V.v. portion of the vertebral plate which will give rise to the vertebral bodies ; x. subnotochordal rod.
 
The intermediate cell mass connects the dorsal and ventral mesoblast ; it is seen on the left-hand side of the figure, between the lines pointing to x and ao.
 
 
(fig- 56, c, m) are the longitudinal muscles which lie on each side of the notochord ; they arise as differentiations of the basal portion of the splanchnic cells of that region.
 
If fig. 56 is compared with figs. 150 and 175, it will be seen that the great lateral muscles of Elasmobranchs are developed from similar splanchnic cells, and the same may be traced in an early stage in the muscle-plates of the Amniota. In the embryo Bird
 
 
Fig. 151. - Horizontal Section through the Trun of an Embryo Fowl.
 
The section passes through the notochord and shows the separation of the cells to form the vertebral bodies from the muscle-plates.
 
ep. epiblast ; l.m. longitudinal muscles differentiated from the splanchnic portion of the muscle-plate, m.p ; nch. notochord ; v.r. vertebral rudiment.
 
 
the first-formed muscles have a longitudinal direction, and are divided into segments.
 
A horizontal section through a portion of the body of an embryo Fowl (fig. 1 51) on the level of the notochord clearly exhibits the segmented character of the dorsal mesoblast. The section is taken at a stage when the splanchnopleur has differentiated into an inner vertebral rudiment (p. 199) and an outer layer of longitudinal muscles, while the somatopleur is unmodified. A comparison of
 
 
192
 
 
THE STUDY OF EMBRYOLOGY.
 
 
this figure with that of Amphioxus brings out the fact that the dorsal portion of the body is characterised by a series of mesoblastic pouches, each of which contains an isolated portion of the body-cavity. This primitive character is masked in most other forms, but in all the Chordata the great lateral muscles are developed therefrom.
 
Balfour terms each mesoblastic pouch a somite, which is the equivalent of a protovertebra of many authors, reserving the name of muscle-plate to the somite after it has given rise to the vertebral rudiment, as it is then entirely metamorphosed into the voluntary muscular system.
 
The muscle-plates increase in size and extend into the ventrolateral wall of the embryo. The splanchnopleur is first converted into muscle-cells, the somatopleur becomes implicated later.
 
The musculature of the limbs early appears as dorsal and ventral bands, which originate from processes from the muscle-plates (fig. 103, mp.l). These become segmented off from the muscleplates, which then pass into the ventral wall of the body.
 
We may conclude that the primitive continuous lateral fin was put in motion by muscular processes from each muscle-plate ; and that when the limbs were differentiated from the fin, some, at least, of the segmental muscles were so grouped as to form the muscles of the limbs.
 
The muscles of the head, including the eye-muscles, arise from the walls of the cephalic somites (p. 140), in the same manner as those of the body.
 
The transformation of an epithelial -cell into a muscle-cell occurs by the differentiation of the protoplasm into the contractile fibrils either at one side or peripherally ; in the former case the original nucleus is lateral, in the latter it is situated in the centre of each cell.
 
Dermal Skeletal Structures. - Invertebrates. - Mesodermal exo-skeletal structures scarcely occur amongst the Invertebrates. The Holothuroidea have thin perforated calcareous plates or spicules imbedded in their skin; all the other Echinoderms are characterised by an extensive development of solid calcareous plates and spines.
 
Chordata. - The dermal skeletal elements of the Chordata may be conveniently reduced to one type, namely, to a placoid scale, the development of which has already been noticed (p. 103). Minute placoid scales or denticles scattered over the skin constitute the shagreen of Elasmobranchs. Each denticle has a basal plate formed of bone.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
193
 
 
The large dermal plates of Ganoids and some Teleosts are by some regarded as formed by the fusion of the basal plates of numerous denticles, the polished surface of the plate being due to a deposit of enamel.
 
The thin scales of Amia, most Teleosts, (and Dipnoi) are undoubtedly the somewhat degraded representatives of the bony plates of their Ganoid ancestors. In many cases the supposed epiblastic portion (enamel) of the scales and dermal plates atrophies or is undeveloped.
 
The dermal plates, which have a purely mesoblastic origin, form the group of bones known as membrane bones (see also p. 2 io). To this category belong the parosteal elements of the skull, and the “ clavicles - of Teleosts.
 
Recent Amphibia are peculiarly deficient in a dermal exo-skeleton. Bony plates occur in skin of the back in Ceratophrys dorsata and Ephippifer aurantiacus, and scutes in the Ctecilians.
 
The scutes (often called scales) of Lacertilia and Crocodilia are formed as ossifications in the derma. The scale-papilla may be best compared to an extremely flattened feather-papilla, which, like the latter, is set at an angle within a follicle. The mesoblastic core ossifies, and the overlying Malpighian layer of the epiblast possibly in some cases deposits a layer of enamel.
 
Among recent Reptilia the Chelonia have by far the most developed dermal exo-skeleton, which forms a dorsal carapace and a ventral plastron. Parosteal riblike bones (splints) occur in the ventral wall of the abdomen of Hatteria and Crocodilia. Similar ossifications are occasionally present in the intermuscular septa of Teleosts.
 
The bony plates which occur in the sclerotic in Birds, Reptiles, and many Pishes belong to this category.
 
No dermal skeletal structures occur in the trunk of Birds, and but few in Mammals, the most noticeable being the extensive scutes of the Armadillos.
 
Mesoblastic Endo- Skeletal Structures. -Invertebrates. -
 
The supporting or endo-skeletal structures of the Invertebrates are almost universally of epiblastic origin. The following are the chief mesoblastic formations.
 
The spicules of Sponges arise from a single mesamoeboid ; when bundles of delicate spicules (trichites) occur, the whole mass is developed from a single cell.
 
The exact origin of the gelatinous supporting tissue of Coelenterates (mesogloea) has not been fully made out. The calcareous skeleton of the Hexacoralla and the calcareous spicules of the Octocoralla are secreted by cells derived from the ectoderm.
 
N
 
 
194
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The horny axial 'skeleton of the Gorgon iidse, the alternate horny and calcareous axis of the Isidinse, and the calcareous stem of Corallium, may prove to be epiblastic, like the horny axis of the Antipatliidse.
 
In the free-swimming larvae (Plutei, &c.) of the Echinoidea, Ophiuroidea, and Crinoidea, a calcareous spicular skeleton is secreted by the mesamceboids (fig. 16, m.s).
 
A cartilaginous axis supports the branchial plume of the Serpulae.
 
True cartilage occurs in the Cephalopods and in connection with the odontophore in Gasteropods ; the former are the only Invertebrates in which the brain is protected by a cartilaginous brain-case.
 
Chordata. - An endo-skeleton which supports the body and grows with its growth is one of the principal characteristics of the Chordata as a whole. It would perhaps be hardly too much to say that the possession of this and the adaptive axial skeleton was probably the main factor in the evolution of the group. The endoskeleton of the Chordata includes an axial and appendicular elements. The former consists primitively of the notochord with its skeletogenous sheath, and secondarily of the vertebral column and the cranium.
 
The appendicular skeleton is derived from the primitive supports of the locomotory organs (fins). These at first were entirely independent of the axial skeleton, but a more or less intimate connection has subsequently been acquired with the latter.
 
Other structures have appeared in the walls of the body which have all come to be connected with the axial skeleton; for example, the ribs in the somatopleur of the trunk, the internal branchial visceral bars in the splanchnopleur of the pharynx, and the labial cartilages of the face.
 
Vertebral Column. - The notochord with its sheath persists as the axial skeleton in Amphioxus, the Cyclostomes, Dipnoi, and Selachian Ganoids. In all the higher Vertebrates a skeletogenous sheath is developed round the notochord.
 
Skeletogenous Sheath of Notochord. - The skeletogenous or cartilaginous sheath of the notochord is developed from a layer of mesoblast cells which range themselves round the elastica limitans interna (fig. 152, b). The layer increases in thickness, and forms a continuous unsegmented tube of fibrous tissue with flattened concentrically arranged nuclei. Outside this layer another sheath is developed, variously known as the elastica limitans externa or outer sheath of the notochord.
 
This unconstricted condition of the notochord is retained by the adult Cartilaginous Ganoids and Dipnoi (fig. 153, A). In Chimsera there are added thin calcareous rings, which bear no relation to
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
195
 
 
the neural arches, and are more numerous. In some Elasmobranchs true vertebrae are imperfectly developed.
 
In all other forms the notochord is serially constricted by the development of true vertebral centra, and is eventually partially or entirely replaced by the mesoblastic vertebral column.
 
Vertebral Arches and Vertebral Bodies. - In Amphioxus the neural canal is merely protected by a sheath of connective tissue ; but in the true Vertebrates a series of cartilaginous bars, neural
 
 
Fro. 152 . - Notochord of Lepidosteus. [ After Balfour and Parker .]
 
A. Transverse section through the anterior part of the trunk of an embryo about a month after hatching, showing the connection of the air-bladder with the throat. B.
 
Portion of transverse section through the vertebral column of a larva of 5.5 cm.
 
(through the vertebral region).
 
a.b. air-bladder; c.sh. cuticular sheath of notochord; h.a. haemal arch; i.s. interspinous bone; l.l. ligamentum longitudinale superius; m.e. membrana elastica externa ; n.a. neural arch ; n.a' . dorsal element of same ; n.c. neural canal ; nch. notochord; ces. oesophagus \ pc. pericardium; p.g. pectoral girdle; pr.n. pronephros; sh. sheath of notochord (elastica limitans interna) ; v. ventricle.
 
The cartilage is dotted ; its bony sheath is left blank in B.
 
arches, are developed, which at first laterally, and then dorsally as well, protect the neural canal.
 
In the lowest true Vertebrates, the Cyclostomi, the neural arches are irregularly arranged bars of cartilage which do not meet over the neural canal.
 
Fishes. - In other forms the neural arches first appear as a pair of continuous cellular ridges resting on the skeletogenous sheath of the notochord. A similar ventral ridge, which is better developed in the caudal region, is known as the haemal ridge (fig. 1 52).
 
 
196
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The neural ridges become enlarged at each intermuscular septum in Fishes. These enlargements are converted into cartilage and form the neural arches. The haemal arches develop in a similar manner ; but it is only in the region of the tail that the haemal bars unite in the median ventral line to form a true haemal arch.
 
 
In developing and young, and a few adult Elasmobranchs, in certain young and adult Ganoids (Sturgeon, Polyodon, Amia), and in Chimaera, intervertebral or intercalary neural arches are developed. Interhaemal arches are developed in some cases.
 
The neural arches always make their appearance between the spinal nerves. The interneural arches, when present, usually arise between the dorsal and ventral roots of the nerves.
 
In the adult Scyllium the dorsal root of a spinal nerve passes through the intercalated cartilage, and the ventral root traverses the neural arch immediately in front.
 
 
Fig. 153. - Diagram Representing the Various Types of Vertebral Columns in Longitudinal Section. [ From. Gegenbaur .]
 
A. Primitive type, with no vertebral segmentation. B. Type of Fishes, with vertebral constrictions of the notoehord. C. Amphibian type, intervertebral constrictions of the notochord by intervertebral rings of the cellular sheath. D. Intervertebral constriction of the notochord as in Sauropsida. E. Vertebral constriction of the notochord of Mammals, the intervertebral regions of the cartilaginous sheath being converted into intervertebral ligaments.
 
c. notochord; cs. cuticular sheath of notochord; g. intervertebral articulations; iv. intervertebral regions; s. cellular or cartilaginous sheath ; v. vertebral regions or bodies of the vertebrae.
 
 
The skeletogenous sheath of the notochord also undergoes segmentation, and an annular thickening occurs in the vertebral region (fig. 153, b). This ring becomes converted into hyaline cartilage and encroaches on the notochord, which becomes considerably constricted at these points, but not in the intervals. In the intervertebral regions the sheath of the notochord assumes a fibrous character.
 
From their mode of formation the vertebrae of Fislies are biconcave (amphicoelous). The gelatinous intervertebral spheres are the degraded remnants of the unconstricted portions of the notochord. Lepidosteus is the only Fish in which the centra of the vertebrae directly articulate with one another, the faces of the bodies or centra of the vertebrae being convex in front and concave behind (opisthocoelous). In this form the bases of the neural and haemal arches extend into the intervertebral regions*
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
197
 
 
forming cartilaginous rings. Each intervertebral ring becomes divided into two parts, which will respectively form the anterior face of a given vertebral centrum and the posterior face of that in front of it. There is thus in this Ganoid a secondary intervertebral constriction of the notochord ; the latter entirely disappears, except in the tail.
 
The greater part of the bodies of the vertebrae and of the arches are ossified in Lepidosteus and Teleosts from the membranous perichondrium.
 
The neural arches rarely unite with their fellows in Fishes, the neural arch being completed above by accessory cartilages and a longitudinal elastic band.
 
The various forms of Fishes - tails are described later (p. 203).
 
 
Fig. 154. - Longitudinal Section through the Vertebral Column oe Various Urodeles. [After Wiedersheim.]
 
A. Ranodon sibericus; JB. Amblystoma tigrinum; C. Gyrinophilus porphyriticus (Vertebrae, i, 2, 3). D. Salamandrina perspicillata.
 
a.h. articular head, and a.s. articular socket of vertebral body ; b. peripheral bony covering of centrum ; cli. notochord; i.c.s. intervertebral thickening of cartilaginous sheath; ligt. intervertebral ligament ; m. c. marrow cavity ; t. transverse processes and ribs; v.c. vertebral cartilage and fat cells; x. vertebral constriction of notochord in Amblystoma tigrinum without cartilage and fat cells.
 
The cartilage is dotted and the bone is left white.
 
Amphibia. - The Amphibia present us with an interesting series of phases in the development of the vertebral column.
 
At first, in IJrodele larvae, as in most Fishes, the notochord is vertebrally constricted, and the cellular sheath, which is the equivalent of the skeletogenous sheath of Fishes, is early surrounded by a delicate layer of bone which is formed in the investing connective tissue. This biconcave character of the vertebrae is retained by the Csecilians and the gilled Urodeles.
 
Later, in the intervertebral regions the sheath becomes greatly thickened, forming deep cartilaginous rings, which constrict and ultimately obliterate the notochord (fig. 154, c).
 
 
198
 
 
THE STUDY OF EMBEYOLOGY.
 
 
Finally, an articular cavity is produced by absorption in each intervertebral region, in such a manner that the convex cartilaginous anterior extremity of one vertebra articulates with a corresponding concavity in the preceding vertebra. Thus the caducibranch Urodeles have opisthocoelous vertebrae.
 
Three stages can be distinguished in the development of the vertebral column of Urodeles - (i) a connection of the vertebrae by means of the intervertebrally expanded notochord, as in Fishes generally ; (2) a union of the centra by means of intervertebral masses of cartilage; (3) an articular condition. An ossification of the articular surfaces of the centra of the vertebrae occurs in Lepidosteus, Anura, and most Amniota.
 
It may be noted that the articular facets appear to be the only cartilaginous portions of the vertebrae of Urodeles, their vertebrae being ossified from membrane (connective tissue), as in Lepidosteus and Teleosts. In the Anura the vertebrae ossify from cartilage as in the Amniota. The notochord persists in a cartilaginous form within the centra of the vertebrae for a long time, and may even be found in adult Frogs. The articular facets of the vertebral bodies are mostly concave in front and convex behind (procoelous) in Anura.
 
Haemal arches are present in the tail of Urodeles, as are also transverse processes which may bear ribs. In the Anura the urostyle is formed by the fusion of the two anterior caudal vertebrae with the cellular sheath of the notochord.
 
Sauropsida. - -The cellular sheath of the notochord and the neural arches from the first form a continuous structure.
 
In Hatteria and the Geckos, alone of living Eeptiles, are the vertebrae biconcave, owing to the vertebral constriction of the notochord. This condition was common amongst the extinct forms. All the other Sauropsida agree in the sheath encroaching on the notochord in the intervertebral regions (fig. 153, d). A split occurs in the centre of each intervertebral enlargement, as in Amphibia, which forms the interarticular cavity. In Eeptiles the articular facets of the centra are usually procoelous ; they are saddleshaped in at least the cervical region of Birds. Intervertebral discs or menisci occur between the vertebrae of Crocodiles, and, except in the cervical region, of Birds also.
 
Mammalia. - The view that Mammals have arisen from some group of unspecialised Eeptiles receives additional support from the mode of origin of the vertebrae, as the notochord is from the first vertebrally constricted. The intervertebral regions become wholly converted into the fibro-cartilaginous menisci, intervertebral ligaments, in the centre of which the notochord persists in a degraded form as the nucleus pulposus or gelatinous pulp of
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
199
 
 
the intervertebral disc. Articular surfaces are never developed between the bodies of the vertebrae, although they occur on the neural arches. Vertebral epiphyses are peculiar to Mammals ; they are found amongst Monotremes only in the caudal region, but are universally present in other Mammals, except the Sirenia.
 
Evolution of the Vertebral Column - It is interesting to note that at its first appearance the foundation tissue of the skeletogenous sheath is segmented (fig. 1 5 1, v.r), the segments corresponding with the muscle-plates ; but this segmentation is soon lost.
 
The final segmentation of the vertebral column is alternate to that of the muscleplates, so that the centre of each vertebra is opposite to the intermuscular septa.
 
As Balfour says, “The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebrae had not appeared. As soon, however, as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments.
 
“ The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each myotome shall be capable of acting on two vertebrae, and this condition can only be fulfilled when the myotomes are opposite the intervals between the vertebrae. For this reason, when the vertebrae became formed, their centres were opposite, not the middle of the myotomes, but the intermuscular septa.-
 
The stages of evolution were thus - (1) the formation of axial skeletal mesoblast round the notochord by the segmented muscleplates ; (2) the fusion of these elements to form a flexible continuous sheath round the notochord and nervous axis; (3) the secondary segmentation of the vertebral column above described. The last stage consists of two phases - (a) cartilaginous, ( b ) osseous.
 
Ribs. - In most Ganoids and Teleosts the ribs arise as the cutoff extremities of the haemal processes; in the caudal region, where the haemal processes approach one another, the key of the arch is formed by the fused ribs. The same probably occurs in the Dipnoi.
 
The differentiation of the ribs is independent of that of the haemal processes in Elasmobranchs, in which group they arise as cartilaginous bars in the connective tissue of the intermuscular septa, eventually they become connected with the haemal processes.
 
The ribs appear to develop in Amphibia and Amniota much in the same way as in the Elasmobranchs, but in these groups they are attached to the neural arches or to the transverse processes.
 
Ribs are present in the embryos of all Amniotes throughout the vertebral column except in the tail. In the Amniota the cervical
 
 
200
 
 
• THE STUDY OF EMBRYOLOGY.
 
 
ribs usually fuse with the transverse processes, but one or more (rarely all) may remain free. Several ribs unite to form the sternum ; their ventral moities are often incompletely or entirely unossified, and constitute the sternal ribs. Behind these “ true - ribs there are usually others, often termed “ false,- which do not reach the sternum.
 
In all Vertebrates the pelvis is always supported by sacral ribs ; these may remain distinct, as in Urodeles, or may fuse with the transverse processes of their sacral vertebrae.
 
The occasional presence of abdominal parosteal splints has already been noticed (p. 193). They have been erroneously termed “ abdominal ribs - by some authors.
 
As a matter of fact, but little is really known concerning the development of ribs, and our knowledge must be increased before it is possible to satisfactorily determine the homologies of these structures.
 
Sternum. - The sternum is derived from a fusion from before backwards of the ventral extremities of the ribs. The pair of cartilaginous bars thus formed fuse together to form a central plate which is later segmented off from the ribs. In Mammals especially the sternum ossifies from a series of paired centres. It is doubtful how far the so-called sternum of Amphibia is strictly homologous with the sternum of the Amniota.
 
Miss Lindsay has come to the conclusion that the sternum of Birds has undergone an anterior shortening, consequent upon the lengthening of the neck and the shortening of the trunk in the Avian as compared with the Reptilian type, owing to which the sternum has been severed from the ribs that formed it. The “ manubrium - or “ rostrum - of the Avian sternum has nothing in common with the manubrium sterni of Mammals ; it is a secondary outgrowth for the attachment of the sterno-clavicular ligaments. Miss Lindsay gives the following classification of the parts of the sternum. A. Part common to Sauropsida and Mammalia : Costal sternum arising in two bands ; connected with sternal ribs in the adult, but often losing its connection with the ribs which took part in its early formation. B. Part common to Ratitae and Carinatae, but wanting in early embryos of the former, but never of the latter : Metasternum. C. Part apparently common to both Ratitse and Carinatae, but really of different origin : Anterior lateral process ; added to costal sternum in the Ostrich, formed by atrophy of anterior ribs in the Fowl and Gannet. D. Part absent in Ratitae, but common to all Carinatae : lceel ; the median ventral outgrowth of B. The posterior lateral process is common to some Ratitae and to most Carinatae. The accessory processes of metasternum, the rostrum, and the xiphoid ends of posterior processes are variable in Carinatae.
 
Pectoral Girdle. - Two distinct elements occur in the pectoral girdle, the one being the primitive cartilaginous element, the other consisting of superadded dermal bones (clavicles).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
201
 
 
Without entering upon disputed details, it may be asserted in general terms that the primitive girdle consisted of a pair of laterally placed cartilaginous bars, each of which supported a pectoral fin, and which possibly arose by the fusion or extension of the basal elements of the fin itself.
 
In most Vertebrates the girdle is developed from such a pair of plates, which subsequently are segmented into certain pieces. Taking the articulation of the fore-limb as a starting-point, the dorsal portion is known as the scapula, and the ventral as the coracoid element. The latter is usually divisible into an anterior bar or pre-coracoid, and into a posterior coracoid proper. The girdle always becomes connected with the sternum.
 
Balfour found that in Elasmobranclis the girdle developed external to the muscleplate.
 
The clavicles first appear in the Ganoids as large dermal scutes which have become applied to the cartilaginous girdle. In the Sturgeon there are three pairs of these scutes, the dorsal or supraclavicles, which are connected with the otic capsules of the cranium by the intervention of the post-temporal bones ; the lateral elements are the clavicles, while the infraclavicles (interclavicles) meet each other in the median ventral line.
 
In Teleosts the dermal scutes have become subdermal bones ; the interclavicles are replaced by a single median element, and postclavicles may be added. In* these fishes the clavicles have, so to speak, usurped the place of the original girdle, so that while the limb is borne by the scapular and coracoid, the latter are supported by the enormously developed clavicles.
 
According to Gotte, the interclavicles are segmented oft' from the ventral ends of the clavicles in Birds, and, extending between the inner edges of the two halves of the sternum, give rise when the latter unite to the keel (crista sterni). It is most probable that the keel is a new structure, secondarily acquired in response to the need of increased surface of attachment for the pectoral muscles. It may ossify from a single or a pair of centres. The clavicles fuse in the middle line to form the furculum.
 
In a recent paper Howes homologises the two small coracoid ossifications so constantly present in the Eutheria with the coracoid and epicoracoid of Prototheria (Monotremes), the former being the “ coracoid epiphysis - and the latter the “ coracoid - of human anatomists. These two elements are readily seen in the young Rabbit, which is in this respect in an intermediate condition between the Prototherian and the Eutherian type of shoulder girdle. The Mammalian “ clavicles - may now be definitely regarded as ossifications around pre-existing bars of cartilage which are at first continuous with the scapulae. The “ clavicles - thus correspond with the precoracoid of Anura.
 
Gotte has shown that the cartilaginous predecessor of the Mammalian clavicle early unites with its fellow in the median line ; the tract resulting from this coalescence eventually segments into five pieces, viz., paired clavicular bars, two small nodules which represent the “ lateral episterna- of Gegenbaur or the “omosterna- of Parker and a median episternum. The lateral episterna are stated by him to become attached to the clavicle or converted into the sterno-clavicular ligament. The middle piece enters into connection with the omosternum, and either becomes confluent therewith (Mole) or undergoes a retrogressive metamorphosis within its perichondrium (Lepus). Thus if the lateral bars represent, as unquestionably they appear to do, the
 
 
202
 
 
THE STUDY OF EMBKYOLOGY.
 
 
primary predecessors of the clavicles, this median episternum can only represent that of the interclavicle. This being so, all the elements of the Prototherian shoulder girdle are represented in that of the Eutheria.
 
There is so much contradiction in the accounts of the development of the clavicular elements in the Amniota that it is at present difficult to determine their precise homology.
 
It is possible that the “ clavicles - of the Ganoids and Teleosts form a series by themselves, and that the “ clavicle - of Amphibia and Amniota is merely an ossified precoracoid.
 
Pelvic Girdle. - The pelvic girdle arises as a pair of cartilaginous bars much in the same way as the pectoral girdle develops.
 
Dorsal to the articulation for the hind-limb is a single element, the ilium ; but ventrally there are two elements, an anterior pubis and a posterior ischium. The space between them is known as the obturator foramen.
 
Locomotory Appendages. - Throughout the animal kingdom, when distinct organs for locomotion occur, apart from ciliated areas, they always develop as folds of the epiblast supported by an axial layer of mesoblast.
 
The epidermal surface may not be specially modified, but the mesoblast is differentiated into muscles, often numerous and complex in their action, which serve to put the appendage or limb in motion. Nerves are always, and sense-cells usually, present.
 
