Book - Text-Book of the Embryology of Man and Mammals 17

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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.

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The Organs of the Intermediate Layer or Mesenchyme

THE grounds which made it appear necessary to distinguish in addition to the four epithelial germ-layers a special intermediate layer or mesenchyme have already been given in the first part of this text-book. This distinction is also warranted by the further progress of development. For all the various tissues and organs which are derived in many ways from the intermediate layer allow, even subsequently, a recognition of their close relationship. Histologically the various kinds of connective substance have been for a long time considered as constituting a single family of tissues.

It will be my endeavor to emphasise the relationship of the organs of the intermediate layer, and whatever is characteristic of them from a morphological point of view, more than has been hithei'to customary in text-books, and to do the same in a formal way by embracing these organs in a chapter by themselves and discussing them apart from the organs of the inner, middle, and outer germ-layers.

It is the original province of the intermediate layer to form a packing and sustentative substance between the epithelial layers, a fact which stands out with the greatest distinctness particularly in the lower groups, as for example in the Ccelenterates. It is therefore closely dependent upon the epithelial layers in the matter of its distribution. When the germ-layers are raised up into folds, it penetrates between the layers of the fold as a sustentative lamella ; when the germ-layers are folded inwards, it receives the parts that are being differentiated as for example in the Vertebrates, the neural tube, the masses of the transversely striped muscles, the secretory parenchyma of glands, the optic cups, and the auditory vesicles and provides them with a special envelopment that adjusts itself to them (the membranes of the brain, the perimysium, and the connective-tissue substance of the glands). In consequence of this the intermediate layer, in the same proportion as the germ-layers become more fully organised, becomes itself converted into an extraordinarily complicated framework, and resolved into the most divergent organs, by the formation of evaginations and invaginations and the constricting off of parts.

The form of the intermediate layer thus produced is of a secondary nature, for it is dependent upon the metamorphosis of the germlayers, with which it is most intimately connected. But in addition, the intermediate layer, owing to its own great power of metamorphosis, acquires in all higher organisms, particularly in the Vertebrates, an intricate structure, especially in the way of liistological differentiation or metcvnwrphosis. In this way it gives rise to a long series of various organs the cartilaginous and bony skeletal parts, the fasciae, aponeuroses, and tendons, the blood-vessels and lymphatic glands, etc.

It is therefore fitting to enter here some\vhat more particularry upon a discussion of the principle of histoloyical differentiation, and especially to inquire in what manner it is concerned in the origin of organs differentiated in the mesenchyme.

The most primitive and simplest form of mesenchyme is gelatinous tissue. Not only does it predominate in the lower groups of animals, but it is also the first to be developed in all Vertebrates, out of the embryonic cells of the intermediate layer, and is here the forerunner and the foundation of all the remaining forms of sustentative substance.

In a homogeneous, soft, quite transparent matrix, which chemically considered contains mucous substance or mucin, and therefore does not swell in warm water or acetic acid, there lie at short and regular intervals from one another numerous cells, which send out in all directions abundantly branched protoplasmic processes and by means of these are joined to each other in a network.

In the lower Vertebrates the gelatinous tissue persists at many places, even when the animals are fully grown ; in Man and other Mammals it early disappears, being converted into two higher forms of connective substance, either intojlbrillar connective tissue or into cartilaginous tissue. The first-named arises in the gelatinous matrix by the differentiation of connective-tissue fibres on the part of the cells, which are sometimes close together, sometimes widely scattered. These fibres consist of collagen and upon boiling produce glue. At first sparsely represented, these glue-producing fibres continually increase in volume in older animals. Thus transitional forms, which are designated as foetal or immature connective tissue, lead from gelatinous tissue to mature connective tissue, which consists almost exclusively of fibres and the cells which have produced them. This is capable of a great variety of uses in the organism, according as its fibres cross one another in various directions without order, or are arranged parallel to one another and grouped into special cords and strands. Thus, in connection with other parts derived from the germlayers, it gives rise to a great variety of organs. In some places it forms the foundation for epithelial layers of great superficial extent ; together with them it produces the integument, composed of epidermis, corium, and subcutaneous connective tissue, and the various mucous and serous membranes ; in others it unites with masses of transversely striped muscle, and arranges itself under the influence of their traction into parallel bundles of tense fibres, furnishing tendons and aponeuroses. Again at other places it takes the form of firm sheets of connective tissue, which serve for the separation or enveloping of masses of muscle, the intermuscular ligaments and the fasciae of muscles.

The second metamorphic product of the primary mesenchyme, cartilage, is developed in the following manner : At certain places the embryonic gelatinous tissue acquires as a result of proliferation a greater number of cells, and the cells secrete between them a cartilaginous matrix, chondrin. The parts which have resulted from the process of chondrification exceed in rigidity to a considerable extent the remaining kinds of sustentative substance, the gelatinous and the glue-producing intermediate tissue ; they are sharply differentiated from their softer surroundings, and become adapted, in consequence of their peculiar physical properties, to the assumption of special functions. Cartilage serves in part to keep canals open (cartilage of the larynx and bronchial tree), in part for the protection of vital organs, around which they form a firm envelope (cartilaginous cranium, capsule of the labyrinth, vertebral canal, etc.), and in part for the support of structures projecting from the surface of the body (cartilage of the limbs, branchial rays, etc.). At the same time they afford firm points of attachment for the masses of muscle imbedded in the mesenchyine, neighboring parts of the muscles entering into firm union with them. In this manner there has arisen through histological metamorphosis a differentiated skeletal apparatus, which increases in complication in the same proportion as it acquires more manifold relations with the musculature.

Cartilaginous and connective tissues, finally, are capable of a further histological metamorphosis, since the last form of sustentative substance, osseous tissue, is developed from them in connection with the secretion of salts of lime. There are therefore bones that have arisen from a cartilaginous matrix ami others from one of connective tissue. With the appearance of bone, the skeletal apparatus of Vertebrates has reached its highest perfection.

Even if the rnesenchyme has by these processes experienced an extraordinarily high degree of differentiation and a great diversity of form, the histological processes of differentiation which take place in it are nevertheless not yet exhausted. In the gelatinous or connective-tissue matrix canals and spaces arise in which blood and lymph move in accomplishing their function of intermediating in the metastasis of the organism, not only conveying the nutritive fluids to the individual organs, but also conducting away both the substances which owing to the chemical processes in the tissues -have become useless and the superfluous fluids. Out of these first beginnings arises a very complicated organic apparatus. The larger cavities constitute arteries and veins, and acquire peculiarly constructed thick walls, provided with non-stria te muscle-cells and elastic fibres, in which three different layers can be distinguished as tunica intinia, media, and adventitia. A small part of the blood-passages, especially distinguished by an abundance of muscle-cells, is converted into a propulsive apparatus for the fluid -the heart. The elementary corpuscles that circulate in the currents of the fluid, the blood-cells and lymph-cells, demand renewal the more frequently the more complex the metastasis becomes. This leads to the formation of special breeding places, as it were, for the lymph-corpuscles. In the course of the lymphatic vessels and spaces there takes place at certain points in the connective tissue an especially active proliferation of cells. The substance of the connective-tissue framework assumes here the special modification of reticular or adenoid tissue. The surplus of cells produced enters into the lymphatic current as it sweeps past. According as these lymphoid organs exhibit a simple or a complicated structure, they are distinguished as solitary or aggregated follicles, as lymphatic ganglia and spleen.

Finally there are formed at very many places in the intermediate layer, as especially in the whole course of the intestinal canal, organic [non-striate] muscles.

After this brief survey of the processes of differentiation in the intermediate layer, which are primarily of an histological nature, I turn to the special history of the development of the organs which arise from it, the blood-vessel and skeletal systems.

I. The Development of the Blood-vessel System.

The very first fundament of the blood-vessels and the blood has already been treated of in the first part of this text-book. We will therefore here concern ourselves with the special conditions of the vascular system, with the origin of the heart and chief blood-vessels, and with the special forms which the circulation presents in the various stages of development, and which are dependent on the formation of the foetal membranes. In this I shall treat separately, both for the heart and for the rest of the vascular system, the first fundamental processes of development and the succeeding alterations, from which the ultimate condition is finally evolved.

A. The first DeveloyMientcd Conditions of the Vascular System.

(a) Of the Heart.

The vascular system of Vertebrates can be referred back to a very simple fundamental form namely, to two blood-vessel trunks of which the one runs above and the other below the intestine in the direction of the longitudinal axis of the body. The dorsal trunk, the aorta, lies in the attachment of the dorsal mesentery, by means of which the intestine is connected to the vertebral column ; the other trunk, on the contrary, is imbedded in the ventral mesentery, as far, at least, as such a structure is ever established in the Vertebrates ; it is almost completely metamorphosed into the heart. The latter is therefore nothing else than a peculiarly developed part of a main blood-vessel provided with especially strong muscular walls.

In the first fundament of the heart there are two different types to be distinguished, one of which is present in Selachians, Ganoids, Amphibia, and Cyclostomes, the other in Bony Fishes and the higher Vertebrates Reptiles, Birds, and Mammals.

In the description of the first type, I select as an example the

ep

Fig. 297. Cross section through the region of the heart of an embryo of Salamandra maculosa, in which the fourth visceral arch is indicated, after RABL. d, Epithelium of the intestine ; cm, visceral middle layer ; ep, epidermis ; Ih, anterior part of the body-cavity (pericardio-thoracic cavity) ; end, endocardium ; p, pericardium ; vhg, meso cardium anterius.

development of the heart in the Amphibia, concerning which a detailed account has very recently been published by RABL.

In Amphibia the heart is established very far forward in the embryonic body, underneath the pharynx or cavity of the head-gut (figs. 297, 298). The embryonic body-cavity (IK) reaches into this region, and in cross sections appears upon both sides of the median plane as a narrow fissure. The lateral halves of the body-cavity are separated from each other by a ventral mesentery (vhy), by means of which the under surface of the pharynx is united with the wall of the body. If we examine the ventral mesentery more closely, we observe that in its middle the two mesodermic layers from which it has been developed separate from each other and allow a small cavity (h) to appear, the primitive cardiac cavity. This is stir rounded by a single layer of cells, which is afterwards developed into the endocardium (end).* Outside of the latter the adjacent cells of the middle germ-layer are thickened ; they furnish the material out of which the cardiac musculature (the myocardium) and the superficial membrane of the heart (pericardium viscerale) arise. The fundament of the heart is attached above [dorsally] to the pharynx (d) and below to the body-wall by the remnant of the mesentery, which persists as a thin membrane. We designate these two parts as the suspensory ligaments of the heart, as back [dorsal] and front [ventral] cardiac mesenteries (hhg, vhy), or as mesocardium posterius and anterius. At this time there is nothing to be seen of a pericardial sac, unless we should designate as such the anterior [ventral] region of the bodycavity, from which, as the further course of development will show, the pericardium is chiefly derived.

In the second type, the heart arises from distinct and widely separated halves, as the conditions in the Chick and the Rabbit most distinctly teach.

In the Chick the first traces of the fundament may be demonstrated at an early period, in embryos with four to six primitive segments. They appear here at a time when the various germ-layers are still spread out flat, at a time when the front part of the embryonic fundament first begins to be elevated as the small cephalic protuberance, and the cephalic portion of the intestine is still in the first phases of development. As has already been stated, the intestinal cavity in the Chick is developed by the folding together and fusion of the intestinal plates [splanchnopleure]. If one examines carefully the ridge of an intestinal fold in the very process of being formed (fig. 299 A df) t one observes that its visceral middle layer is somewhat thickened, composed of large cells, and separated from the entoblast by a space filled with a jelly-like matrix. In the latter there lie a few isolated cells, which subsequently

• Relative to the origin of the endothelial sac of the heart, compare the observations given on page 186.

Fig. 298. Cross section from the same series as that from which fig. 297 was drawn, after KABL.

</, Epithelium of the intestine ; vm, visceral, pm, parietal middle layer ; hhg, posterior, vlig, anterior mesocardium ; end, endocardium ; h, cavity of the heart ; Ik, ventral part of the body-cavity ; ep, epidermis.

Fig. 299. Three diagrams to illustrate the formation of the heart in the Chick.

.', Xeural tube; m, mesenchyma of the head ; d, intestinal cavity ; df, folds of the intestinal plate [splanchnopleure], in which the endothelial sacs of the heart are established ; h, endothelial sac of the heart ; ch, chorda ; Ih, bodycavity ; ale, outer, ik, inner germ-layer ; 'nilc 1 , parietal middle layer ; ink", visceral middle layer, from the thickened portion of which the musculature of the heart is developed ; dn, intestinal suture, in which the two intestinal folds are fused ; db, part of the entoblast which has become detached from the epithelium of the cephalic portion of the intestine at the intestinal suture and lies on the yolk ; + dorsal mesocardium ; * ventral mesocardium.

A, The youngest stage shows the infolding of the splanchnopleure, by means of which the cephalic part of the intestine is formed. In the angles of the intestinal folds the two endothelial sacs of the heart have been established between the inner germ-layer and the visceral middle layer.

B, Somewhat older stage. The two folds (A df) have met in the intestinal suture (dn), so that the two endothelial sacs of the heart lie close together in the median plane below the head-gut.

C, Oldest stage. The part of the entoblast which lines the head-gut (<?) has become separated at the intestinal suture (B dn) from the remaining part of the entoblast, which (J6) lies upon

the yolk, so that the two endothelial sacs of the heart are in contact ; they subsequently fuse. They lie in a cardiac suspensorhun formed by the visceral middle layers, the mesocardium, on which one can distinguish an upper [dorsal] and an under part mesocardium superius(-f )and inferius (*). By means of this mesocardium the primitive body-cavity is temporarily divided into two portions.

surround n, small cavity, tho primitive cardiac cavity (A). These cells .issuine more of an endothelial character. While the intestinal folds grow toward each other, the two endothelial tubes become enlarged and ])iish the thickened part of the visceral middle layer before them, so that the latter forms a low, ridge-like elevation into the primitive body-cavity. hi the embryos of higher Vertebrates also, just as in the Amphibia, this stretches forward into the embryonic fundament as far as the last visceral arch, and has here received the special name of neck-cavity or parietal cavity.

In older embryos (fig. 299 7>) the edges of the two folds have met in the median plane, and consequently the two cardiac tubes have moved close together. A process of fusion then takes place between the corresponding parts of the two intestinal folds.

First the entoblastic layers fuse, and in this way is produced (fig. 299 7>) beneath the chorda dorsalis (ck) the cavhVy of the head-gut (d), which then detaches itself from the remaining part of the entoblast (fig. 299 G db) ; the latter is left lying on the yolk and becomes the yolk-sac. Under the cavity of the head-gut the two cardiac sacs have come close together, so that their cavities are separated from each other by their own endothelial walls only. By the breaking through of these there soon arises from them (7i) a single cardiac tube. On the side toward the body-cavity this is covered by the visceral middle layer (mk 2 ), the cells of which are distinguished in the region of the fundament of the heart by their great length and furnish the material for the cardiac musculature, while the inner endothelial membrane becomes only the endocardium.

The whole fundament of the heart lies, as in the Amphibia, in a ventral mesentery, the upper [dorsal] part of which, extending from the heart to the head-gut (fig. 299 G +), can here also be called the dorsal suspensory of the heart or mesocardium postering, and the lower [ventral] part (*) mesocardium anterius. In the Chick, when the cardiac tube begins to be elongated and bent into an S- shaped form, the mesocardium anterius quickly disappears.

Similar conditions are furnished by cross sections through Rabbit embryos 8 or 9 days old. In the latter the paired fundaments of the heart are indeed developed still earlier and more distinctly than in the Chick, even at a time when the entoderm is still spread out flat and has not yet begun to be infolded. Upon cross sections one sees (fig. 301), in a small region at some distance from the median plane, the splanchnopleure separated from the somatopleure by a small fissure (pfi), which is the front end of the primitive body-cavity. At this place the visceral middle layer (ahh) is also raised up somewhat from the entoderm (sw), so that it causes a projection into the bodycavity (ph). Here there is developed between the two layers a small cavity, which is surrounded by an endothelial membrane (lhh\ the primitive cardiac sac. At their first appearance the halves of the heart lie very far apart. They are to be seen both in the very slightly magnified cross section (fig. 300) and also in the surface view of an embryo Rabbit (fig. 302) at the place indicated by h. They

Fig. 300.

Fig. 301.

Figs. 300, 301. Cross section through the head of an embryo Rabbit of the same age as that shown in fig. 302. From KOLLIKER. Fig. 301 is a part of fig. 300 more highly magnified.

Fig. 300. h, h', Fundaments of the heart ; sr, cesophageal groove.

.Fig. 301. rf, Dorsal groove; mp, medullary plate; no, medullary ridge ; h, outer germ-layer; d<l, inner germ-layer ; dd', its chordal thickening ; sp, undivided middle layer ; hp, parietal, dfp, visceral middle layer ; -ph, pericardial part of the body-cavity ; ahh, muscular wall of the heart ; ihh, endothelial layer of the heart ; mes, lateral undivided part of the middle layer ; sw, intestinal fold, from which the ventral wall of the pharynx is formed.

afterwards move toward each other in the same manner as in the Chick by the infolding of the splanchnopleure, and come to lie on the under side of the head-gut, where they fuse and are temporarily attached above and below by means of a dorsal and ventral mesentery. Concerning the processes of development just sketched the question may be raised : What relation do the paired and the unpaired fundaments of the heart sustain to each other ? It is to be answered to this, that the impaired fundament of the heart, ivhich is present in the lower Vertebrates, is to be regarded as the original form. The double

heart-formation, however aberrant it at f,rst si </Ji f appears, can be easily referred back to this.

A single cardiac tube cannot bn developed in tlie higher Vertebrates, because at the time of its formation a headgut does not yet exist, but only the fundament of it is formed in the still flat entoderm. The parts which will subsequently form the ventral wall of the head -gut, and in which the heart is developed, are still two separated territories ; they still lie at some distance from the median plane at the right and at the left. If therefore it is necessary for the heart to be formed at this early period, it must arise in the separated regions, which by the process of infolding are joined into a single ventral tract. The vessel must arise as two parts, which, like the two intestinal folds, subsequently fuse.

Whether the heart is formed Fig. 302. Embryo Rabbit of the ninth day, seen j n one wav or t} ie other, in n it ^ i _ i _ _ *i_ T T" _ _ _ TUT .-, *

a

from the dorsal side, after KOLLIKER. Magnified 21 diameters.

The axial (stem-) zone (stz) and the parietal zone (2?z) are to be distinguished. In the former 8 pairs of primitive segments have been formed at the side of the chorda and neural tube.

p, Area pellncida ; rf, dorsal groove ; r/t, fore brain; ab, optic vesicle; mil, mid -brain; Mi, hind-brain ; uw, primitive segment ; stz, axial zone ; pz, parietal zone ; h, heart ; ph, pericardial part of the body-cavity ; rd, margin of the anterior intestinal portal showing through the overlying structures ; af, fold of the aninion ; vo, \ena omphalomesenterica.

either case it has for a time the form of a straight sac lying ventral to the head -gut and composed of two tubes one within the other, which are separated by a large space assumably filled with a gelatinous matrix. The inner, endothelial tube becomes the

endocardium ; the outer tube, which is derived from the visceral middle layer, furnishes the foundation for the myocardium and the pericardial membrane that immediately invests the surface of the heart.

(/>) The First Developmental Conditions of the Large Vessels. Vitelline Circulation, Allantoic and Placental Circulation.

At both ends, in front and behind, the heart is continuous with the trunks of blood-vessels, which have been established at the same time with it. The anterior or arterial end of the cardiac tube is elongated into an unpaired vessel, the truncus arteriosus, which continues the forward course under the head-gut, and is divided in the region of the first visceral arch into two arms, which embrace the head-gut on the right and left and ascend within the arch to the dorsal surface of the embryo. Here they bend around and run backward in the longitudinal axis of the body to the tail-end. These two vessels are the primitive aortcv (figs. 107, 116 ao) ; they take their course on either side of the chorda dorsalis, above the entoderm and below the primitive segments. They give off lateral branches, among which the arterice omplialomesentericw are in the Amniota distinguished by their great size. These betake themselves to the yolk-sac and conduct the greatest portion of the blood from the two primitive aortas into the area vasculosa, where it goes through the vitelline circulation.

In the Chick, the conditions of which form the basis of the following account (fig. 303), the two vitelline arteries (R.Of.A, L.Of.A) quit the aortaj at some distance from their tail-ends, and pass out laterally from the embryonic fundament between entoderm and visceral middle layer into the area pellucida, traverse the latter, and distribute themselves in the vascular area. They are here resolved into a fine network of vessels, which lie, as a cross section (fig. 116) shows, in the mesenchyme between the entoderm and the visceral middle layer, and which are sharply bounded at their outer edge (toward the vitelline area) by a large marginal vessel (fig. 303 S.T), the sinus terminalis. The latter forms a ring which is everywhere closed, with the exception of a small region which lies in front, at the place where the anterior amniotic sheath has been developed.

From the vascular area the blood is collected into several large venous trunks, by means of which it is conducted back to the heart. From the front part of the marginal sinus it returns in the two vence vitellinai anter lores, which run in a straight line from in front backwards and also receive lateral branches from the vascular network. From the hind part of the sinus terminalis the blood is taken up by the venae vitellinie posteriores, of which the one of the left side is larger than the one of the right ; the latter afterwards degenerates more and more. From the sides likewise there come still larger collecting vessels, the vense vitellinre laterales. All the vitelline veins of either side now unite in the middle of the embryonic body to form a single large trunk, the vena omphalo

Vitelline ;nva.

Vitelline area.

SJF.

S.CnV.

4.0

SX,

Fig. 303. Diagram of the vascular system of the yolk-sac at the end of the third day of incubation, after BALFOUB.

Tlie whole blastoderm has been removed from the egg and is represented as seen from below. Hence what is really at the right appears at the left, and vice vtrsd. The part of the area opaca in which the close vascular network has been formed is sharply terminated at its periphery by the sinus terminalis, and forms the vascular area ; outside of the latter lies the vitelline area. The immediate neighborhood of the embryo is free from a vascular network, and now, as previously, is distinguished by the name area pellucida.

H. Heart; A A, aortic arches; Ao, dorsal aorta; L.Of.A, left, R.Of.A, right vitelline artery; S. T, sinus terminalis ; L.Of, left, R.Of, right vitelline vein ; S. V, sinus venosus ; D.C, diictus Cuvieri ; S.Ca.V, superior, V. Ca, inferior cardinal vein. The veins are left in outline; the arteries are black.

mesenterica (A'.O/and L.Of), which enters the posterior end of the heart (//).

The motion of the blood begins to be visible in the case of the Chick as early as the second day of incubation. At this time the blood is still a clear fluid, which contains only few formed

THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 551 components. For the most of the blood-corpuscles still continue to lie in groups on the walls of the tubes, where they constitute the previously described blood-islands (fig. 114), which cause the redbesprinkled appearance of the vascular area. The contractions of the heart, by which the blood is set in motion, are at first slow and then become more and more rapid. On the average, according to PREYER, the strokes then amount to 130 150 per minute. However, the frequency of pulsations is largely dependent upon external influences; it increases with the elevation of the temperature of incubation and diminishes at every depression of it, as well as when the egg is opened for study. At the time when the heart begins to pulsate, no muscle-fibrillse have been demonstrated in the myocardium ; from this results the interesting fact that purely protoplasmic, still undifierentiated cells are in a condition to make strong rhythmical contractions.

At the end of the third or fourth day the vitelline circulation in the Chick is at its highest development ; it has undergone some slight changes. We find instead of a single vascular network a double one, an arterial and a venous. The arterial network, which receives the blood from the vitelline arteries, lies deeper, nearer to the yolk, while the venous spreads itself out above the former and is adjacent to the visceral middle layer. The circulating blood is distinguished by the abundance of its blood-corpuscles, the bloodislands having entirely disappeared.

The function of the vitelline circulation is twofold. First it serves to provide the blood with oxygen, opportunity for acquiring which is afforded by the whole vascular network being spread out at the surface of the egg. Secondly it serves to bring nutritive substances to the embryo. The yolk-el einents below the entoblast are disassociated, liquefied, and taken up into the blood-vessels, by which they are carried to the embryo, where they serve as nutrition for the rapidly dividing cells. Thus far the embryonic body increases in size at the expense of the yolk-material in the yolksac, which becomes liquefied and absorbed.

The system of vitelline blood-vessels in Mammals agrees in general with that of the Chick, and is distinguished from the latter only in some unimportant points, which do not need to be discussed. However, this question certainly arises* What signification has a vitelline circulation in Mammals (fig. 134 ds) in which the egg is furnished with only a small amount of yolk-material ? Two things are here to be kept in mind ; first, that the eggs of

Mammals were originally provided with abundant yolk-material, like those of Reptiles (compare p. 222), and, secondly, that the blastodermic vesicle, which arises after the process of cleavage, becomes greatly distended by the accumulation within it of a fluid very rich in albumen, furnished by the walls of the uterus. Out of this vesicle likewise the vitelline blood-vessels undoubtedly take up nutritive material and convey it to the embryo, until a more ample nutrition is provided by means of the placenta.

In addition to the vitelline blood-vessels there arises in the higher Vertebrates a second system of vessels, which is distributed in the foetal membranes outside the embryo and for a time is more developed than the remaining vessels of the embryo. It serves for the allantoic circulation of Birds and Reptiles and the placenta! circulation of Mammals.

When in the Chick the allantois (PI. I., fig. 5 al] is evaginated from the front [ventral] wall of the hind-gut, and as an ever increasing sac soon grows out of the body-cavity through the dermal umbilicus into the coalom of the blastodermic vesicle between the serosa and the yolk-sac, there appear in its walls two blood-vessels, which grow forth from the ends of the two primitive aortse the umbilical vessels, or arterice umbilicales. The blood is again collected from the fine capillary network, into which these vessels have been resolved, into the two umbilical veins (veme \ umbilicales), which, after having arrived at the navel, pass on to the two Cuvierian ducts (see p. 577) and pour their blood into these near the entrance of the latter into the sinus venosus. The terminal part of the right vein soon atrophies, whereas the left receives the lateral branches of the right side and is correspondingly developed into a larger trunk. This now also loses its original connection with the ductus Cuvieri, since it effects with the left hepatic vein (vena hepatica revehens) an anastomosis, which continually becomes larger and finally carries the whole stream of blood. Together with the left hepatic vein the left umbilical vein then empties directly into the sinus venosus at the posterior margin of the liver (HOCHSTETTER).