The appendages of the Craniata are always productions from the body-wall, and, being solid, never contain a diverticulum from the body-cavity; the reverse is the rule in Invertebrates. The skeletal elements of the appendages are axial in the Craniata, and, as a rule, external in Invertebrates.
 
Invertebrates. - The “ arms - of the Starfishes are mere prolongations of the body ; but owing to the reduction of the body-cavity in them, the arms of the Ophiuroidea and Crinoidea have a superficial resemblance to mere appendages. In all, the calcareous axial skeleton is of mesoblastic origin.
 
The parapodia of the Chsetopoda are segmentally paired lateral prolongations of the body-wall, the cavity of which communicates with the body-cavity. The walls are usually greatly thickened, and normally bear setae, and often scales, cirri, and gills. The setae are of epiblastic origin ; often a pair are immersed so deeply within each parapodium that they must serve to give a certain amount of rigidity to the structure, and thus function as a skeletal element. The parapodia may be rudimentary, and even absent (Earthworm).
 
The appendages of the Arthropoda are jointed tubular paired processes from the ventro-lateral aspect of each segment. The mainly chitinous exoskeleton is secreted by the epiblast ; the muscles are entirely internal. The limbs at first develop as hollow buds, their cavity freely communicating with the body-cavity; but in
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
203
 
 
most cases the limb subsequently becomes solid. In several Arachnoids the alimentary canal sends prolongations into the limbs.
 
The larval velum and the adult locomotory organs of the Mollusca call for no special mention.
 
Chordata. - The locomotory appendages of the Chordata fall into two classes, the median and the paired.
 
Unpaired Limbs. - A median unpaired fin is characteristic of all the Ichthyopsida ; in its fullest development it extends along the dorsal side of the body, commencing behind the head, passing round the tail, and, running forward along the ventral aspect of the tail, it terminates just behind the anus. The median ventral fin, however, extends in front of the anus in the adult Amphioxus and in embryo Teleosts.
 
Median Fin. - Usually the median fin is interrupted, above and below, in front of the end of the tail, so that definite regions are established which are known as the dorsal, caudal, and anal fins. The dorsal fin is frequently further subdivided.
 
Its development is very simple, since the fin arises as a lamellar fold of the epiblast, within which the mesoblast is modified to form muscles ; and, later, fine supporting rods or fin-rays are developed, which are quite independent of the axial skeleton, although they may subsequently be closely connected with the neural and hsemal spines. The fin-rays never occur in the unpaired fin of Amphioxus or Amphibia.
 
The median fin is found in all larval Amphibians, and it is more or less developed in those adult Urodeles which retain an aquatic mode of life. The males of the ISTewt have it largely developed during the breeding season.
 
A dorsal fin occurs in many Cetacea. Here it is a fold of the skin which is supported by fibrous and fatty tissue, but without any skeletal elements. It, of course, has no connection with the dorsal fin of Fishes, but has been independently acquired.
 
Caudal Fin. - There can be no doubt that primitively the notochord extended as a straight tapering rod to the extreme posterior end of the animal, and that the caudal fin passed symmetrically round it. Such a protocercal or diphycercal tail is found in Amphioxus, Cyclostomi, Dipnoi, and larval Elasmobranchs, Ganoids, Teleosts, and Amphibia.
 
The next stage in the development of the tail in Teleosts is characterised by the greatly increased size of the ventral lobe, resulting in the dorsal flexion of the notochord. This is the permanent condition in most Elasmobranchs, and is known as a heterocercal tail.
 
The ventral lobe projects still farther, and the dorsal portion which contains the notochord dwindles away, merely forming a kind of dorsal border to the permanent caudal fin, a condition which is characteristic of Ganoids.
 
 
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THE STUDY OF EMBRYOLOGY.
 
 
Finally the tail becomes symmetrical externally ; the fin-rays are supported by one or two greatly developed haemal arches (hypural bones). The now ossified vertebral column apparently ends abruptly, but a rodlike bone, the urostyle, can usually be detected, which extends obliquely into the upper part of the base of the fin. The urostyle is the unsegmented ossified sheath of the upturned posterior extremity of the notochord. This is usually, but not invariably, the condition which obtains in the tails of Teleosts. The tail of the ordinary adult Teleost is, strictly speaking, as heterocereal as that of Elasmobranchs or Ganoids ; but having a superficial symmetry, it is usually termed homocercal.
 
The protocercal nature of the larval tail is retained in Urodele Amphibia, but the notochord is replaced by the segmented vertebral column.
 
Paired Limbs. - Paired limbs are developed in all Craniata higher than the Cyclostomi, except in a few groups in which they have become lost.
 
Dohrn believes that he has found a rudiment of the pelvic fins of the Lamprey in the longitudinal folds bordering the anus and rudiments of muscles in the Ammocoete-stage.
 
In the Elasmobranchs, and to a less extent in Birds, the paired limbs are developed from a larval lateral ridge, which extends from behind the gill-clefts to the anus.
 
The ridge consists of a fold of epiblast with a core of mesoblast. It is rapidly produced into an anterior and posterior process ; the intervening portion (Wolffian ridge) disappears, leaving the fore and hind pair of limbs.
 
In most animals the lateral ridge is not visible, each pair of limbs being apparently independent of the other. It is now generally held that the paired limbs are to be regarded as special developments of a pair of posteriorly converging lateral fins, which had essentially the same structure as the median fin.
 
The axial mesoblast of the limbs differentiates into cartilage, and forms the skeleton of the appendages.
 
Two main types of limb occur in the Craniata : the one found in Eishes is known as the ichthyopterygium; the other, peculiar to Amphibia and Amniota, is termed the cheiropterygium.
 
There is much controversy respecting the nature of the ichthyopterygium, based largely on speculation, but with very little positive embryological evidence ; the subject is therefore quite beside the scope of this book.
 
The relation of the ichthyopterygium to the cheiropterygium is also at present obscure ; the structure of the latter is fundamentally identical in all those animals in which it occurs. The main differences are attributable to modifications in accordance with the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
205
 
 
habits of the animal, to the loss of certain elements, and to the fusion of parts primarily distinct.
 
Skull. - The skull is a composite structure, and in order to gain a clear conception of it as a whole it is necessary to bear in mind the distinctness of the parts involved. The morphology of the skull is one of the most intricate of zoological problems, consequently only the main points can be touched upon here, and these but lightly.
 
The old view of the segmentation of the skull, which regarded it as composed of four modified vertebrae, is now entirely abandoned, thanks to the labours of Huxley, Parker, Gegenbaur, and others. In that view the radical distinction between membrane bone and cartilage, with the bones ossified- from it, was entirely overlooked, and no recourse had been made to embryology.
 
According to the now generally received opinion, without itself being distinctly segmented, the head corresponds to some dozen or so of the anterior segments of the body, excluding an unsegmented portion in front of the mouth, the pre-oral lobe.
 
The skull is essentially composed of an axial brain-box or cranium, and of three pairs of sense-capsules, and various bars which surround the mouth and visceral clefts, and which collectively form what are termed the visceral arches. To the primitive cartilaginous cranium, and the bones which may develop within it, are usually added a large number of dermal bones. Por the sake of simplicity, the cranium, the visceral arches, and the dermal bones will be considered more or less separately.
 
In Amphioxus (Cephalochordata) the notochord extends in front of the neural tube ; in all the Craniata the notochord terminates anteriorly immediately behind the infundibulum (fig. 94) ; its extremity being usually bent downwards, being probably acted upon by the cranial flexure or by the down-growth of the infundibulum.
 
Cranium. - A layer of mesoblast at first surrounds the brain and constitutes what is known as the membranous cranium, the notochord extending along its floor as far as the infundibulum.
 
A continuous tract of cartilage is next developed on each side of the notochord, hence termed parachordal ; and a separate pair of bowed rods appears in front, the trabeculae cranii. The posterior extremities of the trabeculae embrace the apex of the notochord (fig. 155, a). The curved trabeculae enclose a space known as the primitive pituitary space; in front they usually fuse
 
 
206
 
 
THE STUDY OF EMBRYOLOGY.
 
 
together below the nasal capsules. A median rod of cartilage, the prenasal rostrum, is often present between the anterior ends of the trabeculse.
 
The parachordals early fuse with each other, and entirely enclose the notochord, with its skeletogenous sheath, to form the basilar plate.
 
The cartilaginous auditory capsule also unites with the basilar plate, which forms a ventral support for the posterior half of the brain. The notochord gradually atrophies, and, as a rule, entirely disappears.
 
The trabeculse enlarge in size and fuse with the basilar plate ; in the nasal region a considerable amount of cartilage is formed, and the pituitary space is reduced (fig. 155, b).
 
 
A. Early stage, with the trabeculse and parachordals as simple bars and membranous sense capsules. B. Later stage, in which a fusion of the above elements has occurred and the cartilaginous nasal and auditory capsules are incorporated in the cranium. C. Side view of about same stage as B.
 
a.o. antorbital process ; au. auditory capsule ; hr. branchial arches ; c. cornua trabeculse ; hy. hyoid arch ; mn. Meckel -s cartilage, mandibular arch ; na. nasal capsule ; n.ar. neural arch ; nch. notochord ; op. optic capsule ; p. ch. parachordal : pl.pt. palato-pterygoid arch; p.o. post-orbital process ; p.t.s. pituitary space ; q. quadrate ; r. rostrum ; tr. trabeculse.
 
The nasal capsule is supported anteriorly by the outwardly curved extremities of the trabeculse, the cornua trabeculse; and posteriorly by a spur of cartilage, the preorbital process. Thus, like the auditory capsule, the nasal capsule is early engrafted into the cranium. The optic capsules or eyeballs always remain free.
 
The floor of the cranium being thus laid, the walls are raised by vertical upgrowths from the sides of the basal cartilage. Between the auditory capsules the walls usually meet above the brain and form the posterior cranial roof ; and a solid upgrowth of cartilage often occurs anteriorly between the nasal capsules.
 
The primitive cartilaginous cranium (chondro-cranium) thus consists of a ventral plate and lateral walls of cartilage, which enclose the auditory and olfactory capsules, and a posterior roof. The
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
207
 
 
floor is perforated by the pituitary space, through which also the internal carotid artery at first passes. The cranial nerves pass through apertures (foramina) left during the extension of the cartilage.
 
Definite regions can be made out in the cliondro-cranium at this stage, which may now be enumerated.
 
The posterior roofed extremity of the skull, occipital region, articulates with the anterior vertebra (except in Cyclostomes, some Elasmobranchs, Ganoids (except Lepidosteus), and Dipnoids, in which forms the persistent notochord is continued into the skull, or the occipital region is fused with more or fewer of the anterior vertebrae. In front of the occipital is the auditory region, and between them is the aperture (foramen lacerum posterius) for the glosso-pharyngeal (ix.) and vagus (x.) nerves (fig. 156).
 
The sphenoidal region extends from the auditory to the nasal capsule ; an anterior and posterior pair of cartilages usually grow up from the basi-sphenoidal cartilage, which are respectively known as the orbito- and ali-sphenoid plates. A large slit-like orifice (foramen lacerum medius) is left between the auditory capsule and the ali-sphenoid; in it is lodged the Gasserian ganglion, and through it emerges the trigeminal (v.) nerve. Between the ali- and orbitosphenoid is a cleft (foramen lacerum anterius or sphenoidal fissure) through which the optic nerve (11.) and the motor nerves of the eyeball (in., iv., vi.) pass ; the fourth nerve sometimes passes out independently above the optic foramen. The cartilage at the base of the ali-sphenoids (basi-sphenoid) is continuous with that below the orbito-sphenoids (pre-sphenoid).
 
Between the nasal capsules is the ethmoid region.
 
Such a chondro-cranium as that described above is practically the permanent condition of the crania of all Fishes, except the Teleosts and bony Ganoids. In the Ganoids ossification commences in more or fewer of these cartilaginous areas, and, with some variation, the bones which result from these centres of ossification occur all through the Vertebrate series.
 
It not unfrequently happens that cartilage extends beyond its primitive area and encroaches on other regions or surrounds certain nerves or blood-vessels, or two or more ossifying tracts may fuse to form a compound bone. On the other hand, portions of the chondro-cranium may atrophy, or even not be developed at all.
 
Visceral Arches. - Cartilaginous bars are early developed in the lateral walls of the pharynx between the visceral clefts. These
 
 
208
 
 
THE STUDY OF EMBRYOLOGY.
 
 
visceral arches, as they are termed, are primitively very similar, and each consists of a simple bar of cartilage, which later may become segmented, and usually more or less ossified. The greatest number occurring in any animal are found in the Cyclostomi and Notidanus, where there are nine in all : as a rule, there are seven in the Ichthyopsida and fewer in the Amniota.
 
The first is the mandibular arch, the second is the hyoid, and the remainder are known as branchials.
 
 
It was formerly thought that the branchial basket-work of the Cyclostomi belonged t o a different series of cartilages from the visceral arches of Gnathostomatous Crani
 
 
Fig. 156. - The Chondro-Cranium and Visceral Skeleton with the Anterior Part of the Vertebral Column of a Dog-Fish (Scyllium canicula). Seen from the right side; the labial cartilages are omitted. [ Ajttr A. M. Marshall .]
 
A. auditory capsule ; B. post-orbital groove ; c. inter-orbital canal ; D. pre-spiracular (meta-pterygoid) ligament, with the pre-spiracular cartilage ; e. upper jaw (pterygoquadrate arcade) ; f. lower jaw (Meckel -s cartilage) ; G. hyo-mandibular cartilage ; h. cerato-hyal; 1. pharyngo-branchial ; K. epi-branchial ; L. cerato-branchial ; M. extra-branchial ; n. vertebral neural arch ; no. olfactory capsule ; 0. centrum of vertebra ; p. intervertebral neural arch ; R. neural spine ; s. foramen for the ventral root of a spinal nerve ; t. foramen for the dorsal root of the preceding nerve ; u. orbital grooves, lodging the ophthalmic branches of the fifth and seventh nerves ; w. aperture at end of orbital groove through which the above-mentioned branches leave the orbit ; z. ethmo-palatine (palato-trabecular) ligament. 11. optic foramen ; in. foramen for third nerve ; iv. foramen for fourth nerve ; V. foramen for the main branches of the fifth and seventh nerves and for the sixth nerve ; va. foramen for the ophthalmic branch of the fifth nerve ; vna. foramen for the ophthalmic branch of the seventh nerve ; ix. foramen for the ninth or glosso-pharyngeal nerve.
 
 
ates, the former being supposed to be developed on the outer wall of the so-called head-cavities, while the latter arose from their inner wall. The extra-branchial cartilages of Elasmobranchs (fig. 156) were also supposed to be rudiments of the external series. Dohrn has, however, shown that there is no real distinction between these elements, and that the branchial skeleton of Lampreys is as truly internal as that of other Craniates, the main distinction being that in Cyclostomes the visceral arches are unsegmented. Dohrn also finds that the extra-branchials of Elasmobranchs are merely the dorsal and ventral cartilaginous branchial rays of their respective arches, which early shift their position.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
209
 
 
Mandibular Arch. - From the mandibular arch a bud grows forward on each side in front of the mouth, and a separation occurs in the arch at the angle of the mouth, so that an upper and a lower jaw cartilage result, which articulate together. The upper jaw arch is termed the palato-quadrate or pterygo-quadrate arcade; the lower bar forms Meckel -s cartilage. The portion of the primitive arch above the pteryogoid bud is the metapterygoid, and possibly constitutes the primitive means of attachment of the jaw with the cranium.
 
The mandibular arch may posteriorly be supported solely by the proximal element of the hyoid arch (hyomandibular), or partially by the latter and partly by its own proximal portion (metapterygoid ?), or the mandibular arch is directly attached to the cranium without the intervention of the hyoid arch. The first mode of attachment, known as hyostylic, occurs in many Elasmobranchs, and in most Ganoids and Teleosts ; the second or amphistylic is found in the Notidanidse and Cestracion ; the last, autostylic, is peculiar to Holocephali, Dipnoi, Amphibia, and Amniota.
 
The upper jaw arch may anteriorly be quite independent of the cranium, or attached by a ligament ethmo-palatine or palatotrabecular ligament, or by a cartilaginous bar, the palatine. In the Holocephali and Dipnoi the whole of the pterygo-quadrate bar is fused with the base of the cranium.
 
The quadrate, or that region on which the lower jaw articulates, is usually cut off as a distinct element, and serves, in the Sauropsida, as the support (suspensorium) for the mandible. In Mammals it is pressed into the service of the internal ear as the incus (p. 1 5 1).
 
In the cartilaginous Fishes, Meckel -s cartilage, or the primitive cartilage of the lower jaw, is very massive ; but in other forms, although always present in early life, its place is generally usurped by membrane bones.
 
The proximal articulating element is segmented off in Mammals, and now generally regarded as the malleus.
 
Ossifications occur in certain centres of the cartilage, or in the perichondrium, but the details of these ossifications in this and the succeeding visceral arches do not fall within the scope of this book.
 
Hyoid Arch. - The upper portion of the hyoid arch segments off in Fishes as a distinct cartilage, the hyomandibular, to which allusion has just been made. The inferior moiety becomes divided into-several rod-like pieces, which may become ossified.
 
 
0
 
 
210
 
 
THE STUDY OF EMBRYOLOGY.
 
 
From his researches on the development of Fishes, Dohrn finds that the problem of the original number of visceral clefts and arches is not so simple as is generally imagined. He is satisfied that what is usually regarded as the hyoid arch is certainly a double structure. He also regards the spiracular cartilage as being the rudiment of another arch, and he is inclined to believe that both the upper and the lower jaws are cartilages belonging to distinct arches. According to him, the enumeration of the visceral arches of the jaw and hyoid regions would be : I. upper jaw; 2. lower jaw ; 3. spiracular cartilage ; 4. hyomandibular ; 5. hyoid. The clefts between the mandibular and hyoid arches have become difficult to recognise as such ; the median thyroid body may perhaps represent the coalesced rudiments of one pair.
 
Branchial Arches. - The greatest number of branchial arches obtains in Heptanchus, Notidanus ; where there are seven, in most Fish there are five (fig. 156); this number may be considerably reduced in Teleosts. The originally continuous bars become jointed, and may ossify.
 
Basi-hyoid and basi-branchial cartilages are universally present. A cartilage which Huxley believes may represent a basi-mandibular element is present in the Cyclostomi.
 
In the adults of the Caducibranchiate Amphibia and Amniota the post-oral visceral skeleton is greatly reduced, and is represented by the so-called “ hyoid.- In reality this composite structure consists of a flat plate or body, which results from the fusion of the median pieces of the hyoid and first branchial arch. The anterior or greater cornua are the persistent hyoid arches, and the posterior or lesser cornua are the degenerate first branchials. What appears to be a vestige of the second branchial arch has been described by Howes for Phocsena, and several branchial elements enter into the “ hyoid - in most Amphibia.
 
Dermal Bones. - The ehondro-cranium of Elasmobranchs is simply covered by the skin of the head. In Ganoids the brain is further protected by large bony plates, which assume a more or less regular disposition. Certain of these plates persist in Teleosts as the dermal bones of the skull, and similar bones with an analogous distribution are found in higher animals.
 
An irregular median series is sometimes present, but these are crowded out by a paired series, which form the roof-bones of the skull.
 
Membrane bones are developed on the side of the face, along the upper and lower jaw, on the roof of the mouth, and outside the hyoid arch.
 
Those parosteal bones (ex. parasphenoid, vomers), which are developed within, or in some cases at the side of the mouth, appear to be primarily due to the fusion of the basal portion of teeth.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
211
 
 
All the above elements collectively constitute the skull.
 
Body-Cavity. - The mode of formation of the body-cavity is necessarily dependent upon the development of the mesoblast, and has already been incidentally dealt with ; there is, therefore, no need to repeat the former descriptions or inferences.
 
It is necessary to remember that, with the exception of the lower Worms and Molluscs, diverticula or pouches (somites) grow out from the archenteron and become separated from it. The sacs thus formed increase in size and surround the alimentary canal. Their outer wall (somatic mesoblast) becomes applied to the epiblast to form the body-wall (somatopleur) and their inner wall (splanchnic mesoblast) together with the hypoblast constitutes the somatopleur ; their cavity is the coelom or true body-cavity.
 
The mesothelium which forms the walls of the somites may differentiate into various structures, but it nearly always gives rise to a delicate epithelium (peritoneum or serous membrane) on the surface facing the body-cavity. The somatic epithelium is known as the parietal layer, the splanchnic as the visceral layer of the peritoneum.
 
As the visceral or splanchnic walls of each pair of somites approach one another they form a double-layered membrane, the mesentery. In some animals the primitive dorsal and ventral mesentery may persist, but usually the mesentery is largely absorbed leaving strands of tissue (mesenteries) which sling the alimentary canal.
 
In Cyclops, according to Urbanovics, the body-cavity is formed by a fusion of paired excavations of a mesoblastic band ; the disepiments between only disappear very late. The dorsal and ventral mesenteries persist ; the dorsal mesentery contains a space which is a remnant of the blastocoel, and plays an important part in the circulation in the absence of the heart. It is difficult to understand why this should not be regarded as a true heart ; it may be a rudimentary structure, but the development is similar to that of a heart (p. 215).
 
The behaviour of the somites in Amphioxus gives us a key to the original mode of formation of the body-cavity in the ChordatP. The segmented somites are developed from the dorso-lateral angles of the archenteron (fig. 56) ; subsequently they extend ventralwards forming the somatic and splanchnic mesoblast, finally the upper portion loses its central cavity and becomes converted into the lateral muscles of the body, which always retain their original segmentation. The ventral portions of the somites not only fuse with their fellows, but form a continuous body-cavity which extends along the whole length of the body.
 
 
212
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In all Chordata the primitive segmentation of the body is retained solely by the dorsal moieties of the somites - primarily in the muscles, secondarily in the vertebral column, and partially in the excretory organ. The two former are developed from the main portion of the dorsal halves of the somites, which eventually are entirely separated from the ventral halves. Before this is effected they are connected by what is known as the “ intermediate cell mass - (figs. 1 50, 174, 178*). This tissue gives rise to the excretory organ (p. 243).
 
At the origin of the mesentery, the peritoneun is columnar throughout a considerable length of the body-cavity, and constitutes the germinal epithelium (fig. 175, p.o).
 
Mesentery. - As the alimentary canal of Vertebrates is at first a simple straight tube, so the mesentery which slings it forms a simple fold. With the appearance of distinct regions in the alimentary canal, those portions of the mesentery which suspend them receive corresponding names; thus the mesogastrium, the mesocolon, and the mesorectum.
 
In Man the stomach is at first an antero-posterior dilation of the mesenteron, as is permanently the case in most of the lower Vertebrates. The stomach soon turns over towards the right side, so that the mesogastric border is turned to the left, but the stomach still retains its longitudinal direction, as in some adult Mammals. The new left border bulges out to form the greater curvature, and the stomach assumes by degrees a transverse direction, carrying the mesogastrium with it. As a result of this rotation of the stomach, a mesogastric sac is formed which is the commencement of the omentum ; the orifice of the sac is the foramen of Winslow. The omentum increases in size and extends down to the colon.
 
In Fishes the kidneys remain above the dorsal wall of the bodycavity ; in Amphibia and higher forms they project slightly into the coelom, being more or less suspended by folds of the peritoneun.
 
The generative glands are suspended within distinct folds of the peritoneun, which are known as the mesorchium for the testis, and mesoarium for the ovary.
 
It must be borne in mind that the viscera which are described as lying within the body-cavity are all, morphologically speaking, outside it. The body-cavity or coelom is a closed sac lined by a serous membrane. Various viscera may sink into the contained cavity, but they always push before them the serous membrane, which thus forms a fold round them. All structures such! as blood-vessels or nerves pass to and from , the viscus between the laminae of the fold of the serous membrane.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
213
 
 
Pericardium. - The anterior portion of the primitive body-cavity undergoes certain changes. A horizontal septum is formed, connecting the splanchnopleur with the somatopleur of each side on a level with the ductus Cuvieri at the spot where they enter the sinus venosus, and really serving to support these vessels.
 
The transverse septum extends anteriorly and posteriorly ; below lies the heart, and above is the alimentary canal. As the septum stretches from the body-wall to what may be termed the dorsal mesocardium (fig. 159), it naturally divides the anterior region of the body-cavity into a ventral pericardium and a pair of dorso-lateral cavities ; these all communicate anteriorly and posteriorly. By further growth forwards of the septum, the pericardium is cut off from the anterior dorsal horns of the body-cavity. The septum extends posteriorly along the under side of the liver till it reaches the ventral wall of the body, where the liver is attached by its ventral mesentery (falciform ligament) ; but a posterior canal, usually opening into the general body-cavity by two orifices, persists in Elasmobranchs.
 
The pericardium is thus a specialised portion of the body- cavity, and therefore it is lined by its serous membrane, which was primitively continuous with that of the coelom. As its viscus, the heart, depends into the pericardial cavity in the same manner as the mesenteron depends into the body-cavity, so it is covered by the reflected or visceral portion of the pericardium, the outer being known as the parietal portion.
 
In air-breathers the developing lungs project on each side of the throat into the dorso-lateral extensions of the body-cavity above the pericardium. This condition is practically retained in Amphibia and most Reptiles. The common body- cavity is thus often termed in them the pleuro-peritoneal cavity.
 
Diaphragm. - The diaphragm later makes its appearance by a dorsal extension of the posterior wall of the pericardium, which cuts off the pleural -cavities from the abdominal coelom. The diaphragm is at first tendinous ; the muscle grows in later from the dorsal side, probably from the muscle-plates.
 
Uskow enumerates the following grades of development : -
 
1. The ventral and dorsal portions of the diaphragm are fully developed ; they completely divide the coelom, and have muscles. The diaphragm is entirely separated from the pericardium, except two thin lamellae (Rabbit).
 
2. Similar, but a part of the diaphragm remains united with the pericardium (Man).
 
3. Same as 2, but the diaphragm contains no muscles, and its ventral part is completely fused with the pericardium (Fowl).
 
4. Similar to 3, but the dorsal part is not completely developed, remaining in a primitive condition (Lizard) or in an early stage (Frog).
 
5. Like 4, the diaphragm is not separated from the pericardium, persisting at the stage of the septum transversum (Myxinoids and Ammocoete).
 
6. The Teleosts form a distinct type ; although, as in the Salmon, there is a certain separation of the diaphragm from the pericardium, even more than in Birds, yet the dorsal portion is completely wanting.
 
Pleurae. - The serous membrane of the pleural cavities is termed the pleura, and,
 
' as in the case of the pericardium, a parietal layer (costal pleura) and a visceral layer (pulmonary pleura) are present. The mediastinal space is that space which occurs between the closed pulmonary serous sacs, the mediastinum itself being formed by the junction of the parietal pleura of each side.
 
In the adult males of the higher Eutheria the primitive coelom is divided into the following perfectly distinct serous sacs : - The two pleurae and the pericardium, which together form the thoracic cavity, the abdominal cavity, and the paired tunica vaginalis (p. 262).
 
Abdominal Pores. - A pair of apertures, by means of which the abdominal cavity is placed in direct communication with the
 
 
214
 
 
THE STUDY OF EMBRYOLOGY.
 
 
exterior, occurs in Cyclostomi, Elasmobranchii, Ganoidei, a few Teleostei, Dipnoi, Chelonia, and Crocodilia. Occasionally there is only a single pore.
 
These abdominal pores, as they are termed, usually open into the cloaca on each side of the urogenital aperture, but they may occur outside the cloaca, and either in front or behind.
 
 
In Cyclostomes, Scott states they are developed from the hypoblastic section of the cloaca ; in other forms they arise as epiblastic pits, but the pores in Cyclostomes may not be homologous with those of other animals. The abdominal pores of most Teleosts have also been regarded as not homologous with those of other fish (see p. 258).
 
They serve for the egress of the generative products in Cyclostomes and a few Teleosts.
 
Abdominal pores are entirely absent in Amphibia and Birds, and have not been recognised in Mammals. It is, however, possible that the inguinal canals of Mammals, which have a similar relation to the urogenital orifice, may prove to be remnants of the abdominal pores of their hypotherian ancestors.
 
 
The branchial or atrial pore of Amphioxus is often erroneously termed an abdominal pore; its mode of formation (p. 178) proves these two pores have nothing in common.
 
The Vascular System. - The vascular system consists of a closed network of vessels containing a fluid (plasma), within which float free cells (blood corpuscles). The whole is invariably derived from the mesoblast.
 
There are yet numerous gaps in our knowledge of the development of blood-vessels in various animals. Two modes of formation have been described for both Invertebrates and Chordata.
 
Development of Blood-Vessels. - In the vascular area of the blastoderm of Amniotes, the mesoblast cells form a protoplasmic network. Some of the nuclei of these cells rapidly divide and form masses of nuclei. The protoplasm round each nucleus acquires a red colour (haemoglobin), and, on the deliquescence of the central portion of the protoplasmic network become liberated as red-blood corpuscles. The peripheral nuclei form the nuclei of the walls of the vessels.
 
A similar mode of formation of blood-vessels has been described by Lankester in the adult Leech, and it is probably of wide occurrence.
 
The process may be summed up as a liquid vacuolation of certain reticular mesoblastic tissue. Some of the nuclei remain in the walls of the channels, others (red blood-corpuscles) with free mesoblastic elements (white blood-corpuscles) are suspended in
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
215
 
 
the fluid (plasma) thus formed, and, on the assumption of contractility by the walls of the main vessels, they are hurried along in the general circulation.
 
The second mode of vascular development consists in linear masses of mesoblast cells being formed, the outermost of which arrange themselves into a tube containing the central free cells or corpuscles. This occurs in the trunk of Vertebrate embryos, and is usually described for Invertebrates generally.
 
In the lower Invertebrates the vascular system is either not at all or very imperfectly developed. The Chsetopod Worms have a large dorsal (abneural) blood-vessel, which is very contractile and drives the blood from behind forwards ; some of the lateral branches are also contractile. The Mollusca and Arthropoda possess a distinct heart, which in the latter may be considered as a concentration of the elongated dorsal vessel of the higher worms. Although many of the blood-vessels in Amphioxus are contrac Fig. 157. - Diagrams Illustrating the Formation of the a Heart of (A) Inverte- A
 
BRATES AND (B) CHORDATA.
 
 
The neural aspects are placed the £'/ same way in both diagrams to faci- Of litate comparisons. U\
 
al. mesenteron ; cor. coelom or vx body-cavity ; ep. epidermis ; ht. \ cavity of heart ; me. mesocardium ; m.p. muscle-plate; n. central nervous system ; nch. notochord ; so. peritoneum (somatic mesoblast) ; v.m. ventral mesentery (of Invertebrates).
 
tile, no distinct heart is present. In the true Vertebrates a heart is always present, and the blood-vessels retain their contractibility to a greater or less extent.
 
Formation of the Heart. - The origin of the heart in many Invertebrates is still a matter of some uncertainty. From the recent investigations of Biitschli, Schimkewitsch, and others, it would appear that the cavity of the heart, at least in certain of the Annelida and Arthropoda, is a persistent portion of the segmentation-cavity which has been enclosed between the vertical walls of the archenteric diverticula where they join one another to form the dorsal (abneural) mesentery (fig. 157, a).
 
Patten, on the other hand, maintains that although in the Cockroach (Blatta) the heart is formed by the junction of the two folds of mesoblast, the cavity of the heart is not the space included between the two folds, but is in reality an enclosed portion of the true body-cavity. The folds of the mesoblast pulsate long before a special heart is formed, and a circulation occurs through the irregular sinuses of the body-cavity. Blood corpuscles arise before the formation of the heart by the liberation of indifferent cells, and afterwards from the walls of the heart itself.
 
 
216
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In the Spider [Balfour, but not Schimkewitsch] and in some of the higher Crustacea (Asellus [Dohrn], Astacus and Palsemon
 
 
Fig. 158. - Transverse Section through the Head of a Rabbit of Eight Days Fourteen Hours. [ From Kolliker.]
 
A. Magnified 48 diameters. - h.h. paired rudiment of heart ; sr. cavity of mesenteron.
 
B. Part of A magnified 152 diameters. - ahh. muscular wall of heart; dd. hypoblast; dd'. thickening of hypoblast to form the notochord ; dfp. splanchnic mesoblast; h. epiblast ; hp. somatic mesoblast ; ihh. epithelioid layer (endothelium) of heart ; mes. lateral undivided mesoblast ; mp. neural plate ; ph. pericardial section of body-cavity ; rf. neural groove ; rw. neural fold ; sp. intermediate cell-mass ; sw. part of the hypoblast which will form the ventral wall of the pharynx.
 
 
[Bobretzky] ) the heart is said to arise from a solid rod of mesoblast cells, of which the central portion becomes the corpuscles. This may, however, prove to be only a secondary mode of formation.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
217
 
 
The formation of the heart in Vertebrata appears to be essentially identical with that in Invertebrates, the cavity of the heart being that space which is left between the median walls of the lateral halves of the body-cavity as they approach one another below the throat (fig. 157, b).
 
 
Fig. 159. -' Transverse Section through the Cardiac Region of an Embryo Fowl of Thirty-nine Hours. Magnified 61 diameters. [ From Kolliker .]
 
a. aortic arches; dfp'. somatic mesoblast of throat ; Ent. epiblast of wall of throat; g. vessels of the internal border of the area opaca ; h. epiblast ; hh. body-cavity of neck ; hp. dorsal somatic mesoblast ; hzp. muscular wall of heart ; ihh. endothelium of heart ; m. neural canal ; ph. pharynx ; s. septum formed by the junction of the two endothelial tubes; uhg. inferior cardiac mesentery (mesocardium) formed by the meeting of the splanchnopleur below the developing heart ; the corresponding though widely separated folds between the heart and the pharynx may he termed the dorsal, and the former the ventral, mesocardium.
 
Shipley has very recently shown that the heart of the embryo Lamprey develops in the same manner, the endothelium being derived by the splitting of the approaching walls of the splanchnopleur. The blood-corpuscles originate from the free edges of the lateral plates of mesoblast.
 
 
218
 
 
THE STUDY OE EMBRYOLOGY.
 
 
As Balfour has shown, the heart will from the first appear as single or double, according to the relative time of its formation.
 
In Elasmobranchs and Amphibia the throat early becomes constricted off from the yolk, and in these groups the heart appears as a single tube in the ventral (abneural) mesentery (mesocardium) ; but in those forms (ex. Teleosts, Birds, figs. 159, 160, and Mammals, fig. 158) in which the ventral wall of the throat is formed later than the first appearance of the heart, the latter necessarily develops as two tubes.
 
 
Fig. 160. - Transverse Section through the Cardiac Region of an Embryo Fowl of Thirty-nine Hours. Magnified about 95 diameters. [ From Kolliker.]
 
The section passes through the point where the omphalo-mesenteric veins open into the heart, and therefore behind fig. 159.
 
a. aortic arch ; ch. notochord ; dfp. splanchnic mesoblast ; dfp'. somatic mesoblast of throat ; ent. epiblast of wall of throat ; h. epiblast ; h'. thickened portion of epiblast where the auditory sacs will be formed ; hh. body-cavity of neck ; hp. somatic mesoblast; hzp. muscular wall of heart; ihh. endothelium of heart; m. neural canal; ph. pharynx ; uhg. inferior cardiac mesentery.
 
The Fowl occupies a somewhat intermediate position, since the extreme anterior end of its heart arises as an almost single tube ; but it diverges posteriorly, each limb of the A thus formed being one of the veins which bring the blood back from the yolk (vitelline veins) (fig. 161).
 
The internal epithelium (endothelium) of the heart is single or double like the heart itself ; but when the two tubes unite to form the single heart, the endothelial tubes also coalesce ; but just at first there is a median septum left (fig. 159, s), indicating where the two tubes have joined ; this soon breaks through, and a single
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
219
 
 
tube results, the thick walls of which early become very muscular.
 
It follows from what has been stated concerning the mode of development of the Fowl -s heart, that at an early stage an anterior section would show the imperfect coalescence of the lateral endothelial tubes, whereas one taken a little farther behind would only exhibit the approximation of these tubes. Thus in the same embryo several stages in the development of the heart would be illustrated. The anterior section would be the most advanced, and the approaching vitelline veins would represent a much earlier period (see figs. 159-160).
 
The primitively straight tubular heart of the Chordata undergoes
 
 
Fig. 161. - Ventral View of Embryo Fowl at the End of the Second Day. 4.27 mm. long; removed from the blastoderm. [From Kolliker.]
 
Ab. optic vesicle ; Ch. notochord ; H. heart ; om. omphalo-mesenteric or vitelline vein; Vd. indicates the backward extension of the headfold : in front of this point the pharynx is interiorly completed ; while behind, the alimentary tract is still open below to the yolk.
 
 
a sigmoid flexure, at first slight (fig. 161, h), but eventually the S-like flexure is complete (fig. 162, 3). The dorsal limb constitutes the auricular portion (atrium), and the ventral forms the ventricular part. The dorsal and ventral portions are separated by a constriction.
 
There is present in Fishes in connection with the atrium a posterior thin-walled sac, the sinus venosus, into which the collecting veins (ductus Cuvieri) open, and into the single auricle. A pair of valves guard the orifice leading to the ventricle.
 
The posterior region of the ventricular moiety becomes the ventricle of the adult, while the anterior portion is divided into a
 
 
220
 
 
THE STUDY OF EMBRYOLOGY.
 
 
posterior conus arteriosus and an anterior bulbus arteriosus. The conus is long, and provided with several transverse rows of valves, except in most Teleosts, in which group it is rudimentary or absent. The non- valvular bulbus leads to the branchial arteries.
 
Among the Dipnoi the blood-vessel bringing blood from the air-bladder (lungs) to the heart opens into a small second (left) auricle. The conus and ventricle may also be partially divided
 
 
in two by an imperfect longitudinal septum.
 
 
Fig. 162. - Development of the Mammalian Heart. [From Landois and Stirling.']
 
 
1. Heart with slight curvature. 2. Sigmoid flexure of the heart. 3. Formation of the auricular appendages, and external furrow in the ventricle. 4. Commencing division of the truncus arteriosus. 5. Dorsal view ; the auricle has been opened to show the ventricular septum ; the aorta (a) and pulmonary artery open into their respective ventricles. 6. Diagrammatic view from above of the mode in which venae cavae open into the auricle. 7. Ventral view of heart of full-time foetus.
 
A. auricular portion of heart; a. aorta; B. ductus arteriosus Botalli; 5 . bulbus arteriosus; c. carotid; c,c. innominate; Ci. inferior vena cava; Cs. superior vena cava; L. left ventricle ; o. 0.1. auricular appendages; p. pulmonary artery; R. right ventricle; s. left subclavian artery; V. ventricular portion of heart; v. auricle and veins entering the heart ; x. arrow showing the flow of blood from the superior vena cava through the valve into the right auricle, and y that of the inferior vena cava through the valve into the left auricle ; 1 and 2. right and left pulmonary arteries.
 
The single auricle of the primitively piscine heart of the Amphibia is early provided with a pair of lateral appendages, and an. oblique septum is developed which divides the single auricle (atrium) into a right and left chamber. The ventricle remains single ; the conus arteriosus (pylangium) has a longitudinal valve and a row of valves at each end ; there is also a bulbus arteriosus (synangium), which, however, is rudimentary in the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
221
 
 
Anura. The conus and bulbus are usually collectively known as the truncus arteriosus. A sinus venosus is also present.
 
The flexure of the developing heart is very marked in the Amniota. The auricular portion develops lateral appendages ; the large venous trunk which opens into this region (fig. 162, 3, v) is composed of the superior and inferior venae cavae. This common trunk is later absorbed into the enlarging auricle, and thus arises the separate termination of these vessels (fig. 162, 4, v). The constriction between the auricular and ventricular divisions of the heart is known as the canalis auricularis.
 
The heart begins to divide into a right and left half on the third day in the Fowl and about the fourth week in Man, the division first occurring in the ventricle. The ventricular septum arises from the ventral wall and rapidly extends to the dorsal, dividing
 
 
Fig. 163. - Lateral View op
 
Heart of Human Embryo, y.f 3 7 the Right Side being cut AWAY. [After His.]
 
a. aortic channel ; c.v. coronary vein ; d. diaphragm ; l. liver ; p. pulmonary channel ; s.a. septum aorticum in the bulbus aorta; s.inf. septum inferius ; s.int. septum intermedium; s.r. sinus reuniens; s.s. septum superius ; v.c.i. vena cava inferior; v.c.s.l. vena cava superior (left) ; v.c.s.r. right superior cava ; v.e. Eustachian valve.
 
 
the ventricle into two somewhat curved chambers, one more to the left and above, the other to the right and below. Thus the large undivided auricle communicates by a right and left auriculoventricular opening with the corresponding ventricle (fig. 162, 5).
 
A fold appears on the ventral wall of the auricle, dividing the cavity into a right and left chamber. The fold extends only a short distance, thus forming an incomplete septum (the auricular septum). The right and left auricles communicate throughout embryonic life by means of the aperture thus left, the foramen ovale (fig. 163).
 
The vena cava inferior opens into the right auricle directly opposite the auricular septum, and its blood has a tendency to flow through the foramen ovale into the left auricle. The right vena cava superior joins the vena cava inferior, and its blood also . passes ,into the left auricle. The left vena cava superior opens
 
 
222
 
 
THE STUDY OF EMBRYOLOGY.
 
 
independently into the right auricle, and its blood flows into the right ventricle.
 
A valve, the Eustachian valve, next develops from the dorsal wall of the right auricle to the right of the entrance of the vena cava inferior into the auricle, and between it and the right and left superior venae cavse. This serves to still further direct the blood from the vena cava inferior into the left auricle, and at the same time to retain the blood of the superior venae cavae within the right side of the heart. In many of the higher Mammals, including Man, the right vena cava superior disappears during foetal life.
 
A second fold arises from the dorsal wall in the median line of the auricles ; this projects freely across, and to the left side of, the foramen ovale, thus forming a valve which prevents the blood from flowing back from the left to the right auricle.
 
The left auricle is at first larger than the right. Later the cavities approximate in size, and the foramen ovale is much smaller.
 
Lastly, the truncus arteriosus is longitudinally divided in Birds by a septum, which arises between the fourth and fifth pair of arches and extends in a somewhat spiral manner to close to the ventricular orifice. In Mammals the truncus (fig. 162, 4, a.p) appears to be constricted dorsally and ventrally to form the aorta and pulmonary artery.
 
Semilunar valves are developed in the short interspace between the orifice and the free end of the septum of the truncus. The dorsal and ventral valves first appear, the former as a continuous ridge, the latter as a pair of small processes. The septum of the truncus extends between the latter, and, entirely dividing the ' ventricular orifice, fuses with the ventricular septum.
 
By the division of the truncus in Birds, the fifth pair of arches communicates with the right ventricle, while the third and fourth pairs communicate with the left ventricle; of these, the former becomes the pulmonary arteries, and the two latter the carotid and aortic arches respectively (p. 227). In Mammals, also, the rio-ht ventricle is continuous with the last aortic arch, the four anterior arches, or what remains of them, being connected with the left ventricle.
 
In all Reptiles, except the Crocodiles, the primitively single ventricle is retained. The ventricular septum was independently acquired by Crocodiles, Birds, and Mampaals ; thus in these three groups it is what is termed homoplastic, but not
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
223
 
 
homogenetic. An interesting example of the “falsification of the embryological record- is afforded, as Bell points out, by the development (ontogeny) of the ventricles, as in those forms in which they become distinct the ventricular septum develops prior to the auricular septum, whereas in the true phylogeny the reverse occurred. This is a case of what Haeckel calls cenogeny, and is no doubt dependent on the requirements of the organism.
 
The complicated series of changes undergone in the evolution of the Vertebrate heart is apparently mainly the result of the modifications which have occurred in the respiratory organs. Without going into details, the following facts are worthy of note :
 
- The respiratory tract of Amphioxus is extremely long, the “ heart - is undifferentiated, and the median ventral vessel (subintestinal vein) is contractile ; in Fishes the
pharynx is much shorter, and is increasingly reduced in the more specialised forms ; the flexure of the heart may be related to the shortening of the neck ; the assumption of aerial respiration by the air-bladder, and a change in the origin of its afferent, and in the destination of its efferent blood-vessels ; the necessity for the brain and sense-organs being supplied with well-oxydised blood.
 
Development of the Vascular System in Vertebrates. -
 
In the following brief account of the evolution of the vascular
 
 
Fig. 164. - Diagrammatic Outlines of the Early Arterial System of a Mammal Vertebrate Embryo. [After Allen Thomson.]
 
A. At a period corresponding to the 36th or 38th hour of incubation. B. Later stage, with two pairs of aortic arches.
 
h. bulbus arterious of heart ; v. vitelline arteries; 1-5. the aortic arches; the dotted lines indicate the position of the future arches.
 
 
system in Vertebrates, the plan is adopted of first describing the development of the circulatory system in Amniota, and afterwards that of the Ichthyopsida. A considerable number of minor points are omitted in order to avoid unduly lengthening the section and complicating the subject.
 
Very early in the development of the embryo the inner portion of the area opaca (p. 38) becomes so permeated with a network of blood-vessels as to receive the name of the area vasculosa. This net- work soon becomes connected with the embryonic vascular system, but before this is accomplished the heart has already commenced to beat.
 
The embryonic circulation of Amniotes may be conveniently divided into four sections - (1.) The early stages of the embryonic circulation. (2.) The vitelline circulation. (3.) Later stages of
 
 
224
 
 
THE STUDY OF EMBRYOLOGY.
 
 
foetal circulation. (4.) The allantoic circulation. It is impossible to describe the first without considering the others ; but it is important to bear in mind the essentially secondary character of the second and fourth systems.
 
1. Early Stages of Embryonic Circulation. - It is customary to speak of all those vessels which carry blood away from the heart as arteries, and those which return the blood as veins. The arterial system will be. described before the venous.
 
In its earliest stage the arterial system consists of a parallel pair of arteries, each of which arises from the single bulbus
 
 
Fig. 165. - Diagram of the Embryonic Vascular System.
 
[From Wiedersheim.\
 
A. atrium: A. A. dorsal aorta; Ab. branchial vessels ; Acd. caudal artery ; All. allantoic (hypogastric) arteries ; Am. vitelline arteries ; B. bulbus arteriosus ; c, c.' external and internal carotids ; D. ductus Cuvieri (precaval veins) ; E. external iliac arteries ; HC. posterior cardinal vein ; Ic. common iliac arteries ; KL. gill clefts ; RA. right and left roots of the aorta; S, S'. branchial collecting trunks or veins; Sb. subclavian artery ; Sb'. subclavian vein ; Si. sinus venosus; V. ventricle; VC. anterior cardinal vein ; Vra. viteliine veins.
 
 
arteriosus of the heart, and bends round at the anterior end of the pharynx to its dorsal side. Still remaining distinct, each aorta, as it is termed, runs backwards along either side of the notochord below the mesoblastic somites (fig. 176). About half-way down an artery is given off at right angles by each aorta, which, as it passes to the yolk-sac (area vasculosa), is called the vitelline artery (comp. fig. 166, R.Of.A, L.Of.A).
 
Somewhat later the two aortse unite together to form a short dorsal aorta, which lies beneath the notochord. The two aortae soon
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
225
 
 
separate and dwindle away in the tail. The vitelline arteries arise from each trunk behind the median fusion, and are so large that nearly all the blood passes through them. The arteries which arise from the heart running forwards, upwards, and backwards are known as arches. Thus the dorsal aorta is produced by the junction of a pair of aortic arches. Very shortly afterwards a second and a third pair (figs. 164-167) are developed behind the primitive pair.
 
The embryonic venous system at this stage consists of an anterior
 
 
AA
 
 
Fig. 166. -Diagram of the Circulation of the Yolk-Sac of the Fowl at the End of the Third Day of Incubation. [ From Foster and Balfour . ]
 
The veins are marked in outline, and the arteries are black. The whole blastoderm has been removed from the egg, and is supposed to be viewed from below, hence the apparent reversal of the sides.
 
A A. the second, third, and fourth aortic arches; the first has become obliterated in its median portion, but is continued at its proximal end as the external carotid, and at its distal end as the internal carotid ; AO. dorsal aorta ; D.C. ductus Cuvieri ;
 
H. heart; L.of.A. left vitelline artery; L.of. left vitelline vein; R.Of. right vitelline vein; R.of.A. right vitelline artery; S.Ca.V. superior (anterior) cardinal vein; S.T. sinus terminalis ; S. V. sinus venosus.
 
and posterior pair of longitudinal veins (cardinal veins), which run superficial to the aorta. The anterior (superior) cardinal or jugular veins unite with the inferior or posterior cardinals to form a common trunk, ductus Cuvieri (figs. 166, 169), which returns the blood to the heart. Posteriorly the blood is collected from the yolk-sac by the vitelline veins (fig. 166, L.Of R.Of), and transmitted to the heart by the median sinus venosus.
 
P
 
 
226
 
 
THE STUDY OF EMBKYOLOGY.
 
 
2. Vitelline Circulation. - The vascular supply of the yolk-sac may be conveniently described here. The area vasculosa extends to some distance round the embryo, but it is at first undeveloped in the median line in front of the embryo ; it is thus somewhat U-shaped. When the vitelline circulation is first established, the blood enters through the two large vitelline arteries previously mentioned. These arteries divide and subdivide until they terminate in capillary vessels.
 
The lateral periphery of the area vasculosa is bounded by a blood-vessel, the sinus terminalis, which also extends round the anterior horns of the area and down their inner side. The blood from the capillaries flows in three directions : ( i ) most of it is collected by the large vitelline veins and conveyed straight to the heart ; (2) part flows forward along the anterior portion of the sinus terminalis, round the anterior prolongation, and back along the inner margin of the notch, where it enters the root of the vitelline vein ; and (3) lastly, a small quantity proceeds along the posterior half of the sinus terminalis, and is lost in small capillaries, but it ultimately returns by the vitelline veins.
 
When the vitelline circulation is fully developed (fig. 166), the flow of the blood differs slightly from the condition just described. The sinus terminalis forms a
 
 
Fig. 167. - Diagrams of the Aortic Arches of a Mammal.
 
[From Landois and Stirling after Rathke.]
 