The umbilical and vitelline veins undergo opposite changes in calibre during development : while the vitelline circulation is well developed, the umbilical veins are inconspicuous stems ; afterwards, however, with the increase in the size of the allantois they enlarge, whereas the venae omplialomesentericae undergo degeneration and in the same proportion as the yolk-sac by the absorption of the yolk becomes smaller and loses in significance.

So far as regards the purpose of the umbilical circulation, it subserves in Reptiles and Birds ihe function of respiration. For the allantois, when it has become larger, in the Chick for example, applies itself closely to the serosa and spreads itself out in the vicinity of the air-chamber and underneath the shell, so that the blood circulating in it can enter into an exchange of gases with the atmospheric air. It loses its importance for respiration in the egg only at the moment \vhen the Chick with its beak breaks through the surrounding embryonic membranes, and breathes directly the air contained in the air-chamber. For the conditions of the circulation are now altered throughout the whole body, since with the beginning of the process of respiration the lungs are in a condition to take up a greater quantity of blood, resulting in a degeneration of the umbilical vessels (compare also p. 584).

The umbilical or placental circulation in Mammals (fig. 139 Al) plays a still more important role ; for here the tw r o umbilical arteries convey the blood to the placenta. After the blood has been laden in this organ with oxygen and nutritive substances, it flows back again to the heart, at first through two, afterwards through a single umbilical vein (p. 584).

B. The further Development of the Vascular System up to the Mature Condition.

(a) The Metamorphosis of the Tubular Heart into a Heart vnth Chambers.

As has been .shown in a preceding section, the heart of a Vertebrate originally has for a short time the form of a straight sac, which sends off at its anterior end the two primitive aortic arches, while it receives at its posterior end the two omphalomesenteric veins. The sac lies far forward immediately behind the head on the ventral side of the neck (fig. 304 /*,), in a prolongation of the body-cavity (the parietal or cervical cavity). It is here attached by means of a mesentery of only brief duration, which stretches from the alimentary canal to the ventral wall of the throat, and which is divided by the cardiac sac itself into an upper [dorsal] and an under part, or mesocarclium posterius and anterius.

During the first period of embryonic development the heart is distinguished by a very considerable growth, especially in the longitudinal direction ; consequently it soon ceases to find the necessary room for itself as a straight sac, and is therefore compelled to bend itself into an S-shaped loop (lig. 304). It then takes such a position in the neck that one of the bends of the S, which receives the vitelline veins or, let us say briefly, the venous portion, conies to lie behind and at the left ; the other or arterial portion, which sends off' the aortic arches, in front and at the right (fig. 305).

But this initial position is soon altered (figs. 305, 313) by the two curves of the S assuming another relation to each other. The venous portion moves headwards, the arterial, on the contrary, in the opposite direction, until both lie approximately in the same transverse plane. At the same time they become turned around the longitudinal axis of the embryo, the venous loop moving dorsally, the arterial, on the contrary, ventrally. Seen from in front [ventral aspect] one hides the other, so that it is only in a side view that the S-shaped curvature of the cardiac sac is distinctly recognisable.

By the increase in the size of this viscus the anterior part of the bodycavity is already greatly distended, and becomes still more so in later stages, when there is produced a very thin-walled elevation, that projects out to a great distance (figs. 157 h, 314). Inasmuch as the heart completely fills the cavity, and is covered in by only the thin, transparent, and closely applied wall of the trunk, the niembrana reuniens inferior of KATHKE, it appeal's as though at this time the heart were located entirely outside of the body of the embryo.

After the completion of the twisting, there is effected a division of the S-shaped sac into several successive compartments (figs. 306, 308). The venous portion, which has become broader, and the arterial part are separated from each other by a deep constriction (ok} and can now be distinguished as atrium (vli) and ventricle, while the constricted region between the two may be indicated, by a designation introduced

Fig. 304. Head of a Chick incubated 58 hours, seen from the dorsal face, after MIHALKOVICS. Magnified 40 diameters.

The brain is divided into 4 vesicles: prli, primary fore-brain vesicle ; iiih, mid-brain vesicle ; kh, hindbrain vesicle ; nil, after-brain vesicle; an, optic vesicle ; k, heart (seen through the Jast brainvesicle) ; -co, vena omphalomesenterica ; us, primitive segment ; rm, spinal cord ; x, anterior wall of brain, which is evanii'ated to form the cerebrum.

by HALLER, as auricular canal (ok). The atrium thereby acquires a striking form, since its two lateral walls develop large out-pocketings (ho), the auricles of the heart (auriculae corclis) ; the free edges of the latter, which in addition soon acquire notches, are turned forward, and subsequently enfold more and more the arterial part of the heart, the truncus arteriosus (Ta), and a part of the surface of the ventricle.

The auricular canal (fig. 308 o)is in embryos a well-distinguished narrowed place in the cardiac tube. Owing to the great flattening of its endothelial tube in the sagittal direction, its walls almost

Ta

Fig. 305.

306.

Fig. 305. Heart of a human embryo, the body of which was 2 - 15 mm. long (embryo Lg), after His. [Compare fig. 313.] K, Ventricle ; Ta, truncus arteriosus ; V, venous end of the S-shaped cardiac sac.

Fig. 306. Heart of a human embryo that was 4'3 mm. long, neck measurement (embryo 1), after His. k, Ventricle ; Ta, truncus arteriosus ; ok, canalis auricularis ; vh, atrium with the heart-auricles ho (auriculas cordis).

coming into contact, the passage between atrium and ventricle is reduced to a narrow transverse fissure. It is here that the atrioventricular valves are afterwards developed.

The fundament of the ventricle at first presents the form of a curved tube (figs. 305, 306 k), which however soon changes its form. For at a very early period there is observable on its anterior [ventral] and posterior surfaces a shallow furrow running from above downward, the sulcus interventricularis (fig. 307 si), which allows a left and a right half of the ventricle to be distinguished externally. The latter is the narrower, and is continued upward into the truncus arteriosus (Ta), the beginning of which is somewhat enlarged and designated as bulbus. Between bulbus and ventricle lies a place that is only slightly constricted, called the /return Halleri ; it was recognised even by the older anatomists, then remained for a time little regarded, and now has been again described as noteworthy by His. For it marks the place at which subsequently the semilunar valves are established.

During the externally visible changes of form, some alterations are also progressing in the finer structure of the walls of the heart. As previously remarked, the fundament of the heart consists in the beginning of two sacs, one within the other an inner endothelial tube lined with flat cells, and an outer muscular sac consisting of cells with abundant protoplasm and derived from the middle germ-layer. The two are completely separated from each other by a considerable space, which is probably filled with gelatinous substance.

The endothelial tube is in general a tolerably faithful copy of the muscular sac, yet the narrower and wider regions are more sharply marked off from one another in the former than in the latter ; "as regards its form, it sustains such a relation to the whole heart as it would if it were a greatly shrivelled, internal cast of it " (His). In the muscular sac distinct traces of muscle-fibres can be recognised even at the time when the S-shaped curvature makes its appearance. At later stages in the development differences appear between atrium and ventricle. In the atrium the muscular wall is uniformly thickened into a compact plate, with the inside of which the endothelial tube is in immediate contact. In the ventricle, on the contrary, there occurs a loosening, as it were, of the muscular wall. There are formed numerous small trabeculse of muscular cells, which project into the previously mentioned space between the two sacs and become united to one another, forming a large-meshed network (fig. 311 A). The endothelial tube of the heart, by forming out-pocketings,

SI rk Ik

Fig. 307. Heart of a human embryo of the fifth week, after His. rk t Right, Ik, left ventricle ; si, sulcus interventricn laris ; Ta, truncus arteriosus ; Iho, left, rho, right auricle of the heart.

soon comes into intimate contact with the trabeculse, and envelops each one of them with a special covering (His). Thus there arise in the spongy wall of the ventricle numerous spaces lined with endothelium, which toward the surface of the heart end blindly, but which communicate with the central cavity and like this receive into them the stream of blood.

The embryonic heart of Man and Mammals resembles in its first condition that which has been described up to this point the heart of the lowest Vertebrates, the Fishes. In the former as in the latter it consists of a region, the atrium, which receives the venous blood from the body, and of another, the ventricle, which drives the blood into the arterial vessels. Corresponding to this condition of the heart, the whole circulation in embryos of this stage and in Fishes is still a single and a single one. This becomes changed in the evolution of Vertebrates, as in the embryonic life of the individual, with the development of the lungs, upon the appearance of which a doubling of the heart and of the blood-circulation is introduced.

The cause of such a change is clear, from the topographical relation of the two lungs to the heart, the former arising in the immediate vicinity of the heart by evagination of the fore-gut (fig. 314 la). The lungs therefore receive their blood from an arterial stem lying very near the heart, from the fifth [sixth] pair of aortic arches that arise from the trimcus arteriosus. Similarly they give back again the venous pulmonary blood directly to the heart through short stems, the pulmonary veins, which, originally united into a single collecting trunk (BoRN, ROSE), open into the atrium at the left of the great venous trunks. Therefore the blood that flows directly out of the heart into the lungs also flows directly back again to the heart. Herein is furnished the prerequisite for a double circulation. This comes into existence when the pulmonary and the body currents are separated from each other by means of partitions throughout the short course of the vascular system which both traverse in common (viz., atrium, ventricle, and trimcus arteriosus).

The process of separation begins in the vertebrate phylum with the Dipnoi and Amphibia, in which pulmonary respiration appears for the first time and supplants bronchial respiration. In the amniotic Vertebrates it is accomplished during their embryonic development. Therefore we now have to follow out further the manner in which, in the case of Mammals and especially of Man, according to the recent investigations of His, BORN, and ROSE, the partitions are formed how atrium and ventricle are each divided into right and left compartments, and the truncus arteriosus into arteria pulmonalis and aorta, and how in this way the heart attains its definite form.

The partitions arise independently in each of the three divisions of the heart mentioned.

Let us first take into consideration the atrium, which is for a time the largest and most capacious region of the cardiac sac (fig. 308). In Man a separation into left and right halves (Iv and rv) is observable even in the fourth week, since there is then formed on its hinder [dorsal] and upper wall a perpendicular projection inward, the first trace of the atrial partition (vs) or septum atriorum.

The halves are even now distinguished by the fact that they receive different venous trunks. The vitelline and umbilical veins, as well as the Cuvierian ducts to be discussed later, empty their blood into the right compartment, not directly, however, and by means of separate orifices, but after they have united with one another in the vicinity of the heart to form a large venous sinus (sr) the sinus venosus or s. reunions. This is immediately adjacent to the atrium and communicates with it by means of a large opening in its posterior [dorsal] wall, which is flanked on the right and on the left by a large venous valve (*). Only one small vessel, which traverses the musculature of the heart obliquely, opens, near the atrial partition, into the left compartment ; it is the previously mentioned unpaired pulmonary vein, which is formed immediately outside the atrium by the union of four branches, two of which come from each of the two wings of the lung now being established.

In the further course of development the atrial partition grows

Ps IS sr rv Iv ok

rk ks Ik

Fig. 308. Heart of a human embryo 10 mm. long, neck measurement ; posterior [dorsal] half of the heart, the front walls of which have been removed. After His.

/.s 1 , 1'artitiou of the ventricle ; Ik, left, rk, right ventricle ; ok, auricular canal ; Iv, left, re, right atrhini ; sr, mouth of the sinus reunions ; vs, partition of the atrium (atrial crescent, His ; septam primuin, BORN) ; * Eustachiau valve ; Ps, septum spurium.

from above downward until it reaches the middle of the atrial canal (fig. 309 si). In this manner two completely separated atria would have come into existence at a very early period, if there had not been formed in the upper part of the partition, while it was still growing downward, an opening, the future foramen ovale, which maintains a connection between the two chambers (fig. 309) up to the time of birth. The opening has arisen either from the septum atriorum having become thin and having broken through at a certain region, or from its having been incomplete at this place from the very beginning, as is the case with the Chick for example, where it is traversed by numerous small orifices. Afterwards the foramen ovale, adapting itself to the conditions of the circulation existing at the time, becomes

still larger.

do\vngrowth

The of the atrial partition has, moreover, the immediate result of separating the auricular canal into the left and right atrio

Ps vs sr rv Iv

SI rk ks lie

Fig. 309. Posterior [dorsal] half of the heart of a human embryo of the fifth week, cut open, after His.

ks, Ventricular partition ; Ik, left, rk, right ventricle ; si, lower [posterior] part of the atrial partition (septum intermedium, His) ; to, left, rv, right atrium ; sr, mouth of the sinus reunions ; vs, atrial partition (atrial crescent, His ; septum secundum, BORX) ; Ps, septum spurium ; * E\istachian valve.

ventricular orifices (compare fig. 308 ok with fig. 309). The auricular canal, even very soon after its formation, undergoes important alterations both from without and within. At first visible from the outside (fig. 308 o&), it afterwards disappears from view (fig. 309) by being in a manner overgrown on all sides by the ventricle, and thereby incorporated in its walls, which enlarge upward and, in consequence of a vigorous growth of the musculature, acquire considerable thickness. The opening of the atrial canal into the ventricle, or the foramen atrioventriculare commune (fig. 310 A F.av.c), now has the form of a fissure extending from left to right, which is bounded on either side by two ridge-like lips (o.ek and u.ek}the atrioventricular lips of LINDES, or the endotbelial cushions of SCHMIDT. The ridges have arisen from a growth of the endocardium, and consist of a, gelatinous connective substance and an endothelial investment. The atrial partition, when it has grown down to the auricular canal, soon fuses along its free lower margin with these lips (fig. 309 si) ; the auricular canal is thereby divided into a left and a right atrioventricular opening, ostium atrioventriculare sinistruin and clextrum (fig. 310 B F.av.s and F.av.d], and at the same time both the dorsal and ventral endocardial ridges, which originally bound the opening, are divided in the middle (o.ek and n.ek). The dorsal components soon fuse with the corresponding pieces of the opposite [ventral] side, and thus there arise at the lower margin of the atrial partition (fig. 309 si) two new ridges, one of which projects into the left, the other into the right atrioventricular opening, which furnish the foundation of the median cuspidate valves.

The development of the atrial partition and the division of the auricular canal into the two atrioventricular openings are closely related processes, the former being the cause of the latter. This is clearly proved by pathological -anatomical conditions of arrested development of the heart. In all cases in which the formation of the atrial partition has been for any reason w r hatever interrupted and the lower part of it has been altogether wanting, there has always been only one atrioventricular opening (an ostium venosum commune) present (ARNOLD).

Before we progress further in the history of the development of the atrium, we must add an account of the metamorphoses which have taken place meanwhile in the territory of the ventricle and truncus arteriosus.

The ventricle begins to acquire its partition not much later than the atrium. By the end of the first month its musculature has become considerably thickened (fig. 311 A). Muscular trabeculre have arisen, which project far into the interior of the chamber and are joined to one another, so as to constitute a spongy tissue, the numerous fissures in which are continuous with the narrowed cavity of the heart and likewise allow the current of the blood to pass through them. At one place the musculature is especially thickened and forms a crescent-shaped fold projecting inward, the fundament of the ventricular partition (septum ventriculorum) (figs. 308, 309, 310 ks). This takes its origin from the lower and posterior [dorsal] wall of the ventricle, in the region which is marked externally by the previously mentioned sulcus interventricularis (fig. 307 si). Its free edge is directed upwards and grows toward the bulbus arteriosus and the atrioventricnlar opening. The latter originally lies more in the left half of the ventricle (fig. 310 A F.av.c), but it gradually moves over more to the right, and finally assumes such a position that the ventricular partition by its growth upwards meets it exactly

Oi

o.ek F.av.s

- Ik

Fig. 310. Two diagrams (after BORX) to elucidate the changes in the mutual relations of the ostium atrioventriculare and the ostium interventriculare, as well as the division of the ventricle and large arteries. The ventricles are imagined to have been divided into halves ; one looks into the posterior [dorsal] halves, in which, moreover, the cardiac trabeculse, etc., have been omitted for the sake of simplifying the view.

J, Heart of an embryo Rabbit, in which the head is 3-5 5-8 mm. long. The ventricle is divided by the ventricular partition (ks) into a left and a right half as far as the ostium interventriculare (Oi). The right end of the foramen atrioventriculare commune (F.av.c) extends into the right ventricle ; the endocardia! cushions (o.ek, u.ek) are developed.

B, Heart of an embryo Rabbit, head 7'5 mm. long. The endocardial cushions (o.ek, u.ek) of the foramen atrioventriculare commune are fused, and thereby the for. atrioventr. com. is now separated into a for. atrioventr. dextrum (F.av.d) and sinistrum (F.av.s). The ventricular partition (ks) has likewise fused with the endocardial cushions, and has grown forward as far as the partition (s) of the trunciis arteriosus. By the closure of the remnant of the ostium interventriculare (Oi) the septum membranaceum is formed.

rk, Right, Ik, left ventricle ; ks, ventricular partition ; Pu, arteria pulmonalis ; Ao, aorta ; s, partition of the truncus arteriosus ; Oi, ostium interventriculare ; F.av.c, foramen atrioventriculare commune ; F.ae.d and F.av.s, foramen atrioventriculare dextrum and sinistrum ; o.ek, u.ek, upper and lower endothelial or endocardial cushions.

in the middle and fuses with its edges directly opposite the atrial partition (figs. 309, 310 B).

The division of the ventricle in Man is completed as early as the seventh week. From the atrium, the two compartments of which are united by the foramen ovale, the blood is now conducted through a right and a left ostium atrioventriculare into completely separated right and left ventricles.

The two atrioventricular openings are narrow at the time of their origin ; they are in part surrounded by the previous!} mentioned endocardial ridges that project from the partition, in part by corresponding growths of the endocardium at their lateral circumference. The membranous projections are comparable with primitive pocketvalves, such as are also established in the bulbus arteriosus (GEGENBAUR) ; they constitute the starting-point for the development of the large atrioventricular valves, but furnish, as GEGENBAUR and BERNAYS have shown, only a part the membranous marginal thickening (mk l ) which subsequently disappears almost completely, whereas the compact main part of the valve arises from that portion of the thickened muscular wall of the ventricle itself that surrounds the atrioventricular opening (fig. 311 B mfc).

As was previously stated, in the case of Man the wall of the ventricle during the first months consists of a close spongy network

mi 1

Fig. 311. Diagrammatic representation of the formation of the atrioventricular valves. A , Earlier, B, later condition. After GEGENBAUR. mk, Membranous valve ; mk\ the primitive part of the same ; cht, chordae tendinese ; v, cavity of the ventricle ; b, trabecular network of cardiac musculature ; 21111, papillary muscles ; tc, trabeculfe carnese.

of muscular trabeculae, which are invested by the endocardium and the interstices of which communicate with the small central cavity (fig. 311 A). Such a spongy condition of the wall of the heart persists permanently in Fishes and Amphibia ; in the higher Vertebrates and Man, on the contrary, metamorphoses occur. Toward its external surface the wall of the heart becomes more compact, in that the muscular trabeculae become thicker and the spaces between them narrower, in some parts even disappearing entirely (fig. 311 B tc). The reverse of this process takes place toward the inside. In the vicinity of the atrioventricular opening the trabeculse become thinner and the interstices larger. In this way a part of the thick wall of the ventricle, which looks toward the atrium and encloses the opening, is undermined, as it were, by the blood-current. In this part the muscle-fibres afterwards become entirely rudimentary; there are formed from the interstitial connective-tissue substance tendinous plates, which with the endocardial cushions attached to their margins become the permanent atrioventricular valves (fig. 311 B ink). The latter therefore arise from a part of the spongy wall of the ventricle.

The remnants of the shrivelled muscular trabecuke (fig. 311 B cht), which are attached to the valve from below, become still more rudimentary in the immediate vicinity of the attachment : here also a part of the muscular fibres disappears entirely ; the connective tissue, on the contrary, is preserved, and is converted into the tendinous cords which, known under the name of chordca tendinete, serve to hold in place the valves. At some distance from the latter the trabeculse projecting into the ventricle preserve their fleshy condition and become the papillary muscles (pvi), from the apices of which the chordae tendinere arise. " Whatever of the primitive trabecular network still persists on the inner surface of the ventricle forms a more or less stout ineshwork of muscles, the fleshy pillars of the heart (tc), or trabeculfe carnese." In consequence of all these alterations the originally small cavity of the ventricle has become considerably enlarged at the expense of a part of its spongy wall. For the whole of the space which in fig. 311 B lies below the valves has been produced from the system of originally narrow spaces (fig. 311 A), and has been employed for the enlargement of the central cavity by the degeneration of the fleshy columns into slender tendinous cords.

It still remains for us to investigate the division of the truncus arteriosus and the final metamorphosis of the atrium.

At about the time when the formation of the partition in the ventricle takes place, the truncus arteriosus, which arises from it, becomes somewhat flattened, and thus acquires a fissure-like lumen. On the flat sides two ridge-like thickenings make their appearance (fig. 310 A and B s), grow toward each other, and by their fusion divide the cavity into two passages which are triangular in cross section. Now, too, the beginning of the internal separation makes itself visible externally as two longitudinal furrows, in the same way that the formation of a partition in the ventricle is indicated by the sulcus interventricularis. The two canals resulting from the division are the aorta and the pulmonary artery (Ao and Pu). For a time they continue to be surrounded by a common adventitia, then they become widely separated and also externally detached from each other. The whole process of separation in the truncus arteriosus takes place independently of the development of a partition in the ventricle, beginning as it does at first above and advancing from there downwards. Finally the aortic septum penetrates also into the cavity of the ventricle itself (fig. 310 It s and ks), there unites with the independently developed ventricular partition, furnishes the part known as pars membranacea (Oi), and thus completes the separation of the vessels leading out from the heart, the aorta falling to the lot of the left ventricle, the art. pulmonalis to the right.

The pars membranacea indicates therefore in the finished heart the place at which the separation between the right and left halves of tlu heart is completed (fig. 310 B Oi}. "It is, as it were, the keystone in the final separation of the primitive simple cardiac sac into the four secondary cardiac cavities, as they are formed in Birds and Mammals " (ROSE). From a comparative-anatomical point of view this place presents a special interest from the fact that in Eeptiles there exists here a permanent opening between the two ventricles, the foramen Pannizzse.

Even before the division of the truncus Fig. 3112. Diagram of the ar- arteriosus, the semilunar valves have become rangement of the arterial ,, ., ... ,.

valves. From GEGENBAUR. established as Jour ridges, consisting ot A, Undivided truncus arteriosus gelatinous tissue with a covering of enclo with four fundaments of , , i i valves. B, Division into pui- thelium, at the contracted place which is monaiis (p) and aorta (>, designated as the /return Halleri. Two of each of which possesses three . , , . .

valves. them are halved at the time ot tne divi sion of the truncus into aorta and art.

pulmonalis. For each vessel, therefore, there are now three ridges, which, owing to a shrivelling of the gelatinous tissue, assume the form of pockets. Their arrangement, to which GEGENBAUR has called attention, is intelligible from their method of development, as the accompanying diagram (fig. 312) shows. "By the division of the originally single bulbus arteriosus (A) into two canals (7>), the nodule-like fundaments of the four original valves are distributed in such a manner that the anterior [ventral] one and the anterior halves of the two lateral ones fall to the anterior arterial trunk (pulmonalis), the posterior and the posterior halves of the lateral ones to the posterior arterial trunk (aorta)." Finally, as regards the atrium, it is to be said that the sinus venosus, mentioned at p. 558, the mouth of the pulmonary vein, and the foramen ovale undergo important alterations.

The sinus venosus disappears as an independent structure, since it

THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 565 is gradually merged into the wall of the atrium. In consequence of this the great venous trunks, which originally emptied their blood into it and which have meanwhile been converted into the superior and inferior vense cavre and into the sinus coronarius (the details of which are given in section d), empty directly into the right half of the atrium, and here gradually separate farther and farther from one another. Of the two valves which surround, as was previously stated, the mouth of the sinus venosus, the left becomes rudimentary (figs. 308, 309) ; the right (*), on the contrary, persists at the mouth of the inferior vena cava and of the sinus coronarius, and is divided, corresponding to these, into a larger and a smaller portion, of which the former becomes the valvula Eustachii, the latter the valvula Thebesii.

The four pulmonary veins are united for a time into a common short trunk, which empties into the left half of the atrium. Subsequently the common terminal portion becomes greatly enlarged and merged with the wall of the heart, in the same way as the sinus venosus does. In consequence the four pulmonary veins then open separately and directly into the atrium.

The foramen ovale, the formation of whicli was previously described, maintains a broad communication between the two sides of the atrium during the entire embryonic life. It is bounded behind and below by the atrial partition, a connective-tissue membrane that subsequently receives the name of valvula foraminis ovalis (fig. 309 si). Also from above and in front there is formed a sharp limitation, since a muscular ridge projects inward from the atrial partition, the anterior atrial crescent or the limbus Vieussenii (?;s). Even in the third month all of these parts are distinctly developed ; the valvula foraminis ovalis already reaches nearly to the thickened margin of the anterior muscular crescent, but is deflected obliquely into the left half of the atrium, so that a broad fissure remains open and permits the blood of the inferior vena cava to enter into the left part of the atrium. After birth the margins of the anterior and posterior folds come into contact, and, with occasional exceptions, fuse completely. The posterior fold furnishes the membranous partition of the foramen ovale ; the anterior, with its thickened muscular margin, produces above and in front the linibus Vieussenii. With this the heart has attained its permanent structure.

While the cardiac sac undergoes these complicated differentiations, it changes its position in the body of the embryo and acquires at an

566

EMBRYOLOGY.

period a. special investment, the pericardium. Tn connection with llic latter the diaphragm is formed as a partition between the thoracic and abdominal cavities. This is consequently the most suitable place at which to acquaint ourselves better with these important processes, a part of which are not easily understood. The most of the discoveries in this field we owe to the investigations of CADIAT, His, BALFOUR, USKOW, and others.