1. Arterial trunk with one pair of arches, and an indication where the second and third pair will develop. 2. Ideal stage of five complete arches ; the four clefts are shown on the left side. 3. The two anterior pairs of arches have disappeared.
 
4. Transition to the final stage.
 
A. aortic arch; ad. dorsal aorta; ax. subclavian or axillary artery ; Ce. external carotid ; Ci. internal carotid ; dB. ductus arteriosus Botalli ; P. pulmonary artery ;
 
5. subclavian artery ; ta. truncus arteriosus ; v. vertebral artery.
 
 
complete ring round the area. The distribution of the vitelline arteries and veins is mainly the same, but there is a slight alteration in the second and third channels for the return of the blood. Of the anterior recurrent veins the left is always the larger, and sometimes the right is aborted, so that the blood from the anterior region of the area vasculosa is returned solely by the left anterior recurrent vein. Of course in this case a fusion of the anterior limbs of the area vasculosa has occurred in front of the embryo (fig. 76). On the junction of the lateral halves of the sinus terminalis behind the embryo, the blood is returned by a single median posterior recurrent vein into the left vitelline vein (fig. 1 66, L.of).
 
 
3. Later Stages of Foetal Circulation. - Five pairs of aortic arches make their appearance (figs. 146, 165, 167, 168), hut usually the first two have atrophied before the last is formed. The arteries lie towards the inner side of each visceral arch ; there is one for the mandibular, hyoid, and each of the three branchial arches.
 
The common ventral trunk (ventral aorta) is continued beyond
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
227
 
 
the mandibular arch, as the external carotid and the internal carotid is a similar anterior extension of each dorsal aortic trunk (figs. 167, 168, 170). After the disappearance of the first two aortic arches, the aortic trunk connecting the dorsal end of the third arch with the fourth disappears, except in Lizards, but a rudiment, the ductus Botalli, can be traced in some Beptiles. In this manner the internal and external carotids arise from the ventral aorta of the third arch (common carotid), as shown on figs. 167, 168.
 
The fourth arch always gives rise, as in Amphibia, to the dorsal aorta. This pair of arches persists in Beptiles ; but on the longitudinal division of the truncus arteriosus, the channel leading from the left side of the ventricle is continuous with the right
 
 
Fig. 168. - Diagram Illustrating the Transformations of the Aortic Arches in a Lizard, A ; a Snake, B ; a Bird, C ; a Mammal, D. Seen from below. [ After Itathke .]
 
a. internal carotid; b. external carotid; c. common carotid. A. -d. ductus Botalli between the third and fourth arches; e. right aortic arch; /. subclavian ; g. dorsal aorta ; h. left aortic arch; i. pulmonary artery; k. rudiment of the ductus Botalli between the pulmonary artery and the aortic arches. B. - d. right aortic arch ; e. vertebral artery ; /. left aortic arch ; h. pulmonary artery ; i. ductus Botalli of the latter. C. - d. origin of aorta ; e. fourth arch of the right side (root of dorsal aorta) ; /. right subclavian ; g. dorsal aorta ; h. left subclavian (fourth arch of the left side) ; i. pulmonary artery ; k. and l. right and left ductus Botalli of the pulmonary arteries. D. - d. origin of aorta ; e. fourth arch of the left side (root of dorsal aorta) ; /. dorsal aorta ; g. left vertebral artery; h. left subclavian ; i. right subclavian (fourth arch of the right side) ; k. right vertebral artery ; l. continuation of the right subclavian; m. pulmonary artery ; n. ductus Botalli of the latter (usually termed ductus arteriosus).
 
 
 
fourth arch (fig. 168, A, e; B, d ), and from it also arise the carotids (c). The left fourth arch (a, h; B ,/), is connected with the right side of the ventricle, but it unites with its fellow to form the dorsal aorta (g).
 
In Birds the right (fourth) aortic arch alone retains its connection with the aorta, the left arch persists as the left subclavian artery (fig. 168, C, h). The reverse occurs in Mammals (fig. 1 68, d ). In both there is a single aortic arch, which springs from the left side of the ventricle.
 
The fifth arch is known as the pulmonary, as it invariably supplies the lungs ; it arises from the right side of the ventricle. In all the Sauropsida the right and left arches persist as the right
 
 
228
 
 
THE STUDY OF EMBRYOLOGY.
 
 
and left pulmonary arteries respectively, except in Snakes, in which the left alone persists (fig. 168, B, h). In Mammals the left arch disappears, and the right goes to the lungs (fig. 168, D, m).
 
In some forms traces of the communication between the fourth and fifth arches may remain as the ductus Botalli. A comparison of' figs. 164-168 will render the development of the adult from the embryonic condition perfectly comprehensible.
 
The development of the main venous trunks, as it occurs in Birds, will be briefly described as a standard of comparison with other Amniota.
 
As the embryo increases in size new veins appear, and an anterior (superior) vertebral vein, bringing back blood from the head and neck, and a subclavian from the wing (fig. 169, A, s.c) open into the anterior cardinal or jugular vein. The two ductus Cuvieri persist as the superior venae cavae. In the lower Mammals there are two superior venae cavae, as in Sauropsida, but more usually an anastomosis (left brachio-cephalic or innominate vein) is developed between the right and left jugular veins (fig. 172), and eventually the whole of the blood of the left superior vena cava is conveyed to the right side. The base of the left superior vena cava remains as the coronary sinus.
 
The posterior or inferior cardinal veins which pass along the outer border of the kidneys unite behind with the caudal veins, and anteriorly they open into the ductus Cuvieri. The intercostal veins begin to be connected with a new longitudinal trunk (posterior vertebral vein), which is continuous with the anterior vertebral, and gradually lose their connection with the posterior cardinals. Owing to their diminished function, the anterior portions of the posterior cardinals disappear ; their posterior moieties become the venae renales advehentes. As a result of this change, the blood from each side of the wall of the body of the embryo, instead of entering the heart through the posterior cardinal, is collected by the posterior vertebral, and, together with the anterior vertebral, passes into the jugular and the ductus Cuvieri (superior vena cava or precaval) (fig. 169, B, c).
 
The two posterior vertebrals are at first symmetrical, but in Beptiles, when transverse anastomoses develop between them, the right becomes the larger. In Mammals (fig. 171) the left posterior vertebral usually becomes rudimentary, and is known as the hemiazygos vein ; it is connected by a transverse anastomosis with the right posterior vertebral or azygos vein.
 
 
ORGANS DERIVED FROM THE MESOBLAST. 22D
 
While these changes have been going on, a new and important vein, the vena cava inferior, has made its appearance. At first it is a small vein arising in two roots from the inner border of the kidneys, and unites with the allantoic vein (to be described shortly) before it enters the heart. The atrophy of the anterior portion of the posterior cardinals is doubtless due to the newly developed vena cava inferior carrying the venous blood of the kidney direct to the heart. On its way to the heart the vena cava inferior passes through the liver, from which it receives a few vessels, venae reheventes (fig. 169, b).
 
Renal Portal System. - In Reptiles the blood from the caudal veins and the posterior portion of the posterior cardinal veins (venae renales advehentes) is broken up into capillaries in the
 
 
Fig. 169. - Diagram of Three Stages in the Development of the Venous Circulation of the Fowl. [ After Balfour .]
 
A. At the commencement of the fifth day. B. During the later days of incubation.
 
C. At the commencement of respiration by means of the lungs.
 
all. allantoic (anterior abdominal) vein; a.v. anterior (superior) vertebral vein; cr. crural veins; c.v. caudal vein; cy.m. coccygeo-mesenteric vein ; d.C. ductus Cuvieri; d.v , ductus venosus; h. heart; hy. hypogastric veins; h.v. hepatic vein; i.v. inferior vertebral vein ; j. jugular vein (superior or anterior cardinal) ; k. kidney ; l. liver ; m. mesenteric vein; p.c. posterior (inferior) cardinal vein; pul. pulmonary vein; p.v. portal vein; s.c. subclavian vein; v. vitelline vein ; v.c.i. vena cava inferior ; v.x.r. right superior vena cava. The ductus venosus passes through the liver in A and B.
 
kidneys, and passes thence to the heart by the vena cava inferior. This is known as the renal portal system.
 
In Birds and Mammals this does not occur ; the blood from the tail and hind-limbs passes directly into the vena cava inferior, and not indirectly through the kidneys. This comes about in Mammals by the development of the common iliac veins, which collect the blood from the hind-quarters; the posterior portion of the cardinal veins enter the common iliac as the hypogastric (fig. 169, 0, hy).
 
Hepatic Portal System. - As has already been described, the blood from the yolk-sac is conveyed by the vitelline veins direct to the heart. A small vein early appears in connection with
 
 
230
 
 
THE STUDY OF EMBRYOLOGY.
 
 
developing mesenteron. This mesenteric vein (fig. 169, B and c, m) joins the vitelline vein ; their common trunk (ductus venosus or omphalo-mesenteric trunk) becomes enveloped within the rapidly growing liver, and sends off branches into that viscus. As these branches increase in size they convey more and more blood, and the ductus venosus, which originally passed directly into the heart, is proportionately diminished, until eventually all the blood from the yolk-sac and mesentery passes into the hepatic branches, venae advehentes, and is collected by the vense reheventes and transmitted to the vena cava inferior. There is nothing remarkable in the association of the vitelline and mesenteric veins, as it has been already shown that the yolk-sac is practically merely the hypertrophied ventral wall of the mesenteron, consequent upon the occurrence of food-yolk. It may be stated in another way by saying that the vessels from the digestive tract break up in the liver into capillaries before entering the heart.
 
In Birds and Mammals the right vitelline vein soon disappears.
 
4. Allantoic Circulation. - There is in Amphibia a vein, anterior abdominal, which receives blood from the hind-limbs and from the urocyst (bladder), and passing along the median ventral wall of the abdomen it enters the liver.
 
There are in Beptiles, as in Anura (p. 234), at first two anterior abdominal veins developed. These run along the anterior abdominal wall and enter the ductus Cuvieri; posteriorly they are connected with the system of the posterior cardinal by the epigastric veins, and also with the bladder. On account of the precocious development of the bladder to form the allantois, these veins are known as allantoic veins. The left disappears, so that a single allantoic vein enters the heart after having been joined by the inferior vena cava. Later the two unite nearer the liver, and finally the anterior abdominal (allantoic) vein joins the portal system.
 
In Birds the two anterior abdominal veins unite and fall into the ductus venosus (fig. 169, b) ; the single stem comes to be very long, owing to the rapid growth of the allantois, and it forms the allantoic vein. The right anterior abdominal disappears ; the left bifurcates on reaching the allantois (fig. 169, b, all).
 
The vitelline veins are at first very large (fig. 166), and the allantoic vein quite small, but their relative size is reversed as the allantois increases and the yolk-sac diminishes in importance. The mesenteric vein joins these two, and thus the large portal
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
231
 
 
vein is formed. Although the allantoic vein disappears before hatching, the caudal and posterior pelvic veins are connected with the portal vein in the adult by the coccvgeo-mesenteric vein (fig. 169, C, cy, m).
 
In Mammals the two primitive anterior abdominal (allantoic) veins are very early developed, and unite in front with the vitelline vein. The right allantoic vein (fig. 171, b, u), like the right vitelline vein (o'), soon disappears. The long common trunk of
 
 
Ci S
 
 
Fig. 170. - Diagram of the Arrangement of the Principal Vessels in a Human Foetus.
 
[From Claus after Fcker.]
 
Ao. aortic trunk; Am. amnion; Aod. dorsal aorta; Az. azygos vein ; C. anterior cardinal vein ; Cc. common carotid; Ce. external carotid; Ci. internal carotid ; X>. ductus venosus Arantii ; DC. ductus Cuvieri ; Dv. vitelline duct ; H. ventricle ; L. liver ; N. umbilical vesicle (yolk-sac) ; O. vitelline (omphalomesenteric) ar tery ; O -, vitelline vein ; P. lung ; S. subclavian artery; U. allantoic (umbilical) arteries with their placental ramifications, U"; TJ -. allantoic vein; V. auricle; V.c. vena cava inferior; Vp. portal vein ; i, 2, 3, 4, 5. the arterial arches - the persistent left aortic arch is not visible.
 
 
O'
 
the (left) allantoic and vitelline veins (ductus venosus) passes through the liver.
 
In its passage through the liver, according to Kolliker, the ductus venosus gives off branches near its entrance, and receives branches from the anterior end of the liver (fig. 171, b). The main duct, unlike what occurs in the Sauropsida, persists throughout life as the ductus venosus Arantii (fig. .171, D , 1 ).
 
When the placenta is developed, the allantoic circulation becomes extremely important. The vitelline vein, on the other hand, is greatly reduced, and, with the larger mesenteric vein, it constitutes the portal vein. Later the portal vein (fig. 17 1, D, p) enters one of the venae advehentes of the allantoic vein (p').
 
The vena cava inferior and the ductus venosus at first unite together and enter the heart by a common trunk (fig. 17 1, A, ci, l) Owing to the increased size of the former, the venae reheventes or hepatic veins open into it, and not into the ductus venosus. The
 
 
232
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ductus venosus itself (ductus venosus Arantii) comes to be a small branch of the vena cava (fig. 171, d).
 
The allantoic vein degenerates at the end of foetal life into the solid cord known as the round ligament, and all the venous supply of the liver comes from the portal vein.
 
 
Beddard finds that there is in the adult Echidna an anterior abdominal (allantoic) vein, which arises from the under surface of the bladder, and passing along the ventral wall of the body, is distributed to the left half of the liver.
 
 
An anastomosis between the iliac and portal veins is not established in Mammals.
 
The allantoic arteries arise from the dorsal aorta as branches of
 
 
 
 
Fig. 171 - Diagrams Illustrating the Development oe the Great Veins in Mammals. [From Quain after Kiilliker.]
 
A. Plan of the principal veins of the human embiro of about four weeks, or soon after the first formation of the vessels of the liver and the vena cava inferior.
 
B. Hepatic circulation at a somewhat earlier stage.
 
C. Principal veins of the foetus at the time of the first establishment of the placental circulation. D. Hepatic circulation at the same period.
 
az. azygos vein, above p (in C) - the oblique line is the vein by which the hemiazygos joins the azygos vein ; ca. posterior cardinal veins; ca'. (inC) the remains of the left cardinal vein by which the superior intercostal veins fall into the left innominate vein ; cr. external iliac or crural veins ; ci. vena cava inferior ; dc. ductus Cuvieri ; h. hypogastric or internal iliac veins, in the line of continuation of the primitive cardinal veins; il. the division of the vena cava inferior into the common iliac veins ; j. jugular or anterior cardinal veins ; l. ductus venosus ; V. hepatic veins ; li. (in C) in dotted lines, the transverse branch of communication between the jugular vein which forms the left innominate vein; m. mesenteric veins ; o. vitelline or omphalomesenteric vein ; </. right vitelline vein ; p. portal vein ; p'p'. venae advehentes ; ri. right innominate vein ; s. subclavian vein ; u. allantoic, umbilical or (left) anterior abdominal vein ; u\ (in B) the temporary right allantoic vein.
 
 
the common iliac arteries (figs. 165, Ic , 17 1, U). On the disappearance of the allantois they remain as the hypogastric arteries.
 
Circulation in Ichthyopsida. - Having now described the development of the circulation in the Amniota, it will be necessary to briefly refer to the circulation in Ichthyopsida.
 
Fig. 165, which represents the embryonic circulation of an Amniote in a diagrammatic manner, will, with a few alterations, serve to illustrate the circulation in Fishes. The vitelline arteries {Am) and the allantoic arteries {All) are not present, and the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
233
 
 
blood from behind is returned to the heart by the subintestinal vein, and not by the vitelline veins (Vm).
 
The first or mandibular arterial arch is represented by a small vessel which arises from the branchial vein of the hyoid arch, and supplies the rudimentary gill (pseudobranch) of the spiracle. In other Fish this artery of the first arch disappears.
 
The second or hyoid arterial arch is functional throughout life in Elasmobranchs ; usually it remains as a small vessel which goes to the pseudobranch of the hyoid. The artery is said to persist in Protopterus amongst the Dipnoi.
 
The air-bladder is supplied with arterial blood from the caeliac artery or direct from the aorta, except in some Ganoids (Polypterus
 
I.
 
 
Fig. 172. - Venous Circulation in Mammalian Embryo. [From Landois and Stirling .]
 
I. Early arrangement of veins. II. Final disposition.
 
Ad. right innominate vein; ,4s. left innominate vein; Az. azygos vein ; b. subclavian veins ; Ci. vena cava inferior ; ci. posterior vertebral veins; Cs. vena cava inferior; cs. anterior cardinal vein ; DC. ductus Cuvieri (superior venae cavge) ; /. external iliac vein; h. hypogastric vein ; Hz. hemiazygos vein ; Ie. external jugular vein ; Ji. internal jugular vein; om. vitelline or omphalo-mesenteric vein ; U. umbilical or allantoic vein ; V. ventricle ; Vc. vena cava inferior.
 
 
and Amia) and Dipnoids, where the last branchial arch sends an artery direct to the air-bladder.
 
In Amphibia the first aortic arch (mandibular) is never developed, and the second (hyoid) arises later than the succeeding arches ; it never unites dorsally with the latter, and only gives rise in part to the lingual artery.
 
Of the four branchial aortic arches present in larval Amphibia, only the second, in the Anura, retains its connection with the dorsal aorta. The first becomes the carotid arch, and gives rise to the carotids ; the second forms the systemic arch ; the third is rudimentary or absent (Anura) in the adult, while the fourth or pulmonary supplies the lungs. A narrow anastomosis or ductus
 
 
234
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Botalli may unite the second, third, and fourth arches in adult Urodeles.
 
The venous system of Fishes primitively consists of a median unpaired subintestinal vessel extending from the end of the tail to the heart ; indeed, the heart may be considered as a specialised portion of this vessel. Later, cardinal veins are developed, as in Embryonic Amniotes, but in Fishes they persist as the main venous trunks. The caudal portion of the subintestinal vessel acquires a secondary connection with the posterior cardinal veins. In some cases this, its anastomosis, breaks up into capillaries in the mesonephros, thus forming a renal portal system.
 
After the appearance of the cardinal veins the main portion of the subintestinal vein disappears, but a remnant of one of its branches occurs in some Elasmobranchs as the vein of the spiral valve, and it also leaves its trace in the hepatic portal system.
 
A branch from the subintestinal goes to the yolk-sac, and the common trunk is imbedded in the developing liver. Later, vessels from the alimentary viscera are developed, which break up in the liver. The hepatic veins convey blood from the liver to the sinus venosus of the heart.
 
In some Fishes vessels from the anterior abdominal wall enter into the portal circulation. These may be regarded as the forerunners of the paired anterior abdominal veins.
 
The ductus venosus and the caudal vein may be regarded as the representatives of the subintestinal vein in Amniota.
 
In Fishes the air-bladder ranks as an ordinary viscus of the mesenteric series, as its blood enters into the hepatic portal system before being returned to the heart ; the only exception occurring is in the Dipnoi, where the pulmonary vein, as it may now be called, carries the blood direct to the left auricle. The same obtains in Amphibia.
 
The Amphibia initiate a new departure in the development of a vena cava inferior, which functionally replaces the larval posterior cardinal veins. The hepatic veins enter into the vena cava inferior. On the disappearance of the posterior cardinals the ductus Cuvieri (superior venae cavae) are connected only with the anterior cardinals (jugular veins).
 
At first two anterior abdominal veins occur, and open anteriorly into the sinus venosus, having previously united with a vein from the truncus arteriosus. An epigastric branch from the iliac vein and veins from the urocyst or bladder (allantois) join them, after
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
235
 
 
which they unite into a single vessel. The atrophy of the right vein is said to result in a single anterior abdominal vein. A secondary connection occurs between the anterior abdominal and the portal system, which persists in the adult.
 
In other respects the Amphibia are essentially piscine in their vascular system.
 
Summary of the History of the Aortic Arches. - As there is still some uncertainty concerning the* fate of some of the aortic arches in the various groups of Vertebrates, it may not be superfluous to briefly recapitulate the facts as at present known. In this summary, as in the foregoing account, the view is adopted which is most generally current, viz., that there is one prehyoid aortic arch, usually termed the mandibular or first aortic arch, the hyoid is the second, while in most Fishes there are four branchial aortic arches. Dohrn terms the aortic arch immediately in front of the hyoid the arteria thyreoidea mandibularis, or shortly the thyroid artery (the mandibular of Balfour), which, in Elasmobranchs, after receiving a venous commissure from the hyoid arch, is called the spiracular artery, as it supplies the spiracle.
 
First aortic arch (mandibular?), present in all embryonic Vertebrata, except the Amphibia, only persisting in Elasmobranchii, and that imperfectly, as the spiracular artery.
 
Second aortic arch (hyoid), present in all embryonic Vertebrata, but imperfect in larval Amphibia. Persistent in Elasmobranchii, usually so in Ganoidei, rudimentary in Teleostei (as artery of pseudobranch), may disappear in some Dipnoi, and partially persists as the lingual artery in Amphibia and Amniota.
 
Third aortic arch (first branchial), present in all larval forms, and persists as a complete arch in all Fishes. In adult Amphibia and Amniota it loses its connection with the other arches and. gives rise to the common carotid trunks.
 
Fourth aortic arch (second branchial), retains its connection with the dorsal aorta throughout the Vertebrate series.
 
Fifth aortic arch (third branchial), persists in all adult Fishes, and to a diminished degree in adult Urodela (still uniting with the dorsal aorta), is lost during the metamorphosis of Anura [Boas], and disappears in the Amniota.
 
Sixth aortic arch (fourth branchial), persists throughout the Vertebrate series. In some Ganoidei (Polypterus, Amia) and Dipnoi also giving a branch to the airbladder, and in adult Amphibia and Amniota supplying the lungs. In adult Urodela alone is a connection still left with the dorsal aorta, and in Anura a large cutaneous branch is given off.
 
It is usually stated that the pulmonary artery of Anura and Amniota is the third branchial aortic arch, and that the fourth disappears. The subject requires reinvestigation, as probably there is a fusion of these two arches, both of them losing their connection with the dorsal aorta, but the fourth branchial still giving origin to the pulmonary artery. Boas has shown that this is actually the case in the young Frog, and in Salamandra the third branchial arch has the appearance of a diminishing artery. It is, moreover, very improbable that the arterial supply of the lungs should shift from the last arch to the one in front of it. If this be admitted, the term “ fifth aortic arch - in the above description of the development of the arterial arches in Amniota must be understood as implying fifth + sixth aortic arch, making seven arches in all.
 
Changes Undergone in the Circulation of Foetal Mammals. - The earliest phases in the circulation have already been described. Later all the venous blood passes directly into the right auricle. The venous blood from the head and upper portion of the body is returned by the two venae cavae superiores (innominate veins). In most Mammals the proximal portion of the left superior vena cava atrophies ; so all
 
 
236
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the blood from the right and left sides of the anterior region of the body comes t6 be returned by the single (right) superior vena cava.
 
The primitive posterior cardinal veins, and later the posterior vertebrals (azygos and hemiazygos), convey blood from the latero-dorsal walls of the trunk to the superior vena cava. The venous blood of the cardiac circulation passes by the coronary vein into the right auricle.
 
The main portion of the blood from the hinder region of the body is brought back by the vena cava inferior, which is by this time rapidly rising into importance. The decreasing blood from the yolk-sac and the gradually increasing mesenteric venous blood passes by the portal vein into the allantoic vein (here known as the ductus venosus), which passes straight through the liver and enters the right auricle along with the vena cava inferior. At its entrance into the liver the ductus venosus gives rise to a few veins (vense advehentes), and receives again a small number of veins (venae reheventes) before leaving that viscus. The liver is also supplied with arterial blood by a branch (hepatic artery) from the dorsal aorta. As the vena cava inferior increases in size the hepatic veins (venae reheventes) open into it.
 
The blood of the superior vena cava passes ventral and to the right side of the Eustachian valve, and, together with a small quantity of blood from the inferior vena cava, passes into the right ventricle, and thence along the pulmonary artery to the lungs. During foetal life the latter are not distended ; consequently only a very small quantity of blood is concerned in the pulmonary circulation : this is returned by the pulmonary veins to the left auricle. The remaining blood passes through the wide ductus arteriosus (Botalli) (figs. 167, 168) into the dorsal aorta, just beyond the spot where the carotid and subclavian arteries arise.
 
Only a small portion of the blood returned by the vena cava inferior passes into the right ventricle ; by far the greater portion is diverted by the Eustachian valve through the foramen ovale into the left auricle, and thence, together with the small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle, then it passes along the ascending arch of the aorta (fourth aortic arch of the left side), and is mainly distributed to the head and fore-part of the body by the carotid and subclavian arteries. A small quantity probably passes along with the blood from the ductus arteriosus down the descending or dorsal aorta.
 
To recapitulate, and omitting minor details : - The blood from the anterior region of the body enters the right auricle by the superior vena cava, thence to the right ventricle and pulmonary artery. A small quantity passes to the lungs and back to the right auricle (pulmonary circulation) ; the greater portion flows through the ductus arteriosus to the dorsal aorta, and thence to the posterior region of the body. This blood is returned by the vena cava inferior to the right auricle, where it is diverted by the Eustachian valve to the left auricle, and, entering the left ventricle, passes by the aortic arch to the anterior region of the body.
 