(I>) The Development of the Pericardial Sac and the Diaphragm. The Differentiation of the Primary Body-cavity into Pericardial, Thoracic, and Abdominal Cavities.

Originally the body-cavity is widely extended in the body of the embryo, for it can be traced in the lower Vertebrates into the fundament of the head, where it furnishes the cavities of the visceral arches. After the latter have become closed, during which muscles arise from the cells composing their walls, the body-cavity extends forward as far as the last visceral arch and constitutes a large space (fig. 313), in which the heart is developed within the ventral mesentery (mesocardium anterius and posterius). REMAK and KOLLIKER named this space throatcavity ; His introduced the name parietal cavity. But it will be most appropriate if one designates it, after the permanent organs which are derived from it, as the pericardio - thoracic cavity. The more the cardiac tube is thrown into curves, the more extensive this cavity becomes, and it soon acquires in the embryo a comparatively enormous size. By this its front wall is protruded ventrally like a hernia between the head and the navel of the embryo (figs. 314, 157).

Fig. 313. Human embryo (Lg of His) 2 15 mm. long, neck measurement. Reconstruction figure, after His (" Menschliche Embryonen "). Magnified 40 diameters.

M/>, Oral sinus ; Ab, aortic bulb ; Vm, middle part of the ventricle ; Vc, vena cava superior or ductus Cuvieri ; Sr, sinus reunions ; Vii, vena nmbilicalis ; VI, left part of the ventricle ; //o, auricle of the heart ; D, diaphragm ; V.om, vena omphalomesenterica ; Lb, solid fundament of the liver ; Lbg, hepatic duct.

The peiicardio-thoracic cavity begins very early to be sharply marked ofT from the future abdominal cavity by a transverse fold (figs. 313, 314 s-f 0> which begins at the front [ventral] and Literal walls of the trunk, and the free edge of which projects dorsalwards and median wards (fig. 314 z-\-l) into the primitive body-cavity. It marks the course which the terminal part of the vena omphalomesenterica takes in order to reach the heart. Subsequently there are found imbedded in the fold all of the venous trunks which empty into the atrial sinus of the heart (figs. 313, 314), the omphalomesenteric and umbilical veins and the Cuvierian ducts (dc), which collect the blood from the walls of the trunk. Therefore the formation of the transverse fold is most intimately connected ivith the development of the veins. It takes the name of septum transversum (massa transversa, USKOW), and has the form of a transverse bridge of substance uniting the two lateral walls of the trunk (fig. 313), which inserts itself between the sinus venosus and the stomach, and is united with both as well as with the ventral mesentery. Its posterior portion (fig. 314 z + l) contains abundant embryonic connective tissue and blood-vessels, and constitutes a mass described as prekepatieus (Vorleber), since the two liver-sacs (fig. 313 Lb + Lbg) grow out from the duodenum into it and produce the hepatic cylinders. In proportion as this takes place, and the hepatic cylinders spread out from the ventral mesentery laterally into the septum transversum, the latter increases in thickness and now embraces two different fundaments, in front, a plate of substance in which the Cuvierian ducts and other veins run to the heart (the primary diaphragm) ; behind, the two lobes of the liver, which produce ridges that project into the body cavity.

By means of the septum transversum the pericardio-thoracic and the abdominal cavities are almost completely separated (fig. 314). There remain only two narrow canals (brh) (thoracic prolongations of the abdominal cavity, His), which establish a connection behind with the abdominal cavity at either side of the intestinal tube and its dorsal mesentery. The two canals (brh) receive the two fundaments of the lungs (Ig) when they grow out from the ventral wall of the intestinal tube. They afterwards become the two thoracic or pleura! cavities (brh), whereas the larger cavity communicating with them (hh), in which the heart has developed, becomes the pericardial chamber. The latter takes up the whole ventral side of the embryo ; the thoracic cavities, on the contrary, lie quite dorsal next to the posterior wall of the trunk.

How does the closure of these three originally communicating spaces take place, and how do they attain their altered, final position in relation to one another? The pericardia! sac is the first to be separated off. The impulse to separation is furnished by the Cuvierian ducts (fig. 314 dc). One portion of the latter runs down from the dorsum, where it arises by the confluence of the jugular and cardinal veins, along the lateral walls of the trunk to the transverse septum (fig. 314 dc} ; it thereby

Fig. 314. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo R, His), to elucidate the development of the pericardio-thoracic cavity and the diaphragm, after His. al>, Bnlbus arberiosus ; brh, thoracic cavity (recessus parietalis, His) ; hh, pericardia! cavity ; dc, ductus Cuvieri ; dv, vena omphalomesenterica ; nr, umbilical vein ; vca, cardinal vein ; rj, jugular vein ; lg. lung ; z + I, fundament of the diaphragm and liver ; ilk, mandible.

crowds the pleura into the pericardio-thoracic cavity, and in this manner produces the pleuro-pericardial fold. Since the latter is carried farther and farther inward, it continues to narrow the communication between the pericardial cavity (hli) and the two pleural cavities (brh} ; finally, it cuts off the communication entirely, when its free edge has grown [median wards] as far as, and has fused with, the mediastinum posterius, in which the rcsophagus lies. By this migration of the Cuvierian ducts is also explained the position of the superior vena cava, which later opens into the atrium from above, for it is derived from the Cuvierian duct. Originally located in the lateral wall of the trunk, its terminal part is afterwards enclosed in the mediastinum.

After the closure of the pericardia! sac, the narrow, tubular thoracic cavities (fig. 314 Mi) continue for a time to remain in communication behind with the abdominal cavity. The fundaments of the lungs (ly] meantime grow farther into them, and their tips finally come in contact with the upper surface of the liver, which also has now become larger. Then a closure is effected at these places also. From the lateral and posterior walls of the trunk project folds (the pillars of USKOW), which fuse with the septum transversum, and thus form the dorsal part of the diaphragm. One can therefore distinguish a ventral older part and a dorsal younger one.

As GEGENBAUR points out, this explains the course of the phrenic nerve, which runs in front of [ventral to] the heart and lungs and approaches the diaphragm from in front.

Occasionally the fusion of the dorsal and ventral fundaments is interrupted on one side. The consequence of such arrested development is a diaphragmatic hernia i.e., a permanent connection between abdominal and thoracic cavities by means of a hernial orifice, through which loops of the intestine can pass into the thoracic chamber.

When the four large serous spaces of the body have been completely shut off from one another, the individual organs must still undergo extensive alterations of position, in order to attain their ultimate condition. The pericardial sac at first takes up the whole ventral side of the breast, and over a large area is connected with the anterior wall of the thorax and with the upper wall of the diaphragm. Moreover, the latter is united with the liver along its whole under surface. The lungs lie hidden in narrow tubes at the dorsal side of the embryo.

There are two factors that come into the account in this connection (fig. 315). With the increase in the extent of the lungs (/(/), the thoracic cavities (pl.p) extend farther ventrally, and thereby detach the wall of the pericardial sac (jpc), or the pericardium, on the one hand from the lateral and anterior walls of the thorax, and on the other from the surface of the diaphragm. Thus the heart (ht\ with its pericardial sac, is displaced step by step toward the median plane, where, together with the large blood-vessels (ao), the oesophagus (al), and the bronchial tubes, it helps to form a partition the mediastinum -between the greatly enlarged thoracic cavities. In front the pericardial sac then remains in contact with the wall of the thorax (st} and below with tho diaphragm for a little distance only.

The, second factor is the separation of the liver from the primary diaphragm, with ivhich it was united to form the septum transversum. This takes place as follows : At the margin of the liver the peritoneum, which originally covered only its under surface, grows over on to its upper surface, separating it from the primary diaphragm. A connection is retained near the wall of the trunk only. Thus is explained the development of the ligamentum coronarium hepatis,

Fig. 315. Cross section through an advanced embryo of a Rabbit, to show how the pericardial cavity becomes surrounded by the pleural cavities, from BALFOUR. ht, Heart ; pc, pericardial cavity ; pl.p, thoracic or pleural cavity ; Ig, lung ; al, alimentary canal ; no, dorsal aorta ; ch. chorda ; r'p, rib ; st, sternum ; sp.c, spinal cord.

which was disregarded in the section which treated of the ligamentous supports of the liver (p. 330).

The diaphragm finally acquires its permanent condition by the ingrowth of muscles from the wall of the trunk into the connectivetissue lamella.

(c) The Metamorphoses of the Arterial System.

The development of the large arterial trunks lying in the vicinity of the heart is of great interest from a comparative-anatomical point of view. As in all Vertebrates at least five pairs of visceral arches are established on the two sides of the fore-gut (permanently in the gill-breathing Fishes, Dipnoi, and a part of the Amphibia, transitorily in the higher Vertebrates), so also there are developed at the corresponding places on the part of the vascular system five pairs of vascular arches* (fig. 316 1 ' 5 ). They take their origin from the truncus arteriosus (figs. 316, 317), which runs forward under the fore-gut, then follow along the visceral arches up to the dorsal surface of the embryo, and here unite on either side of the vertebral column into longitudinal vessels, the two primitive aortse (fig. 317 ad}. On this account they are called aortic arches, but they are more appropriately designated as visceral-arch vessels.

In the Vertebrates that breathe by means of gills, the vessels of the visceral arches become of importance in the process of respiration, and early lose their simple structure. From their ventral initial portions there arise numerous lateral branches running to the branchial lamella?, which have arisen in large numbers from the mucous membrane investing the visceral arches ; here they are resolved into fine capillary networks. From these the blood is re-collected into venous branches, which open into the upper end of the visceral-arch vessels. The larger the ventral and dorsal lateral branches, the more inconspicuous does the middle part of the vessel of the visceral arch become. At length it has separated into an initial part, the branchial artery, which is distributed to the branchial lamellae in numerous branches, and an upper part, the branchial vein, into which the blood is re-collected. The two are connected with each other by means of the close network only, which, from its superficial position in the mucous membrane, presents a suitable condition for the removal of the gases from the blood.

Since in the Amniota there are no branchial lamellae produced, branchial arteries and veins also fail to be developed, the vessels of

• [The existence of six pairs of vascular arches has recently been shown to be the typical condition, the newly discovered pair, situated between the fourth and fifth pairs of RATHKE'S scheme (fig. 316), being of short duration in Amniota.]

Fig. 316. Diagram of the arrangement of the vessels of the visceral arches from an embryo of an amniotic Vertebrate.

1 5, First to fifth aortic arches ; a<l, aorta dorsalis ; ci, carotis interna ; ce, carotis externa ; v, vertebralis ; s, subclavia ; p, pulmonalis.

the visceral arches retaining their original simple condition. But thoy are in part of only short duration; they soon suffer, by the complete degeneration of extensive portions, a profound metamorphosis, which is effected in a somewhat different manner in Reptiles, Birds, and Mammals. An exposition of the changes in the case of Man only will be given here.

In human embryos only a few millimetres long, the truncus arteriosus, which emerges from the still single cardiac tube, is divided in the vicinity of the first visceral arch into a left and a right branch, which surround the pharynx, and are continuous above with the two primitive aortse. It is the first pair of aortic arches. In

Fig. 317. Development of the large arterial trunks, represented from embryos of a Lizard (A), the Chick (), and the Pig (C), cifter RATHKE. The first two pairs of arterial arches have in all cases disappeared . In A and B the third, fourth, and fifth pairs are still fully preserved ; in C only the two latter are still complete. p, Pulmonary artery arising from the fifth arch, but still joined to the dorsal aorta by means of a ductus Botalli ; c, external, c', internal carotid ; aJ, dorsal aorta ; o, atrium ; r, ventricle ; n, nasal pit ; m, fundament of the anterior limb.

only slightly older embryos their number is rapidly increased by the formation of new connections between the ventral truncus arteriosus and the dorsal primitive aorta?. Soon a second, a third, a fourth, and, finally, a fifth pair make their appearance in the same sequence in which the visceral arches are established in the case of Man as well as the remaining Vertebrates.

The five pairs of vascular arches give off lateral branches to the neighboring organs at a very early period ; of these several acquire a great importance and become carotis externa and interna, vertebralis and subclavia as well as pulrnonalis. The carotis externa (fig. 316 ce and fig. 317 c) arises from the beginning of the first vascular arch, and is distributed to the region of the upper and lower jaws. The carotis iiiterna (tigs. 316 ci, 317 c') likewise arises from the first arch, but farther dorsally, at the point where the arch bends around to become continuous with the root of the aorta ; it conducts the blood to the embryonic brain and to the developing eye-ball (arteria ophthalmica). From the dorsal region of the fourth vascular arch (fig. 316 4 ) a branch is given off which is soon divided into two branches, one of which goes headwards to the medulla oblongata and the brain, the arteria vertebralis (v), whereas the other (s) supplies the upper limb (arteria subclavia). In the course of development these two arteries interchange relations in respect to calibre. In young embryos the vertebralis is by far the more important, while the subclavia is only a small inconspicuous lateral branch. But the more the upper extremity increases in size, the more the subclavia is elevated into the position of" the main trunk, and the more the vertebralis sinks to the rank of an accessory branch. Finally, from the fifth [sixth] arch there bud forth branches to the developing lungs (figs. 316, 317 p}.

As the simple diagram shows, the fundament of the arterial trunks which arise from the heart is originally strictly symmetrical. But at an early period there occur reductions of certain vascular tracts even to their complete disappearance ; in this way the symmetrical arrangement is (jradually converted into an unsymmetrical one.

The accompanying diagram (fig. 318) in which the parts of the vascular course that degenerate are left free, and those which continue to be functional are marked by a heavy central line will serve to illustrate this metamorphosis.

First, as early as the beginning of the nuchal flexure, the first and second vascular arches with the exception of the connecting portions through which the blood flows to the carotis externa (b) disappear.

The third arch (c) persists, but loses its connection with the dorsal end of the fourth, and therefore now conveys all its blood toward the head into the carotis iiiterna (), of which it has now become the initial part.

The chief role in the metamorphosis is assumed by the fourth arid fifth arches (fig. 317 C). They soon exceed all other vessels in size, and as they lie nearest to the heart, they are converted into the two chief arteries which arise from it, the aortic arch and the arteria pulmonalis. An important modification is effected at the place of their origin from the tnmcus arteriosus when the latter is divided lengthwise by means of the development of the partition previously mentioned. The fourth arch (fig. 318 e) then remains in connection with the trunk (d) which arises from the left ventricle and receives blood exclusively from that source. The fifth arch (n), on the contrary, forms the continuation of that half (m) of the truncus arteriosus which emerges from the right ventricle. Thus the division of the blood into two separate currents initiated in the heart is also continued into the nearest vessels, but for a short distance only, since the fourth and fifth pairs of vascular arches (fig. 317) still empty their blood together into the aorta cominunis (ad), with the exception of a certain portion which runs through their accessory branches, in part to the head (c.c) and upper limbs, in part to the still diminutive lungs. Gradually, however, the, process of separation thus introduced is continued still farther into the region of the peripheral vessels and finally leads to the establishment of the entirely distinct major and minor circulations. The final condition is attained by the degeneration of certain portions of the vessels and the enlargement of others.

A preponderance of the vascular arches of the left side over those of the right is soon recognisable (fig. 318). The former continually increase in size, while those of the right side become less and less apparent and finally in places disappear altogether. They are retained only in so far as they conduct the blood to the lateral branches which, arising from them, go to the head, the upper limbs, and the lungs. Consequently of the right aortic arch there remains only the tract which gives rise to the right carotis cominunis (c) and the right subclavia (i-\-l). We designate its initial part as the arteria anonyma brachiocephalica. With this the permanent condition is now established. The remnant of the right fourth vascular arch appears as a side branch only of the aorta (e), which forms an arch 011 the left side of the body, and here gives rise to the carotis cominunis sinistra (c) and the subclavia sin. (A) as additional lateral branches.

The right half of the fifth [sixth] pair of vascular arches likewise undergot'H degeneration, except for the portion that conveys blood

m

Fig. 318. Diagrammatic representation of the metamorphosis of the bloodvessels of the visceral arches in a Mammal, after RATHKE.

a, Carotis intei'na ; b, carotis externa ; c, carotis communis ; d, body or systemic aorta ; e, fourth arch of the left side ; /, dorsal aorta ; g, left, k, right vertebral artery ; /i, left subclavian artery ; i, right subclavian (fourth arch of the right side) ; I, continuation of the right subclavian ; m, pulmonary artery ; n, its ductus Botalli.

to the right lung. On the left side of the body, on the contrary, the pulmonary arch still persists for a long time and conducts blood into the left lung and also through the ductus arteriosus Botalli (n), into the aorta. After birth, in connection with pulmonary respiration, the duct of BOTALLI also degenerates. For the lungs, when they are expanded by the first act of inspiration, are in a condition to receive a greater quantity of blood. The consequence is that blood no longer flows into the ductus Botalli, and that the latter is converted into a connective-tissue cord, which extends between aorta and art. pulmonalis.

In addition to the regressive changes mentioned, there are effected meantime alterations of position in the large vascular trunks that arise from the heart. They move at the same time with the heart from the neck region into the thoracic cavity. In this fact lies the explanation of the peculiar course of the nervus laryngeus inf. or recurrens. At the time when the fourth vascular arch still lies forward in the region

Fig. 319. Diagrammatic representation of the metamorphosis of the arterial arches in Birds, after RATHKE.

, Intez-nal, b, external, c, common carotid ; d, systemic aorta ; e, fourth arch of the right side (root of the aorta) ; /, rightsubclavian; g, dorsal aorta ; h, left subclavian (fourth arch of the left side) ; i, pulmonary artery ; k and I, right and left ductus Botalli of the pulmonary arteries.

of its formation in the fourth visceral arch, the vagus sends to the larynx a small nerve branch, which, to reach its destination, passes below [caudad of] the vascular arch.

When the latter migrates downwards, the nervus laryngeus must thereby be carried down with it into the thoracic cavity, and must form a loop, one portion of which, arising in the thoracic cavity from the vagus, bends around the arch of the aorta on the left side of the body (but around the subclavia on the right side of the body) to become continuous with the second portion, which takes the opposite or upward course to the region of its distribution.

The processes of development discussed also throw light on a series of abnormalities which are quite frequently observed in the large vascular trunks. I shall cite and explain two of the most important of these cases.

Occasionally in the territory of the vessels of the fourth visceral arches the original symmetrical condition is retained. The aorta is then divided in the adult into right and left vascular arches, which convey the blood into the unpaired aorta. From each of them there arises, as in the embryo, a separate carotis commnnis and subclavia.

Another abnormality is brought about by the development of the aortic arch of the right side of the body instead of that of the left, a condition which is met with in the class of Birds (fig. 319) as the normal state. This malformation is always connected with an altered position of the organs of the chest, a situs inversus viscerum. Of the other changes in the region of the arterial system the metamorphosis of the primitive aorta is to be mentioned before all others. As in the other Vertebrates (fig. 127 o), so in Man, there are formed a right and a left aorta ; but they subsequently move close together and fuse. This, again, explains an abnormality, which, it is true, has very rarely been observed in Man. The aorta is divided into right and left halves by means of a longitudinal partition ; the process of fusion, therefore, has not been fully effected.

The aorta gives off at an early period as branches the unpaired mesenterica sup. and rnesenterica inf. to the intestinal canal ; furthermore, near its posterior end, the two voluminous navel vessels, arteries unibilicales (fig. 139 Al). These run from the dorsal wall of the trunk along the sides of the pelvic cavity ventrally to that part of the allantois which is subsequently differentiated into urinary bladder and urachus, here bend upward and pass on either side of the latter in the abdominal wall to the navel, enter the umbilical cord, and are resolved in the placenta into a capillary network, from which the blood is re-collected into the veme unibilicales. During their passage through the pelvic cavity the umbilical arteries give off lateral branches that are at first inconspicuous, the iliacae internee, to the pelvic viscera, the iliacae externse to the posterior limbs now sprouting forth from the trunk as small knobs. The more the latter increase in size in older embryos, the larger do the iliacse externse and their continuations, the femorales, become.

After giving off the two umbilical arteries, the aorta becomes smaller and is continued to the end of the vertebral column as an inconspicuous vessel, the aorta caudalis or sacralis media.

At birth an important alteration occurs in this part of the arterial system also. With the detachment of the umbilical cord, the umbilical arteries can 110 longer receive blood ; they therefore waste away with the exception of the proximal portion, which has given off as lateral branches the internal and external iliacs, and is now designated as the iliaca communis. However, two connectivetissue cords result from the degenerating vessels, the ligamenta vesico-umbilicalia lateralia, which run to the navel on. the right and left of the bladder.

(cZ) Meta/niorphoses of the Venous System.

The older excellent works of KATHKE and the more recent meritorious investigations of His and HOCHSTETTER constitute the foundation of our knowledge in the difficult field with which we are now concerned. They show us that originally all of the chief trunks of the venous system, with the exception of the inferior vena cava, are established in pairs and sij HI metrically. This holds true not only for the vessels which collect the blood from the walls of the trunk and from the head, but also for the veins of the intestinal tube and the embryonic appendages which arise from it.

In the first place, so far as regards the veins of the body, the venous blood is collected from the head into the two jugular veins (fig. 320 vj and fig. 321 A je, ji), which run downwards along the dorsal side of the visceral clefts and unite in the vicinity of the heart with the cardinal veins (fig. 320 vca and fig. 321 A ca). The latter advance in the opposite direction, from below upwards, in the dorsal wall of the trunk, and collect the blood especially from the niesonephros. There arise from the confluence of the two veins the Cuvierian ducts (figs. 320, 321 A dc), from which are subsequently developed the two superior venae cavse. The veins of the trunk in Fishes exhibit a symmetrical arrangement like this throughout life.

In the earliest stages the Cuvierian ducts lie for some distance in the lateral wall of the pericardio-pleural cavity, where they run downwards from the dorsum to the front [ventral] wall of the trunk (fig. 320). On arriving at this point, they enter into the septum transversum, KOLLIKER'S mesocardiuni laterale, in order to reach the atrium of the heart. This important embryonic structure forms a point of collection for all the venous trunks emptying into the heart. In it there are joined to the Cuvierian ducts the veins from the viscera (fig. 313 V.om and Vu, fig. 320 dv and nv), the paired yolk veins and umbilical veins, all of which are joined into the common sinus venosus, which was previously (p. 558) mentioned apropos of the development of the heart, and which is situated directly between atrium and septum transversum.

The two vitelline veins (v. omphalomeseiitericre) return the blood from the yolk-sac ; they are the two oldest and largest venous trunks of the body, but they become inconspicuous in the same ratio as the yolk-sac shrinks to an umbilical vesicle. They run close together along the intestinal tube, and come to lie at the sides of the duodenum and stomach, where they are united to each other by transverse anastomoses even at a very early period.

The navel veins (vena? umbilicales) are also originally double. At first very small, they subsequently become, in contrast with the vitelline veins, more and more voluminous, as the placenta, from

ab uk Ith

Fig. 320. Sagittal reconstruction of a human embryo 5 mm. long, neck measurement (embryo R, His), to illustrate the development of the pericardio-thoracic cavity and the diaphragm, after His.

ab, Aortic bulb ; brh, thoracic cavity (recessus parietalis,jHis) ; hit, pericardial cavity ; tie, ductus Cuvieri ; dc, vitelline vein (v. omphalomesenterica) ; nv, umbilical vein ; vca, cardinal vein ; vj, jugular vein ; Ig, lung ; z + I, fundament of the diaphragm and the liver ; uk, lower jaw.

which they convey the blood back to the body of the embryo, is further developed. At the time of their first appearance the umbilical veins are found to be imbedded in the lateral wall of the abdomen (fig. 313 Vu), in which they make their way to the septum transversuni and the sinus venosus (sr).

The inferior vena cava (fig. 321 A ci] is established later than any of these paired trunks. It makes its appearance as an inconspicuous, from the beginning unpaired, vessel (in the Rabbit on the twelfth day, HOCHSTETTER) on the right side of the aorta in the tissue between the two primitive kidneys ; caudalwards it is connected by

lateral anastomoses with the cardinal veins. At the heart it opens into the sinus venosus.

From this primitive form of the venous system (fig. 321 A) is derived the ultimate condition in Man. There are three changes which are conspicuous in this connection. (1) The veins empty directly into the atrium instead of a venous sinus. (2) The symmetrical arrangement in the region of the Cuvierian ducts and the jugular and cardinal veins gives place to an unsymmetrical arrange

Fig. 321. Diagram of the development of the venous system of the body.

dc, Ductus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subclavia ; ch, v. hepatica revehens ; U, v. umbilicalis ; ci (ci 2 ), v. cava inferior ; ca (ca l , cor, ca 3 ), v. cardinalis ; ilcd, ilcs, v. iliaca communis dextra, sinistra ; ad, us, v. anonym a brachiocephalica dextra, sinistra ; cs, v. cava superior ; csd, v. cava superior dextra ; ess, rudimentary portion of v. cava superior sinistra ; cc, v. coronaria cordis ; o.z, v. azygos ; liz (kz 1 ), v. hemiazygos ; He, v. iliaca externa ; ill, v. iliaca interna ; /, v. renalis.

meiit accompanied by a degeneration or stunting of some of the chief trunks. (3) With the development of the liver there is formed a special portal system.

The alteration first mentioned is accomplished by the incorporation of the sinus venosus in the atrium. At first enclosed in the septum transversum, the sinus elevates itself above the upper surface of the latter, from which it detaches itself, and conies to lie as an appendage to the atrium in the anterior trunk-cavity. Finally it fuses completely with the heart and furnishes the smooth region of the atrial wall, which is destitute of the pectinate muscles (His).

There are in it separate openings for the two Cuvierian ducts the future venae cavae superiores and an opening distinct from them for the veins coming from the viscera below (the future cava inferior).

The metamorphoses in the region of the Cuvierian ducts begin with a change in their position. Their course from above downward becomes more direct. At the same time, like the sinus venosus, they emerge from the niveau of the transverse septum and lateral walls of the trunk into the body-cavity and carry before them the serous membrane, with which they are covered, as a crescentshaped fold, which contributes to the formation of the pericardial sac, and has been already described as the j)leuro-j)ericardial fold. By fusing with the mediastinum the Cuvierian ducts pass from the walls of the trunk into the latter and come to lie nearer together in the median plane. Of their affluents the jugular veins gradually predominate over the cardinal veins (fig. 322 B). There are three reasons for this. First, the anterior part of the body, and especially the brain, far outstrips in growth the posterior part ; secondly, there arises in this region a competitor of the cardinal veins, the inferior vena cava, which assumes in place of them the function of returning the blood. Thirdly, when the anterior limbs are established, the venae subclavise (s) empty into the jugulares. Consequently the lower portion of the jugular, from the entrance of the subclavia onward, now appears as the immediate continuation of the Cuvierian duct, and together with it is designated as superior vena cava (fig. 322 B csd).

There exists between the right and left sides a difference in the course of the superior venae cavae, which, as GEGENBAUR has pointed out, is the cause of the asymmetry that is developed in Man. While the right vena cava superior (fig. 322 B csd) descends more directly to the heart, the left (ess) describes a somewhat longer course. Its terminal portion is bent from the right to the left around the posterior [dorsal] wall of the atrium, where it is imbedded in the coronal furrow and receives the blood from the coronal vein (cc) of the heart.

In Reptiles, Birds, and many Mammals a stage of this kind, with two venae cavae superiores, becomes permanent ; in Man it exists only during the first months. Then there is a partial degeneration of the left vena cava superior. The degeneration is initiated by the formation of a transverse anastomosis (fig. 322 B as) between the right and left trunks. This conveys the blood from the left to the right side, where the conditions are more favorable for the return of the blood to the heart. In consequence of this the proximal end of the right cava becomes much larger, the left, on the contrary, proportionately smaller. Finally, there is a complete wasting away of the latter blood course (fig. 322 ess] as far as the terminal part (cc), which is lodged in the coronal groove. This part remains open, because the cardiac veins convey blood to it, and is now distinguished as sinus coronarius.

A process in many respects similar to this is repeated in the case

Fig. 322. Diagram of the development of the venous system of the body.

ilc, Ductus Cuvieri ; je, ji, vena jugularis externa, interna ; s, v. subclavia ; r/<, v. hepatica revehens ; U, v. umbilical is ; cl (cl~), v. cava inferior; ca (ca l , ca' 2 , ccr), v. cardinal is ; ilcd, ilcs, v. iliaca communis dextra, sinistra ; ail, *, v. anonyma brachiocephalica dextra, sinistra ; cs, v. cava superior ; csd, v. cava superior dextra ; c.s-.s, nidimentary portion of v. cava superior sinistra ; cc, v. coronaria cord is ; az, v. azygos ; hz (/<:'), v. hemiazygos ; He, v. iliaca externa ; Hi, v. iliaca iuterna ; r, v. renalis.

of the cardinal veins (fig. 322 A ca). The latter collect the blood from the primitive kidneys and the posterior wall of the trunk, from the pelvic cavity and the posterior limbs. From the pelvic cavity they receive the vente hypogastrica? (Hi), and from the limbs the v. iliacae externae (He) and their continuation, the v. crurales. In this way the cardinal veins are at first, as in Fishes, the chief collecting trunks of the lower half of the body. Subsequently, however, they diminish in importance, since the inferior vena cava becomes the main collecting trunk instead of them.

It is only within the last few years that the development of the inferior vena cava has been (by HOCHSTETTER) explained. According to his investigations there are to be distinguished two tracts which are of dill'erent origin, a shorter anterior and a longer posterior. The former, as previously mentioned, makes its appearance as an inconspicuous vessel on the right side of the aorta in the tissue between the two primitive kidneys (fig. 322 A and B ci) ; the latter, on the contrary, is developed subsequently out of the posterior region of the right cardinal vein (fig. 322 B ci 2 ). The anterior, independently arising part of the inferior vena cava, soon after its establishment, unites with the two cardinal veins by means of transverse branches in the vicinity of the vena renalis (?*). In consequence of this increase of drainage territory, it soon increases considerably in calibre, and since it presents more favorable conditions for the conveyance of blood from the lower half of the body than the upper portion of the cardinal veins does, it finally becomes the chief conduit.

If the stage thus far described were to become the permanent condition (fig. 322 It), we should have an inferior vena cava, which forks in the region of the renal veins (r) into two parallel trunks, that descend at both sides of the aorta to the pelvis. Such cases, as is known, are found among the varieties of the venous system ; they are derived from the previously described stages of development as malformations by arrested growth. However, they are only rarely observed, for in the normal course of development there is established at an early period an asymmetry between the lower portions of the two cardinal veins, from the moment, indeed, when they have united themselves to the lower part of the inferior vena cava by means of anastomoses. The right portion acquires a preponderance, becomes enlarged, and finally alone persists (fig. 322 B, C), whereas the left lags behind in growth and withers. This results from two conditions. First, the right cardinal vein (ci 2 ) lies more in the direct prolongation of the vena cava inferior than does the left, and thus occupies a more favorable situation ; secondly, there is formed in the pelvic region an anastomosis (ilcs) between the two cardinal veins, which conducts the blood of the left hypogastrica and the left iliaca externa and cruralis to the right side. Owing to this anastomosis, which becomes the vena iliaca communis sinistra, the portion of the left cardinal vein lying between the renal veins and the pelvis (fig. 322 C c 3 ) is rendered functionless, and with the degeneration of the primitive kidney disappears. The right cardinal vein has now become for a certain distance a direct continuation of the inferior vena cava; it furnishes that portion of the latter which is situated between the renal veins and the division into the two ven;i> iliacse cornmunis (fig. 322 B and C ci 2 }.

While the abdominal part of the left cardinal vein (fig. 322 C e 3 ) succumbs and the corresponding region of the right cardinal vein produces the lower part of the inferior vena cava (ci 2 ), their thoracic portions persist in a reduced form, since they receive the blood from, the intercostal spaces (fig. 322 7? c). In this region occurs still another and last metamorphosis, by which likewise an asymmetry is brought about between the halves of the body. In the thoracic part of the body the original conditions of the circulation are altered by the degeneration of the left cava superior (fig. 322 C ess). The direct flow of the left cardinal vein to the atrium is thereby rendered more difficult, and finally ceases altogether, the tract designated by ca 2 undergoing complete degeneration. Meanwhile a transverse anastomosis (hz l ], which has been formed in front of the vertebral column and behind the aorta between the corresponding vessels of both sides, receives the blood of the left side of the body and transports it to the right side. In this manner the thoracic part of the left cardinal vein and its anastomosis become the left hemiazygos (hz and hz l ) ; the right and larger cardinal vein becomes the azygos (az).

Thus by many indirect ways, is attained the permanent condition of the venous system of the trunk, with its asymmetry and its preponderance of the venous trunks in the right half of the body.

A third series of metamorphoses, which we shall now take into consideration, concerns the development of a liver circulation.

The liver receives its blood in different stages of the embryonic development from various sources : for a time from the vitelline veins ; during a second period from the umbilical veins ; after birth, finally, from, the veins of the intestines the portal vein. This threefold alteration finds its explanation in the conditions of growth of the liver, the yolk-sac, and the placenta. As long as the liver is small, the blood corning from the volk-sac suffices for its O v nourishment. But when it increases greatly in size the yolk-sac, on the contrary, growing smaller other blood-vessels at this time, the umbilical veins, must supply the deficiency. When, finally, at birth the placental circulation ceases, the venous trunks of the intestinal canal, which meanwhile have become very large, can supply the needs.

These circumstances must be kept in mind, in order to comprehend the shifting conditions of circulation in the liver and the profound altrr.il ions lo which the venous trunks connected with it the vitelline, imihilic:il, and portal veins are naturally subjected in the changing supply of blood.

When the hepatic ducts grow out from the duodenum into the ventral mesentery and septum transversnm and send out shoots, they enclose the two vitelline veins accompanying the intestine, which are at tins place connected with each other by ring-like anastomoses (sinus annularis, His) which surround the duodenum (fig. 320 dv). They are supplied with blood by lateral branches given oft' from these veins. The more the liver increases in size, the larger do the lateral branches (venae hepaticse advehentes) become. Between the network of hepatic cylinders (fig. 187 lc] they are resolved into a capillary network (<y), from which at the dorsal margin of the liver large efferent vessels (vena? hepaticse revehentes) re-collect the blood and convey it back into the terminal portion of the vitelline vein, which empties into the atrium. In consequence of this the portion of the vitelline vein which lies between the vena3 hepaticse advehentes and revehentes continually becomes smaller, and finally atrophies altogether, since all the blood from the yolk-sac is employed for the hepatic circulation. The same process in the main is accomplished here as in the vessels of the visceral arches of gill -breathing Vertebrates, which upon the formation of branchial lamellae are converted into branchial arteries, branchial veins, and a capillary network interpolated between the two.

The two umbilical veins participate, even at an early period, in the hepatic circulation. Originally they run from the umbilical cord in the front [ventral] wall of the abdomen (fig. 313 Vu], from which they receive lateral branches, and then enter the sinus venosus (/Sr) above the fundament of the liver. They pursue, therefore, an entirely different course from that which they do later, when the terminal part of the umbilical vein is situated under the liver. According to His, this change in their course takes place in the following manner : The right umbilical vein in part atrophies (as also in the Chick, p. 552) and becomes, as far as it persists, a vein of the ventral wall of the abdomen. The left umbilical vein, on the contrary, gives off anastomoses in the septum transversum to neighboring veins, one of which makes its way under the liver to the sinus annularis of the vitelline veins, and thereby conducts a part of the placental blood into the hepatic circulation. Since by its rapid growth the liver demands a large accession of blood, the anastomosis soon becomes the chief course, and finally with the degeneration of the original tract receives all the blood of the umbilical veins. This, mingled with the blood of the yolk-sac, circulates through the liver in the vessels which took their origin from the vitelline veins in the venae hepaticre advehentes and revehentes. Then it flows into the atrium through the terminal part of the vitelline vein. The latter also receives the inferior vena cava, which at this time is still inconspicuous, and can therefore be designated even now, in view of the ultimate condition, as the cardiac end of the inferior vena cava.

During a brief period all of the placental blood must first traverse the hepatic circuit in order to reach the heart. A direct passage to the. inferior vena cava through the ductus venosus Arantii does not yet exist. But such an outlet becomes necessary from the moment when, by the growth of the embryo and the placenta, the blood of the umbilical veins has so increased in amount that the hepatic circu

c.i'

- r.le

n.v

lation is no longer able

Fig. 323. Liver of an 8-months human embryo, seen from the under surface, from GKGENBAUR. Lie, Left lobe of the liver ; r.le, right lobe ; n.r, umbilical vein ; d.A, ductus venosus Arantii ; j>f.a, portal vein ; ha. .v, Jta.il, vena hepatica advehens sinistra and dextra ; /(./, vena hepatica revehens ; c.i', cava inferior; c.i", terminal part of the cava inferior, which receives the vente hepaticae revehentes (/<./).

to contain it. There is then developed on the under surface of the liver out of anastomoses a more direct connecting branch, the ductus venosus Arantii (fig. 323 d.A), between umbilical vein (n.v) and inferior vena cava (c.i"). Thus is established and it persists until birth a condition by which the placental blood (n.v) is divided at the porta into two currents : one passing through the ductus venosus Arantii (d.A) into the inferior vena cava (c.i ") ; the other pursuing a round-about way, passing through the venae hepaticse advehentes (ha.s and ha.d) into the liver, here mingling with the blood brought to the liver through the vitelline vein (pf.a) from the yolk-sac and from the intestinal canal, which has in the meantime become enlarged, and finally passing through the venae hepaticse revehentes (h.r), also to reach the inferior vena cava (c.i").

There is still something to be added concerning the development of tin' portal vein. It is to be seen in fig. 323 as an unpaired vessel (/>/'.}. It. empties into the rig-lit aflerent hep.-itie vein, derives its roots from the region of the intestinal canal, and conveys the venous blood from the latter into the right lobe of the liver. It takes its origin from the two primitive vitelline veins.

According to the account of His, the two vitolline veins fuse along the tract where they run close together on the intestinal canal ; on the contrary, in the region where they run to the liver and are connected with each other to form two ring-like anastomoses, each of which encircles the duodenum, an unpaired trunk is formed by the atrophy of the right half of the lower [posterior] ring and the left half of the upper one. The portal vein thus formed therefore runs first to the left and backward [dorsad] around the duodenum, and then emerges on the right side of it ; it draws its blood partly from the yolk-sac and partly from the intestinal canal through the vena mesenterica. Afterwards the first source is exhausted with the regressive metamorphosis of the yolk-sac, but the other becomes more and more productive with the enlargement of the intestine, the pancreas, and the spleen, and during the last months of pregnancy conveys a large stream of blood to the liver.

The additional changes, which occur at birth, are easily comprehended (fig. 323). With the detachment of the after-birth the placental circulation ceases. The umbilical vein (n.v) conveys no more blood to the liver. That portion of its tract which extends from the umbilicus to the porta hepatis degenerates and becomes a fibrous ligament (the lig. hepato-umbilicale or lig. teres hepatis), Likewise the ductus Arantii (d.A) produces the well-known ligament enclosed in the left sagittal fissure (lig. venosum). The right and left venas hepaticre advehentes (ha.d, ha.s) again receive their blood, as in the beginning of the development, from the intestinal canal through the portal vein (pf.a).

Now that we have become acquainted with the details of the morphological changes, I close this section on the vascular system with a short sketch of the fcetal circulation of the blood. It is characteristic of this that no division into two separate circulations, into the major or systemic and the minor or pulmonary, has yet taken place ; further, that in most of the vessels neither purely arterial nor purely venous blood circulates, but a mixture of the two. Purely arterial blood is contained only in the umbilical veins as they come from the placenta, whence we will follow the circulation.

Having arrived at the liver, the current of the umbilical veins is divided into two branches. One stream goes directly through the ductns Arantii into the inferior vena cava, and is here mingled with the venous blood which returns to the heart from the posterior limbs and the kidneys. The other stream passes through the liver, where there is added to it the venous blood of the portal vein coming from the intestine ; by this circuitous course it also reaches, through the venae hepaticse revehentes, the inferior vena cava. From the latter the mixed blood flows into the right atrium, but, in consecjuence of the position of the Eustachian valve and because the foramen ovale is still open, the greater part of it passes through the latter into the left atrium. The other smaller part is again mingled with venous blood, which has been collected by the superior vena cava from the head, the upper limbs, and (through the azygos) from the walls of the trunk, and is propelled into the right ventricle and from there into the pulmonaiis. The latter sends a part of its highly venous blood to the lungs, the other part through the ductns Botalli to the aorta, where it is added to the arterial current coming from the left ventricle.

The blood of the left ventricle comes principally from the inferior cava, only a small part of it from the lungs, which pour their blood, which at this time is venous, into the left atrium. It is driven into the aortic arch and part of it is given off through lateral branches to the head and upper limbs (carotis communis, subclavia) ; the rest is carried on downwards in the aorta descendens, where the venous current of blood from the right atrium by the way of the cluctus Botalli is united with it. The mixed blood is distributed to the intestinal canal and the lower limbs, but the most of it reaches the placenta through the umbilical veins, where it is again made arterial.

In the distribution of the blood in the anterior and the posterior regions of the body a noteworthy difference is easily recognised. The former receives through the carotis and subclavia a more arterial blood, since to the stream in the aorta descendens is added the venous blood of the right ventricle through the ductus Botalli. Especially in the middle of pregnancy is this difference important. There has been an endeavor to refer to this fact the more rapid growth of the upper part of the body in comparison with the lower.

As this sketch has shown, there is everywhere a mingling of the different kinds of blood. This, it is true, is not uniform in the different months of embryonic life, because, indeed, the separate organs do not alter in size uniformly, and especially because the lungs during the later stages are in a condition to receive more blood, and finally because the foramen ovale and the ductus Botalli become narrower during the last months. On account of theso facts, less blood passes, even before birth, from the inferior vena, cava into the left atrium, and likewise less from the pulmonary artery into the descending aorta, than was the case in earlier months. Thus there is gradually introduced toward the end of pregnancy a separation into a right and a left heart, with their separate blood-currents (HASSE). But it is almost at a single stroke that this separation, in consequence of birth, becomes complete.

Great alterations are now brought about by the beginning of pulmonary respiration and by the cessation of the placental circulation. Both events cooperate to increase the blood-pressure in the left heart, and to diminish that in the right. The blood -pressure becomes reduced because no more blood runs into the right atrium from the umbilical vein and because the right ventricle must furnish more blood to the expanding lungs. In consequence of this the ductus Botalli (fig. 318 n) is closed and then converted into the ligamentuiu Botalli. Since, moreover, a greater quantity of blood now flows from the lungs into the left atrium, the pressure in the latter is increased, and since at the same time the pressure is diminished in the right atrium, the closure of the foramen ovale, owing to the peculiar valvular arrangements, is now effected. For the margin of the valvula foraminis ovalis applies itself firmly to the limbus Vieussenii and fuses with it.

By the closure of the oval foramen and the Botallian duct the division of the blood -current into a major, systemic circuit and a minor, pulmonary circuit, which was initiated before birth, is now completed.

Summary

Development of the Heart.

1. In the first fundament of the heart two different types can be distinguished in Vertebrates.

First Type. In Gyclostomes, Selachians, Ganoids, and Amphibia the heart is developed from the beginning as an unpaired structure on the under [ventral] surface of the cavity of the head-gut, in the ventral mesentery, which is thereby divided into a mesocardium anterius and posterius.

Second Type. In Birds and Mammals the heart is developed out of separate halves, which afterwards fuse with each other into a single tube, which then has the same position as in the first type.

2. The second type is to be derived from the first, and is explainable as an adaptation to the great size of the yolk, in that the heart is established at a time when the splanchnopleure is still spread out flat upon the yolk and is not yet folded together to form the headgut.

3. The cells which are united to form the endothelium of the heart are split off from a proliferating, thickened place of the entoderm.

4. The heart is first established in all Vertebrates in the cervicocephalic region behind the last visceral arch.

5. The posterior or venous end of the single cardiac tube receives the blood from the body through the oniphalomesenteric veins ; the anterior or arterial end gives off the blood to the body through the truncus arteriosus.

6. In the arnniotic Vertebrates the single cardiac sac is converted by a series of metamorphoses (1) by flexures, constrictions, and changes of position, and (2) by the formation of partitions inside of it into a heart composed of two ventricles and two atria.

7. The straight sac assumes the form of a letter S.

8. The venous portion of the 8 comes to lie more dorsal, the arterial more ventral ; the two are marked off from each other by a constriction, the auricular canal, and are now to be distinguished as atrium and ventricle.

9. The venous portion or the atrium forms lateral evaginations, the auricles of the heart, which surround from behind the truncus arteriosus.

10. The formation of partitions, by which atrium, ventricle, and truncus arteriosus are divided into right and left halves, begins at three different places.

(a) First of all, the atrium is divided by an atrial partition into a right and a left half ; but the separation is incomplete, since there exists a passage in the partition, the foramen ovale, which remains open up to the time of birth.

(6) By its downward growth the atrial partition reaches the auricular canal (septum intermedium of His) and divides the opening in it into a right and left ostium atrioventriculare.

(c) The ventricle is divided into right and left halves by a partition (septum ventriculi) beginning at the apex of the heart ; the division is also indicated externally by the sulcus interventricularis,

(d) The truncus arteriosus is divided into pulmonary artery and aorta by the development of a special partition, which begins above, grows downward, arid joins the ventricular partition.

() The complete separation of the atria first takes place after birth by the permanent closure of the foramen ovaie.

11. At the ostium atrioventriculare and at the ostium arteriosum the first fundaments of the valves are formed as thickenings of the endocardium (endocardia! cushions) projecting inward.

Development of the C kief Arterial Trunks of Man and Mammals.

12. From, the truncus arteriosus there arise five pairs of visceral arch vessels (aortic arches), which run along the visceral arches, embrace the head-gut laterally, and unite dorsally to form the two primitive aortas.

13. The two vessels fuse at an early period to form the unpaired aorta lying under the vertebral column.

14. In Mammals, of the five pairs of visceral-arch vessels the first and second degenerate ; the third furnishes the proximal part of the carotis interna ; the fourth arch becomes on the left side the aortic arch, on the right side the arteria anonyma brachiocephalica and the proximal part of the subclavia ; [the fifth early disappears ;] the fifth [sixth] arch gives off branches to the lungs, and becomes the pulmonary artery, but on the left side remains until the time of birth in open communication with the aortic arch through the ductus Botalli, whereas the corresponding portion 011 the right side atrophies.

15. After birth the ductus Botalli is closed and converted into the ligament of the same name.

16. From the aorta two pairs of large arterial trunks go to the fcetal membranes to the yolk-sac the vitelline arteries (arterise omphalornesentericse), to the allantois and placenta the umbilical arteries.

17. The vitelline arteries subserve the vitelline circulation, and afterwards, with the reduction of the umbilical vesicle, degenerate.

18. The umbilical arteries, which continually become larger with the increasing development of the placenta, arise from the lumbar portion of the aorta, pass forward [ventral] in the lateral wall of the pelvis, then at the side of the bladder and along the inner surface of the abdominal wall to the umbilicus and umbilical cord.

19. The umbilical arteries give off the iliaca interna to the cavity of the pelvis, the iliaca externa to the lower limbs.

20. After birth the umbilical artery degenerates into the ligamentuni vesico-unibilicale laterale, with the exception of its proximal part, which persists as the iliaca comniunis.

Development of the Chief Venous Trunks.

21. With the exception of the inferior vena cava, all venous trunks are established in pairs.

22. The two jugulars collect the blood from the head, the two cardinals from the trunk, but especially from the primitive kidneys.

23. The jugular and cardinal veins of either side unite to form the Cuvierian ducts, which pass transversely from, the lateral wall of the trunk to the posterior end of the heart, imbedded in a transverse fold of the front wall of the trunk, the septum transversum.

24. The two vitelline veins collect the blood from the yolk-sac ; from the navel onward they run in the ventral mesentery to the septum transversum.

25. The two umbilical veins collect the blood from the placenta ; from the attachment of the umbilical cord they run at first in the abdominal wall to the transverse septum.

26. In the septum transversum the Cuvierian ducts and the vitelline and umbilical veins unite to form the sinus reunions, which subsequently disappears as an independent structure and is incorporated in the atrium.

27. The cardinal veins diminish in importance (1) in consequence of the degeneration of the primitive kidneys, and (2) from the fact that the blood from the lower half of the body is conveyed back to the heart by the inferior vena cava.

28. The upper part of the inferior vena cava arises as an unpaired, independent vessel between the two cardinal veins, and then, at the place where the renal veins empty in, unites with the right cardinal vein. The latter is in this way converted into the lower portion of the inferior cava.

29. The Cuvierian ducts with the beginning of the jugular veins are designated as the venae cavse superiores.

30. An asymmetry in the embryonic venous trunks, which are established in pairs, is brought about by the fact that the two superior vena3 cavse, and also at their middle the remnants of the two cardinal veins, are joined together by transverse trunks:

31. Since through these cross anastomoses more and more of the blood, and finally the whole of it, is conveyed from the trunks of the left half of the body into those of the right half, the proximal part of the left superior vena cava, except a small portion, v/hich lies in the coronary groove of the heart, degenerates, receives the cardiac veins, and becomes the sinus.coronarius cordis. Likewise the cardiac end of the left cardinal vein disappears.

32. From the paired fundaments of the venous trunks are formed the single superior vena cava, the sinus coronarius cordis, and the vena azygos and hemiazygos.

33. The vitelline veins, which afterwards become unpaired, give rise, when the liver is developed, to the portal circulation (the venae hepaticee advehentes and revehentes).

34. The umbilical veins, of which the right early degenerates, originally run in the abdominal wall above the liver to the sinus reunions ; then the left forms an anastomosis with the vitelline vein under the liver, whereby its current shares in the portal circulation.

35. There arises out of an anastomosis between the umbilical vein and the cardiac end of the inferior vena cava on the under surface of the liver the ductus venosus Arantii, which results in the division of the blood of the umbilical vein into two currents.

36. After birth the umbilical vein degenerates into the ligamentum teres hepatis, and the ductus venosus Arantii is obliterated ; the veme hepaticse advehentes now receive their blood from the terminal part of the original vitelline vein or the portal vein only, which collects the blood from the intestinal canal.

37. The septum transversum, in which run the venous trunks on their way to the heart, is the starting-point for the development of the diaphragm and the pericardial sac, and forms at first an incomplete partition between the abdominal cavity and pleuro-pericardial cavity, which still communicate with each other on either side of the vertebral column.

38. The pericardial sac is separated off from the thoracic cavity as follows : (1) the Cuvierian ducts or future superior vense cavse, instead of running transversely, run more and more obliquely from above downward, detach themselves from the septum transversum, and elevate the pleura into pericardial folds, which run from above downward and project inward ; (2) the margin of the pericardial fold fuses with the mediastinum posterius, in which are enclosed ossophagus and aorta, whereby the superior venaB cavee are transferred to the mediastinum.

39. The thoracic cavities have for a time the form of tubular spaces lying on the dorsal side of the heart and on either side of the spinal column ; they receive the developing lungs, and still communicate caudad with the abdominal cavity.

40. The two thoracic cavities are separated from the abdominal cavity by the fusion of the dorsal rim of the septum transversum with peritoneal folds of the dorsal wall of the trunk (the pillars of USKOW).

41. The diaphragm is composed of two parts, the ventral septum transversum, and a dorsal part, the pillars.

42. Upon its first establishment the liver grows into the septum transversum, but subsequently detaches itself from the latter and remains united to the diaphragm by means of its peritoneal covering only, the coronal ligament.

II. The Development of the Skeleton.

With the exception of the chorda dorsalis, which takes its origin from the inner germ-layer, the skeleton of Vertebrates is a product of the intermediate layer, resulting from a series of histological differentiations, a general survey of which has already (p. 540) been given. There have appeared many articles treating on this very complicated apparatus in the higher Vertebrates from a developmental and also especially from a comparative-anatomical standpoint. By an exhaustive treatment of this subject this part of the work would attain to greater proportions than the plan of the present textbook permits. I shall therefore limit myself to the more important conditions of organisation and for what remains refer to the textbooks of comparative anatomy.

Two chief parts are distinguishable in the skeleton of Vertebrates : (1) the axial skeleton, which is in turn divisible into that of the trunk and that of the head, and (2) the skeleton of the limbs. The former is the older and more primitive, being possessed by all Vertebrates ; the latter has been developed later, and is entirely wanting in the lower groups (Amphioxus, Cyclostomes).