It will be evident from the above that the blood returned by the allantoic veins is distributed to the anterior region of the body after passing through the liver. Thus the large developing brain is supplied with the most nutritious and aerated blood available, while the grosser organs have distributed to them the blood which has already circulated through the anterior region of the body. A large portion of the blood from the dorsal aorta passes into the allantoic (placental) circulation, and becomes partially purified in the placental villi by diffusion of gases with the maternal blood. In the embryo, as in the adult, it is the right ventricle which pumps the blood into the respiratory organ ( i.e ., placenta or lungs).
 
During the later portion of intra-uterine existence, the blood returned by the inferior vena cava increasingly mixes with that of the superior vena cava, and a gradual approach to the adult arrangement is observable.
 
The rapid dilatation of the lungs and the loss of the placenta at birth result in a considerable modification in the circulation.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
237
 
 
The vessels of the distended lungs become filled with a large quantity of blood, which, being returned into the left auricle, equalises the pressure of the blood on each side of the auricular septum, and no blood passes from one auricle into the other. The free fold of the foramen ovale gradually becomes fused with the margin of the foramen, and thus permanently completes the septum. As was previously mentioned, this valvular fold of the auricular septum was so arranged that, even during foetal life, blood could only flow from the right into the left auricle. A larger or smaller portion of the foramen ovale may remain unclosed for a long period, or even throughout life.
 
The ductus arteriosus rapidly diminishes in size, and normally entirely disappears ; the same fate also befalls the allantoic (umbilical) arteries. The allantoic (umbilical) vein is obliterated as far as its entrance into the liver, and the ductus venosus disappears within that organ.
 
 
Excretory Organs. - An excretory organ consists essentially of a tube or duct which leads from the interior of the animal to the exterior ; such a tube is termed a nephridium.
 
The internal orifice of a nephridium opens into the archicoel (Platyhelminths) or coelom (Coelomata) ; in the latter case the special part of the coelom into which it enters may be more or less separated from the general body-cavity; thus the nephridium may open into the pericardium or into a Malpighian body (Vertebrates). The orifice itself (nephrostome) may be funnel-shaped and richly ciliated, the cilia working outwards ; or there may be a single long cilium, which has a screw-like action, lying within the nephrostome (Platyhelminths, Rotifers).
 
The tube itself may be straight, bent upon itself, or coiled. Each tube may either open independently to the exterior (Invertebrates), or the nephridia of each side may communicate with a common duct which opens posteriorly (Vertebrates).
 
A pair of nephridia only may be present (Platyhelminths, Rotifers, Nematodes, Gephyrea, Polyzoa, Brachiopoda, Mollusca), or numerous pairs may occur, in which case there may be a single pair (most Chsetopoda, a few Vertebrata) or several pairs of nephridia for each segment of the body in which they occur.
 
In addition to carrying away nitrogenous waste, the nephridia, or some of them, may also act as the efferent ducts of the generative organs (Brachiopods and some Chsetopods, Molluscs, and vasa efferent] - a of Vertebrates).
 
Invertebrates. - Our knowledge of the development of the excretory organs of Invertebrates is in a very unsatisfactory condition.
 
The excretory system of Platyhelminths and Rotifers consists in the main of a pair of lateral longitudinal vessels, from which
 
 
238
 
 
THE STUDY OF EMBRYOLOGY.
 
 
numerous fine branches arise which open into the interstices of the spongy mesenchyme (archicoel), into the blood-vessels in some Nemerteans (according to Oudemans), or into the “ body-cavity - of Rotifers. The longitudinal trunks may open anteriorly or posteriorly either independently or by a common orifice ; in the latter case the conjoint vessels may expand into a contractile vesicle. In Nemerteans the nephridial canals communicate with the exterior by one or numerous ducts, which are always situated above the nerve-trunks [Oudemans].
 
The only observations on the development of this system are those of Hubrecht -s for the Nemertean Worm (Lineus obscurus). He finds that a pair of vesicular outgrowths arise from the hypoblastic oesophagus ; although their further development could not be traced, he believes they are the rudiments of the nephridia.
 
The paired segmental organs or nephridia of Chsetopods appear to be developed from the peritoneal epithelium of the body-cavity, either on the posterior wall of the transverse septa or on the bodywall. The external opening is secondarily acquired.
 
There are several forms of excretory organs amongst Arthropods. Peripatus possesses segmental organs similar to those of Annelids, except that, from Balfour -s account, they appear to be devoid of cilia. The Amphipod Crustacea have hypoblastic intestinal cseca (pp. 169, 186), while the Insects have epiblastic rectal Malpighian tubules (p. ill). The excretory organs of the Decapod Crustacea are the green glands situated in the basal joint of the antennae, the outer chamber of whic^ appears to be developed as an epiblastic invagination. The so-called shell-glands of Crustacea may also be excretory organs.
 
Provisional renal organs are developed in the embryos of most of the groups of Odontophorous Mollusca. A pair of V-shaped tubes, with an internal opening into the cavity of the head and an external orifice on the ventral surface behind the mouth, is present in the aquatic Pulmonata, and possibly in some other forms. Rabl and Hatschek ascribe to them a mesoblastic origin, but Fol states that they arise as epiblastic invaginations. Certain epiblastic larval excretory organs have already been described ( P . 108).
 
The adult renal organ (organ of Bojanus) has been variously described to have an epiblastic and a mesoblastic origin. Rabl states that in Planorbis a mass of mesoblast cells appears near the end of the intestine, which, becoming vesicular, attaches itself to
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
239
 
 
the epiblast to the left of the anus, and acquires an external opening. The internal pericardial orifice does not appear to be acquired till after the formation of the heart.
 
Chordata. - No distinct urinary organs occur in AscidiaUs, unless the neural gland has this function.
 
Hatschek has recently discovered a true kidney in Amphioxus, which has the structure and development of a nephridium. It develops in the larva as a mesodermal ciliated funnel and canal on the left side only of the mouth in the region of the first somite. In the adult the nephridium lies in a narrow portion of the bodycavity, near the ventral body of the notochord, overlying the left carotid (which is a continuation of the left aorta). It appears to open into the pharynx.
 
The Vertebrate excretory system consists of three parts - (i.) Head-kidney or Pronephros; (2.) Wolffian body or Mesonephros; (3.) Kidney proper or Metanephros. These three portions are never functional at the same time, and are to be regarded as differentiations of the primitive kidney which have occurred in the evolution of the Vertebrates.
 
1. Pronephros. - The first part of the excretory system to develop is the duct - variously termed segmental duct, pronephric duct, the duct of the primitive kidney, or archinephric duct. The pronephros, when present, is always connected with the anterior extremity of this duct.
 
The following is the generally received account of the development of the segmental duct, but the duct has been shown by several investigators to have an ejoiblastic origin (fig. 178*, s.d). The significance of this will be shortly pointed out (p. 249).
 
I11 the Amphibia the segmental duct appears as a groove (fig. 173, A, s.d) along the outer angle of the dorsal region of the body- cavity, which commences just behind the branchial region. The groove is continuously constricted off from before backwards so as to form a canal or duct ; except anteriorly, where the constriction only takes place at intervals, leaving two (Urodela), three (Anura), or four (Ceecilia) openings. The short tubes connecting these openings or nephrostomata with the segmental duct increase in length and form the segmental tubes, which correspond in number with the segments which the pronephros occupies.
 
The duct immediately behind these tubules becomes coiled. A vascular process from the peritoneum, the glomerulus, projects on each side of the aorta into a dilated section of the body-cavity,
 
 
240
 
 
THE STUDY OF EMBRYOLOGY.
 
 
which becomes partially cut off from the rest of the coelom (fig! 173, b). The whole of these structures collectively constitutes the pronephros.
 
The segmental duct eventually opens posteriorly into the cloaca.
 
The pronephros develops in the Teleostei in a similar manner, except that there is only one anterior opening (nephrostome) ; and the part of the body-cavity into which it opens, and in which the glomerulus lies, becomes completely constricted off, so as to form what is practically an enormous Malpighian body (fig. 173, c).
 
 
A. Transverse section through a very young Tadpole of a Toad (Bombinator) at the middle of the body. [After Giitte .] B. Diagram illustrating the partial isolation of the glomerulus within a pouch of the body-cavity. C. Transverse section through the pronephros of a Trout ten days before hatching. [After Balfour .] D. Diagram of the pronephros of Lepidosteus. [After Balfour and Parker .] ao. dorsal aorta; b.c. body-cavity; ep. two-layered epiblast ; /. peritoneal funnel; gl. glomerulus ; m. mesenteron ; rn.p. muscle-plate ; n. neural tube ; ncli. notochord ; n.s. nephrostome; p.o. opening of pronephric tubule into the isolated portion of the body-cavity : s.d. segmental duct ; so.rn. somatic, and sp.m. splanchnic, mesoblast ; t. pronephric tubule ; x. subnotochordal rod ; y.hy. yolk hypoblast.
 
The same arrangement occurs in young larvse of the Ganoid Lepidosteus (Balfour and Parker), except that a tubular communication with the body-cavity (fig. 173, d) is retained for some time.
 
 
It is usually stated that in some Teleosts the head-kidney (pronephros) is the only excretory organ of the adult ; in most it occurs together with the Wolffian body (mesonophros), and in a few it disappears altogether. Balfour, however, has shown that in certain typical forms (and therefore probably in all) the pronephros, when it persists, loses its excretory function and degenerates into a lymphatic gland. In those specialised Teleosts {e.g., Lophius) in which the pronephros only is supposed to occur, the mesonephros has probably been mistaken for that organ. Weldon suggests that the head-kidney of Teleosts may be regarded as a suprarenal body.
 
 
The pronephros occurs in all the Ichthyopsida, except the Elasmobranchii, but only functions during a period intervening between hatching and the attainment of full maturity ; in other words, the pronephros is always a larval organ, and never con
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
241
 
 
statutes an active part of the excretory system in the adult state. It is either absent or imperfectly developed in those types (Elasmobranchii and Amniota), which undergo the greater part of their development within the egg or before birth.
 
In the Elasmobranchs the segmental duct arises anteriorly as a solid ridge of cells from the somatic layer of the intermediate cellmass. Erom this ridge a solid column of cells grows back to the cloaca without coming into contact with any neighbouring structures. A central cavity or lumen soon appears, and the duct opens widely into the body-cavity anteriorly.
 
The development of the pronephros has been most carefully studied in the Fowl. In this form the segmental duct arises as a solid ridge from the parietal mesoderm, just ventral to the muscleplates. The ridge, which extends to five segments, is constricted
 
 
Fig. 174. - Diagrams Illustrating the Development oe the Pronephros in the Fowl.
 
ao. aorta ; b.c. body-cavity ; ep. epiblast with its epitrichial layer ; hy. hypoblast ; m.s. mesoblastic somite; n.c. neural canal; nch. notochord; p.t. pronephric tubule; so. somatic, and sp. splanchnic, mesoblast.
 
off at intervals from the intermediate cell-mass, but remains attached at certain points. The duct grows backwards as in Elasmobranchs. The further history of this duct will be described later. The pronephros extends in the Fowl over the seventh to the eleventh segments inclusive, the most anterior mesoblastic somite behind the auditory involution being counted as the first.
 
As the pronephros is the first part of the excretory system to be developed, and often is the sole excretory organ for a considerable period, it is usually concluded that it and its duct (the segmental duct) are the most primitive parts of the vertebrate excretory system. The mode of its development in the Amphibia may also be regarded as primitive, especially since Shipley has shown that the anterior portion of the pronephros of the Lamprey develops in a similar manner.
 
2. Mesonephros. - The Wolffian body or mesonephros is largely
 
Q
 
 
242
 
 
THE STUDY OF EMBRYOLOGY.
 
 
developed in all Vertebrates, but it does not persist as an excretory organ in adult Amniota.
 
The mesonephros consists of a number of serially arranged primary tubules, segmental or Wolffian tubules, which may be segmentally arranged (Elasmobranchs, some Amphibia, and at first in Reptiles), but usually a variable number of tubules are formed in each segment. Each tubule opens on the one hand into the segmental duct, and on the other into a Malpighian body. The latter sometimes (Elasmobranchs and Amphibia) communicates with the body-cavity by a short tube (peritoneal funnel). In addition to the primary tubules there may be an inconstant number of dorsally placed secondary, tertiary, &c., tubules, which correspond with and are developed from the primary tubules.
 
 
Fig. 175. - Transverse Section
 
THROUGH THE TRUNK OF A
 
Young Embryo Elasmobranch
 
(Scyllium). [ From, Balfour .]
 
ao. dorsal aorta ; ch. notochord ; mp. somatic, and mp'. splanchnic, layer of muscle-plate; p.o. primitive germinal cells ; pr. dorsal root of spinal nerve ; sd. segmental duct ; sp.c. neural canal ; sp.v. spiral valve of intestine ; v. subintestinal vein : vr. rudiment of vertebral body ; W. white matter of spinal cord; x. subnotochordal rod.
 
 
These dorsal secondary tubules resemble in their structure the primary tubules, and usually open into the latter just before they enter the segmental duct. In the larval Amphibia only, the secondary and other tubules are known to have peritoneal funnels arising from their Malpighian bodies. It is worthy of note that the nephrostomata are connected with the Wolffian tubules in larval Anura, but that later on they become separated from them, and open into the renal-portal vein [Wiedersheim].
 
The primary Wolffian tubules are usually stated to be derived as solid ingrowths from the peritoneum towards the segmental duct ; but Sedgwick has shown that in Elasmobranchs they have the following development. It lias previously been stated (p. 212) that the muscle-plates of Elasmobranchs are dorsal extensions of
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
243
 
 
the body-cavity, which become cut off (fig. 1 50, m.p) by the coming together of the somatic and splanchnic mesoblast. The continuous lion-segmented band of cells connecting the non-segmented muscleplates with the peritoneal epithelium being known as the intermediate cell-mass. Sedgwick found that the passage connecting the body-cavity with that of the muscle-plates persists for some time. Its connection with the ventral dilation of the muscle-plate cavity is carried 'ventral wards as far as the outer dorsal corner of the segmental duct, so that it appears as a canal opening into the body- cavity just internal to the segmental duct, and thence curling
 
 
Fig. 176. - Transverse Section through the Trunk of a Duck Embryo with about Twenty-Four Mesoblastic Somites. [From Balfour.']
 
vm. amnion : ao. aorta ; ca.v. cardinal vein ; ch. notochord ; hy. hypoblast ; m.s. muscle-plate ; so. somatopleur ; sp. splanchnopleur ; sp.c. spinal cord : sp.g. spinal ganglion : st. segmental tube ; wd. Wolffian (segmental) duct.
 
round its dorsal wall, opens into the muscle-plate cavity. The ventral wall of this passage is formed of large columnar cells, the inner and dorsal wall of much flatter cells.
 
At the next stage of development the passage becomes quite separated from the muscle-plate cavity, and now lies as a blind tube (fig. 175, st) opening into the body-cavity internal to the segmental duct, with which it soon unites and forms a segmental tubule.
 
Sedgwick has also further shown that in the Fowl, in the region of the body between the twelfth and fifteenth somites inclusive, the segmental tubes (Wolffian tubules) have a double origin: (1)
 
 
244
 
 
THE STUDY OF EMBRYOLOGY.
 
 
from outgrowths from the Wolffian or segmental duct; (2) as parts of the intermediate cell-mass.
 
The intermediate cell-mass is at first continuous with the peritoneal epithelium in every section, but this connection soon becomes lost at certain points and maintained at others. At the points where the continuity is retained, a peritoneal funnel is subsequently formed by the development of a lumen extending from the body-cavity into the intermediate cell-mass.
 
The tubules have at this stage their characteristic and wellknown S-shape (fig. 176). They consist of the following parts: -
 
 
A - C. A series of successive sections through the thirteenth segment of an embryo with thirty-one or thirty-two segments, A being the most anterior. In A and B the tubule is connected with the peritoneal epithelium ; and a lumen has appeared in it, which is continued behind into the part of the tubule separated from the peritoneal epithelium, as in C.
 
D - E. Sections through the thirteenth or fourteenth segment of an embryo with thirty-four or more segments, showing the first appearance of the external and internal glomeruli, D and E correspond to, and are further developments of, B and C.
 
E. Diagrammatic longitudinal vertical section, showing the relations of the further developed external and internal glomerulus.
 
b.c. body-cavity ; c.v. cardinal vein ; e.gl. external glomerulus ; gl. glomerulus ; i.c.m. intermediate cell-mass ; i.gl. internal glomerulus ; me. mesentery ; p.f. peritoneal funnel ; W.d. Wolffian duct.
 
(1) The now hollow Wolffian duct; (2) the outgrowth from it to the intermediate cell-mass forming the upper limb of the S ; (3) the intermediate cell-mass with the commencing lumen from the body-cavity.
 
At a slightly later stage (fig. 177, A-c) there is a distinct lumen opening into the body- cavity, which is continued behind into the part of the intermediate cell-mass which has separated from the peritoneal epithelium (c, i.c.m). This part will in the next stage (fig. 177, e) become converted into that part of a tubule in which a Malpighian body is developed, while the anterior part will form a much wider peritoneal funnel (nephrostome).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
245
 
 
A glomerulus is formed about this time on the anterior wall of the peritoneal funnel of each segmental tubule. The glomerulus increases in size, and its lower portion hangs down freely into the body-cavity (fig. ijy, f, gl). Before the period of the greatest development of the glomerulus the mouth of the peritoneal funnel becomes closed, thus dividing the glomerulus into an anterior lower or external portion and into a posterior upper or internal portion, the latter persists as the glomerulus of the Malpighian body of the Wolffian body. The external portion afterwards disappears.
 
Behind the fifteenth segment the segmental tubules develop entirely from the cells of the intermediate cell-mass. At first the intermediate. cell-mass is at points distinctly continuous with the peritoneal epithelium; at others it is less so. It soon breaks away and occurs as a solid cord of cells, connected at intervals with the peritoneal epithelium.
 
At the next stage the intermediate cell-mass entirely breaks away from the peritoneal epithelium, and lies as a cellular blastema (the Wolffian blastema) just internal to the Wolffian duct. The Wolffian blastema almost directly breaks up into the structures constituting the first rudiments of the Wolffian tubules.
 
Posteriorly, from about the twentieth segment, the intermediate cell-mass has never any connection with the peritoneal epithelium, and gives rise to the Wolffian blastema quite independently of the peritoneal epithelium.
 
The cells of the blastema group themselves into tubules, one end of which forms the Malpighian body, and the other opens into the Wolffian duct. There appear to be outgrowths from the duct to meet the tubules.
 
Although the Wolffian blastema extends as far back as the thirty-fourth segment, it does not break up into Wolffian tubules behind the thirtieth segment. From the thirty-first to the thirtyfourth segment it undergoes a different fate, and is known as the kidney blastema.
 
In the anterior region of the mesonephros there appears to be only one primary tubule (Wolffian tubule) for each segment of the body, but the number increases up to the twentieth (counting from the auditory involution). All the segments from the twentieth to the thirtieth inclusive contain five or six primary tubules.
 
The secondary or dorsal tubules are also more numerous behind than in front, the most anterior segment being about the twenty-first. Some primary tubules, according to Sedgwick, have as many as four secondary tubules ; thus in the twenty-eighth segment there are twenty secondary tubules (five sets of four).
 
Balfour has shown that the secondary tubules^develop in Elasmobranchs in con
 
246
 
 
THE STUDY OF EMBRYOLOGY.
 
 
nection with the Malpighian bodies of the primary tubules. A process from one Malpighian body grows forward and unites with the preceding tubule just before it. enters into the Wolffian duct. The stalk of origin degrades into a fibrous band or is aborted. The tertiary, &c., tubules probably arise from the same rudiment.
 
The secondary Malpighian body is produced in the Fowl, according to Sedgwick, by the division of the primary glomerulus into two parts, the upper one forming the secondary and the lower the primary glomerulus ; and by the simultaneous development of certain folds which separates the dorsal secondary tubule from the ventral primary tubule.
 
The somewhat later origin of the posterior tubules of the mesonephros in the Fowl, and their development from a blastema, is a distinct approach towards the mode of origin of the metanephros, now to be described.
 
It appears to be probable that in the Teleosts and Amphibia the segmental tubules of the mesonephros develop in situ from a blastema analogous to that in the posterior region of the Fowl. The tubules subsequently acquire openings into the Wolffian tube on the one hand, and into the body-cavity on the other.
 
3. Metanephros. - The kidney proper or metanephros, as a gland distinct from the mesonephros, only occurs in Amniota. In the Fowl it develops from a blastema which is at first perfectly continuous with, and indistinguishable from, that which gives rise to the posterior portion of the Wolffian body. Although the kidney blastema arises at a comparatively early stage in development, still it is not till a much later stage that it shifts its position and begins to show signs of developing into the segmental tubules. This retarded development is analogous with the late appearance in Amphibia of the mesonephros as compared with the pronephros.
 
The first distinct structure to develop is the ureter, which arises as a dorsal outgrowth from the hinder part of the Wolffian duct. The ureter grows forward in close connection with the abovementioned blastema, which has by this time broken away from the mesonephric blastema and assumed a position dorsal to it (fig. 178, c).
 
The metanephric blastema extends in the Fowl from the thirtyfirst to the thirty-fourth segments, and collects round swellings of the ureter from which kidney tubules grow out. These tubules burrow into the blastema, and they are increased by segregation of the blastema cells.
 
The ureter soon loses its connection with the Wolffian duct, and acquires an independent opening into the cloaca.
 
The primitive continuity of the metanephric with the mesonephric
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
247
 
 
blastema, together with the general similarity of the development of the renal tubules and the identity of their adult structure, proves that the metanephros is merely a special portion of the primitive Wolffian body, which develops late.
 
The acquisition by the posterior portion of the Wolffian body in some Elasmobranchs and Amphibia of efferent ducts opening into
 
 
Fig. 178. - Development oe Metanepheos in the Fowl. {Adapted from Sedgwick.}
 
A. Transverse section through an embryo at the end of the fourth day. B. Longitudinal vertical section through an embryo of about the same age, showing the absolute continuity of the kidney blastema with the hindermost part of the Wolffian blastema, in which the development of Wolffian tubules is taking place. C. Transverse section through an embryo at the end of the sixth day.
 
ao. dorsal aorta: b.c. body-cavity ; c.v. cardinal vein; k.b. kidney blastema; k.t. kidney tubule; M.d. Mullerian duct; mes. mesentery; vch. notochord; pe peritoneum ; T. testis ; u. ureter ; v.c. vertebral centrum ; W.B. Wolffian body ; Wd. Wolffian duct ; Wt. 1 primary, and Wt . 2 secondary, Wolffian tubule.
 
 
the urogenital sinus or into the extreme posterior end of the Wolffian duct, is, as Balfour pointed out, a definite step towards the formation of a metanephros. According to Mikalovics, the mesonephros remains functional till the second year in Lizards, and thus is functional at the same time as the metanephors.
 
 
248
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Summary of the Development of the Vertebrate Excretory Organ. - From his
 
investigations on the development and phytogeny of the vertebrate excretory organs, Sedgwick has arrived at the following conclusions. For the facts and arguments upon which they are based, recourse must be had to his papers.
 
The pronephros attains a functional development in all the Ichthyopsida (except the Elasmobranchii), but usually only during larval life.
 
The segmental duct arises first as a ridge from the parietal peritoneum. This ridge usually contains a diverticulum from the body-cavity, and is continuously constricted off to form a duct.
 
Except anteriorly, where the constriction only takes place at intervals, leaving the openings of the pronephros (except in Teleostei, where there is only one opening).
 
These openings correspond in number with the segments which the pronephros occupies.
 
A vascular structure, called a glomerulus, is formed, projecting on each side of the aorta into a special dilatation of the anterior part of the body-cavity. (Myxine forms a peculiar exception to this otherwise universal fact.)
 
This dilated part of the body-cavity may become partially or completely separated off to form a capsule, into which the glomerulus projects and the anterior end of the segmental duct opens.
 
The development of the pronephros in the Fowl is essentially identical with the above, except in the absence of a continuous glomerulus opposite the nephrostomata ; but that in the Elasmobranch is greatly modified and reduced.
 
In those animals which possess a functional larval pronephros, the mesonephros develops from a blastema ; this is undoubtedly an abbreviated method. The lateness and consequent modification of the development of the mesonephros in these Ichthyopsida is due to the fact that the larva already possessed a functional excretory organ, and devoting all its energy in developing those organs which it will really require as a larva, it leaves over the development of the organs not so required until later ; and in order that it may not be burdened by useless organs, the cells, which will give rise to the tubules, are so reduced as hitherto to have escaped observation. If the phylogenetic order had been adhered to, these cells would have arisen quite early in embryonic life, and from the parietal mesoblast in the normal manner. In the Amphibia the mesonephros increases in size and complexity with the growth of the larva.
 
On the other hand, the mesonephric tubules develop in what is clearly a more primitive manner in those forms in which the pronephros is functionless. In Elasmobranch s, and in the anterior region in the Fowl, the tubules are practically persistent tubular portions of the body-cavity (since the intermediate cell-mass is a continuation of the coelomic epithelium), which soon acquire an opening into the segmental duct. The early discontinuity of the tubules with the duct is, however, a secondary feature. In Birds a segmental glomerulus is developed in connection with each nephrostome, part of which is converted into the glomerulus of the Malpighian body. In Elasmobranchs only internal Malpighian bodies are formed.
 