A. The Development of the Axial /Skeleton.

The original foundation of the axial skeleton of all Vertebrates is the notochord or chorda dorsalis. By this is understood a flexible, rod-like structure, which is situated in the axis of the body below the neural tube and above the intestine and aorta. It reaches from the front end of the base of the mid-brain to the end of the tail.

For a time after its establishment the front end of the chorda remains in union jit a small pla.ee with the epithelium of the fore-gut. This place is immediately behind the upper attachment of the primitive pharyngeal membrane (Kachenhaut). There is here found, a little behind the hypophysial pocket, a slight depression in the epithelial lining of the fore-gut SEESSEL'S pocket or the palatal pocket of SELENKA. It is only some time after the rupture of the pharyngeal membrane that the chorda becomes detached from the intestinal epithelium and terminates free in the mesenchyma, often with a hook-like end (KEIBEL, KANN, CARIUS).

In the case of Arnphioxus the chorda is the only skeletal structure present in the whole of the soft body; in the lower Vertebrates (Cyclostomes, Fishes, Amphibia) it exists even in the adult animals as a more or less important organ ; but in the Amniota it is reduced almost to obliteration, and only in the earliest stages of development plays a role as the forerunner, as it were, of the higher form of axial skeleton which finally Tig 324 Cross section takes its place. While reference is made through the vertebral to previous portions of the text-book for inSaimon, after GEGEN- formation about the first development of the BAUR - chorda, its further metamorphosis may be o*. Sheath of the chorda; k, neural arch; ', treated or here more at length. 1 his varies according as the chorda becomes a really functional organ or soon begins to degenerate.

In the first instance, when the band of chordal cells has been constricted off from the inner germ-layer, it becomes more sharply limited at its periphery by the secretion of a firm, homogeneous envelope, the sheath of the chorda (fig. 324 cs). Then the cells increase in size by the accumulation of fluid within their protoplasm, which finally exists in the form of a thin superficial layer only ; the cells become enveloped in firm membranes, thus acquiring exactly the appearance of plant cells. But directly beneath the sheath of the chorda (fig. 324) the cells remain small and protoplasmic and constitute a special layer, the chordal epithelium, which by proliferation and metamorphosis of its elements causes an increase of the substance of the chorda.

haemal arch ; m, spinal cord ; a, dorsal aorta ; r, cardinal veius.

Immediately after its formation the chorda is in contact above with the neural tube, below with the entoderm. and laterally with the primitive segments. This relation is altered as soon as the intermediate layer makes its appearance between the first embryonic fundaments. Then a layer of cells grows around the chorda (fig. 262), spreads itself out from here around the neural tube above, and furnishes the foundation from which are developed the segmented vertebral column and in front, in the region of the five brain-vesicles, the cranial capsule ; it has therefore received the name of membranous vertebral column and of membranous cranial capsule (membranous primordial cranium) ; it is also appropriately designated as skeletogenous layer, the envelope which invests the chorda being called the skeletogenous sheath of the chorda. (Compare p. 172 for an account of the first formation of it.) The mesenchyme also spreads out laterally in the embryo, penetrates into the spaces between primitive segments, and is. converted into thin plates of connective tissue (ligamenta interinuscularia), by means of which the musculature of the trunk is parted into separate muscle segments (myomeres). The muscle-fibres find attachment and support upon both the anterior and posterior faces of these plates.

Such a condition is permanently preserved in Amphioxus lanceolatus. The chorda, with its sheath, is the only firm skeletal structure. Fibrous connective tissue (membranous vertebral column) envelops it and the neural tube, and sends out into the musculature of the trunk the intermuscular ligaments.

When the originally membranous tissue surrounding the chorda and neural tube is followed in its further development in the embryos of the higher Vertebrates, it is to be seen that it successively undergoes two metamorphoses : that at first it is partially chondrified, and that subsequently the cartilaginous pieces are converted into osseous tissue ; or, in other words, the first-established membranous vertebral column is soon converted into a cartilaginous, and this in turn is replaced by a bony one, and in the same manner the membranous primordial cranium is transformed into a cartilaginous, and this in turn into a bony cranial capsule.

The three stages which succeed one another in the development of the higher Vertebrates are also encountered in a comparativeanatomical investigation of the axial skeleton in the series of Vertebrates, and in such a manner that the condition, which in many classes appears only as a transitory embryonic one, is i-etained permanently in the lower classes. As Amphioxus possesses a membranous axial skeleton, so the Selachians and certain of the Ganoids are representatives of the stage with cartilaginous vertebral column. The third stage in the evolution of the axial skeleton is more or less completely attained by all the higher Vertebrates.

Tli is, again, is a very instructive example of which the embryology of the skeleton presents many others of the parallelism which exists between the development of the individual and that of the race ; it teaches how embryological and comparative-anatomical investigations are mutually complemental. In the detailed description of the conditions which are observed in the development of the cartilaginous and bony axial skeleton, I shall limit myself to Man and Mammals, and since great differences exist between the posterior region, which encloses the spinal cord, and the anterior, which envelops the vesicles of the brain, I shall treat of them separately.

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Fig. 325. Longitudinal [frontal] section through the thoracic region of the vertebral column of a human embryo 8 weeks old, after KOLLIKEB.

v, C a r t i 1 a g i n o u s body of vertebra,; U, intervertebral 1 i g a m e n t ; cJt, chorda.

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() Development of the Vertebral Column.

The process of chondrification commences in Man at the beginning of the second month. At certain places in the tissue enveloping the chorda the cells secrete between themselves a cartilaginous matrix, and move farther apart, whereas at other intervening and narrower tracts the character of the tissue is not altered (fig. 325). In this manner the skeletogenous layer is differentiated into numerous vertebral bodies (v), which in longitudinal sections are the more translucent, and into the intervertebral discs (ligamenta intervertebralia) which separate them (li).

The process of chondrification, as FEOEIEP has followed it in the case of the embryo calf, proceeds as follows : there arise on both sides of the chorda masses of cartilage which are united on the ventral side of it by a thinner layer. Somewhat later the cartilaginous half-cylinder is closed on the dorsal side also.

Upon the appearance of a segmented vertebral column the chorda loses its function of a supporting skeletal rod. From this time forward it therefore suffers a gradual obliteration. The parts enclosed in the bodies of the vertebrae are restricted in their growth,

THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 597 whereas the shorter portions lying in. the soft intervertebral discs continue to enlarge (fig. 325 ch). Thus the chorda now acquires the appearance of a string of beads, since thickened spheroidal portions are joined to one another by small connecting thread-like portions. Subsequently it entirely disappears in the bodies of the vertebrae, especially when the latter begin to ossify (fig. 326) ; the intervertebral portion (li) alone persists, although indistinctly limited from the

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Pig. 326. Longitudinal [sagittal] section through the intervertebral ligament and the adjacent parts of two vertebrae from the thoracic region of an advanced embryo Sheep, after KOLLIKEB.

la, Ligament longitudinale anterius ; ^>, lig. long, posterius ; li, lig. inter vertebrale ; k, k', cartilaginous caps (epiphyses) of the vertebrae ; w and w', anterior and posterior vertebrae ; c, inter vertebral, c' and c", vertebral enlargements of the chorda.

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surrounding tissue, and produces by the proliferation of its cells the gelatinous core of the intervertebral disc.

Soon after the appearance of the bodies of the vertebrae the fundaments of the corresponding arches are observable. According to FRORIEP'S account, there arise small, independent pieces of cartilage in the membrane enveloping the spinal cord, in the immediate vicinity of the bodies of the vertebrae, with which they soon fuse. Their growth is rather slow. During the eighth week they still appear in Man as short processes from the bodies of the vertebrae, so that the spinal cord is still covered dorsal ly by the membranous skeleton. In the third month they grow into contact with each other at the dorsuni ; however, it is only in the following month that a complete fusion takes place, and that cartilaginous neural spines are formed. The part of the membrane which lies between the cartilaginous arches furnishes the ligamentous apparatus.

In the process of chondrification the nascent bodies of the vertebrae have a fixed position relative to the primitive or muscle-segments ; it is such that on either side of the body they are adjacent to two of the latter, one half to a preceding segment, the other half to a following one ; or, in other words, the ladies of the vertebras and the muscle-segments do not coincide, but in their 'positions alternate with each other.

The necessity of such an arrangement follows from the very function which vertebral column and musculature together have to fulfil. The axial skeleton must possess two opposite properties united : it must be firm, but also flexible, firm, in order to serve as a support for the trunk ; flexible, so as not to impede the motions of the latter. Since a continuous cartilaginous rod would not have possessed sufficient flexibility, the process of chondrification could not take place throughout the whole extent of the skeletogenous layer, but there must be left more elastic tracts, which allow a movement of the cartilaginous pieces on one another. But a movement of the cartilaginous pieces would obviously be impossible if they should lie so that the muscle fibres had their origin and insertion on one and the same vertebral element. In order that the fibres of a musclesegment may operate upon two vertebra?, the muscular and vertebral segments must alternate in position.

This process, which is easily intelligible in the way in which it has been outlined, has given occasion for the assumption of a " resegmentation of the vertebral column." This conception originated with EEMAK, and since then has been for a long time tenaciously held to in the literature.

REMAK, like other einbryologists before him (BAER), perceived in the primitive segments of the Chick the material for the establishment of the vertebral column, and therefore gave them the name " protovertebrre." But inasmuch as he found that the cartilaginous vertebrse did not afterwards correspond in position with the protovertebrse, he announced the proposition that a new " segmentation of the vertebral column takes place, from which arise the secondary, permanent bodies of the vertebra 5 ." Both the name " protovertebra " and the assumption of a resegmentation of the vertebral column should be dropped, and for the following reasons :

The signification of the primitive segments consists, if not exclusively, at least principally, in this, that they are the fundaments of the musculature of the body. But in the arrangement of the musculature is expressed the original and oldest segmentation of the vertebrate body. It is present even in Amphioxus and the Cyclostomes. The segmentation of the vertebral column, on the contrary, ivas acquired much later, and has resulted, as was explained above, from a necessary dependence on the segmentation of the musculature. A primary segmentation of the vertebral column as understood by REMAK and his followers has never existed, for the cartilaginous vertebrie are formed from an unsegmented mass of tissue enveloping the chorda from the skeletogenous layer. One cannot speak of a segmentation of the vertebral column until the beginning of the process of chondrification, by reason of which alone it became necessary.

Even before the cartilaginous vertebral column has been completely established, it enters in Mammals upon the third stage, which begins in Man at the end of the second month.

The ossification of every cartilage takes place in the main in a corresponding, typical manner. Blood-vessels at one or several places grow from the surface into its interior, dissolve the matrix of the cartilage of a limited region, so that there arises a small cavity filled with vascular capillaries and marrow-cells. In the vicinity of this salts of lime are deposited in the cartilage. By a portion of the proliferated medullary cells, which become osteoblasts, bone substance is then secreted (fig. 326 w). In this manner there arises in the midst of the cartilaginous tissue a so-called bone nucleus or centre of ossification, around which the destruction of the cartilage and its replacement by osseous tissue advance further and further.

The places where the separate bone nuclei are formed, as well as their number, are tolerably uniform for the different cartilages.

In general the ossification of each vertebra proceeds from three points. At first a centre of ossification is established in the base of each half of the vertebral arch, to which there is added somewhat later a third centre in the middle of the body of the vertebra. In the fifth month the ossification has advanced up to the surface of the cartilage. Each vertebra is now distinctly composed of three pieces of bone, which for a, long time continue to be joined to one another by bridges of cartilage at the base of each half of the arch and at the union of the latter with the vertebral spines. The last remnants of cartilage do not ossify until after birth. During the first year with the development of a bony spinous process the halves of the arch are fused. Each vertebra is then separable after destruction of the soft parts into two pieces, into the body and the arch. These are united between the third and eighth years.

In addition to the pieces of bone just described, accessory centres of ossification appear on the vertebras in subsequent years ; it is in this way that there arise the epiphysial plates at the end-surfaces of the body and the small bony pieces at the ends of the vertebral processes (the spinous processes and the transverse processes). SCHWEGEL gives detailed information concerning the time of their appearance and their fusion.

Cartilaginous skeletal parts, which serve for the support of the lateral and ventral walls of the body, the ribs and the breast bone, contribute to the completion of the axial skeleton.

The ribs are developed independently of the vertebral column, in Man during the second month, by the chondrification of strips of tissue in the mterrnuscular ligaments between the successive musclesegments. They are at first visible as small bent rods in the immediate vicinity of the body of the vertebra, and from here they rapidly extend vent rally.

In early stages of development ribs are established from the first to the last segment of the vertebral column (the coccyx in Man excepted), but only in the case of the lower Vertebrates (Fishes, many Amphibia, and Reptiles) are they developed into large bows supporting the wall of the trunk in a uniform manner in all regions, whereas in Mammals and in Man they exhibit in the separate regions of the vertebral column different conditions. In the neck, lumbar and sacral regions, they appear from the beginning in a rudimentary condition only, and undergo metamorphoses to be described later. It is exclusively in the thoracic region that they attain important dimensions, and here at the same time they give rise to a new skeletal part the breast bone, or sternum.

The sternum, which is wanting in Fishes and Dipnoi, but is present in Amphibia, Reptiles, Birds, and Mammals, is a formation derived from the thoracic ribs, and is originally established, as RATHKE was the first to discover, as a paired structure, 'which early fuses into an unpaired skeletal piece.

HUGE has followed the development of the sternum in Man in a very thorough manner, and has found that in embryos 3 cm. long the first five to seven thoracic ribs have become prolonged into the ventral surface of the breast and by a broadening of their ends have united at some distance from the median plane to form a cartilaginous band, whereas the following ribs end free and at a greater distance from the median plane. The two sternal bars are separated from each other by membranous tissue ; later they approach each other in the median plane, and commencing in front, begin to fuse together into an unpaired piece, from which the individual ribs which gave rise to them are afterwards separated by the formation of joints.

The paired origin of the sternum serves to explain some of its abnormalities. For example, in the adult there is sometimes seen a fissure, which, although closed by connective tissue, passes quite through the sternum (fissura sterni), or a few larger or smaller gaps are found in the body and xyphoid process of the sternum. All these abnormal cases are explained by the complete or partial failure of the two sternal bars to fuse in the usual way during embryonic life.

The ossification of ribs and sternum is in part accomplished by the development of special centres of ossification, that of the ribs beginning as early as the second month, the sternum somewhat late, in the sixth foetal month.

Each rib contains at lirst one centre of ossification, through the enlargement of which the bony part is formed, while next to the sternum a portion remains cartilaginous throughout life. In the eighth to the fourteenth year there appear in the capitulum and tuberculum of the rib, according to SCHWEGEL and KOLLIKER, accessory centres, which fuse with the main piece between the fourteenth and the twentyfifth year.

The sternum (fig. 327) ossifies from numerous centres, of which one arises in the manubrium, and from six to twelve in its body. Between the sixth and twelfth years the latter begin to fuse together into the three or four large pieces of which the body of the sternum is composed. The xyphoid process remains partly cartilaginous, but acquires a centre of ossification during childhood.

Regarding the episternal pieces which appear on the manubrium, the textbooks of comparative anatomy and the article by RuG-E should be consulted.

Through inequalities in the development of the separate vertebral and costal fundaments and through the fusions which take place here and there are produced the different regions of the skeleton of the trunk : the cervical, dorsal, and lumbar regions of the vertebral column, the sacrum and coccyx. A correct understanding of these skeletal parts is to be acquired only through embryology.

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Fig. 327. Cartilaginous sternum, with portions of the ribs attached and with several centres of ossification (kk\ from a child two years old.

k, Cartilage ; kk, centres of ossification ; sch, xyphoid process.

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The rudimentary fundaments of the cervical ribs at their first appearance fuse with the cervical vertebra, at one end with the body of the vertebra, at the other with an outgrowth of the neural arch, and with the latter enclose an opening through which the vertebral artery runs the foramen transversarium. The so-called transverse process of the cervical vertebra is therefore a compound structure, and were better designated lateral fwocess, for the bony rod that lies dorsad of the foramen transversum is formed by an. outgrowth from the vertebra and alone corresponds to the transverse process of a dorsal vertebra ; the ventral rod, on the contrary, is a rudimentary rib, which possesses in fact a separate centre of ossification.

The fundament of the rib of the seventh cervical vertebra occasionally attains greater size, does not fuse with the vertebra which consequently does not possess any foramen transversarium and is described under the abnormalities of the skeleton as free cervical rib. Its presence is explained therefore as being the result of a more voluminous development of a part which in all cases exists as a fundament.

The transverse fwocess of the himbar vertebrce is also better designated as lateral process, because it encloses the rudiment of a rib. This explains the phenomenon of a thirteenth or small lumbar rib occasionally observed in Man.

The sacral region is the one that is most modified. A large number of vertebrae in this region by becoming firmly united with the pelvic girdle have lost the power of moving on one another, and are fused together into a large bone : the sacrum. This consists in human embryos of five separate cartilaginous vertebras, the first three of which especially are characterised by very broad, well-developed lateral processes.

I say lateral processes because comparative-anatomical grounds and embryological evidence both indicate that there are included in them rudimentary sacral ribs, such as in lower Vertebrates make their appearance as independent structures. On the embryological side, the method of their ossification favors this view, for each sacral vertebra undergoes ossification from five centres. To the three typical centres, those of the body and the neural arches, are added in the lateral processes large bone-nuclei (centres), which are comparable with the centres of ossification of a rib. They produce the well-known lateral masses of the sacrum (massse late rales), which bear the articular surfaces for union with the ilium.

The fusion of the five bony pieces of a sacral vertebra, at first separated by strips of cartilage, takes place later than in other parts of the vertebral column, namely, between the second and the sixth year after birth. For a long time the five sacral vertebrae remain separated from one another by their intervertebral discs, which begin to ossify in the eighteenth year ; the process has usually come to an end by the twenty -fifth year.

Behind the sacrum there follow four or five rudimentary coccygeal vertebra3, which represent the caudal skeleton of Mammals and do not acquire centres of ossification until very late. In the thirtieth year or later they may fuse with one another, and sometimes with the sacrum.

Atlas and episiropheus (axis) now demand special mention. These vertebra acquire their peculiarities of form by an early fusion of the cartilaginous body of the atlas (fig. 3'2Sa) with the epistropheus (e) to form the odontoid process of the latter. The one therefore contains less, the other more than a normally developed vertebra.

That the odontoid process is the real body of the atlas is recognisable even later by means of two facts. First, like every other vertebral , body, it is traversed, as long as it remains cartilaginous, by the chorda, which at the tip Fig. 328. Median section of the process is continued into the ligamentuni through the body and suspensoriuni and from this into the base of the odontoid pr cess of the epistropheus.

cranium. Secondly, it acquires in the fifth iu the cartilage two ceamonth of development a separate centre of tra of ossification (e and a) are to be seen.

ossification (fig. 328 a), which is not completely fused with the body of the epistropheus until the seventh year.

The neural arches of the atlas, which have remained independent, are joined together on the ventral side of the odontoid process by a tract of tissue in which an independent piece of cartilage is formed (hypochordal cartilage-rod of FRORIEP) a structure which, according to FRORIEP, is present in every vertebra in the case of Birds. This piece of cartilage develops in the first year after birth a special centre of ossification, fuses between the fifth and the sixth year with the lateral halves, and constitutes the. anterior [ventral] arch (KOLLIKEK).

(b) Development of the Head-Skelcton.

From its position the skeleton of the head appears as the most anterior part of the axial skeleton, but it is on the whole very unlike the posterior part, the vertebral column, because it is adapted to

peculiar purposes. For in the morphological plan of Vertebrates the head takes, in comparison with the trunk, a preeminent position ; it is furnished with especially numerous and highly developed organs concentrated into a short space.

The neural tube has here become differentiated into the voluminous brain, with its dissimilar regions. In its immediate vicinity have arisen complicated sensory organs such as nose, eye, and ear. Likewise the part of the digestive tube enclosed within the head bears in many ways its peculiar stamp, since it contains the mouth op ening and is provided with organs for the reception and trituration of the food, and is pierced by visceral clefts. All of these parts exercise a determining influence on the form of the skeleton, which adapts itself most accurately to the brain, to the sensory organs, and to the functions of the head-gut, and thereby becomes a very complicated apparatus, especially in the higher Vertebrates.

Embryology sheds a flood of light on the method of the origin of the cephalic skeleton of Vertebrates ; it shows the relations to one another of widely different lower and higher forms, and also answers the question, What relation do the vertebral column and head -skeleton sustain to each other in the plan of organisation of Vertebrates ? Consequently the development of the cephalic skeleton proves to be an especially interesting subject, which has always attracted rnorphologists, and which has incited to careful investigation.

During the account some comparative-anatomical digressions will be made, which will contribute to the better comprehension of certain facts, especially those treated of in the final section, in which the vertebral theory of the skull will be briefly discussed.

As in the case of the vertebral column, there are to be distinguished three stages of development according to the histological character of the sustentative substance : a membranous, a cartilaginous, and a bony.

The chorda serves as the foundation for the membranous skeleton of the head, and extends forward to the between-brain. At its anterior end there is formed in Amniota the cephalic flexure, by which the axis of the first two brain-vesicles makes an acute angle with the three following ones (fig. 153). Here also the mesenchyme early grows around the chorda and envelops it in a skeletogenous layer, which spreads out from this region laterad and dorsad, enveloping the five brain-vesicles, and is subsequently differentiated into the membranes of the brain and a layer of tissue, which becomes the foundation of the cranial capsule, and has received the name of membranous primordial cranium.

Thus far there is an agreement in the development of the vertebral column and of the cranium. With the beginning of the process of chondrification the conditions become more peculiar. Whereas in the region of the spinal cord the skeletogenous layer undergoes a regular differentiation into cartilaginous and connectivetissue parts into vertebrae and vertebral ligaments and is thereby divided into successive movable segments, such a segmentation does not take place in the head.

The layer of tissue called membranous primordial cranium undergoes continuous chondrification into a non-articulate capsule enveloping the brain-vesicles. If we go through the whole series of Vertebrates down to the lowest, in no one of them is there exhibited a separation into movable segments corresponding to vertebrae. Therefore the anterior part and the remaining part of the axial skeleton pursue from an early period different directions in their development.

The contrast is intelligible in view of the different duties to be fulfilled in the two regions, and especially in consideration of the different influences which the action of the muscles exercises upon the form of the skeleton.

In water-inhabiting animals the trunk-musculature is the most important organ of locomotion, for it bends the trunk now in this direction, now in that, and thereby propels it forwards through the water. If, however, the head region were likewise flexible and movable, it would be disadvantageous for forward motion, inasmuch as a rigid part operates as a cut-water. Moreover, the musculature developed on the head assumes a different function, inasmuch as in the grasping of food and in the process of respiration which is accompanied by an enlargement and reduction of the respiratory tract of the alimentary tube it now adducts and then abducts the ventrally situated parts of the axial skeleton. Besides, it is advantageous here to have the skeletal axis present firm points of attachment for the muscles. Finally, the voluminous development of the brain and the higher sensory organs is likewise a participating influence tending to make the part of the head that serves for their reception an inflexible region.

In view of these various factors working in the same direction, it becomes intelligible that in the head a segmentation of the axial skeleton is wanting from the beginning.

In other respects there prevails a great agreement with the vertebral column, especially in the manner in which the metamorphosis into cartilaginous tissue takes place in the membranous primordial cranium. In both the chondrifi cation first begins at the surface of the chorda dorsalis (fig. 329 A).

As a foundation for the base of the skull there arise two pairs of elongated cartilages : behind, on either side of the chorda, the two parachordal cartilages (PS) ; in front, the two trabeculce cranii (Tr) of RATHKE, which begin at the tip of the chorda and from there run forward beneath the between- and the fore-brain.

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Fig. 329 A and B. First fundament of the cartilaginous primordial cranium, from WIEDERS TIKIM.

A, First stage. C, Chorda ; PE, parachordal cartilage ; Tr, RATHKE'S trabeculse cranii ; PR, passage for the hypophysis ; N, A, 0, nasal pit, optic vesicle, otocyst.

B, Second xtaye. C, Chorda ; B, basilar plate ; T, trabeculse cranii, which have become united in front to constitute the nasal septum (S) and the ethmoid plate ; Ct, AF, processes of the ethmoid plate enclosing the nasal organ ; 01, foramina olfactoria for the passage of the olfactory nerves; PF, post-orbital process; NK, nasal pit; A, 0, optic and labyrinthine vesicles.

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The four pieces soon fuse with one another (fig. 329 B}. The two parachordal elements grow around the chorda, first below, then above, thus enveloping it and producing the basilar plate (B). Its anterior margin rises far up into the angle of the flexure between mid -brain and between-brain and corresponds to the future clorsum sellre. The trabeculce cranii (T] spread out at their anterior ends, which become fused to constitute the ethmoid plate (>S'), the foundation of the anterior portion of the cranium, which acquires its particular stamp through its reception of the organ of smell. In the middle of their length they remain separate a long time, and enclose an opening, which corresponds to the sella tnrcica, and has been caused by the formation of the hypophysial pocket from the oral sinus and by its growing through the membranous basis of the cranium toward the infundibulum of the brain. Rather late there is also formed, as the floor of the sella turcica, beneath the hypophysis, a thin cartilaginous plate, which is pierced only by the holes for the internal carotids.

After the base of the cranium has been developed, the process of chondrification involves the side walls and at last the roof of the membranous primordial cranium, precisely as the halves of the neural arch grow out from the body of the vertebra and finally terminate in the dorsal spine.

In this manner there is developed around the brain in the case of the lower Vertebrates, in which the axial skeleton remains in the cartilaginous condition throughout life (fig. 330), a closed, tolerably thick-walled capsule, the cartilaginous primordial cranium,.

In the higher Vertebrates, in which to a greater or less degree processes of ossification occur later, the primordial cranium attains a less complete development, as is shown by the fact that its walls remain thinner, and indeed acquire at some places openings, which are closed by connective -tissue membranes. In Mammals the latter condition occurs very extensively in the roof of the skull, which becomes cartilaginous only around the foramen magnum, whereas in the region in which afterwards the frontal and parietal bones are located the cranium remains membranous. The cartilage attains a greater thickness only at the base of the cranium and in the regions of the olfactory organ and the membranous labyrinth, where it gives rise to the nasal and ear capsules.

For the sake of better orientation, it is useful to distinguish in the primordial cranium different regions. There are two different principles of division that may be employed in this connection.

Following GEGENBAUR, one can divide the primordial cranium, in accordance with its relation to the chorda dorsalis, into a posterior and an anterior portion.

The posterior region reaches up to the dorsum sillse and encloses in its basal portion the chorda, which in Man enters into it from the odontoid process through the ligamentum suspensorium dentis. The anterior portion is developed in front of the pointed end of the chorda out of RATHKE'S cranial trabeculae. GEGENBAUR designates the two as vertebral and evertebral regions (for which KOLLIKER employs the names chordal and prechordaT) ; he shows that the vertebral region must be, on account of its relation to the chorda, the older part and alone comparable with the remainder of the axial skeleton, that the non- vertebral part, on the contrary, is a later acquisition and constitutes a new structure, which has boen caused by the forward extension of the fore-brain vesicle and by the development of the ovgjin of smell, to the enclosing of which (nasal capsule) it contributes.

The second method of division is based upon the different appearance which the individual regions of the primordial cranium acquire through their relations to the sense orc/ans. The anterior end of

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Fig. 330. Diagrammatic representation of the cartilaginous cranial capsule and the cartilaginous visceral skeleton of a Selachian and of the larger nerve trunks of the head.

N, Nasal capsule (ethmoid region of the prin.ordial cranium) ; Au, cavity for the eye (orbital region) ; la, region of the labyrinth ; Oc, occipital region of the cranium ; O, palato-quadratum ; U, lower jaw (mandibulare) ; Ik, labial cartilage ; zb, hyoid arch ; kb, first to fifth branchial arches ; Tr, nervus trigeminus ; Fa, facialis ; Gl, glosso-pharyngeus ; Fa, vagus ; rl, ramus lateralis of the vagus ; rb, rami branchiales of the vagus.

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the cartilaginous capsule (fig. 330) receives the organ of smell ; a following portion contains depressions for the eyeballs ; in a third are imbedded the membranous auditory labyrinths ; finally, a fourth effects a union with the vertebral column. Consequently one may distinguish an ethmoidal, an orbital, a labyrinthine, and an occipital region.

In addition to the cartilaginous primordial cranium, there are developed in the head numerous cartilaginous pieces (which serve as supports to the walls of the head-gut) in a manner similar, although not directly comparable, to that in which the ribs (fig. 330) have arisen in the walls of the trunk in the region of the vertebral column. Together they constitute a skeletal apparatus which undergoes in the series of Vertebrates very profound and interesting metamorphoses. Whereas it attains in the lower Vertebrates a great development, it becomes in part rudimentary in Reptiles, Birds, and Mammals. The part, however, which remains furnishes the foundation for the facial skeleton. I begin with a short sketch of the original conditions in the lower Vertebrates, especially in the Selachians.

As has been described in a previous chapter, the lateral walls of the head-gut are traversed by the visceral clefts, of w r hich there are ordinarily as many as six in Sharks (fig. 331). The bands of substance intervening between the clefts are called the membranous throat- or visceral arches. They consist of a connective-tissue foundation invested with epithelium, of transversely striped muscle -fibres, and of the visceral-arch bloodvessels (see p. 571). Inasmuch as they have different functions to fulfil, and consequently acquire different forms, they are distinguished as jaw-, hyoid, and branchial arches. The most anterior of them is the jaw-arch, which serves to bound the oral opening. Following this, and separated from it by only a rudimentary visceral cleft, the spiracle, is the hyoid arch, which is connected with the origin of the tongue. Ordinarily this is followed by five branchial arches.

At the time when the membranous primordial cranium is converted into cartilage, chondrification also takes place in the connective tissue of the membranous visceral arches, thus producing the cartilaginous visceral arches (fig. 331). These exhibit a regular segmentation into several pieces, placed end to end and articulated with one another by connective tissue.

The jaw-arch is divided on either side into a cartilaginous palatoquadratuin (fig. 330 0) and a lower jaw (mandibulare). These 39

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Fig. 331. Head of a Shark embryo 11 lines long.

From PARKER AND BI;TTANV. Tr, RATHKK'S trabecuhe cnmii ; PI. PI, ptcrygo-quad rntum ; jl//', mandibular cartilage; Hy, hyoiil arch; Er.l, first branchial arch; Sj>, spiracle; C/', first bran cliia I cleft; Lch, groove under the eye; JK'a, fundament of the nose; E, eyeball; An, auditory vesicle; C.I, C.2, C.3, brain-vesicles; Hut, cerebral hemispheres; /.>'./, fronto-nasal process.

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carry, in the mucous membrane investing them, the teeth of the jaws. The two mandibular elements are united to each other in the median plane by means of a mass of tense connective tissue. The following visceral arches, on the contrary, are alike in having their lateral halves, which are divided into several pieces, joined ventrally by means of an unpaired connecting piece, the copula, in a manner similar to that in which the ribs are united by the sternum. The pieces of the hyoid arch are designated, in sequence from the dorsal to the ventral side, hyomandibular, hyoid, and (the copula) os entoglossum.

In Mammals and Man (figs. 154, 157) structures similar to those of the Selachians are formed in the membranous stage, but subsequently they are only in part converted into cartilaginous pieces, which in turn never acquire a great size, having meantime lost their original function. They help to form the facial part of the head-skeleton, and have already been treated of partially in previous chapters in the discussion of the head-gut and of the organ of smell. I am therefore compelled for the sake of continuity to repeat much that has already been presented concerning the visceral skeleton.

In very young human arid mammalian embryos the mouth-opening is bounded on the sides and below by the paired maxillary and mandibular processes (tig. 156, compare p. 284). The former are widely separated from each other, because the unpaired frontal process, in the form of a broad, rounded projection, is at first inserted from above between them. Afterwards this projection becomes divided by the development, on its rounded surface, of the two nasal pits with the nasal grooves leading down to the upper margin of the mouth (compare p. 513); it is then divided into the outer and inner nasal processes. The former are separated from the maxillary process by a groove, which runs from the eye to the nasal furrow, and is the first fundament of the lachrymal duct.

Behind the first visceral arch comes the hyoid arch (figs. 157, 158 &), the two being separated by a small visceral cleft, which becomes the tympanic cavity and Eustachian tube. This is followed by three additional visceral arches with three visceral furrows (or clefts), which are of only short duration.

During a later stage fusions take place between the .processes that surround the oral opening (fig. 332).

The maxillary processes, by growing farther inward, meet the inner nasal processes, fuse with them, and produce a continuous

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Fig. 332. Roof of the oral cavity of a human embryo with fundaments of the palatal processes, after His. Magnified 10 diameters.

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upper boundary to the mouth. In this way each olfactory pit with its nasal groove is converted into a canal, which leads into the oral cavity through an inner opening close behind the margin of the upper jaw. The membranous margins of the upper and lower jaws also lose their superficial positions, because the skin that covers them is raised up into externally projecting folds, and forms the lips, which from this time forward constitute the boundary of the oral opening.

A third stage, with the development of the palate, practically completes the formation of the face. (Compare pp. 515-17.) From the membranous upper jaw there arise two ridges projecting into the mouth-cavity (fig. 290) ; these become enlarged into the palatal plates, which grow horizontally.

The plates meet in the median plane and fuse with each other and with the median part of the frontal process, which has meantime become reduced by the enlargement of the olfactory organ to the thin nasal septum. Thus there is cat off from the primary oral cavity an upper chamber, which contributes to the enlargement of the nasal cavity, and which opens into the pharynx through the posterior nares ; at the same time [as the result of this growth] there has arisen a new roof of the mouth-cavity, the palate, which is afterwards differentiated into hard and soft palate.

A further differentiation of the face, which is now in the membranous stage of development, is brought about by the process of chondrification. This produces, however, in Mammals, as compared with Selachians, only small and unimportant skeletal structures. Some of these structures undergo degeneration (MECKEL'S cartilage), some are utilised as auditory ossicles in the function of hearing, and others are united to form the fundament of the hyoid bone. They arise from the soft tissue of the first, second, and third visceral arches ; in the case of the fourth and fifth arches there is not even a process of chondrification in Mammals, so that with the closure of the fissures they are no longer recognisable as distinct parts, perhaps the thyroid cartilage is to be referred to them (DUBOIS). I will describe the conditions in detail, first in the case of sheep embryos of different stages of development, and then in the case of a human embryo.

In a sheep embryo 2 cm, long there are to be found, according to the account of SALENSKY (fig. 333), two long and slender cylindrical cartilaginous rods, one in front, the other behind the first visceral cleft ; their posterior (proximal) ends abut upon the labyrinthregion of the primordial cranium, and are here united to each other by means of embryonic connective tissue. In older embryos (fig. 334) the first visceral arch becomes at its upper st hah zb [proximal] end more and more distinctly Figs. 333, 334. The dissected-out cartilages of MECKEL and Segmented by means

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Fig. 333.

am' am ha

nik

EEICHERT with the fundament of the auditory ossicles, from a sheep embryo 2'7 cm. long. After SALENSKY. Fig. 333. mk, MECKEL'S cartilage ; ha, hammer (malleus) ; am, aiivil (inc\is) (long process) ; am', its short process ; zb, cartilaginous hyoid arch. Fig. 334. am, Anvil; am', its short process; ha, hammer; /Kill, hammer-handle; si, stirrup (stapes); mk, MECKEL'S cartilage ; zb, cartilaginous hyoid arch.

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of constrictions, into two smaller pieces and a larger one. The first small piece, the one lying next to the wall of the labyrinth, gradually assumes the form of the incus (ani) with its processes, the second becomes the malleus (7ia) ; the two are joined by means of a mass of connective tissue. The third piece (ink} is of considerable length, and haw the form of a cylindrical rod ; it is enclosed in the membranous lower jaw, and is designated in honor of its discoverer as MECKEL'S cartilage. It remains for a long time in union with the fundament of the malleus by means of a narrow cartilaginous bridge, upon which the long process (pr. gracilis) of the malleus is afterwards developed by periosteal ossification. The second visceral arch (zb) becomes incorporated in the hyoid bone.

In a human embryo of the fifth month one observes structures similar to those just described, only somewhat further developed. Figure 335 exhibits the incus (am\ easily recognised by its form, lying on the wall of the labyrinth ; with it is articulated the malleus (ha\ the long process of which is continuous with MECKEL'S cartilage (MK). This extends ventrally as far as the median line, where it is united with the cartilage of the opposite side by means of connective tissue a kind of symphysis.

The second visceral cartilage, called also REICHERT'S cartilage, has become divided into three portions. The uppermost portion is fused with the labyrinth-region the petrous portion of the temporal bone and constitutes the fundament of the processus styloideus (yrf) ; the middle portion has become fibrous tissue in Man, and forms a strong ligament, the lig. stylohyoideum [Ist/t], whereas in many Mammals it becomes a large cartilage ; the third and lowest portion produces the lesser cornu (M) of the hyoid bone. This sometimes becomes developed to a great length by the chondrification of the lower part of the ligamentum stylohyoideum, and reaches up very close to the lower end of the stylohyoid process.

In the third visceral arch chondrification takes place only in the ventral tracts, producing upon the sides of the neck the greater cornua of the hyoid bone (yh). Greater and lesser cornua are attached to an unpaired median piece of cartilage, which corresponds to a copula of the visceral skeleton of Selachians and becomes the body of the hyoid bone.

The third auditory ossicle, the stapes (fig. 335 st), also belongs to the visceral apparatus ; it has been left unmentioned until now, because there is, even at present, a wide difference of opinion concerning its development. According to the original view of REICHERT, which GEGENBAUR is also inclined to adopt, the stapes arises from the uppermost end of the hyoid arch. KOLLIKER refers it to the first visceral arch. ' According to GRUBER and PARKER, on the contrary, it arises in connection with the fenestra ovalis, as though it were cut directly out of the outer wall of the labyrinth.

According to the recent investigations of SALENSKY, GRADENIGO, and RABL, it appears to me that the stapes has a double origin, arising from two different parts.

The plate of the stapes, which is let into the fenestra ovalis, is differentiated in the manner first emphasised by GRUBER and PARKER, and now again by GRADENIGO, out of the cartilaginous capsule of the labyrinth. Its development therefore agrees with that of the operculum of the Amphibia, as described by STOHR. The ring-like part of the stapes, on the contrary, comes from the upper end of the second visceral [hyoid] arch, which lies in contact with the capsule of the labyrinth (GRADENIGO, EABL). Its ring-like condition results

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Fig. 335. Head and neck of a human embryo 18 weeks old with the visceral skeleton exposed, after KOLLIKER. Magnified. The lower jaw is somewhat depressed in order to show MECKEL'S cartilage, which extends to the malleus. The tympanic membrane is removed arid the annulus tympanicus is visible. 1m, Malleus, which passes uninterruptedly into MECKEL'S cartilage, MK ; v.k, bony lower jaw (dentale), with its condyloid process articulating with the temporal bone ; am, incus ;

• , stapes \ pr, annulus tympanicus ; c/rf, processus styloideus ; Ixth, ligamentum stylo

hyoideum ; ///, lesser cornu of the hyoid bone ; gli, its greater cornu.

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from the fact that the tissue from which it is formed is traversed by a small branch of the carotis interna, the arteria mandibularis or perforans stapedia. In Man and certain of the Mammals this subsequently degenerates entirely, whereas in others (Rodents, Insectivores, etc.) it remains as a vessel of considerable size.

Both fundaments of the stapes fuse with each other very early and form a small cartilage, which on the one hand articulates with the incus by means of a lenticular connecting element (os lentiforme), and on the other reposes with its plate -like base in the fenestra ovalis.

The view here adopted that the stapes belongs to the second, the malleus and incus to the first visceral arch is supported by the important relation of the nerves in their distribution to the musculus stapedius and to the tensor tympani, as has recently been rightly pointed out by RABL. The muscle of the stapes is supplied from the nerve of the second visceral arch, the nervus facialis ; it forms part of a group embracing the in. stylohyoideus, and the posterior belly of the digastric; the muscle of the malleus receives a branch of the trigeminus, which is the nerve of the mandibular arch.

The separation of the territories of innervation prevails, moreover, with the muscles of the palate, one of which the tensor veil palatini arises in front of the Eustachian tube the remnant of the first visceral cleft and is therefore supplied by the n. trigerninus, whereas the levator veil palatini and azygos uvulae lie behind it, and, because belonging to the hyoid arch, receive branches from the n. facialis (EABL).

At first all the auditory ossicles lie imbedded in a soft gelatinous tissue outside the tympanic cavity, which still has the form of a narrow fissure. These conditions are not altered until after birth. The tympanic cavity, taking in air, then becomes enlarged, its mucous membrane is evaginated between the auditory ossicles, and the gelatinous tissue just mentioned undergoes a process of shrinkage. Auditory ossicles and chorda tympani thus come to lie apparently free in the tympanic cavity ; accurately considered, however, they are only crowded out into it, for even in the adult they are enclosed in folds of the mucous membrane, and by means of these they preserve their original and genetically established connection with the wall of the tympanic cavity.

Up to the present stage the construction of the head-skeleton is, on the whole, simple. In the third stage of development, on the contrary, upon the beginning of the process of ossification, it attains in a short time a high degree of complication, which is effected especially by the development of two entirely different kinds of bone, one of which has been called primordial bone, the other covering bone (Deck- oder Belegknochen).

Primordial bones are such as are developed out of the cartilaginous skeleton. Either there arise centres of ossification within the cartilage after softening and dissolution of its matrix, as was described in the ossification of the vertebral column, the ribs, and the sternum, or the perichondrium alters its formative activity, and secretes, in place of layers of cartilage, bony tissue upon tin- already formed cartilage. In the first instance one can speak of an endochondral, in the second instance of a perichondral ossification. The cartilaginous primordial skeleton can be crowded out and replaced by a bony one in both ways, remnants of cartilage of greater or less magnitude being preserved in the several classes of Vertebrates.

The covering bones, on the contrary, arise outside the primordial cranium in the connective tissue enveloping it, either in the skin which covers its surface or in the mucous membrane that lines the head-gut. They are therefore ossifications which do not occur on any other part of the axial skeleton and which are also at first foreign to the skeleton of the head. Consequently in early stages of development, and in many classes of Vertebrates even in the adult animal, they can be dissected off without in any way injuring the primordial cranium. It is otherwise with the primary bones, the removal of which always causes a partial destruction of the cartilaginous skeleton.

If, as just now stated, the covering bones are at first foreign to the skeleton of the head, there arises the question of their source. To answer this I must go back a little.

In lower Vertebrates there is developed, besides the internal cartilaginous axial skeleton, an external or dermal skeleton, which serves for the protection of the surface of the body, and is also continued at the mouth for some distance into the cavity of the head-gut, where it may be designated as mucous-membrane skeleton. In the simplest condition it consists, like the scaly armor of the Selachians, of small close-set denticles, the placoid scales, which have arisen from ossifications of dermal and mucous-membrane papillse. In other groups of the Fishes the dermal armor is composed of larger or smaller bony plates, which bear upon their surfaces numerous denticles or simple spines. They are described according to their form and size as scales, scutes, plates, or dermal bones ; they are explainable in a very simple manner as derivatives from the Selachian armor of placoid scales, by the fusion at their bases of larger or smaller groups of denticles, which thus produce larger or smaller skeletal pieces. The larger bony pieces arise principally in the region of the head, and especially at the places where cartilaginous parts of the cranial capsule or of the visceral arches approach close to the surface. Thus in many Ganoids and Teleosts the brain is found to be enveloped by a double capsule an inner capsule, either purely cartilaginous or provided with centres of ossification, and a bony armor lying directly upon it.

In the higher Vertebrates the most of the dermal skeleton has completely degenerated, but on the head it is in large part preserved, and furnishes the previously mentioned covering bones, which serve to supplement and complete the internal skeleton.

An interesting insight into the original method of the development of covering bones can still be acquired in many of the Amphibians (fig. 336). For example, the vorner and the palatinum, which are covering bones, arise in very young Triton larvae by the formation of small denticles (z'} in the mucous membrane of the oral cavity, and by the fusion of their bases to form small tooth-bearing plates of bone (z, z). These plates increase in size for a time, owing to the establishment in the Fig. 336. Vomer of an Axoioti IT- i PIT larva 1 - 3 cm. long.

neighboring mucous membrane of addi- By the fnsiou of t * th (=< 2) a tioiial dental spines, which become attached tooth-bearing plate of bone P. P . h;is arisen in the mucous to their margins; afterwards they often ln , m brane. c', Apices of

lose the equipment of denticles, which are teeth in P TOC ess of . ment, which are subsequently destroyed by being resorbed. attached to the margin of tiu It may be said that the original process bon ^ i )late aml contribute to 4-1 ! l C i its growth.

in the development or covering bones here described is abbreviated in most of the Amphibia. For at the places in the mucous membrane which the vorner and the palatinum occupy, the tips of denticles are not even begun ; but in the layer of tissue in which otherwise the bases of the denticles would have been fused, a process of direct ossification takes place. In the same abbreviated way the covering bones arise in all Reptiles, Birds, and Mammals.

The skulls of many Amphibia (Frog, Axolotl) likewise afford the best explanation of the original relation of the covering bones to the primordial skeleton (fig. 337). The covering bones are found to be loosely superposed upon the primordial cranium, from which they can be easily removed. Thus upon the left side of the accompanying figure the premaxillaria (Pmx), maxillaria (M), vorner (To), palatinum (Pal), pterygoid (Ft], and parasphenoid (Ps) have been detached, whereas upon the right side they have been retained. After their detachment there is left the inner head-skeleton proper a capsule still consisting in great part of the original cartilaginous tissue (N, N l , PP, Qu], into which, however, there are introduced at some places bony pieces : the occipitalia (Olat), petrosa (Pro), sphenoidea [sphenethmoid] (E), etc.

In the higher Vertebrates, especially in Mammals, the primordial cranium, the primary ossifications, ;intl lln 1 covering bones, which in Fishes and Amphibia aro easily distinguishable from one another even in the adult animals, are to be recognised as separate parts only in very early stages of development ; later it becomes more difficult to distinguish them, at last impossible. This is due to several things : First, the cartilaginous primordial cranium is laid down from the beginning in a rudimentary condition: then, too, a large part of the roof is wanting, the opening being closed by a connective -tissue membrane.

Secondly, the cartilaginous primordial cranium subsequently disappears almost entirely, partly by being dissolved, partly byconversion into primordial bones. There persist small remnants, which have been retained only in the cartilaginous septum narium and the cartilages of the outer

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Fig. 337. Skull of a Frog (Rana esculenta). View from beneath. After ECKER.

The lower jaw is removed. On the left side of the figure the covering bones have been removed from the cartilaginous part of the skull .

C'occ, Condyli occipitales ; Olat, occipitale laterals ; GK, auditory capsule ; Qu, quadratum ; Qjcj, quadrate.jugale ; Pro, prooticum ; P.s 1 , parasphenoid ; As, alisphenoid; Ft, osseous pterygoid ; PP, palato-quadratum; FP, fronto-parietale ; E, ethmoid (os en ceinture) ; Pal, palatinum ; Vo, vomer ; M, maxilla ; Pnix, premaxillare ; N, N 1 , cartilaginous nasal framework ; //, V, VI, places of emergence of n. opticus, n. trigeminus, and n. abdiicens.

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nose connected with it.

Thirdly, in the fully developed skull the primordial bones and the covering bones are no longer distinguishable ; for the latter lose their superficial position, become intimately united to the bones derived from the primordial cranium, and with them, filling up the gaps, constitute a firm, closed, bony receptacle of mixed origin.

Fourthly, in the adult animal, bones which in the embryo are formed separately, and in lower Vertebrates always remain thus, are often fused. There is a fusion not only between bones of like origin, but also between primordial and covering bones, whereby it finally becomes altogether impossible to distinguish them. Many of the bones of the human cranium are consequently bone -complexes.

It may be stated as a general rule that the ossifications on the base and sides of the cranium are primordial, but that on the roof and in the face covering bones make their appearance.

The following parts of the human skull belong to the primordial elements : (1) occipitale, except the upper part of the squauious portion ; (2) the sphenoidale, except the internal pterygoid plate ; (3) ethmoidale and turbiiiatum ; (4) petrosum and raastoid portions of the temporale ; (5) the auditory ossicles malleus, incus, and stapes ; (6) the body of the hyoicles, with its greater and lesser cornua.

The following are covering bones: (1) the upper part of the squamous portion of the occipitale ; (2) the parietale ; (3) the frontale; (4) the squamous portion of the temporale ; (5) the internal pterygoid plate of the sphenoidale ; (G) the annulus tympanicus; (7) palatinum ; (8) voiner ; (9) nasale ; (10) lachrymale ; (11) zygomaticum; (12) maxillse sup. ; (13) maxillse inf.

I will now, after this survey, give a somewhat more detailed account of the development of the bones of the head enumerated above.

I. f>ones of the Cranial Capsule.

(1) The occipitale is at first a cartilaginous ring surrounding the foramen magnum ; it begins to ossify early in the third month at four points. One centre of ossification is formed below the foramen, another above, and two more at its sides. In this way there arise four bones, which are joined by broader or narrower bands of cartilage, according to the degree of their development. In the lower Vertebrates Fishes, Amphibia (fig. 337 Olat) they remain in this condition as separate bones, and are designated as occipitale basilare, oc. superius, and oc. laterale.

To these are added in Mammals and Man a covering bone, which arises from two centres of ossification in the connective tissue farther above the foramen the interparietale. This begins, even in the third foetal month, to fuse with the superior occipital bone to constitute the squama ; however, up to the time of birth furrows running in from right and from left mark the boundary of the two genetically different parts. In the new-born child squama, occipitalia lateralia and oc. basilare are still separated from each other by thin remnants of cartilage. Then in the first year the squama fuses with the lateral parts (partes condyloidese), and finally there is united with these, in the third or fourth year, the pars basilaris. The occipitale is therefore a complex that has originated from five separate bones.

(2) The sphenoidale also arises from numerous centres of ossification, which appear in the base of the primordial cranium, and which in the lower classes of Vertebrates represent parts of the cranial capsule that remain separate. In the anterior prolongation of the pars basilaris of the occipitale there appear in the vicinity of the sella turcica an anterior and a posterior pair of centres, which constitute the fundaments of the bodies of the anterior and posterior sphenoidea. At the sides of these there are developed special centres of ossification for the lesser and for the greater wings.

In most Mammals the lesser wings fuse with the anterior, the greater with the posterior body. Thus there are formed two sphenoidea, an anterior and a posterior, which are placed in front of the occipitale, and are separated from each other by a thin strip of cartilage. In Man these two bones become joined together, by the ossification of the cartilaginous strip mentioned, to constitute the unpaired single sphenoidale, with its many processes. The fusions of the numerous separate ossifications take place in the following order. In the sixth foetal month the lesser wings of the sphenoid fuse with the anterior body ; shortly before birth the latter unites with the posterior body, and in the first year after birth the greater wings are united with the rest. From the latter the outer pterygoid plates grow downward, whereas the inner ptery //aid plates are formed as covering bones. For in the connective tissue of the lateral wall of the oral cavity there is developed a special region of ossification ; this furnishes a thin bony lamella, which is preserved in many Mammals as a special skeletal element (os pterygoideurn) lying on the pterygoid process of the sphenoidale. In Man it early fuses with the sphenoidale, notwithstanding it has an entirely different origin from the latter.

(3) The temporale is a complex of various bones, the greater part of which are still separate in the new-born infant. The os petrosum with the mastoid process is developed from numerous centres of ossification in that part of the primordial cranium which encloses the organ of hearing, and has therefore been designated as cartilaginous ear-capsule. With it is united after birth the styloid process, which in the embryo is a cartilaginous rod that is derived from the upper end of the second visceral arch and that ossifies from its own independent centre.