It may fairly be assumed that the Wolffian tubules were primitively segmentally arranged (as still occurs in the development of Elasmobranchs, Csecilia, and at first also in the Lizard). A shifting of position has, however, occurred, probably partly owing to the shortening up of the organ, so that the number of tubules may exceed that of the segments over which the mesonephros extends. The number of tubules in a segment usually increases with the growth of the embryo, and at the same time the organ is complicated by the development of secondary tubules.
 
In Birds the pronephros is continuous with the mesonephros. The discontinuity in Amphibia is due to the causes mentioned above, but it may not really be so great as it appears.
 
The segmental tubules of the Ichthyopsidan pronephros open into a special recess of the body-cavity, into which the elongated glomerulus projects (fig. 173) ; imme
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
249
 
 
diatelv above this lies the muscle-plate. A comparison of figs. 173- 17 7, will demonstrate that the intermediate cell-mass corresponds to this region. It has been shown that in Birds the peritoneal funnels, the Malpighian body, and a portion at least of the mesonephric tubules are derived from the intermediate cell-mass. The external and internal glomeruli of the Avian Wolffian body are developed from the same region, and all the secondary glomeruli are derived from the internal glomerulus. The internal glomeruli of Elasmobranclis are clearly homologous with those of Birds. According to this view, a Malpighian body is to be regarded as an isolated portion of the body- cavity, comparable with the condition which obtains in the Teleostean pronephros. Therefore the mesonephros, in all particulars, is merely a continuation posteriorly of the primitive vertebrate excretory organ, which, for various reasons, has acquired a more or less independent and modified origin.
 
What applies to the mesonephros also holds good for the metanepliros. The distinction of the latter organ from the former is more apparent than real, as Sedgwick has fully demonstrated. As a matter of fact, the tubules of the liindermost region of the mesonephros develop in an almost similar manner to those of the metanepliros. We have therefore a complete series in the mode of origin of the excretory tubules, from the primitive condition in the pronephros of Amphibia to the modified method by the rearrangement of the cells of a blastema, as occurs in the metanepliros of Amniotes.
 
From the foregoing brief summary it will be seen that the excretory organs of the adult are usually developed from the walls of the body-cavity in the invertebrate and vertebrate Coelomata (and therefore the same possibly occurred in the lower Chordata). It must not, however, be rashly concluded that these organs are necessarily homologous. It is possible that a similar and homologous simple renal organ (arcliinephridium) occurred in the unsegmented vermian ancestors of the Chsetopoda and Chordata ; but the segmental organs of the one are probably homoplastic, rather than strictly homologous with the segmental tubules of the other.
 
It is tempting to regard the origin of the Nemertean excretory organ, as described by Hubrecht, as a degenerate form of the production of a true body-cavity (coelom) by archenteric diverticula, which, in this case, solely develop into nephridia. If this be granted, a further step may be taken, and, accepting Rabl -s account of the development of the Molluscan excretory organ, we may assume that the formative cells of the mesoblastic vesicle actually arose from the archenteron. Should this prove to be the case, the Molluscan nephridia would be comparable with those of the majority of other animals. It is also difficult to believe that the Molluscan pericardium is not a true coelomic cavity.
 
 
Epiblastic Origin of the Segmental Duct. - Since the above account of the development of the vertebrate excretory septem was in type, a preliminary note by Yon Perenyi has appeared, in which he confirms and extends the discovery of the epiblastic origin of the segmental (archinephric) duct. Hensen, Graf Spee, and Flemming have demonstrated that in the Eabbit and Guinea-pig the primitive nephric duct (probably not the whole excretory system, as they assume, although without evidence to support them) arises by delamination from the epiblast at the level of the intermediate cell-mass, with which it later becomes associated. Afterwards Van Wijhe found the same held good for Elasmobranchs, and most recently Yon Perenyi asserts that in the Edible Frog the segmental
 
 
250
 
 
THE STUDY OF EMBRYOLOGY.
 
 
duct develops as a canal-like splitting from the inner (nervous) cell layer of the epiblast, and quite close to the place of origin of the developing somites. In the Lizard, also, it appears as a thick cell-mass separating off from the epiblast.
 
There can now be little doubt that the segmental duct arises from the epiblast. This discovery will necessarily lead to a modification of our views concerning the morphology of the vertebrate excretory organs.
 
The segmental tubules (nephridia) appear to be strictly mesoblastic, and the above account of their development may be taken as probably being fairly accurate. The origin of these nephridia may have been primitively similar to those (segmental organs) of the Chsetopod Worms, the main distinction between the two being that each nephridium of the latter opens directly to the exterior. As has been already stated, Hatschek has described a single nephridium in Amphioxus in all respects comparable with a vermian nephridium.
 
We have, then, only to assume that a pair of similar vermian nephridia occurred in each body-segment of the ancestral Vertebrate, and that the nephridia of each side of the body opened externally into a lateral groove. It would further only be
 
 
Fig. 178*. Transverse Section of Embryo Rabbit (4 mm. in length, stage of sixteen somites). [After Flemming .]
 
The section is taken just in front of the posterior termination of the intestine. The right side of the figure is the left of the body. There is a small rupture in the left (right of figure) mesoblastic somite. All the shading is diagrammatic.
 
al. mesenteron (intestine) ; cce. coelom (body-cavity) ; ep. epiblast; hy. hypoblast; i.c.m. intermediate cell mass; n.c. neural canal; s.d. segmental duct ; som. somatic mesoblasb ; sp. splanchnic mesoblast.
 
 
necessary for the groove to deepen and next to form a canal (in the same manner that the neural groove is converted into a canal) to bring about the vertebrate arrangement. Thus in Vertebrates, as in Invertebrates, the nephridia open by epiblastic pores, but in the former the area upon which they open is precociously converted into a canal, which subsequently acquires a secondary opening to the exterior through the cloaca.
 
As we are justified in assuming the persistence of the blastopore as the anus in early Chordata, the nephric groove, if it were continued behind round to the anus, would practically open into the extreme hinder end of the mesenteron - in other words, into the urodseum. Probably about the same time that the nephric groove was being converted into the nephric canal (segmental duct), the proctodseum was being invaginated. The latter would push before it the posterior orifice of the nephric canal along with the primitive anus (blastopore). On the hypothesis just sketched out, the nephridia of Vertebrates always open by their original epiblastic pores, primitively directly to the exterior, secondarily into a canal separated from the epiblast ; also the archinephros could be equally effectually functional throughout the whole period of its modification.
 
Urogenital Ducts of Vertebrates - For the sake of simplicity the ducts of the^Vertebrate renal organs have been referred to as if
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
251
 
 
solely connected with those organs ; as a matter of fact, they become intimately connected with the generative organs. The modifications which occur in the glands and ducts of the primitive excretory system of the Vertebrates may be regarded as being largely due to their secondary connection with the generative organs.
 
Segmental or Archinephric Duct. - The development of the segmental duct has already been described. Tor the sake of clearness it has been assumed that the segmental duct functions first as the duct of the pronephros, and secondly as that of the mesonephros. This is not, however, exactly the case, as in most cases there is a horizontal division or separation of the duct into two tubes. A ventral tube is termed the Mullerian duct ; while the dorsal, from its association with the mesonephros, is known as the mesonephric or Wolffian duct.
 
Mullerian Duct. - The two ducts are formed in Elasmobranchs by the splitting off from before backwards of a nearly solid cord of cells from the ventral wall of the segmental duct. A very small portion of the lumen of the segmental duct may perhaps be continued into the Mullerian duct. The latter soon grows in size, and forms an elongated tube in the female quite distinct from the Wolffian duct. The longitudinal separation from the segmental duct occurs in such a manner that the whole of the anterior extremity, with its peritoneal opening, belongs to the Mullerian duct, which now forms a complete tube opening posteriorly into the cloaca and anteriorly into the body-cavity. In these forms the single, primitively solid, pronephric tubule persists as the peritoneal opening of the Mullerian or oviduct of the adult.
 
The development of the Mullerian duct in Amphibia is very much the same as in Elasmobranchs. In the Salamander the Mullerian duct is split off from the segmental duct behind its anterior extremity, and acquires an independent opening into the body-cavity slightly behind the pronephros. IJnlike what occurs in Elasmobranchs, the undivided anterior extremity of the segmental duct with the pronephros retains its connection with the Wolffian duct.
 
In the Fowl, Balfour and Sedgwick have shown that the anterior end of the Mullerian duct arises as three grooves connected by an internal thickening of the peritoneum of that region. The thickening separates as a solid rod of cells, which, before long, acquires a central lumen. The whole structure now consists of a short tube opening anteriorly into the body-cavity by three short ductules.
 
 
252
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Posteriorly the Miillerian duct is closely connected with the segmental duct. The backward growth of the Mullerian duct takes place at the expense of a thickening of the ventral wall of the segmental duct. In other words, the avian Mullerian duct is formed posteriorly by the splitting of the segmental duct, as in Elasmobranchs and Amphibia. The permanent abdominal opening of the Mullerian duct (oviduct) corresponds with the anterior of the three grooves, the two posterior grooves disappearing.
 
Balfour regarded the three peritoneal funnels of the Mullerian duct as the sole representative of the head-kidney (pronephros) in the Fowl. Sedgwick still adheres to the earlier published view as to the meaning of the peculiar structures at the anterior end of the Mullerian duct, hut supposes them to have been derived from the anterior part of the excretory system after its modification to form the pronephros.
 
The tubules of the Mullerian duct of the Fowl arise behind the anterior end of the segmental duct, and therefore more or less posterior to the pronephros. In Amphibia the single (solid) tubule is situated behind the pronephros. In both cases we must assume either that the Mullerian tubules are modified and backwardlyshifted pronephric tubules, or that they belong to the region between the pronephros and the mesonephros proper. In any case, the Mullerian duct is split off from the segmental duct. Further researches may modify the account given of the development of the pronephros of Elasmobranchs.
 
The Miillerian duct opens at the anterior end of the body-cavity in the lower Vertebrates; in Elasmobranchs, for instance, the conjoint orifice of the two ducts is situated on the ventral wall of the oesophagus just behind the pericardium. In the higher Vertebrates the Mullerian ducts are situated in the posterior abdominal region. The hydatid (fig. 181, A, h") which is sometimes present near the coelom ic orifice of the oviduct is probably a degraded rudiment of a primitive tubule.
 
In those Ichthyopsida which possess them, in the Sauropsida and in the Ornithodelphia (Monotremata or Prototheria), the paired Mullerian ducts (oviducts) open into that portion of the cloaca which is known as the urogenital sinus (fig. 179, a). Occasionally only one oviduct maybe developed; in Birds it is usually the right which atrophies.
 
f The Didelphia (Marsupials or Metatheria) have a modification of their Mullerian ducts, which is very different from that of other Mammals. It may be here mentioned that three regions are distinguishable in the Mullerian ducts of these and higher Mammals - an anterior or distal narrow tube (Fallopian tube or “oviduct-), which opens into the body-cavity by usually fimbriated lips ; a median swollen uterus, and a posterior or proximal vagina.
 
In the Didelphia the Mullerian ducts with their three regions are at first perfectly distinct, and practically remain so ; that is, there are two vaginae, uteri and Fallopian tubes. Later, in the young, the anterior (distal) ends of the vaginae
 
 
ORGANS DEBITED FROM THE MESOBLAST.
 
 
253
 
 
approach one another ; at the point where they touch they form a median sac, which grows backwardly towards the urogenital sinus. At first this vaginal caecum is a double tube, corresponding to each vagina ; but the median septum is usually soon absorbed. At this stage the two uteri open into the anterior extremity of the vaginal cul-de-sac, into the upper end of which the two vaginae also open. The blind posterior end of the caecum becomes closely connected with the end of the urogenital sinus, between the posterior vaginal orifices ; and, as Fletcher has proved, the two cavities may communicate even in virgin animals, and they certainly do communicate after the first birth. (Unless very exceptionally, there is, according to Fletcher, no direct communication in Macropus major between the vaginal caecum and the urogenital sinus, even after young have been produced.)
 
In the Monodelpliia (Eutheria) the Mullerian ducts fuse with one another to an increasing extent from behind forwards. In
 
 
Fig. 179. - Various Forms of Mammalian Uteri.
 
A. Ornifchorhynchus [ after Owen]. B. Didelphys dorsigera [ after Brass]. C. Phalangista vulpina [a 'ter Brass]. D. Double uterus and vagina ; Human anomaly [ after Farre], E. Lepus cuniculus (Rabbit), uterus duplex [after T. J. Parker]. F. Uterus bicornis. G. Uterus bipartitus. H. Uterus simplex (Human). [F - H after Wieders heim.]
 
a. anus; cl. cloaca; o.d. oviduct; o.t. os tineas (os uteri); ov. ovary; r. rectum; s. vaginal septum ; n.b. urinary bladder ; ur. ureter ; ur.o. orifice of same ; u.s. urogenital sinus ; ut. uterus; v. vagina; v.c. vaginal caecum.
 
all there is a single vagina, but in some of the lower forms, e.g ., Rodents, an imperfect vaginal septum may he present. Usually it merely divides the orifice (os uteri or os tincse) of one uterus from that of the other (fig. 179, e). In such forms the uteri are quite distinct.
 
In other Mammals the uteri come together, and by concrescence form a common uterus, which also, in some cases, possesses a short median septum. In these forms there are paired cornua uteri opening into a single corpus uteri, which communicates with the vagina by a single os uteri.
 
Eig. 179 illustrates various forms of uterus met with amongst
 
 
254
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the Eutheria. In the most specialised case, as in Man, the uterus (h, ut) has a pyriform shape, and the Eallopian tubes arise abruptly from its anterior corners.
 
It is interesting to find that anomalies may occur in the human uterus, which illustrate the evolution of that organ. Thus a median septum may partially or wholly extend along the uterus, and the vagina even may be similarly divided (fig.
 
179, D).
 
The Mullerian duct is rudimentary or entirely absent in the adult male. In the former case it may be represented by a solid cord for the whole of its length (Dipnoi and some Amphibia, fig.
 
180, A, mg), or only the anterior portion may remain (Elasmobranchs and some Lizards), which degrades into the so-called hydatid of Morgagni in Man (fig. 181, B, m). The posterior section is usually stated to persist as the uterus masculinus (figs. 183, III. u, 1 84), present in many Mammals, and especially large in the Eabbit and the Horse ; but Kolliker now believes this structure to be a derivative of the Wolffian duct.
 
The oviduct is normally present as a complete duct in the males of the Dipnoi and of some Ganoids [Ayers, Wiedersheim], and abnormally in Lizards [Howes].
 
To recapitulate : - The segmental duct is the duct of the primitive vertebrate excretory organ. The pronephros was either the sole excretory organ, or it has come to be the only functional portion of the kidney in free-living larval forms, owing to the retardation of the posterior region of the primitive excretory organ. At all events, the segmental duct at first functions as the pronephric duct.
 
One or more of the tubules in the anterior region of the primitive kidney acquired the office of carrying ova to the exterior. This probably occurred after the full development of the pronephros, and in the intermediate region between it and the mesonephros. Possibly at one time the segmental duct conveyed ova to the exterior, together with secretions from both pronephros and mesonephros.
 
From certain causes the pronephros atrophied or changed its function and became a lymphatic gland ; the segmental tube then carried ova and mesonephric secretions. The duct gradually became constricted in such a manner that the ova were conveyed in a ventral groove, which subsequently was converted into a canal.
 
In some such manner the segmental duct may have differentiated into a ventral Mullerian duct or oviduct, and into a dorsal mesonephric duct or Wolffian duct.
 
Wolffian Duct or Mesonephric Duct. - In those forms (Ichthyopsida) in which the mesonephros remains functional throughout life its duct naturally persists, although it also acts as the efferent duct of the generative gland in the males of the Elasmobranchii, Lepidosteus, and Amphibia.
 
Branches grow out from the anterior (three or four in Elasmobranchs) segmental or Wolffian tubules (though probably not from their peritoneal openings - Balfour) and enter the testis, where they form a longitudinal canal (fig. 180, a). These branches, vasa
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
255
 
 
efferentia, convey the semen to the Wolffian body after previous uniting into a longitudinal canal, (the longitudinal canal of the Wolffian body) (fig. 180). Branches of this so-called testicular
 
 
B
 
 
 
Fig. 180 --Diagram of the Urogenital Apparatus of a Male (A) and Female (B) TJrodele. Founded on Triton tseniatus. [ From Wiedersheim after J. W. Spengel.]
 
a. collecting tubules of the mesonephros ; GN. anterior sexual portion of kidney (parorchis of the male) ; Ho. testis ; Ig. Wolffian or Ley dig -s duct, urogenital duct in male, A, and urinary duct in female, B, Ur; mg(Od). Mullerian duct, rudimentary in male, mg';
 
N. posterior non-sexual portion of kidney ; Ot. peritoneal aperture of oviduct ; Ov. ovary ;
 
Ve. vasa efferentia of testis which fall into the longitudinal canal (f) of the Wolffian body ; this testicular network (ft) is rudimentary in the female, B.
 
network enter certain Malpighian bodies, and the semen is thence carried by their tubules to the Wolffian duct.
 
The anterior or sexual portion of the Wolffian body in the male is rudimentary so far as excretory purposes are concerned, and, as in the male, a functionless rudiment of the Mullerian duct is present, so a rudimentary testicular network is developed in the
 
 
256
 
 
THE STUDY OF EMBRYOLOGY.
 
 
female Urodeles (fig. 180, b), and the anterior portion of the Wolffian body is also feebly developed.
 
In the Elasmobranchs and Amphibia the collecting tubes of the non-sexual posterior portion of the Wolffian body unite together to form one to two primary tubes (ureters) before entering the posterior extremity of the Wolffian duct. Thus the Wolffian tube acts as a vas deferens, and the posterior portion of the mesonephros is practically an incipient metanephros.
 
In the males of the Amniota, tubules grow out from certain anterior Malpighian bodies of the Wolffian body in the embryo, and come into connection with the seminal tubuli of the testis.
 
 
Fig. i8i. - Generative Organs of Hitman Adult. [After Kobelt.]
 
A. Female. B. Male.
 
The Mullerian duct (31) in the female functions as the oviduct or Fallopian tube ; from below its fimbriated abdominal opening is seen an hydatid, probably the rudiment of a pronephric tubule ; in the male the blind end of the Mullerian duct forms the hydatid of Morgagni, m. The Wolffian body persists in three sections - (i.) the anterior as rudimentary tubes, sometimes forming hydatids, h ; h'. terminal bulb or hydatid in female ; ( 2 .) the middle set of tubes (c) or coni vasculosi, forming the epididymis of the male and the epoophoron of the female, ep ; ( 3 .) the posterior rudimentary tubules, paroophoron of female and vasa aberrentia of male. The fold of mesentery slinging the ovary ( 0 ) is the mesorchium ; t. testis.
 
 
With the exception of two or three, these tubules become detached from the Wolffian body ; those that remain act as vasa efferentia (coni vasculosi of Mammals). Several rudimentary outgrowths from the Malpighian bodies may persist as hydatids or as vasa aberrentia (fig. 181, b, v.a).
 
The Wolffian duct in the male Amniota is transformed into the vas deferens ; its anterior portion becomes extremely convoluted, and forms the canal of the epididymis, the head of the epididymis being formed from the testicular network, which, as has just been described, is secondarily developed from the Malpighian bodies.
 
In the female rudimentary structures of a similar nature occur (figs. 1 8 1, A, h, v.a); the anterior tubules form hydatids, the posterior degenerate into solid cords. These structures are collectively known as the parovarium (epoophoron or Kosenmiiller -s organ).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
257
 
 
The Wolffian duct more or less disappears in the female. In Snakes and several Lizards the posterior portion may remain as a small functionless canal, and in some Mammals (Pig, Ruminants.
 
 
Fig. 182. - Urogenital Organs of a Female Human F<etus of 3 \ Inches Long, or about Fourteen Weeks. [ From Quain after Waldeyer .]
 
e. tubes of the anterior part of the Wolffian body, forming the epoophoron of Waldeyer (parovarium of Kobelt) ; M. Mullerian duct ; m. its anterior fimbriated orifice ; 0. ovary full of primordial ova ; W. posterior part of the Wolffian body, forming the paroophoron of His and Waldeyer ; W. Wolffian duct.
 
 
Fox, Cat, and some Monkeys) the middle portion may persist as Gaertner -s duct.
 
The posterior or non-sexual portion of the Wolffian body de
 
 
I. Ideal undifferentiated condition. ~H. reproductive gland lying on the tubules of the Wolffian body, W : M. Mullerian duct ; S. urogenital sinus. II. Transformations in the female. - F. fimbriated orifice, with hydatid (M) of the Fallopian tube, T; O. ovary ; P. parovarium ; U. uterus. III. Transformations in the male. - a. vas aberrans ; E. epididymis with hydatid, h ; u. uterus masculiuus ; V. vas defereus ; urogenital sinus. 4. Monotrematous, and 5. Eutheriau, stages in development of the posterior passages, -a. allantois ; b. bladder ; u. urachus ; d and M. rectum ; k. cloaca ;
 
&. urogenital sinus; m. perineum.
 
grades into the para-epididymis or organ of Girald&s in the male (fig. 184, w), and into the paroophoron in the female (fig. 185, w).
 
" R
 
 
258
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Generative Ducts of Ganoids and Teleosts. - There are not at present sufficient data upon which to satisfactorily determine the homology of the Teleostean oviduct. Rathke, Balfour, and Huxley have demonstrated that the Teleostei form an extreme of the Ganoid series, and that the oviduct of the Smelt (Osmerus) is in every way identical with that of Amia. Some Teleosts, such as the Salmon, have no oviduct, their ova dehiscing into the body-cavity to pass to the exterior through the abdominal pore. Huxley points out that in the Sturgeons (Sturios) and Lepidosteus the renal are much wider than the generative ducts, and the communication between them is effected far in front of the external aperture ; while in Polypterus and Amia the oviducts are wider than the ureters, and they communicate nearer the external opening ; in Osmerus the common aperture of the oviducts lies in front of the opening of the ureter ; and lastly, in the Salmo the abortion of the oviducts, commenced in Osmerus, is completed, and the so-called “abdominal pore- is the homologue of half of the urogenital opening of the Ganoids, and has nothing to do with the “ abdominal pores - of these fish and of the Selachians. Against this view must be placed the fact, discovered by Rathke and confirmed by Bridge, that in Mormyrus oxyrhynchus the ordinary generative ducts coexist with abdominal pores. There is, unfortunately, no complete account of the development of the Ganoid oviduct ; it is possible that it represents, in part at least, the Mullerian duct of other forms, but Balfour has suggested that it is a modified segmental tubule of the mesonephros.
 
There is also great uncertainty concerning the nature of the duct of the testis in Teleosts. What has been said above for the oviduct also applies largely to the efferent duct of the testis. Balfour has proved that the anterior portion of the Wolffian body in Lepidosteus is connected with the testis as in Elasmobranchs, and thus in that Ganoid the Wolffian duct functions as the vas deferens.
 
Weber, who has very recently investigated the subject, has come to the conclusion that the genital pore in female Salmonidae is the homologue of that of other Teleostei ; it communicates with a pair of peritoneal funnels which open widely into the bodycavity. These may in some instances extend forwards close to the ovary (Mallotus, Osmerus). The peritoneal funnels are incompletely homologous with the oviducts of those Teleostei with so-called enclosed ovaries, and neither are homologous with the oviducts of other Vertebrates. In the male Salmonidae the vasa defferentia of the testes open to the exterior by a pore common to the ureters, precisely as in other Teleostei. In old Salmonids, in males as well as in females, a pair of true abdominal pOres occur, a pore being situated on each side of the anus. They are not concerned in the evacuation of ova; in individual cases one or both may be absent. Weber considers these abdominal pores as rudimentary structures, perhaps as remnants of segmental ducts. He homologises them with the abdominal pores of Holocephali, Elasmobranchii, Ganoidei, and Mormyridse. The so-called abdominal pore of the Cyclostomi and Mursenidse may be compared with the genital pore of the Salmonidse and other Teleostei (c/. p. 214).
 
Metanephric Duct. - The duct of the metanephros or kidney proper is known as the ureter. At first it opens into the Wolffian duct, but it early acquires an independent opening into the cloaca (urogenital sinus).
 
In Sauropsida and Monotremes the ureters open into the urogenital sinus quite independently of the urinary bladder. In the higher Mammals the ureters open directly into the bladder.
 
Suprarenal Bodies. - The suprarenal bodies of Vertebrates were shown by Balfour to have a double origin. The medullary substance is derived from an extension of
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
259
 
 
the ganglia of the sympathetic system, while the cortical substance is of mesoblastic origin.
 
Weldon has lately demonstrated that the cortical substance of the suprarenal bodies arises as a proliferation of the peritoneum, just internal to the segmental tubules, throughout the whole extent of the mesonephros. This blastema subsequently surrounds the outgrowths from the ganglia.
 