To the primordial bones there are added in Man two covering bones, squama and pars tympanicus, which are as foreign to the primordial cranium as the parietal or frontal bones. Of these the pars tympanicus (fig. 335 pr) is at first a narrow bony ring, which serves as a frame for the tympanic membrane. It is developed in connective tissue outside of the auditory ossicles, and, in particular, outside the malleus (ha) and the connected MECKEL'S cartilage (MK). Thus is explained the position of the long process of the malleus in the fissura petrotympanica, when, soon after birth, the primordial and covering bones fuse with each other. For the annulus tympanicus gradually becomes broadened into a bony plate, which serves as a support for the external meatus. This plate then fuses with the petrosal bone, except along a narrow cleft, the fissura petrotympanica or Glaseri, which remains open, because here the chorda tynipani and the long process of the malleus were in the embryo shoved in between the bones, while they were still separate.

In lower Vertebrates, and also in many Mammals, the pieces mentioned remain separate, and are distinguished in comparative anatomy as os petrosum, os tympanicum, and os squamosum.

(4) The ethmoidale and the turbinatum of the nose are primordial bones, which are developed out of the posterior part of the cartilaginous nasal capsule, whereas the anterior part remains cartilaginous and becomes the cartilaginous septum nasoruin and the external nasal cartilages.

" The ossification begins in the lamina papyracea in the fifth month. Then follows the ossification of the lower and middle turbinals. At birth these are united by means of cartilaginous portions of the ethmoidale. After birth the vertical plate with the crista galli is the first to ossify; then follows the ossification of the upper turbinal and of the gradually developed labyrinth, from which the ossification advances to the corresponding halves of the cribriform plate. The union of the two lateral halves with the lamina perpendicularis does not take place until between the fifth and the seventh year." (GEGENBAUR.) Of the covering bones of the primordial cranium, which in general begin to ossify at the beginning of the third month, the following remain separate : the parietale, frontale, nasale, lachrymale, and vomer. Of these the frontale is originally, like the others, a paired structure, and still continues in this condition into the second year after birth, when the closure of the frontal suture begins. Nasale and lachrymale are covering bones of the cartilaginous nasal capsule. The vomer arises as a paired structure at the sides of the cartilaginous septum of the nose in the third month. The two lamellre afterwards fuse, the cartilage between them disappearing.

II. Bones of the Visceral Skeleton.

The remaining bones of the head, which have not been mentioned hitherto, belong to the visceral skeleton, some of them being primordial, others covering bones.

The hyoid bone and the auditory ossicles (perhaps also the thyroid cartilage) are primordial parts ; they are characterised by very diminutive size and occupy a very subordinate position in comparison with the enormously developed covering bones. The hy aides begins toward the end of embryonic life to ossify at several points. The auditory cartilages acquire from the periosteum as early as the fourth month a bony investment, within which here and there remnants of cartilage persist even in the adult. According to recent researches, the malleus is a confound skeletal piece. The long process is developed as a covering bone on that part of MECKEL'S cartilage which penetrates between petrosal and annulus tympanicus. While the cartilage undergoes degeneration, the covering bone fuses with the larger, primordial part of the malleus. It probably corresponds with the os angulare of lower Vertebrates.

The covering bones of the visceral skeleton, the maxillare superius, palatinum, pterygoideum, zygomaticum, and maxillare inferius, are developed in the vicinity of the mouth-opening in the connective tissue of the superior and inferior maxillary processes, The maxillaria superiores are a complex of two pairs of bones, which indeed remain separate in most Vertebrates. One pair is developed on the two superior maxillary processes laterad of the cartilaginous nasal capsule. The other pair appears in the eighth or ninth week, according to TH. KOLLIKER'S detailed investigations, upon the part of the frontal process that lies between the nasal orifices. It corresponds to an actual paired intermaxillary (prernaxillare), and subsequently encloses the fundaments of the four incisors.

The two intermaxillaries in Man early fuse with the fundaments of the two superior maxillaries, the two membranous superior maxillary processes having previously united with the inner nasal processes. The boundary between maxillary and intermaxillary is indicated on the crania of young persons by a suture-like place (sutura incisiva), running transversely outward from the foramen incisivum, which is occasionally retained even in the adult.

There early grow out from the two superior maxillaries into the palatal processes horizontal lamellae which produce the two palatal bones the hard or bony palate.

Palatals and pterygoids are developed in the roof and side walls of the oral cavity they are consequently mucous-membrane bones. The pterygoids apply themselves, as was stated on p. 620, to the cartilaginous downgrowths of the greater wings of the sphenoid. In many Mammals they remain separate from the latter throughout life, but in Man they unite with it and are now distinguished as inner pterygoid plates from the outer plates, which arise by ossification of cartilage.

The development of the visceral skeleton, which has been discussed here and in previous sections (pp. 284, 515), furnishes the basis for the interpretation of the malformations which are quite frequently met with in the maxillary and palatal region in Man. I refer to the labial, maxillary, and palatal fissures, which are simply malformations due to arrested development. They result when the separate fundaments from which are formed the upper lip, the upper jaw, and the palate do not come into normal union (figs. 288-91).

The malformations of arrested development can present very different variations, according as the coalescence is wholly or only partly omitted, and according to whether it affects one or both sides of the face.

In the case of total arrest, in palatal, maxillary, arid labial fissures of both sides, both nasal cavities are broadly in communication with the oral cavity by means of a right and a left fissure running from in front backward. From above there projects free into the oral cavity the nasal septum, which is enlarged in front, and here bears the incompletely developed intermaxillary with its rudimentary incisor teeth. In front of it lies a small dermal ridge, the fundament of the middle part of the upper lip. At the sides of the fissures and the nasal openings, which have not been closed in below, there lie the two separated maxillary processes, with the bony upper jaw and the fundaments of the canine and molar teeth. The horizontal palatal plates project as ridges only a little distance into the oral cavity, and have not effected a junction with the nasal septum. A malformation of this kind is very instructive for the comprehension of the normal processes of development previously described.

When the arrest is only partial, coalescence may fail either on the superior maxillary processes only, or on the palatal plates only, and either on one or on both sides. In the first case there is produced a labio-maxillary fissure, or even a labial fissure (hare-lip) only, while hard and soft palates are formed normally. In the other case the upper jaw is well developed and no external evidence of malformation is visible, while there is a fissure on one or both sides which passes through the soft palate, and sometimes through the hard palate also (cleft palate).

The development of the lower jaw is coupled with fundamental metamorphoses. As has been previously explained, in the youngest embryos the oral cavity is limited below by the membranous inferior maxillary processes. Within this there is developed (fig. 338) MECKEL'S cartilage (MK\ the cranial end of which becomes (compare p. 611) the fundament of the malleus (ha), by means of which MECKEL'S cartilage is articulated with the incus (cmi). At its ventral end in Mammals it unites in the middle line with the corresponding part of the other side, whereas in Man a small space remains between them.

Inasmuch as the small cartilages mentioned have arisen in the first visceral arch, they correspond both in position, and also in their mutual connections and many other relations, to the large cartilaginous elements with which we have already become familiar in the Selachians (fig. 330) as palato-quadraturn (0) and mandibulare (U). In the Selachians the palato-quadratum and mandibulare are functional as a genuine jaw-apparatus, for they bear on their margins the teeth, which are attached in the mucous membrane only, and the masticatory muscles are inserted on their surface.

In Mammals and Man the function of the skeletal parts corresponding to them has become essentially different, for they have entered into the service of the auditory apparatus ; a profound, and in its final results wonderful and highly important metamorphosis has taken place here. In order to explain this it is necessary to touch briefly upon a few comparative-anatomical facts.

With the beginning of ossifications the primary lower jaw loses in Teleosts, Amphibia, and Reptiles its simple condition, and is converted into an apparatus which is often very complicated. The ossifications are here, just as was the case in the other parts of the head-skeleton, of two different kinds, primary and secondary. One bone, which makes its appearance in the articular part of the cartilage and produces the os articulare, is a primary bone. With this are associated several covering bones arising in the surrounding connective tissue, two of which, the angulare and the dentale, acquire special importance. Both are attached to the outer surface of the cartilaginous [Meckelian] rod, the angulare near the joint, the dentale in front of it and extending to the syrnphysis.

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Fig. 338. Head and neck of a human embryo 18 weeks old with the visceral skeleton exposed, after KOLLIKF.R. Magnified. The lower jaw is somewhat depressed in order to show MECKEL'S cartilage, which extends to the malleus. The tympanic membrane is removed and the annnlns tympaiiicus is visible. lie, Malleus, which passes uninterruptedly into M ECKEL'S cartilage, A/A" ; ulc, bony lower jaw (dentale), with its condyloid pmcess articulating with the temporal bone; am, incus; st, stapes; pr, aunulus tympaiiicus; grf, processus styloidens ; lath, ligamentnm stylo liyoideum ; kh, lesser cornu of the hyoid bone ; t/h, its greater cornu. +++++++++++++++++++++++++

The latter is an important skeletal element, which attains a considerable size, receives into its upper margin the teeth, and grows around the cartilage of MECKEL in such a manner that the cartilage is almost completely enclosed in a bony cylinder. The whole complicated apparatus, composed of several bones and the original cartilage enclosed within them, articulates at the primary joint of the jaio between palato-quadratum and os articulare.

The same fundaments are again met with in Mammals and Man.

In tho articular part of the cartilage of the lower jaw, which has assumed the form of the malleus (figs. 334, 338 ha), there arises a special centre of ossification, which corresponds to the articulare of other Vertebrates. In its vicinity appears, as a, covering hone, an exceedingly small angulare, which subsequently fuses with it, producing the long process of the malleus. The second covering bone, the dentale (fig. 338 uk), attains, on the contrary, a great size and alone becomes the subsequently functioning lower jaw, whereas the remaining parts, which in the compound mandibular apparatus of Teleosts, Amphibia, Reptiles, and Birds participate in the function of chewing (palato-quadratum, or quadratum, articulare, angulare, and MECKEL'S cartilage), lose their original function and are employed in another manner.

The most important motive to this profound metamorphosis is to be found in the fact that in Mammals and Man there is developed in place of the primary articulation of the jaw a secondary one. The primary articulation, upon which the tooth-bearing dentale is moved, lies, as we have seen, between palato-quadratum and articulare. Inasmuch as these elements correspond respectively to the incus and malleus of Mammals, the primary articulation of the jaw of lower Vertebrates is to be sought in the incus-malleus articulation of the higher Vertebrates. In Mammals and Man the dentale is no longer moved at this joint, because the dentale itself forms a direct articulation with the cranial capsule by means of a bony projection, -the processus coiidyloideus (fig. 338), which it sends upward, and through which it is united to the squamous portion of the temporal bone at some distance in front of the primary articulation. This union constitutes the secondary articulation of the jaw, in which only covering bones participate.

The natural result of the formation of a new articulation is, that the primary lower-jaw apparatus has become superfluous for the act of mastication, and that its development is restricted. Incus, malleus, and angulare, which is united with the malleus, are converted into parts of the auditory organ (see p. 613). The remaining part of MECKEL'S cartilage (JOT) begins to degenerate, in Man in the sixth month. A portion of it, which is a prolongation of the long process of the malleus, extending from the fissura petrotympanica as far as the entrance into the bony lower jaw at the foramen alveolar e, is converted into a connective -tissue cord, the ligamentum laterale internum maxillae inferioris. A small portion near the front end early acquires a special centre of ossification and fuses with the covering bone. The remainder of that portion of MECKEL'S cartilage which is enclosed in the canal of the lower jaw, from the foramen alveolare onward, is gradually broken down and dissolved ; however, remnants of the cartilage are found even in the new-born infant at the symphysis.

At first the bony lower jaw is a paired structure, consisting of tooth-bearing halves. These remain in many Mammals as separate bones, being united in a symphysis by means of connective tissue. In Man they are united in the first year after birth into a single piece by the ossification of the intervening tissue.

A special peculiarity is exhibited by the articular end of the lower jaw, phylogenetically a covering bone. Instead of beginning to be formed, in the manner of the anterior portion, by direct ossification of the connective-tissue foundation, there first arises here a cartilaginous tissue consisting of large vesicular cells and soft intercelluar substance, which is gradually converted into bone. This presents a certain similarity to the development of the primordial bones. But that the resemblance is only superficial is shown by the difference in the structure of the articulation, to which I shall return in a subsequent section.

(c) Concerning the Relation of the Head-Skeleton to the Trunk-Skeleton.

In different sections of this text-book in discussing the primitive segments, the nervous system, and especially now in the discussion of the axial skeleton reference has been made to many points of agreement that have been recognised between the structural conditions of the head and those of the trunk. In a critical comparison of these two regions of the body there arise many important questions which have for several decades engaged the attention of the best morphologists. It may therefore be well here, after having given the pertinent facts, to take up these questions more particularly, and determine the relation which head and trunk, and especially that which head-skeleton and trunk-skeleton, sustain to each other.

Before I elucidate the present state of the question, I will give a brief survey of the history of these researches, which have been grouped together under the name " The Vertebral Theory of the Skull" The relation which the anterior and posterior parts of the skeleton of the trunk sustain to each other in the morphology of Vertebrates was for the first time subjected to a thorough scientific discussion at the beginning of the present century, when the school of the " Natural Philosophers " began its career. An attempt to solve the problem was made in very similar ways by two persons, by the natural philosopher OKEN and by the poet GOETHE, without either of them having been influenced by the other.

According to the OKEN-GOETHE vertebral theory, the skull is the most anterior part of the vertebral column, and is composed of a small number of modified vertebrae. OKEN distinguished three vertebra? in his " Programme " entitled " Ueber die Bedeutung der Schadelknochen," which appeared in 1807, when he entered upon a professorship conferred upon him in Jena. He named them the ear-, eye-, and jaw-vertebra?.

Each head-vertebra, like a trunk- vertebra, consisted in his opinion of several parts a body, two arch-pieces, and a dorsal spine. OKEN, GOETHE, and their numerous followers believed that this composition was most distinctly recognisable in the last cranial vertebra, the OGcipitcde, the base of which was compared to the body of the vertebra, the condyloid parts to the lateral arches, and the squama to the spine of the vertebra.

A second cranial vertebra was discerned in the body of the %)osterior sphenoidale, which together with its greater wings and the two parietal bones formed a second bony ring around the brain.

A third vertebra was constructed out of the body of the sphenoidale, anterius, the lesser wings and the frontale.

The ethmoidale was cited by many investigators as a fourth the most anterior cranial vertebra. A number of bones, which would not fit into this scheme, were considered to be structures sui generis, and were in part associated with the sensory organs as sensory bones, in part compared with the ribs of the thorax.

In this form, which underwent numerous modifications in details, the OKEN-GOETHE vertebral theory of the cranium dominated morphology for decades and formed the foundation of many investigations. It had a stimidating and fruitful effect until, with a deeper insight into the structure of Vertebrates, it was abandoned as defective and erroneous, giving way before the force of numerous newly discovered facts.

For neither the comparative osteology of the skull nor growing embryological research could point out in a satisfactory way which bones were really to be interpreted as parts of vertebrae. The most dissimilar, and more or less arbitrary, opinions upon this subject made their appearance. An agreement even as to the number of vertebrse contained in the skeleton of the head could not be reached. Some investigators assumed six, others five or four, or even three only.

HUXLEY, in his "Elements of Comparative Anatomy," by a critique based upon facts, was the first to prepare the way for a termination of this unpleasant state of affairs, in which the vertebral theory was held to with tenacity, notwithstanding the contradictions that everywhere arose. In his discussion he argued from a series of facts 'which embryological investigation had brought to light. As such the following, important for the problem of the skull, should be cited before all others.

First, the discovery that the skeleton of the head, like the vertebral column, is developed out of a cartilaginous condition, and that the brain is first enclosed by a primordial cartilaginous cranium (BAER, DUGES, JACOBSON).

Secondly, the doctrine established mainly by KOLLIKER, that the bones of the head-skeleton are separable into two groups according to their development into the primordial bones, which arise in the primordial cranium itself, and the secondary or covering bones, which have their origin in the enveloping connective tissue.

Thirdly, the insight which was acquired, through the important works of RATHKE and REICHERT, into the metamorphoses of the visceral skeleton, and thereby into the development of the palatomaxillary apparatus and the auditory ossicles.

Through an examination of these various facts, HUXLEY was led to the important and fully justified conclusion, that not a single cranial bone can be recognised as a modification of a vertebra, that the skull is no more a modified vertebral column than the vertebral column is a modified skull ; that, rather, both are essentially distinct and different modifications of one and the same structure.

While HUXLEY stopped at the negative standpoint, simply denying the vertebral theory, GEGENBAUR has made the question of the relation of skull and vertebral column, raised by GOETHE and OKEN, but from ignorance of the facts incorrectly answered by them, again the object of profound comparative study. Rightly recognising that the problem can be solved only by detailed investigation of the primordial skeleton, he selects as the object for his studies the cartilaginous skull of the Selachians, and endeavors in his revolutionising work, " Das Kopfskelet der Selachier als Grundlage zur Beurtheilung der Genese des Kopfskelets der Wirbelthiere," to produce the evidence that the primordial cranium has arisen by fusion from a number of segments equivalent to vertebral. Instead of the OKEN-GOETIIE vertebral theory he propounds the seymental theory oj the skull, as I suggest the doctrine of GEGENBAUR be called.

GEGENBAUR proceeds from the correct conception that the segmentation of a region of the body is recognisable not only in the metamerism of the vertebral column, but also in many other structures in the method of the arrangement of the chief nerve-trunks, and in the ventral arch-structures attached to the axial skeleton. He investigates, accordingly, the cranial nerves of the Selachians, and arrives at the conclusion that, with the exception of the olfactory and optic nerves, which are metamorphosed parts of the brain itself, they deport themselves like spinal nerves both in their origin and their peripheral distribution. He determines that there are nine pairs of them ; and therefore concludes that the portion of the headskeleton which is traversed by the nine segmentally arranged cranial nerves must be equivalent to nine vertebral segments, and that it must have arisen by their very early fusion.

The visceral skeleton of Selachians is regarded by GEGENBAUR from the same instructive point of view. He discerns in the maxillary, hyoid, and branchial arches skeletal elements which are represented in the vertebral column by the ribs.

Inasmuch as a vertebral segment belongs to each pair of ribs, a similar relation is also assumed as the original arrangement for the visceral arches. Thus this method of considering the question leads to the same result : that the primordial cranium since at least nine visceral arches belong to it as ventral arch-structures has been produced from at least nine segments.

Such an origin GEGENBAUR accepts for the posterior chordatraversed region of the skull only, in which alone the emerging nerves agree with spinal nerves. He therefore distinguishes this as vertebral from the anterior or non-vertebral portion, which does not allow the recognition of any segmentation, and which begins in front of the anterior end of the chorda. He interprets the latter as a new formation which has been established by the enlargement in front of the vertebral part of the skull.

GEGENBAUR explains the great differences which exist between skull and vertebral column as adaptations, partly to the enormous development of the brain, partly to the sensory organs of the head, which are received into pits and cavities of the primordial cranium.

Since the time when GEGENBAUR with keen discrimination propounded his segmental theory of the skull, the way has been prepared in many directions, chiefly through embryological investigation, for a better comprehension of the skeleton of the head.

Investigations which I undertook on the dermal skeleton of Selachians, Ganoids, and Teleosts, as well as 011 the head-skeleton of Amphibia, showed that the difference between primordial and covering bones is much greater than it was originally assumed to be. For as their development shows, the covering bones are at first structures quite foreign to the axial and head-skeleton, formed at the surface of the body in the skin and mucous membrane. They are parts of a dermal skeleton, which in lower Vertebrates protect the surface of the body as a scaly armor, parts which have entered into union with the superficially located portions of the inner, primordial cartilaginous skeleton. Therefore the covering bones of the lower Vertebrates are often tooth -bearing bony plates, which have originated from a fusion of isolated dental fundaments, a condition which may be regarded for many reasons as the primitive one.

A further acquisition of broad significance is the discovery of the primitive segments of the head, which we owe to BALFOUR, MILNES MARSHALL, GOETTE, WIJHE, and FRORIEP.

By it an important point of agreement between head and trunk has been made out. The two body-sacs penetrate even into the head ; here also the two middle germ -layers are separated into a dorsal portion, lying in contact with the chorda arid neural tube, which is divided into nine pairs of primitive segments,* and into a ventral portion (see p. 351).

The head is therefore segmented similarly to the trunk, even at a time when the first traces of the fundament of a vertebral column or a head-skeleton are not yet present.

Thirdly, the insight into the development of the cranial nerves (BALFOUR, MARSHALL, WIJHE, and others) is important. An agreement with the development of the spinal nerves has been established in so far as some cranial nerves have a dorsal origin from a neural crest, like the sensory roots of spinal nerves, while others grow out ventrally from the brain-vesicles like anterior roots.

Finally, I would mention as a step in advance, which is not without significance for the interpretation of the head-skeleton, the altered conception of the, meaning of the primitive segments which embrijological evidence has compelled us to form.

The primitive segments are the real fundaments of the musculature

• [Sea footnote p. 458.]

of the body. The iirst segmentation of the vertebrate body affects the body -sacs and the musculature arising from them. The formation of the primitive segments is only remotely and indirectly connected with the development and segmentation of the vertebral column. It is only after muscle-segments have existed for a long time that, at a comparatively late stage of development, the fundaments of a segmented vertebral column are established. But these arise, by histological metamorphosis, from an unsegmented connective-tissue matrix, in consequence of the appearance of a process of chondriiication.

All the conditions here only briefly touched upon are of farreaching significance for the question of the relation of the head- and trunk-skeletons to each other. For, as GEGENBAUR rightly points out, since the establishment of his segmental theory " the vertebral theory of the skull has become more and more a problem of the phylogenesis of the whole head." I desire to give briefly and connectedly my own views upon this subject : Theory concerning the Relation of the Head and its Skeleton to the Skeleton of the Trunk.

The segmentation of the vertebrate body begins with the walls of the primary body-sacs, the dorsal portion of which, abutting upon the chorda and neural tube, is divided by the formation of folds into successive compartments, the primitive segments.

Inasmuch as the voluntary musculature is developed from the walls of the primitive segments, it is the first system of organs in Vertebrates to be segmented.

The myomeric condition " myomerism ' -is the direct cause of a segmental arrangement of the peripheral nerve-tracts, for the motor nerves pertaining to a segment unite to form an anterior [ventral] root as they emerge from the spinal cord, and in the same manner the sensory nerves which come from a corresponding part of the skin together constitute a sensory root.

At a time when the segmentation of the musculature and of the peripheral nerve-tracts has already been effected, the skeleton is still unsegmented ; for it is represented by the chorda dorsalis alone. The soft mesenchyme, which envelops the chorda and the neural tube, and which becomes the matrix of the subsequently formed segmented axial skeleton, is still a continuous mass of cells, filling in the spaces between these organs.

At this time the differentiation of head and trunk has already taken place. This is accomplished, first by the establishment of the higher sensory organs in the anterior portion of the body, secondly by the enlargement of the neural tube into the voluminous brainvesicles, thirdly by the formation of a regular series of visceral clefts in the walls of the head-gut, which thus also undergo a kind of segmentation (branchiomerism).

The reyion of the body which is thus metamorphosed into a head is from the beginning segmented, and is composed, as the Selachians show, of at least nine primitive segments.

The development of visceral clefts produces still further differences between head and trunk. By the appearance of visceral clefts, the front part of the body-cavity is divided up into several successive headcavities. By the disappearance of these cavities, parts corresponding to the thoracic and abdominal cavities have become obliterated. Further, there are developed out of the cells composing the walls of the head-cavities important masses of transversely striped muscles for moving and constricting the separate portions of the branchial region of the alimentary canal, whereas in the trunk the voluntary musculature arises exclusively from the primitive segments. In the trunk these masses of muscle spread out both dorsally over the neural tube and also ventrally into the wall of the thorax and abdomen, whereas in the head they remain limited to a small space and do not undergo any extensive development.

It is only after head and trunk have thus already become in a high degree different that the cartilaginous axial skeleton begins to be formed.

The latter Ls therefore a structure of comparatively recent origin, as it also is peculiar to the phylum, Vertebrata, and even here is wanting in the lowest representative, Ainphioxus lanceolatus.

The development of the cartilaginous axial skeleton in the two chief regions of the body is from the beginning partly similar, partly dissimilar.

The development is similar in so far as the process of chondrification begins in both head and trunk in the perichordal connective tissue, then extends around the chorda both above and below, ensheathing it, and finally is continued into the connective-tissue layer that envelops the neural tube.

The dissimilarity is expressed in the occurrence or omission of segmentation. In the trunk under the influence of the musculature there arises a segmentation of the cartilaginous axial skeleton into firm vertebral pieces, alternating with mtervertebral ligaments which remain in the connective-tissue state. In the head there is developed at once a continuous cartilaginous capsule around the brain-vesicles. The segmentation, which in this region is expressed in other systems of organs, in the formation of primitive segments and in the arrangement of the cranial nerves, does not occur in the corresponding part of the axial skeleton. Never in the course of the development of any Vertebrate has there been observed, as the first fundament of the primordial cranium, a succession of cartilaginous pieces, alternating with connective -tissue discs, and there seems to be no ground for assuming that a condition of this kind existed in earlier times. In the slight development of the muscles derived from the primitive segments of the head, and in the voluminous condition attained by the brain and sensory organs, are to be discerned, on the contrary, factors which have converted the head, at an early period, into a more rigid portion than the trunk. The cause, which in the trunk has made the segmentation of the axial skeleton necessary, has been wanting in the head.

During the last few years the opinion has been expressed by a number of persons (ROSENBERG, STOHII, FRORIEP) that in some classes of Vertebrates the occipital region of the primordial cranium is increased by fusion with vertebral fundaments of the neck-region, and thus, as it were, " is constantly advancing caudad." I leave undetermined to what extent this is true. GEGENBAUR combats the interpretation of STOHR, but describes a quite frequently occurring fusion of the cranial capsule with vertebrae in Bony Fishes. One thing only would I point out : the conception of the first unsegmented fundament of the primordial cranium which I have presented is not irreconcilable with the view that subsequently new vertebral segments may be added behind.

Besides the segmented condition of the vertebrce, a segmentation of the axial skeleton is also expressed in the appearance of ventral arches, which are repeated in regular order from before backwards. On the head they are designated as visceral arches, on the trunk as ribs.