In Bdellostoma, as Weldon has shown, the head-kidney has become modified so as to form an organ functionally analogous to the suprarenals ; while in Teleosteans a most remarkable series of modifications, affecting every region of the kidney, has been described by Balfour and Emery.
 
Weldon holds that the same causes which led to the degeneration of the original renal pronephros (causes among which the specialisation of the pericardium and the development of the air-bladder and lungs may have played a considerable part), the same causes which led to the establishment of the mesonephros as the chief seat of renal secretion, may, and indeed must, have rendered advantageous the suppression of any glandular organ in the pronephric region ; and thus when, in consequence of the change of function of the Wolffian duct more and more, the mesonephros became useless as a kidney, it is easy to understand how some of its component parts underwent in their turn the same change of function as had been undergone by tlie anterior part of the renal organ at an earlier period of its evolution.
 
 
Urinary Bladder. - A dilated portion of the Wolffian ducts which occurs in many Fishes is usually termed a urinary bladder. In Amphibia a thin-walled vesicle (urocyst) develops from the ventral wall of the cloacal section of the mesenteron, and is homologous with the urinary bladder of the Amniota. On referring to the mode of development of the Wolffian duct, it will be obvious that the piscine “ urinary bladder - is not in any sense of the term homologous with that of the Amniota.
 
In the Amniota the urinary bladder is a persistent portion of the stalk of the allantois (p. 81), which becomes converted into a vesicle. That portion of the stem of the allantois distal to the bladder which remains within the body-cavity after the formation of the umbilical cord becomes degraded into a solid cord, and is known as the urachus (figs. 143, 184, 185).
 
The bladder opens on the ventral wall of the cloaca in Amphibia and in those Sauropsida in which it persists throughout life (Chelonia and Lacertilia). In these the ureters open independently into the cloaca.
 
In the Monotremes the bladder opens into the anterior end of the urogenital sinus (fig. 179, A, u.s .), into which the ureters and generative ducts also debouch. The urogenital sinus or vestibule may be regarded as the proximal portion of the allantoic stalk.
 
In all higher Mammals the ureters open directly into the bladder itself, owing to the increase in length of the primitively short interspace between the orifices of the ureters and generative
 
 
260
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ducts. This narrow lengthened portion of the urogenital sinus
 
The urethra and generative
 
 
Fig. 184. - Diagram of the Mammalian Type of Male Sexual Organs, [ From Quain .] Compare with fig. 185.
 
C. Cowper -s gland of one side ; cp. corpora cavernosa penis, cut short ; e. caput epididymis ; g. gubernaculum ; i. rectum ; m. hydatid of Morgagni, the persistent anterior end of the Mullerian duct, the conjoint posterior ends of which form the uterus masculinus ; pr. prostate gland ; s. scrotum ; sp. corpus spongiosum urethrae ; t. testis (testicle) in the place of its original formation, the dotted line indicates the direction in which the testis and epididymis change place in their descent from the abdomen into the scrotum; vd. vas deferens ; vh. vas aberrans ; vs. vesicula seminalis; W. remnants of Wolffian body (the organ of Giraldes or paradidymis of Waldeyer), 3, 4, 5, as in fig. 185.
 
 
ducts open into the anterior extremity of the urogenital sinus in Marsupials and many of the lower Eutheria (compare fig. 179).
 
This condition always persists in the male (fig. 184), as the urogenital sinus traverses the penis. In the females, however, of
 
 
is known as the urethra (fig. 184).
 
 
Fig. 185. - Diagram of the Mammalian Type of Female Sexual Organs.
 
[From Quain.]
 
This diagram should be carefully compared with fig. 184, it will be seen that the dotted lines in one indicate functional organs in the other, and help to demonstrate the significance of certain rudimentary structures.
 
C. gland of Bartholin (Cowper -s gland) ; c.c. corpus cavernosum clitoridis ; dG. remains of the left Wolffian duct, which may persist as the duct of Gaertner; /. abdominal opening of left Fallopian tube ; g. round ligament (corresponding to the gubernaculum) ; h. hymen ; i. rectum ; l. labium ; m. cut Fallopian tube (oviduct or Mullerian duct) of the right side ; n. nympha ; 0. left ovary ; po. parovarium : sc. vascular bulb or corpus spongiosum ; u. uterus ; v. vulva ; va. vagina ; W. scattered remains of Wolffian tubes (paroophoron) ; w. cut end of vanished right Wolffian duct : 3. ureter ; 4. bladder passing below into the urethra ; 5. urachus or remnant of stalk of allantois.
 
 
the more specialised Eutheria the urogenital sinus becomes much shortened and flattened out, so that eventually it is merely re
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
2G1
 
 
presented by the space known as the vestibule of the vulva (fig. 185, 189). In the forms in which this occurs the urinary and generative ducts come to have independent openings to the exterior. The accompanying diagrams illustrate the changes undergone in the human female foetus.
 
At an early period (figs. 186 and 143, a) the allantois and Mullerian duct communicate with the rectum, but not with the exterior. The proctodaeum is next developed (fig. 187), and forms a cloaca, into which the urogenital ducts and the rectum open. The cloaca is then divided into an anterior or ventral part, the urogeni
 
 
Fig. 188. Fig. 189.
 
Diagrams Illustrating the Evolution of the Posterior Passages.
 
[From, Landois and Stirling .]
 
Fig. 186. - Allantois continuous with rectum. Fig. 187. - Cloaca formed. Fig. 188. -
 
Early condition in male, before the closure of the folds of the groove on the posterior side of the penis. Fig. 189. - Early female condition.
 
a. commencement of proctodseum ; all. allantois ; b. bladder ; c. penis ; CL. cloaca ; m. Mullerian duct ; R. rectum ; u. urethra ; s. vestibule ; su. urogenital sinus ; v. vas deferens in fig. 188, vagina in fig. 189.
 
tal sinus, and into a posterior or dorsal portion, the anus (fig. 189), by a downgrowth of the tissue between the rectum and Mullerian duct, which forms the perineum. At a latter stage the bladder forms a rounded vesicle, and the urogenital sinus becomes much more shallow. Fig. 188 represents a stage in the male corresponding to fig. 190, B, before the urogenital orifice has become enclosed by the base of the raphe of the penis.
 
Mammalian External Generative Organs. - The external generative organs of the Eutheria develop as follows : - Anteriorly to the cloaca an elevation (genital eminence) appears, and surrounding it in front and on each side is a large cutaneous fold (fig. 190). The anus is next separated from the urogenital sinus by the formation of the perineum. The genital eminence grows rapidly, forming a cylinder, which is grooved on its posterior surface ; the two folds of the groove extend back
 
262
 
 
THE STUDY OF EMBRYOLOGY.
 
 
wards, so as to lie between the urogenital orifice and the large folds (fig. 190, b). So far the development is precisely the same for both sexes.
 
In the female the genital eminence usually remains comparatively small, and is known as the clitoris ; its groove becomes less marked ; the posterior edges of the groove persist as the nymph se or labia minora. The anterior portion of the large cutaneous fold becomes the mons veneris, and the lateral folds greatly increase in size, so as, in most cases, to enclose the clitoris and constitute the labia majora.
 
In the male the genital eminence increases in size to form the penis. The margins of the groove close over, so as to convert it into a canal, the posterior ends at the same time growing over the urogenital orifice, so that the urogenital sinus is directly continued through the penis. The lateral portion of the large cutaneous fold unite together behind the penis, and fusing in the middle line, form the scrotum - the raph^ indicating the line of junction. In some of the lower Mammals {e.g., Rabbit) the scrotal sacs remain distinct. The so-called “ urethra - of the male consists of three distinct regions : (1) the urethra proper, “prostatic portion,- extending from the neck of the bladder to the orifices of the vasa deferentia and the uterus masculinus ; (2) the urogenital sinus, “membranous portion and (3) the canal of the penis or “spongy portion,-
 
On each side of the urogenital sinus corresponding to the large fold is a perforation of the inner wall of the abdomen, which is known as the internal inguinal ring. In
 
 
Fig. 190. - Development of the External Sexual Organs in the Human Male and Female from the Undifferentiated Condition. [After Ecker.]
 
A. Embryo of about nine weeks, in which the external sexual distinction is not yet established and the cloaca still exists. B. An older embryo, without marked sexual distinction ; the anus is now separated from the urogenital aperture. C. Female embryo of about ten weeks. D. Male embryo somewhat more advanced.
 
a. anus ; can. tail : c. clitoris ; cl. cloaca ; l. labium ; Is. undifferentiated sexual fold ; p. penis ; p.c. undifferentiated sexual eminence ; s. scrotum ; ug.o. urogenital opening.
 
 
the male a sac-like diverticulum of the peritoneum, the processus vaginalis, passes through the abdominal ring into the scrotum. In some Eutheria the testes always remain within the abdominal cavity, but in others they temporarily or permanently pass through the abdominal ring, and into the peritoneal pouch within the scrotum. Normally, in those animals in which a permanent descent of the testes occurs, the inguinal rings close, and the testes are enclosed within a serous sac ; when this does not take place, a portion of the intestine may force its way through the ring into the scrotum, and thus produce a hernia.
 
 
Generative Organs. - The sexual cells are usually developed from a distinct epithelium; the Sponges form an apparent exception, as the sexual cells are derived from the mesenchymatous mesoderm, which is itself, however, probably solely derived from the endoderm.
 
Weismann and others have recently shown that, as a rule, the sexual cells arise from the endoderm of the stolon or stems of the fixed Hydroids, and subsequently migrate to what are termed the
 
 
ORGANS DERIVED FROM THE AIESOBLAST.
 
 
263
 
 
generative organs. These latter may be situated within fixed (sporosacs) or detachable (medusoids) lateral buds or gonophores. The sexual cells mature in their secondary location. The sexual cells of Hydra are usually stated to be of ectodermic origin, but the prevalence of the former mode of origin in the marine Hydroids, combined with the fact of the presence of chlorophyll in the ovum of Hydra viridis, render it quite possible that a migration occurs also in this degraded fresli-water form.
 
In all the other Coelenterates the ova and spermatozoa arise from the hypoblast of the mesenteric pouches or canals.
 
Lang states that in certain Turbellarian Worms (Polyclades) the sexual cells are developed at the expense of the epithelium of the gastric diverticula, that is, from the hypoblast.
 
Nothing definite is known concerning the development of the generative glands of Molluscs.
 
In Sagitta, although it belongs to the Coelomata, a pair of primitive sexual cells appears as early as the gastrula stage, subsequently each cell develops into the ovary and testis of its side.
 
It is characteristic of most, if not of all the Coelomata, that the generative organs arise from the epithelium of the body-cavity. There are no precise accounts of the mode of formation of the generative organs, or gonads, as they are more concisely termed by Lankester, amongst the Invertebrates. The structure of such organs is never complicated, and the dehiscence of free epithelial cells, as in the case of ova, is not specially remarkable.
 
The maturation of the ovum and its acquisition of food-yolk, and the difficult problem of spermatogenesis, have already been alluded to (p. 14).
 
In the Vertebrates the germ-cells are modifications of a special linear tract (germinal epithelium) of the peritoneum, between the mesonephros and the insertion of the mesentery (fig. 175, 'p.o). The germinal epithelium may project more or less into the bodycavity to form a germinal ridge (fig. 178, c).
 
It is now possible to make a general statement and affirm that in the great majority of cases, at least, the sexual pells arise from the endoderm (hypoblast) in the Acoelomata ; but in those forms in which the archenteron is produced into radial pouches, chambers, or canals, they occur on the walls of such diverticula.
 
In the Coelomata the gonads are developed from the coelomic epithelium ; but as this is derived primitively from archenteric diverticula, the generative epithelium is practically a homologous tissue throughout the Metazoa.
 
The sexual products may find their way to the exterior by very different means. In some cases it is by the rupture or destruction
 
 
264
 
 
THE STUDY OF EMBEYOLOGY.
 
 
of the parent ; they may migrate through the parental tissues, or dehisce into the body-cavity.
 
From the body-cavity they may pass to the exterior through abdominal pores (Cyclostomi and some Teleosts), or be conveyed by more or less modified nephridia (Chsetopoda, Gephyrea, Brachiopoda, Mollusca, and some Vertebrates (see p. 2 37).
 
External generative or copulatory organs occur in the higher members of many groups, to render more certain the fertilisation of the ovum.
 
==Chapter VIII. General Considerations==
 
Complexity of Embryological Phenomena. - -The phenomena of Embryology are of a very complex nature, owing to abbreviation or precociousness in the development of certain organs, and in the occurrence of a series of transformations which have reference solely to the ancestry of the individual, the latter often bearing no discernible relation whatever to the adult condition.
 
The irrelevance of these metamorphoses to the adult state is in some cases emphasised by the fact of their suppression in certain members of a group, as, for example, amongst the Scyphomedusse. The genus Pelagia, although closely related to Aurelia, develops directly from the egg wuthout the intervention of the Scyphistoma larva; and even Aurelia may abnormally have an abbreviated development. The characteristic larval forms of the Echinozoon Echinoderms are omitted in the development of those forms in which the young are reared in brood-pouches or similar protective chambers. The following will serve as types : - Leptychaster kerguelenensis, Ophiacantha vivipara, Hemiaster cavernosus, and Psolus ephippifer. The direct development of Astacus is an example of the suppression of metamorphoses amongst the Crustacea, but in this Decapod a good deal of food-yolk is present.
 
The passing through of a free larval existence must be considered as constituting a drain upon the energy of the organism, and this loss naturally affects the adult condition. As Sollas points out, when such a larva “ finally reaches the adult state, it has already to a considerable extent worn out its machinery and expended its powers of converting energy. A still more important consequence, however, would seem to follow from the premature aging due to a free larval existence, and that is the comparatively early exhaustion of the powder of undergoing transformational change ; the adult or comparatively stable state is reached sooner than it otherwise would be, and the chances of further development are
 
 
266
 
 
THE STUDY OF EMBRYOLOGY.
 
 
correspondingly diminished.- It has been pointed out by several authors that the individual which is best equipped as an adult is that which has rapidly passed through its embryonic condition under circumstances where it has been extraneously nourished and protected. Again, to quote from Sollas, “ The longer life in the mature state, acquired by those forms which are saved from the drudgery of a larval existence, offers increased opportunities for evolution to the adult animals, so that a progressive development, starting from higher and higher platforms, is directly favoured. But not only is a longer existence assured to the adult - existence in the embryonic state is shortened ; and perhaps here the influence of seclusion is most clearly exhibited, for the energy which would be expended in a free larva in activities other than those involved in producing structural change is here solely devoted to that end, and hence the embryonic stages are passed over by secluded forms with comparative rapidity.-
 
In studying the development of animals, it must always be remembered that what is known as the “ struggle for existence - is continually acting upon the larval form as an individual, and that while the larva has to adapt itself to present conditions and to supply its own wants, the rudiments, or the formative tissue (blastema), of future organs may be precociously formed. This is the main reason for the complications and abbreviations which occur so frequently in the development of animals. Occasionally larval forms, so to speak, run wild, and do not develop into their normal adults, the form known as Leptocephalus amongst Teleosts affording a good example of this vagary.
 
The real nature of many einbryological phenomena must remain unknown until the properties of protoplasm are considerably more elucidated. At present, we can deal only with the results, and not with the causes of changes in organic matter.
 
In the course of this work attempts have been made to indicate how certain organs may have been developed from pre-existing simpler structures in response to definite stimuli or to the requirements of the organism. The further our knowledge extends the more certain it appears that evolution is mainly the result of a mechanical necessity, or, as James Hinton put it, “organic forms are the result of motion in the direction of least resistance.-
 
Suggestions as to the possible significance of observed embryological facts must be held only in the most tentative manner. It is easy to frame plausible theories respecting the evolution of
 
 
GENERAL CONSIDERATIONS.
 
 
267
 
 
organs or of the animals themselves, but great caution is necessary in accepting them, and, at best, they should be regarded as merely working hypotheses.
 
Sketch of a Possible Evolution of the Metazoa. - The Protozoa combine all the essential activities of life within the limits of small independent units of protoplasm, and even in these differentiation may occur to a considerable extent. Those causes which result in the production of complicated organs in the Metazoa also act on unicellular forms, but, having less scope, the result is less evident. The higher organisation of multicellular animals is solely attributable to the large number of aggregated units which constitute their body; the forces acting upon all living beings must be the same.
 
The formation of masses or colonies of cells (aggregates of protoplasmic units) may possibly be primarily due to imperfect fission. Cell-division itself ( ie ., reproduction) is usually regarded as being primitively due to excess of growth consequent upon excess of nutrition ; Geddes, however, suggests a different interpretation of the origin of cell-division (p. 279). Amongst the Protozoa reproduction results in the formation of distinct and independent organisms, each one of which is unicellular like its parent. In only a few forms are individuals aggregated into colonies, and in these but little co-ordination occurs.
 
More precise histological research is now demonstrating that in most, if not in all, animal (and vegetable) tissues the component cells are united together by strands of protoplasm, often of extreme tenuity. There may thus be a protoplasmic continuity extending throughout the whole organism, and possibly all the living cells of an animal are directly or indirectly connected with one another, except the lymph and blood-corpuscles.
 
The observations of Sedgwick on the syncytial segmentation of Peripatus (fig. 19) are in this respect very suggestive, and it may yet be proved that the complete division of an ovum into distinct segmentation spheres (fig. 12) is apparent rather than real.
 
It appears that all the cells of adult Coelenterates are connected together by means of protoplasmic processes, and it might fairly be assumed that the cells of a segmented ccelenterate ovum and of the embryo into which it will develop are similarly united ; but there is at present no definite embryological evidence to support this conclusion. The cellular network of the parenchymula larva
 
 
268
 
 
THE STUDY OF EMBRYOLOGY.
 
 
of Obelia (fig. 46) is, according to Merejkowsky, a secondary condition due to the fusion of the processes of amoeboid cells.
 
Whether directly continuous or not, all the cells of a Metazoon are so grouped as to constitute a co-ordinated whole, the life of the individual being the sum-total of the activities or lives of the constituent cells. Theoretically each one of these cells possesses all the attributes of protoplasm, as, most probably, was actually the case when the ancestral form was passing from the Protozoon to the Metazoon condition, a stage which is now represented by the blastula larva. We may assume that each cell then possessed nutritive, sensory, metabolic, and reproductive functions ; but in process of time specialisation occurred, and the concurrent limitation of function resulted.
 
In unicellular animals one pole or aspect of the body is usually concerned in the ingestion of food, and we are justified in assuming the same for the Protozoon ancestor of the Metazoa.
 
The segmentation of the ovum is stated to occur in two different ways. Either, according to the generally received account, it may from the first divide the cell horizontally into a nutritive (vegetative) and sensory (animal) portion ; or, according to Agassiz and Whitman, the ovum may divide longitudinally (axially), then transversely, and lastly horizontally. In either case a multicellular mass is formed, of which the upper pole is more especially sensory (epiblast) and the lower nutritive (hypoblast). Assuming it to have been flattened, Biitschli has termed this theoretical ancestral form a Plakula (p. 23).
 
The series of stages from an unicellular form to an organism, consisting of two sets or layers of cells, presents us with no special difficulty, and plausible theories have been framed to account for the formation of a double-layered gastrula from the single-layered blastula. It is a matter of some importance to note that embryological evidence, as a whole, supports the conclusion that the future epiblastic (ectodermic) and hypoblastic (endodermic) cells are already practically differentiated in the blastula stage, and that the gastrula was evolved as a result of that differentiation. It is too often assumed that all the cells of the blastula are identical in every respect.
 
Brief History of Mesoblastic Tissues. - The conversion of a diploblastic form to one with three layers (triploblastic) is readily conceivable. It is possible that the third layer (mesoblast) primitively arose as the result of excessive nutrition of the nutritive
 
 
GENERAL CONSIDERATIONS.
 
 
269
 
 
cells (hypoblast). The inner moieties of these cells separating themselves as amoeboid cells (mesamoeboids, or archaeocytes of Sollas), which would then crawl about in the space (segmentation cavity) between the two layers. Similar cells arise in some embryos from the epiblast also. These cells would readily assume the amoeboid condition, as they were not subject to pressure and had sufficient space for migration.
 
Whether originally specially nutritive or not, the wandering cells would readily become modified and change their function ; their contractile power might be emphasised, and thus they might be converted into simple muscle-cells. By the secretion of mineral matter they would form skeletogenous cells. By retaining a free existence others would serve as carriers of matter, or, in other words, become corpuscles of the nutrient fluid.
 
Most of the internal supporting (endoskeletal) elements, with the exception of the notochord of the Chordata and the connective tissues, are, together with the blood-corpuscles and vascular system, developed from the mesoblast. Lankester has associated these series of tissues under the common designation of “ skeletotrophic.- This he regards as a “ natural group of tissues which is divisible into - (i.) Skeletal, including fibrous, adenoid, adipose, bony, and cartilaginous tissues. (2.) Vasifactive, including capillaries and embryonic blood-vessels. (3.) Haemolymph, including the haema or haemaglobinous element and lymph, the colourless element of vascular fluids.-
 
Lankester further points out that “ the mother-cells of all tissues are either ‘ entoplastic - or ‘ ectoplastic, - or both - that is to say, the metamorphosis of their protoplasm is either essentially one occurring at the surface of the protoplasmic corpuscle, or one occurring deeply within its substance, or the two processes may go on in connection with the same cell.- Thus hyaline cartilage is essentially ectoplastic, while notochordal tissue results from a metamorphosis of the cells and is essentially entoplastic. “ Fibrous tissue generally is ectoplastic, as the protoplasmic corpuscles remain more or less intact whilst surrounded by the fibrous and lamellar masses to which they have peripherally or laterally given origin. This is true of ordinary subcutaneous areolar tissue, of tendon, of mucous tissue (umbilical cord), and of corneal tissue. At the same time we find in various Invertebrate groups (not in the Vertebrata) an entoplastic form corresponding chemically aud functionally to the ectoplastic forms just cited. This is the vesi
 
270
 
 
THE STUDY OF EMBRYOLOGY.
 
 
cular connective tissue so abundant in the Mollusca, in the Nemertines, and other Invertebrates. The only tissue which in form represents this among the connective tissues of Vertebrates is adipose tissue.- The vesicular cells of Mollusca contain glycogen ; indeed, a glycogenetic function is now known to be widely distributed in various mesoblastic tissues.
 
“Yet further, the tissues of the connective group which are specially related to the nutrient fluids (such as blood and lymph), and which form the wall of the coelom or of blood-channels, may be entoplastic when they give rise, by internal metamorphosis (liquid vacuolation), to capillary vessels ; or ectoplastic when they constitute spongy or lacuniferous cell aggregates, the cells separated by intercellular channels, such as we find in the ‘ pulp * of lymphglands and the spleen, and in the lacunar tissue of Molluscs.-
 
The formation of gastric pouches (archenteric diverticula) appears to have resulted from the disproportional growth of the hypoblast. In forms higher than the Ccelenterates these pouches were constricted off from the central cavity and formed a true body-cavity or coelom. A nutritive fluid might collect by osmosis within the body-cavity.
 
The progression of the organism in a determinate direction would ensure a bilaterally symmetrical arrangement of the organs of the body, and, consequently, of the archenteric diverticula. A dorsal and ventral mesentery would result from the appression of the inner walls of the confluent lateral coeloms, while transverse mesenteries or septa would occur if the coeloms of the segments remained distinct.
 
The primitive nutritive corpuscles (mesamoeboids) lie within the blastocoel (or, as Hubrecht proposes to term it, the archicoel), and consequently outside the archenteric diverticula. When the coelomic walls were approaching one another, many of the corpuscles would be enclosed between them ; and if a small space was left between the walls of the coeloms, a tube would be formed, lying within the mesentery, containing amoeboid corpuscles. The walls of the coeloms possess actual or incipient muscle-fibres, and are therefore contractile. The contractility of the walls of the mesentery would thus result in a longitudinal contractile tube containing corpuscles, in other words, a vascular system would be initiated. The development of the heart in both Vertebrates and Chsetopoda appears to support this hypothesis of its evolution.
 
Hubrecht claims for the blood- vascular system of the ISTeinertiue
 
 
GENERAL CONSIDERATIONS.
 
 
271
 
 
Worm Linens that it arises merely by the “ connective tissue- not obliterating the archiccel in these places, and that the indifferent mesoblast is modified in situ into the endothelium and walls of the vessels. In most other animals the smaller vessels are formed by the hollowing out of solid cell-rows and cell-groups.
 
It would be rash to hazard a conjecture concerning the evolution of the excretory organs until we have more precise information concerning their development in the lower Metazoa. It is not improbable that there is no genetic connection between the excretory organs (nephridia) of certain groups ; thus it is difficult to see the homology in such organs as the green gland of Decapod with the excretory tubes of Amphipod Crustacea, or these again with the nephridia of Peripatus and the Malpighian tubules of Insects. The Vertebrate excretory organs appear almost certainly to have been evolved from some primitive form of nephridium, from which the nephridia of the Segmented Worms were independently differentiated.
 
Embryonic Digestion. - But little is known concerning digestion and assimilation in embryos. The actual processes must be assumed to be essentially similar to those occurring in adults. The following general features, which alone can be dealt with here, are worthy of notice.
 
As was mentioned very early in this work, an oosperm must be regarded as an amoeboid Protozoon, which multiplies by fission very rapidly, but which is precluded from obtaining fresh nutriment directly. The energy requisite for this enormous activity is provided by the breaking down, through digestion, of the highly nitrogenous food-yolk which is stored up within the body of the ovum.
 
In many cases the stored-up nutrient material, yolk, is really derived from neighbouring ovarian cells which the ovum has swallowed. (This process, which is simply a case of feeding, must not be confounded with the formation of a plasmodium or syncytium by the fusion of previously distinct protoplasmic units.) The ovum has, in fact, gorged itself preparatory to entering upon a stage of rapid cell-division. The telolecithal and centrolecithal distribution of the yolk in the ovum and developing embryo has been already referred to. In the former case the yolk is actually stored up within the primitive hypoblast cells, that is, within those very cells whose function is to digest it. In the second case the yolk is afterwards transferred to those cells.
 
 
272
 
 
THE STUDY OF EMBRYOLOGY.
 
 
i. Hypoblastic Digestion. - The act of digestion is almost entirely performed by the hypoblast. From the nature of the case all Protozoon digestion must be intracellular, that is, must be effected within the cell itself. It is now proved that the digestion of the Coelenterates and of some Turbellarian Worms is largely intracellular, although extracellular digestion also occurs to some extent. Even in some of the lower Vertebrates the epithelial cells of the intestine may send out pseudopodia for the purpose of ingesting fragments of partially digested food. In other words, the lower Metazoa have not yet broken away from the traditions of Protozoon digestion. In this respect early embryos of higher Invertebrates reproduce the ancestral condition ; for we find in the Crustacea (fig. 22) that the hypoblast of the gastrula stage feeds upon the yolk by means of pseudopodia, and the digestion is intracellular.
 
Caldwell states that throughout larval life intracellular digestion occurs in the first stomach of Phoronis, but that this mode of digestion ceases when the metamorphosis takes place.
 
Kollmann has recently shown that in the meroblastic ova of the Lizard and Fowl (fig. 66) the primitive cells of the germinal wall, in the equivalent of the gastrula stage, engulf and digest the yolk spheres and granules like an Amoeba eating its prey.
 
It is probable that extracellular digestion, as it occurs in the more specialised Metazoa, does not take place till ‘‘hepatic- or other secretory cells make their appearance. Most Prosobranch Molluscs, such as Purpura and Fusus, possess a large quantity of food-yolk which is stored up within the hypoblast cells (fig. 18), and the digestion of which is consequently intracellular. It is well known that during the veliger stage these Molluscs are truly cannibals and devour their weaker brethren. This new food passes into the mesenteron (archenteron), and certain of the hypoblast cells acquire a very different appearance from the remainder and constitute true digestive cells. Food in process of digestion is seen within the cavity of the mesenteron. As a matter of fact, the two modes of digestion take place simultaneously until the yolk is quite absorbed.
 
This view is rendered the more probable from the fact that in the Ichthyopsida the distinctive complex digestive glands are either not at all or only slightly developed. Each individual cell of the mesenteron may be regarded as individually digestive, and thus in these forms hypoblastic intracellular digestion occurs.
 
 
GENERAL CONSIDERATIONS.
 
 
273
 
 
Temporary pseudopodia, for the seizure of food particles, are very generally emitted by the cells of the intestinal epithelium in the lower Vertebrates. Such highly differentiated glands as the peptic glands and the glands of Lieberkiihn are found, from the Reptilia upwards, in an increasing degree. Their secretion acts chemically upon the whole or a portion of the food, and digests it within the cavity of the alimentary canal. The liver has been omitted in this connection, as it is not, in the true sense of the term, a digestive gland. As Wiedersheim has pointed out, there is a well-marked correlation between the folds of the mucous membrane and the development of intestinal glands. At first, as in the Cyclostomes, the folds have only a longitudinal direction, but afterwards transverse folds appear and crypts are formed in order to increase the secretory surface of the alimentary canal.
 
In Mammals the embryo is nourished directly by the blood of its mother, and the hypoblast of the foetus has never been functional in digestion ; it consequently requires what Sollas has termed a gastric education before it can digest the food of the adult. (This argument does not apply to those Sharks and Lizards in which there is a slight connection between the yolk-sac of the embryo and the blood-vessels of the wall of the oviduct, as in these forms a large amount of food-yolk is always present.) The secretion of milk by the mother supplies a readily assimilable pabulum, and the peculiar character of the first-formed milk probably renders the education still more gradual. A somewhat similar digestive education occurs in some Birds, such as Pigeons, the Flamingo, and others.
 
2. Epiblastic Digestion. - The epiblast very rarely appears to have a digestive function. Metschnikoff, however, has observed intracellular ingestion by the ectoderm cells of larval Actiniae, and Kollmann states that the epiblastic cells of the blastoderm of certain Sauropsida can take up food by means of pseudopodia and digest it in the intracellular manner. It has been previously noted that villi develop from the epiblast which underlies the yolk-sac in Birds (fig. 75, B, v ), and also from the epiblast of the allantoic folds (c, v) } which absorb the remaining albumen of the egg. In the lower Mammals (fig. 8o) similar villi occur, which must be the means of absorbing nutriment from the uterine wall.
 
3. Mesoblastic Digestion. - The undifferentiated wandering mesoblast cells may also be concerned in digestion, but their ingestion of foreign particles may be due in many cases to the
 
s
 
 
274
 
 
THE STUDY OF EMBRYOLOGY.
 
 
mechanical properties of their protoplasm rather than to an actual selection. Our knowledge concerning the behaviour of these cells in embryo Invertebrates is almost entirely due to Metschnikoff, who has proved that the mesamoeboids are of great physiological importance from their first appearance, and in this respect they offer a marked contrast to the mesothelial mesoblast.
 
In Echinoderm larvae, for instance, which undergo rapid metamorphoses, the disappearing organs break down into albuminoid globules, which are devoured and digested by the mesamoeboids, or phagocytes, as Metschnikoff terms them. The latter also ingest small foreign particles which may be forced into the segmentationcavity. In many cases the mesamoeboids fuse to form a plasmodium or giant cell, in order to effect this more readily ; in some cases the mesamoeboids merely collect round the foreign body in order to isolate it.
 
The lymph-corpuscles (leucocytes) have been shown by Wiedersheim and by Schafer to perform an important part in digestion in adult Vertebrates. These cells have been proved to force their way through the mucous membrane into the cavity of the intestine, and there to devour fat, and probably amyloid particles ; they then return, and, crawling between the epithelium cells, pass into the lacteals. Others, again, merely ingest the food particles which have penetrated through the intestinal epithelium. In all cases, probably, the leucocytes pass into the lymphatics, where their contents are discharged by the disintegration of the cells themselves. The lymphatic fluid or chyle then passes into the general circulation, carrying with it the digested food which has been conveyed from the intestine by the leucocytes, and a large amount of proteid material derived from their dissolved protoplasm and nuclei.
 
It is still an open question whether digestion may not be performed in Sponges by the ectoderm as well as by the endoderm. The wandering mesoderm cells are probably concerned in the conveyance of nutriment and the removal of waste products, in addition to those functions which are more generally regarded as typical of that layer.
 
Closely associated with the subject of embryonic digestion is the part which the foetal membranes of Amniote Vertebrates play in nutrition. The reader is referred to the section which deals with these structures (pp. 78-96) for a summary of the evolution of the foetal membranes of the Amniota. The gradual Requisition
 
 
GENERAL CONSIDERATIONS.
 
 
275
 
 
by the allantois of the whole of the nutrition of the embryo is especially noteworthy.
 
Embryonic Respiration. - The function of respiration must of necessity occur throughout the whole of embryonic and larval life. As a rule, it is more active in larvae than in adults ; at all events, the former always speedily succumb to a deficiency in the supply of oxygen.
 
The true respiratory process, i.e., the assimilation of oxygen and the excretion of carbon dioxide, occurs in the ultimate tissues ; it is only the exchange of the latter gas for the former of the external medium which occurs in what are termed respiratory organs.
 
As Dohrn points out, it is the vascular system which is really respiratory, and the pressure of a blood-vessel against an epithelium would cause an evagination of that tissue, be it epiblastic or hypoblastic. Of course the whole skin of the body and the alimentary tract were the primitive respiratory surfaces. The production of gill-filaments on a given area is the result of the presence of blood-vessels ; it is the latter after all, and not merely epithelial prolongations, which constitute gills.
 
It often happens that in embryos and larval forms the delicacy of the tissues suffices for the interchange of the gases, so that special respiratory surfaces are not required. When protective envelopes are present, they are usually very permeable to gases.
 
The proctodaeum serves as a special respiratory organ in certain larval Arthropoda; as, for instance, in the ISTauplius larvae generally, and in the aquatic larvae of Dragon-flies.
 
The higher organisation of the embryos of Vertebrates necessitates a large supply of oxygen, and, consequently, special provision has to be made by the development of larval respiratory organs, especially in those forms which undergo a secluded development. These may either be (i) the phylogenetic respiratory organs, which are utilised in the ontogeny of the individual, or (2) they may bear no relation either to the ancestral or to the adult respiratory organs. A pair of examples of each of these two cases will illustrate the general principle.
 
1. Utilisation of Phylogenetic Respiratory Organs in Ontogeny. - The ordinary hypoblastic gills of Elasmobranchs appear early in the embryo, but the filaments on the posterior aspect of the archs are greatly elongated, so as to form a very characteristic fringe of gills, which have even been regarded as belonging to a different category from the normal filaments.
 
 
276
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Both external and internal, i.e., epiblastic (?) and hypoblastic gills occur in the newly hatched tadpoles of Frogs. Whatever may be the exact significance of the former, the latter certainly are an example of the utilisation by a larva of the ancestral mode of respiration, as the respiratory organs of the adult, in this case the skin and lungs, have no connection whatever with the former.
 
2. Secondarily Acquired Larval Respiratory Organs. - The embryonic respiration of the Arnniota affords a good example of the second proposition. In none are the walls of the visceral clefts functional as gills at any time, and, as the lungs are only functional after birth, accessory respiratory organs must be provided.
 
In Sauropsida the area vasculosa of the yolk-sac forms the first respiratory surface, this function is next shared with the rapidly developing vascular allantois, and lastly, owing to its enormous size, the allantois becomes the sole respiratory organ. As has been mentioned above (p. 259), the allantois is probably the hypertrophied and precociously developed urinary bladder, and we may assume that the ancestral forms of the Arnniota, like the Amphibia, had a large membranous vascular urocyst, which was capable of being early utilised as a respiratory organ. The topography of the allantoic blood-vessels, and the fact that the proximal portion of the allantois actually persists as the adult bladder, support this view. It is well known that egg-shells are very porous to gases.
 
Respiration in the embryos of the Prototherian Mammals is doubtless perfectly comparable with that in Reptilian embryos, whereas, in the Eutheria, aerial respiration is impossible owing to the embryos being included within the uterus. The foetus in utero has, however, no need for special organs of respiration, as it is supplied with aerated arterial blood direct from the main arterial trunks of the parent. The carbonic acid and other waste products of the embryo are carried away by the maternal venous circulation.
 
Evolution of Nervous System and Sense Organs. - The epi blast naturally forms the protective covering of the organism, and would readily be modified to meet special requirements. From its position it would be directly subjected to every vibration in the external medium, and would therefore be continually receiving numerous stimuli, which would call into play the sensibility of the protoplasm of the cells. It is, then, no wonder that sense-cells originated, or that these became grouped together to form sense
 
 
GENERAL CONSIDERATIONS.
 
 
277
 
 
organs, or that a further differentiation occurred which resulted in the evolution of a highly specialised nervous system.
 
All these obvious facts were sufficiently noticed in the section on the “ Organs derived from the Epiblast,- and therefore need not be reiterated here.
 
Continuity of Germ-Producing Tissue. - The germ-producing tissue is to be regarded as the direct product of the similar tissue of its parent, that is to say, a portion, however minute, of germinal substance is transmitted by the parent to its offspring. The germinal substance in the latter is increased by the ordinary method of nutrition and growth, but it still has the same essential character that was transmitted to it. The offspring in its turn passes on this germinal substance. There is thus a continuity of germinal matter, which, since it is transferred in an extremely minute quantity, must have an inconceivably complex structure, as it possesses the power of transmitting hereditary characters even of the most trivial nature.
 
It is maintained by some that the nucleus is the essential element in the germ-cell, whether ovum or spermatozoon, and that the cell-protoplasm is merely a nutritive basis. The structure of the ovum has already been stated to be similar in many respects to that of ordinary undifferentiated tissue-cells (fig. 5*). The distinguishing feature of the nucleus over the rest of the protoplasm of the cell consists in its possession of chromatin. As the chromatin always takes a conspicuous part in segmentation, we are justified in assuming that the chromatin or nuclein is concerned in the reproductive function. Fertilisation appears to be mainly the fusion of the nuclein elements of a pair of cells which are liberated from usually two parents. The resulting compound oosperm develops by segmentation and ulterior differentiation into an organism resembling, and at the same time differing from, each of its parents both in feature and in inherited tendency.
 
Weismann recently proposed the view that the nucleus of every germ-cell contains “ germ-plasma ,- or that substance which enables the germ-cell to build up a new individual ; and “ histogenetic plasma,- or that substance which enables the germ-cell to accumulate yolk, secrete membranes, or, in short, to develop itself into its characteristic structure as a ripe ovum or spermatozoon.
 
It is the germ-plasma alone that is required for the development of the embryo. The histogenetic plasma, having performed its function of building up the germ-cell, is useless, and has to be got rid off ; so it is extruded as the polar-cells, or as the passive element in the male germ-cells. If the germ-plasma left in the ovum has sufficient vigour (which would probably depend upon its quantity), there is nothing to prevent its further development into a new individual - that is, nothing to hinder the occurrence of parthenogenesis. As a matter of fact, however, this is rarely the case, and it requires the sudden accession of fresh energy in the shape of a spermatozoon to enable the germ-plasma of the ovum to further develop. In this view there is no essential distinction between the nucleus of the ovum and that of the spermatozoon ; the latter, like the former, is merely germ -plasm : the difference being that, as a rule, the germ-plasm of the male cell has an entirely different series of inherited characters, which it can transmit to the segmentation nucleus in the same manner as those of the female cell are transmitted.
 
The essential act of fertilisation, therefore, does not consist in the fusion of elements which differ in kind, but merely in the sudden accession of a store of energy which will enable the ovum to segment and build up a new individual. This brings fertilisation to resemble conjugation yet more closely ; and it further explains how it is that, in those forms in which parthenogenesis is not known to occur, the ovum may segment, and proceed a short way on its development.
 
This theory also agrees well with certain facts concerning the asexual reproduction of animals or plants. During segmentation there is formed in the nuclei of the segmentation-cells fresh histogenetic plasma, which is more especially concerned in the differentiation of the tissues ; but the germ-plasma may be generally diffused, or it may be early localised within certain segmentation-cells. Sponges, the Hydra, some Sea- Anemones, may be taken as examples of the former condition, as in these animals apparently any portion of the body containing ectoderm and endoderm will serve to produce a new individual ; and in the case of the two first-named, the germ-cells themselves appear to arise indiscriminately from the mesoderm in the former, and from the ectoderm (?) in the latter. In other Ccelenterata the germ-cells are of hypoblastic origin. In the second case, where the germ-plasma is localised to a special tissue, those segmentation-cells which will form the epiblast possess no germ-plasma, and, consequently, they can only build up specialised tissue. On the other hand, in most cases at all events, the germ-plasma, which at first is restricted to the nuclei of the hypoblast cells, becomes, as development takes place, still further localised until it is situated solely in that tissue which has for its especial function the reproduction of the individual. In other words, it is restricted to the generative gland. Asexual reproduction in such groups as the Polyzoa and Ascidians, and certain "Worms, is rendered possible by the retention of germ-plasma within certain undifferentiated tissue (funicular tissue, stolon, budding zones, &c.), from which the whole or part of the new individual may be formed ; but it is impossible to reproduce a perfect individual from any fragment containing epiblastic and hypoblastic tissue, as can be done in the case of Sponges or the Hydra. In this connection it is interesting to find, as Gruber has shown, that if an Infusorian be artificially divided, each portion will become a perfect individual. But if the dismembered portion does not possess a fragment of the original nucleus, the animal thus produced lacks the power of reproduction. It is perfect in every respect, except that it is deprived of the germ -plasma, which alone possesses the reproductive function.
 
Geddes has recently discussed the theory of growth, reproduction, sex, and heredity in terms of the metabolism of protoplasm. “ Protoplasm is regarded as an exceedingly complex and unstable compound, undergoing continual molecular change or metabolism. On the one hand, more or less simple dead matter or food passes into life by a series of assimilative ascending changes, with each of which it becomes molecularly more complex and unstable. On the other hand, the resulting protoplasm is continually breaking down into more and more simple compounds, and finally into waste products. The ascending synthetic constructive series of changes are termed anabolic, and the descending disruptive series TcatabolicP
 
Growth. - Herbert Spencer first pointed out that in the growth of- similarly shaped bodies the mass increases as the cube of the dimensions, the surface only as the square, and applies this conception to express the occurrence of cell-division. “Thus,- as Geddes expresses it, “ in the growing cell the nutritive necessities of the increasing mass are ever less adequately supplied by the less rapidly increasing absorbing surface. The early excess of repair over waste secures the growth of the cell, but the necessarily disproportionate increase of surface implies less opportunity for nutrition, respiration, and excretion ; and waste thus overtakes, balances, and threatens to exceed repair. Three alternatives are then possible - (i) a temporary equilibrium may be established and growth ceases, or (2) the increase of waste may bring about dissolution and death, or, still more frequently, (3) the balance of mass and surface may be restored by the division of the cell.-
 
“ Reproduction - (a) Asexual. - Continued surplus of anabolism involves growth ; this growth is sooner or later checked by the preponderance of katabolism, and the most frequent alternative is the restoration of the balance by cell-division. Thus arises discontinuous growth or asexual reproduction. Budding, simple-division, and spore formation, like continuous cell-division, are simply different forms of the necessary separation which must occur at the limit of growth if the continuity of life is to be preserved. Like continuous cell-division, asexual reproduction occurs when waste or katabolic processes are in the ascendant. But what holds true in the growth of the individual cell is valid also in regard to the aggregate. There, too, a limit of growth must eventually be reached, when discontinuous growth in some form becomes inevitable. The essential difference is simply that at first in the unicellular individual the disintegration and reintegration entirely exhaust the organism and conclude its individual existence, while in higher forms the process becomes more and more localised.-
 
(6.) Sexual Reproduction. - A comparative study of the methods of reproduction which occur amongst the lower plants and Protozoa will demonstrate that “the almost mechanical flowing together of exhausted cells, as illustrated in plasmodia, is connected through the known surviving cases of multiple conjugation with normal conjugation ; - the dimorphism which marks the transition from conjugation to fertilisation, making the latter indispensable, appears very gradually. “The very gentleness of the gradation leads one to regard the two processes as analogous responses to the same physiological necessities. The same disturbance of the balance between anabolism and katabolism which results in the occurrence of asexual reproduction leads in more developed forms to the separation of the dimorphic and mutually dependent elements of sexual reproduction. As asexual reproduction occurs at the limit of growth, so a check to the asexual process involves the appearance of the sexual, which is thus still further associated with katabolic preponderance.- The following illustration will suffice : - Under conditions of favourable temperature and abundant food the parthenogenetic reproduction of female Aphides can be indefinitely prolonged, while a lowering of the temperature and diminution of the food at once reintroduce sexual reproduction.
 
“Nature of Sex. - In attempting to define the distinctive characteristics of male and female, it is necessary to begin with the sexual elements themselves. The difference between male and female is there exhibited in its fundamental and most concentrated expression. It is in the sexual elements, indeed, that the continuity ot organic life is secured, the vegetative organs being but appendages to the direct immortal chain of sex- cells. The large quiescent ovum is the result of a continued surplus of anabolism over katabolism, while the growing preponderance of katabolism must find its outward expression in increased activity of movement and in diminished size ; and the natural result is the flagellate sperm-cell.-
 
In multicellular organisms sexual reproduction makes its appearance when nutrition is checked. “ Some of the cells are seen differentiating at the expense of others, accumulating capital from their neighbours ; and if their area of exploitation be sufficiently large, emphatically anabolic cells or ova result ; while if their area is reduced by the presence of numerous competitors struggling to become germ-cells, the result is the formation of smaller, more katabolic, and ultimately male cells. In the same species distinct organisms may, in the same way, become predominantly anabolic or katabolic, and may be distinguishable as completely female or male organisms.-
 
The numerous facts which have now been accumulated prove that “ such conditions as deficient or abnormal food, high temperature, deficient light, moisture, and the like, are obviously such as would tend to induce a preponderance of waste over repair
 
- a Jcatabolic diathesis ; and we have just seen that these conditions tend to result in
the production of males. Similarly, such factors as abundant and rich nutrition, abundant light and moisture, must be allowed to be such as favour constructive processes and make for anabolism ; and we have just seen that these conditions result in the production of females .-


Oogenesis and Spermatogenesis. - In the maturation of the ovum, the formation of polar cells seems rightly interpreted as an extrusion of the katabolic or male elements from the preponderatingly anabolic ovum ; the converse occurs in spermatogenesis.
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Fertilisation. - According to this view of Geddes -, li fertilisation is comparable to mutual digestion, and the reproductive process has arisen from a nutritive want. The essentially katabolic male cell, getting rid of all accessory nutritive material contained in the sperm-blastophore, brings to the ovum a supply of characteristic katastates, which stimulate the latter to division. The profound chemical differences surmised by some between the male and female elements are intelligible as the outcome of the predominant anabolism and katabolism in the two elements. The union of the two sets of products restores the normal balance and rhythm of cellular life.-
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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.

   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B
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This historic 1887 embryology textbook by Haddon was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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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)

An Introduction to the Study of Embryology

Alfred Cort Haddon
Alfred Cort Haddon (1855–1940)

By

Alfred C. Haddon, M.A. (Cantab.), M.R.I.A.

Professor Of Zoology In The Royal College Of Science, Dublin.


Philadelphia : P. Blakiston, Son & Co., 1012 Walnut Street. 1887.

To the memory of

his beloved master and friend,

Francis Maitland Balfour


This Book is dedicated by the Author.

Francis Balfour (1851-1882)
Francis Balfour (1851-1882)


Preface

Although there are at the present time, in addition to the special accounts in various text-books of Human and Comparative Anatomy, two Students - Manuals in the English language solely devoted to the study of Embryology, it has appeared to me that a relatively small work, giving a general review of the subject, might prove of use to students.

A knowledge of the main facts of Comparative Anatomy and Systematic Zoology has been assumed for the reader, the book being especially designed for Medical Students, or for those who already possess a general acquaintance with the Animal Kingdom.

It will be noticed that many of the more difficult problems of Ontology and Phylogeny and special modes of development have either been merely alluded to or entirely ignored - as, for instance, the segmentation of the ovum and the formation of the germinal layers in Insecta and Teleostei. This has been of set purpose, as my main object in writing this book has been to give a brief connected account of the principal organs, omitting or barely mentioning structures and phenomena, which may be regarded as of secondary importance.

The facts of development have been largely supplemented by hypotheses; and an endeavour has been made so to present the latter, that the student could not mistake them for the former.

It is inevitable that, in compiling such an introductory textbook as this, many subjects must be treated in a manner similar to that in which they have been dealt with by previous authors ; and therefore I have not hesitated to borrow from them when occasion required.

In order to facilitate references, very recent, important, or doubtful observations have been associated in many cases with the investigator -s name. It must be distinctly understood that I do not necessarily personally adopt statements or views which have been incorporated in the book; they are merely put forward for what they are worth.

The beginner is advised to pay attention only to the large type in the first reading, as purely theoretical subjects or matters of detail are printed in the smaller type. Most of the figures have been so drawn as to admit of their being coloured ; and the student is recommended to tint each germinal layer and the organs derived from it in a uniform manner throughout the book : thus the epiblast and its derivatives might be coloured pink, and the hypoblast tinted blue. A uniform system of colouration will be found to be of great assistance to the memory.

The sources from which the figures have been taken are in all cases acknowledged, and in the cases where no source is given the illustrations are original. Figs. 40, 41, 44, 45, 80, 81, and 178* have appeared previously in the Proceedings of the Eoyal Dublin Society.

The classification adopted will be found in an Appendix. All the genera mentioned in the text have been inserted, in order that their systematic position may be seen at a glance.


A Bibliography has also been appended, which is designed to serve simply as a guide to the more recent literature, and no attempt has been made to render the list exhaustive. It will be noticed that most of the Memoirs cited are of later date than the year 1880. The more important earlier papers are recorded in the late Professor Balfour -s “Treatise of Comparative Embryology.- As any student who seriously studies Embryology must consult that invaluable work, I have considered it superfluous to repeat the Bibliography given by Balfour. The prevalent custom of authors of giving references to the literature of the subject under discussion renders it comparatively easy to discover what has already been written thereon.

Finally, I would here express my warmest thanks to my friend Professor G. B. Howes, of the Normal School of Science, South Kensington, for his kindness in reading the proofs and in making many valuable suggestions.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
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)
   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2024, April 16) Embryology Book - An Introduction to the Study of Embryology. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Introduction_to_the_Study_of_Embryology

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