The position of these skeletal parts also is dependent upon the first segmentation which affects the organisation of Vertebrates. For the ribs are developed between the muscle-segments by a process of chondrification in the connective-tissue plates separating themthe intermuscular ligaments ; while the visceral arches are dependent upon the visceral clefts, by which the ventral part of the head-region is divided into a number of successive segments.

It cannot be concluded from the existence of ribs and visceral

arches that the corresponding skeletal axis must likewise have been segmented. They are only an indication of the segmentation of the region of the body to which they belong.

That the segmentation of the head which is present in the embryo is more or less obliterated in the adult Vertebrate depends upon two causes. First the primitive segments are only slightly developed, furnishing unimportant muscles, and in part wholly degenerate ; secondly the visceral skeleton is subjected to profound metamorphoses. Especially in the higher Vertebrates it experiences such a degeneration and metamorphosis, that finally nothing of the original seginental arrangement of its parts (palato-maxillary apparatus, auditory ossicles, hyoid bone) is left.

B. The Development of the Skeleton of the Extremities.

A description of the skeleton of the extremities should be preceded by a few words in regard to the fundaments of the limbs themselves. These at first appear as small elevations [limbbuds] at the sides of the trunk in front and behind (tig. 339). That they belong more to the ventral than to the dorsal surface of the body is evident from the fact that they are innervated by the ventral branches of the spinal nerves.

Moreover, the limbs appear to belong to a large number of tr link-segments. This is to be inferred both from the method of the distribution of nerves and also from the source

oe

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Fig. 339. Very young human embryo of the fourth w eek 4 mm long, neck-rump measurement ; taken from the uterus of a suicide 8 hours after her death, after RABL.

au, Eye ; ng, nasal pit ; uk; lower jaw ; zb, hyoid arch ; s", *, third and fourth visceral arches ; li t protrusion of the wall of the trunk caused by the growth of the heart ; us, boundary between two primitive segments ; oc, ue, anterior and posterior linibs.

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of their musculature. For the anterior and posterior limbs always receive their nerves from a large number of spinal nerves. The muscles are derived from the same source as the whole musculature of the trunk from the primitive segments.

It has not yet been possible to establish the derivation of the musculature in Mammals and Man. For the limb-buds consist of a mass of small, closely crowded cells ; it is impossible to state which of these belong to the mesenchyme, which to the musculature, or which to the nerves. The conditions in lower Vertebrates, on the contrary, are much more favorable.

In Selachians the fins, which correspond to the limbs of the higher Vertebrates, contain, even at the time of their formation as small plates, distinctly recognisable embryonic gelatinous tissue, which is covered in by the epidermis. An important discovery by DOHRN has established that there grow into the gelatinous tissue of the fin two buds from each of a large number of primitive segments ; the buds then become detached from their parent tissue and each is divided into a dorsal and a ventral half the fundaments of extensor and flexor musculature. Each fin therefore contains a series of muscular fundaments, which have arisen segmentally and are arranged one behind another, a fact which has its weight in many other questions touching the origin of the limbs.

In Man the fundaments of the limbs take on a definite form as early as the fifth week. The outgrowths have become enlarged and divided into two regions, of which the distal becomes the hand, or foot. In the case of the anterior extremity the front margin of the hand already begins to acquire indentations, by which the first fundaments of the fingers are indicated. In the sixth week the three chief divisions of the limbs are recognisable, for the proximal portion is now marked off by a transverse furrow either into arm and fore-arm or into thigh and leg. Now, too, on the foot the toes are indicated by constrictions, but less distinctly than are the fingers on the hand.

In the seventh week there are to be observed at the tips of the fingers claw-like appendages, consisting of epidermal cells the primitive nails. As HENSEN remarks, " The similarity of the hand at this stage to the anterior extremity of a Carnivore viewed from the sole is striking ; in addition to the toe-like brevity and thickness of the fingers, the pads are well developed." With their enlargement the limbs apply themselves to the ventral surface of the embryo, being directed obliquely from in front backward [and ventracl], the anterior limbs more obliquely than the posterior. In both of them the future extensor side lies dorsal, the flexor side ventral. Both the radial and tibial margins with the thumb and great toe are directed cephalad, the fifth finger and the fifth toe caudad.

By this and by the fact that the limbs belong to several trunksegments are explained certain conditions in the distribution of the nerves of the upper extremity. In the case of the arm "the radial side is supplied with nerves (axillaris, musculo-cutaneus), whose fibres are referable to the fifth, sixth, and seventh cervical nerves. Upon the ulnar side, on the contrary, are found nerves (n. cutaneus medialis, 11. medius, and n. ulnaris) whose origin from the lower secondary trunk of the plexus discloses their derivation from the eighth cervical and first dorsal nerves " (SCIIWALBE).

In the further course of development both limbs alter their original position, the anterior to a greater extent than the posterior, inasmuch as they undergo a torsion around their long axes in opposite directions. In this way the extensor side of the upper arm becomes directed backward [caudad], that of the thigh forward ; radius and thumb are now directed laterad, tibia and great toe mediad. These alterations in position due to torsion are naturally to be taken into account in determining the homologies of the anterior and posterior extremities, so that radius corresponds to tibia and ulna to fibula.

In the originally homogeneous cell-mass the fundaments of the skeleton and musculature are gradually differentiated from each other, owing to the fact that the cells acquire a more definite histological character. In this connection the following phenomenon is to be observed : The parts of the skeleton of the extremity are not all established at the same time, but follow a definite sequence, in somewhat the same manner as, in the development of the axial skeleton, the process of segmentation begins in front and progresses backward. So in the limbs the proximal skeletal elements (i.e., those which are situated nearer to the trunk) are formed sooner than the distal ones.

This is the most strikingly apparent in the case of the fingers and toes. Whereas the first phalanx has been differentiated from the surrounding tissue in embryos of the fifth and sixth week, the second and third are not at that time distinguishable ; the ends of the fundaments of fingers and toes still consist of a mass of small cells in process of growth. In this mass the second phalanx is then differentiated, and at last the third.

Furthermore the formation of the anterior limbs outstrips somewhat that of the posterior.

In the development of the skeleton of the extremities there are to be recognised, as in the vertebral column and the skull, three different stages, the stage of the membranous, that of the cartilaginous, and that of the osseous fundament.

After these general remarks I turn to the detailed description of (1) the pectoral and pelvic girdles, (2) the skeleton of the appendage, which projects free from the surface of the trunk, and (3) the formation of joints.

() Pectoral and Pelvic Girdles.

The fundaments of the girdles of the limbs consist each of a pair of curved pieces of cartilage, which are imbedded under the skin in the muscles of the trunk, and which bear near the middle an articular surface for the reception of the skeleton of the free extremity. By this each cartilage is divided into a dorsal half, near the vertebral column, and a ventral half. The former is converted in Mammals and Man into a broad shovel-shaped piece ; the ventral half, which reaches to, or nearly to, the median plane, is, on the contrary, divided into two diverging processes, an anterior and a posterior. The cartilaginous pieces thus distinguishable ossify from special centres, and thereby acquire a higher degree of independence.

The shoulder-blade (scapula) of Man is at first a cartilage of a form similar to that of the adult, except that the basis scapulae is less developed. In the third month ossification begins at the collum scapulse. However, the margins, the spine, and the acromion remain for a long time cartilaginous, and indeed are in part so even at the time of birth. There arise in them here and there accessory centres during childhood.

From the articular part of the shoulder-blade there runs ventrally a cartilaginoiis process, which is short in Man, but in other Vertebrates is of considerable size and reaches down to the sternum. It corresponds to the posterior of the previously mentioned diverging processes into which the ventral part of the cartilaginous arch is divided, and is known in comparative anatomy as pars coracoidea. In Man it is only slightly developed. Its great independence, however, is made evident by its acquiring in the first year after birth a separate centre of ossification. From this there gradually arises a bony element (os coracoideum), which is joined to the shoulder-blade until the seventeenth year by a strip of cartilage, and may therefore be detached. Afterwards it is united with the scapula by bony substance and constitutes the coracoid process. Still later the fusion of the accessory centres previously mentioned takes place, to which, however, no great morphological importance attaches.

There are two different views concerning the place which the clavicle takes in the shoulder-girdle.

According to GOETTE, HOFFMANN, and others, it belongs to the primordial skeletal parts, which are preformed in cartilage, and corresponds to the anterior ventral process, which was present in the primitive form of the shoulder-girdle. According to GEGENBAUR it is a covering bone which has entered into union with the cartilaginous skeleton in the same way as the covering bones of the skull have with the primordial cranium.

It is the peculiar method of the development of the clavicle that has caused this divergence of opinion. This is the first bone to be formed in Man ; it begins to be ossified as early as the seventh week. The earliest bony piece, as GEGENBAUR was the first to ascertain, is developed out of wholly indifferent tissue. Then there are added at both ends masses of cartilage, which are softer and provided with less intermediate substance than the ordinary embryonic cartilage. They serve, as in other bones that are preformed in cartilage, for the elongation of the clavicle at both ends. There is also developed in the sternal end, between the fifteenth and twentieth years, a kind of epiphysial centre, as KOLLIKER states ; this fuses sometimes as late as the twenty-fifth year with the main piece.

The original conditions are the most faithfully preserved in the pelvic girdle, even in Man and Mammals. The first fundament of the girdle consists of a right and a left pelvic cartilage, which are united ventrally in the symphysis by means of connective tissue, and each of which has at its middle an articular fossa. Each pelvic cartilage is composed of an expanded part extending dorsally from the articular depression, the iliac cartilage, which is joined to the sacral region of the spinal column, and two ventral cartilaginous rods, pubis and ischium, which, meeting in the symphysis, enclose the foramen obturatorium.

It is stated by ROSENBERG that the pubic cartilage is at first formed independently, but that it soon fuses with the other cartilages at the acetabulum.

Ossification begins at the end of the third month in three places, and thus are formed a bony ilium, os puhis, and ischium at the expense of the cartilage, of which, however, considerable remnants are still present at the time of birth. .For tho, whole crest of the ilium, the rim and fimdus of tho acetabulum, and the whole tract from the tnberosity of the ischium to tho spine of the pnbis is still cartilaginous.

After birth the growth of the three bony pieces advances toward tho acetabulum, where they all meet, being however separated, up to the time of puberty, by strips of cartilage, which together form a three-rayed figure. At about the eighth year both the ascending and descending rami of pubis and ischium fuse with each other, so that at this time each hip-bone consists of two pieces joined by cartilage at the acetabulum the ilium and an ischio-pubic bone. These do not become united into one piece until the time of puberty.

As in the pectoral girdle, so also in the pelvic girdle, there occur accessory centres of ossification ; of these one, which sometimes arises in the cartilage of the acetabulum, is the most important, and is described as os acetabuli. Others arise in the cartilaginous crest of the ilium, in the spines and tubercles, and in the tuberosity of the ischium. They are not united with the chief bones until the end of the period of growth.

(b) Skeleton of the Free Extremity.

All skeletal parts of the hand, fore-arm, and arm, as well as of the foot, leg, and thigh, are originally solid pieces of hyaline cartilage, which early acquire the general forms of the bones that subsequently replace them. They are marked off from their surroundings by a special fibrous layer of connective tissue, the perichondrium.

From the beginning of the third month the process of ossification takes place in the larger skeletal pieces, by means of which the cartilaginous tissue is destroyed and replaced by osseous tissue, in the same manner as in the vertebral column. In this process several general phenomena regularly make their appearance ; I shall go somewhat into the details of these, without however taking into account the complicated histological changes, information concerning which is given in text-books of histology.

The process of ossification takes externally a somewhat different turn according as the cartilages are small and uniformly developed in all directions, as in the wrist and ankle, or have become more elongated, In the first case the course of development is more simple. From the perichondriurn vascular, richly cellular connective-tissue processes grow into the cartilage, dissolve its matrix, and unite with one another in its centre. There arises a network of medullary [marrow] cavities, in the vicinity of which there is a deposit of salts of lime (a provisional calcification). The medullary spaces extend farther and farther by destruction of the cartilaginous substance. Then there are secreted by the superficially located medullary cells bone-lamella?, which gradually increase in thickness. The osseous nucleus thus formed slowly increases in size, until finally the cartilage is almost entirely replaced, only a thin layer of it remaining at the surface as a covering to the bone.

The ossification of the wrist- and ankle-bones is therefore purely endochondral, and proceeds ordinarily from one, sometimes from two, centres of ossification. It does not begin until very late in the first year after birth. The only exception occurs in the foot, where the os calcis and astragalus acquire a bony nucleus in the sixth and seventh months, and the cuboid begins to ossify a short time before birth. In the others ossification takes place after birth, and, as KO'LLIKER states, in the following order : I. In the hand. (1) Os magnum and unciform (first year) ; (2) cuneiform (third year) ; (3) trapezium and lunar (fifth year) ; (4) scaphoid and trapezoid (sixth to eighth year) ; (5) pisiform (twelfth year).

II. In the foot. (1) Os scaphoideum (first year) ; (2) internal and middle cuneiform (third year) ; (3) external cuneiform (fourth year).

Concerning the cartilaginous fundaments of a special centrale carpi, which usually is not retained as a separate carpal element (ROSENBERG), as well as a special intermedium tarsi or trigonum (BARDELEBEN), the text-books of comparative anatomy are to be consulted.

The process of ossification is more complicated in the long cartilages, in which, moreover, it begins much earlier, usually even in the third month of embryonic life. The course of ossification is fairly typical.

At first a perichondral ossification takes place midway between the ends of each cartilage in the humerus and femur, tibia and fibula, radius and ulna. From the perichondrium there is deposited upon the already formed cartilage bony tissue instead of a cartilaginous matrix, so that the middle portion of the cartilage becomes ensheathed in a bony cylinder, which is continually increasing in thickness.

The further growth of the skeletal element thus composed of two tissues proceeds in two ways : first by growth of the cartilage, and secondly by increase of bony substance.

The cartilaginous tissue increases at both ends of the skeletal piece and contributes to the increase of the latter both in length and thickness. In the middle, on the contrary, where it is enveloped in a bony cylinder, it ceases to grow. Here there is a continual addition of new bony lamellae upon those already formed ; they are produced by the original perichondrium, or, as one may now more properly say, by the periosteum.

In this process the successive lamellae extend farther and farther toward the two ends of the skeletal piece ; new portions of the cartilage are being continually ensheathed in bone and restricted in their growth.

The periosteal bony sheath assumes in consequence the form of two funnels united at their apices.

The cartilage which fills up the funnels early undergoes a gradual metamorphosis and degeneration. From the osseous sheath there grow into it connective-tissue strands with blood-vessels, which dissolve the matrix and produce larger and smaller marrow-cavities. Then, by the secretion of osseous tissue at the surface of the persisting remnants of cartilage, there is developed a spongy bonesubstance, which fills up the funnel -shaped cavities of the compact bony mantle produced by the periosteum. The spongy bone is, however, only an evanescent structure. It in turn is gradually dissolved, beginning at the middle of the skeletal element, and its place is occupied by a very vascular marrow. In this way there arises in the originally quite compact cartilaginous fundament the large central medullary cavity of the long bones.

During these processes the two ends still remain cartilaginous, and serve for a long time by their growth to increase the length of the skeletal element. They are designated as the two epipliyses, in distinction from the middle piece, which is the first to ossify, and which has received the name diaphysis. The latter increases in size at the expense of the epiphysial cartilages, for the endochondral process of ossification progresses, with a very distinct line of ossification, toward both ends.

A new complication in the development of the tubular (long) bones arises either a short time before or in the first years after birth. There are then developed in the middle of each epiphysis special centres of ossification, the so-called epipliysial nuclei ; there are first produced, in the manner previously described, vascular canals, which arise by the dissolution of the cartilaginous substance ; the canals unite to constitute large medullary spaces, at the surfaces of which osseous tissue is then secreted.

By a slowly progressing enlargement of the bony nucleus, which continues for years, the epiphysial cartilage is gradually converted into a spongy osseous disc, being finally reduced to small remnants. First, there is preserved, as an investment of the free surface, a layer only a few millimetres thick, which constitutes the " articular cartilage." Secondly, there remains for a long time a thin layer of cartilage between the older, bony middle piece and the bony disc-like epiphysis, and this serves to keep up the elongation of the skeletal part. For the cartilage grows vigorously by the proliferation of its cells, and thus is being renewed as fast as its two flat surfaces are dissolved away by the endochondral ossification which takes place at its expense, both by the growth of the bony epiphyses and, to a much greater extent, by that of the more rapidly elongating diaphysis.

Thus it happens that long bones which have not yet ceased .growing can be divided into three pieces, if the organic parts are removed by maceration. A fusion into a single osseous piece does not take place until, at the time of maturity, the increase in the length of the body has ceased. Then the thin plates of cartilage between the diaphysis and its two epiphyses are broken down and converted into bony tissue. From this time forward a further increase in the length of the bone is impossible.

Besides the three typical and chief centres already described, from which the ossification of the cartilaginous fundament of a tubular bone proceeds, there are established in many cases smaller centres of ossification of secondary importance, which are denominated accessory bone-nuclei. They always arise in the later years, when the epiphyses are well developed, and sometimes not until they are in process of fusion with the diaphysis. They then appear at places where the cartilaginous fundament possesses elevations and projections, as in the tubercles of the humerus, in the trochanters of the femur, the epicondyles, etc. They serve for the conversion of these elevations into osseous masses, which are generally the last to fuse with the chief bone.

After this general description, I add some detailed statements about the formation and the number of the move important bony nuclei in the fundaments of the separate tubular bones, concerning which we have the extensive investigations of SCHWEGEL.

1. The diaphysis of the humerus ossifies in the eighth week. Epiphysial nuclei are not formed until after birth, at the end of the first or beginning' of the second year. In the second year there appear accessory nuclei in the tuberculum majus and minus ; during and after the fifth year in the epicondyles also.

2. The diaphyses of the radius and ulna also begin to ossify in the eighth week. Epiphysial nuclei do not appear until between the second and the fifth years. Accessory nuclei are observed rather late in the styloid processes.

3. The metacarpals begin to ossify in the ninth week, but, with the exception of the metacarpal of the thumb, there arises only one epiphysis, which is at the distal end. This acquires in the third year its own centre of ossification.

4. The ossification begins in the phalanges at the same time as in the metacarpals.

5. The femur begins to ossify in the seventh week. A sliort time before birth there is formed in the distal ejiipJiysis a centre of ossification, which is a part of the evidence that a child has been carried to the full time, and therefore possesses a certain importance for forensic purposes. After birth an epiphysial nucleus soon appears in the head of the femur. Accessory nuclei are formed in the fifth year in the trochanter major, in the thirteenth or fourteenth in the trochanter minor.

G. Tibia and fibula acquire epiphysial nuclei in the first and third years after birth, first at the proximal, then at the distal end, the ossification in the fibula occurring about a year later than that in the tibia. GEGENBAUR regards this as indicating a subordination of the functional importance of the fibula in comparison with the tibia.

7. The patella begins to ossify in the third year.

8. To the metatarsals and the phalanges of the toes applies in general all that has been said about the corresponding parts of the hand.

(c) Development of the Joints.

Inasmuch as the separate pieces of cartilage in the body are formed by histological differentiation in the connective-tissue layers, they are at first united to one another by remnants of the parent tissue. This generally acquires a more compact fibrous condition and is converted into a special ligament.

Such a union of the separate skeletal elements is the prevailing method in the lower Vertebrates, as, e.g., in the Sharks. In the higher Vertebrates, including Man, it is retained in many, but not all, place?, as, e.g., in the vertebral column, where the bodies of the vertebrae are joined to each other by intervertebral discs of connective tissue. But at the places where the apposed skeletal parts acquire greater freedom of motion upon each other, there appears, in place of the simpler connective-tissue union, the more complicated articular connection.

In the development of the joints the following general phenomena occur : Young cartilaginous fundaments, as, e.g., those of the thigh and leg, are in early stages separated at the place where the articular cavity is subsequently formed by a very cellular intermediate tissue (the intermediate disc of HENKE UND EEYHER). This subsequently diminishes in extent, because the ends of the cartilages grow at its expense. In many cases it disappears entirely, so that the terminal surfaces of the skeletal parts concerned are for some distance in immediate contact.

The specific curvature of the articular surfaces is by this time more or less well established. This is accomplished at a time when there is as yet no articular cavity, and when, moreover, movements of the skeletal parts cannot be executed, because the muscles are not capable of functioning.

From this it follows that during embryonic life the articular surfaces cannot acquire their specific form under the influence of muscular activity, and that they are not formed, as it were, by attrition and adaptation to each other in consequence of definite recurrent movements in a simply mechanical way, as has been assumed by many. The early appearing typical form of the joint seems therefore to be inherited (BERNAYS). Muscular activity can be effective only for alterations at later stages; it is, however, not without influence in the further development and formation of the articular surfaces.

When, after the disappearance of the intermediate tissue, the surfaces at the ends of the developing cartilages come into immediate contact, there arises between them a narrow fissure as the first fundament of the articular cavity. This is bounded directly by the hyaline articular cartilage, which does not here possess any perichondrium. Then a sharper delimitation of the articular cavity from the surrounding connective tissue gradually takes place, inasmuch as a firmer connective- tissue layer, which becomes the capsular ligament, is developed from one cartilage to the other, and additional fibrous tracts are converted into separate tense articular ligaments.

The process of development takes a somewhat different course when the articular surfaces do not fit into each other. In these cases the ends of the cartilages cannot come into immediate contact in the manner previously described ; they now remain separated by more or less considerable remnants of the richly cellular intermediate tissue, which then assumes more and more the condition of compact fibrous tissue.

When the intermediate tissue is preserved in its whole extent, there arises a fibro- cartilaginous interarticular disc (intermediate or interpolated cartilage), which is inserted as an elastic cushion between the skeletal pieces. There is formed an articular fissure between the ligamentous disc and the terminal surfaces of each of the articular cartilages, or, in other words, there is developed an articular cavity, which is divided into two by means of an interpolated disc.

Finally, a special modification of the joint occurs when the cartilages are partly in contact and partly remain separated by intermediate tissue. In this case there appears at the place of contact a single articular cavity ; laterally, however, this is enlarged by the incongruent parts of the cartilaginous surfaces becoming split off from the intermediate tissue separating them. Thus there arises an articular cavity which, it is true, is single, but into which are thrust from the articular capsule the metamorphosed products of the intermediate tissue, which constitute the so-called semi-lunar fibre-cartilages or the menisci, as in .the case of the knee-joint.

As was previously described in treating of the development of the bones of the extremities, there is preserved, even after the termination of the process of ossification, an exceedingly small remnant of the cartilaginous fundament, which forms on the articular surfaces a cartilaginous covering only a few millimetres thick. The articular ends of all bones that are developed out of a cartilaginous fundament possess such a covering.

It is different when bones that have been produced directly in connective tissue (the covering bones) are united to each other by a veritable joint. Such a case occurs in the articulation of the lower jaw in Mammals. The glenoid process of the lower jaw. as well as the glenoid fossa of the squamous portion of the temporal bone, is in this case covered with a thin layer of unossified tissue. It looks like cartilage, and usually is described as such. But microscopic examination shows that it is composed exclusively of layers of connective-tissue fibres.

As there are bones which are preformed, in cartilage and others which are preformed in connective tissue, so a distinction must be made between joints with a covering of hyaline cartilage and joints witli a covering of fibrous connective substance.

Summary

The Vertebral Column

1. During development the vertebral column passes through several (from lower to higher) morphological conditions, of which the lower are permanently preserved in the inferior classes of Vertebrates, whereas in the higher classes they appear only at the beginning of development and are then replaced.

2. In the axial skeleton three different stages of development are distinguished : (1) As chorda dorsalis (notochord), (2) As cartilaginous and (3) As osseous vertebral column.

3. The chorda is developed out of a tract of cells (chorda-entoblast, fundament of the chorda) lying below the neural tube and belonging to the inner germ-layer, from which it is detached by abstriction. (chordal folds).

4. The chorda is a rod composed of vesiculated cells and bounded superficially by a firm sheath ; it begins with a pointed end beneath the mid-brain vesicle (in the region of the future sella turcica of the cranial floor) and reaches to the blastopore (primitive groove).

5. The chorda persists as a permanent skeletal structure in Amphioxus and the Cyclostomes.

6. A cartilaginous vertebral column is found in the adults of the Selachians and some of the Ganoids, while in the remaining Vertebrates it appears more or less during development as a forerunner of the bony vertebral column.

7. The cartilaginous vertebral column is developed by histological metamorphosis out of embryonic connective tissue, a part of which envelops the chorda as skeletogenous chordal sheath, and a part forms a thin continuous envelope (membranous vertebral arches) around the neural tube.

8. The process of chondrification begins 011 both sides of the chorda, progresses around it both above and below, and thus forms a cartilaginous ring, the body of the vertebra, from which the process of chondrification advances dorsally into the membranous envelope of the neural tubes, producing the arches of the vertebrae and ceasing with the formation of the vertebral spines.

9. It is not until the beginning of the process of chondrification in the unsegmented, connective-tissue, skeletogenous chordal sheath that the axial skeleton undergoes a segmentation into separate like portions, which are situated one behind another ; to accomplish this, remnants of the parental tissue do not chondrify, but become, between the bodies of the vertebrae, the intervertebral discs, and, between the arches, the ligamenta intercruralia, etc.

10. The segmentation of the vertebral column has been dependent in its origin upon the segmentation of the musculature, and has been effected in such a way that skeletal segments and muscular segments alternate with one another, and that the longitudinal muscle-fibres, which lie alongside the axial skeleton, are attached by their anterior and posterior ends to two [adjacent] vertebrae and are capable of moving them upon each other.

11. The chorda is more or less restrained in its growth by the cartilaginous bodies of the vertebrae surrounding it, and degenerates in different ways in the different classes of Vertebrates ; in Mammals the part located in the body of tne vertebra is completely obliterated, whereas a remnant of it is preserved between vertebrae and becomes the jelly-core of the intervertebral disc.

12. The cartilaginous vertebral column is converted in most Vertebrates into a bony one, by the breaking down of the cartilaginous tissue, which begins at different places, and its replacement by bony tissue. (Formation of bone-nuclei or centres of ossification.) 13. The ossification of each cartilaginous vertebral fundament in Mammals and Man proceeds from three centres, from one in the body and one in each half of the arch, to which subsequently certain accessory centres are added.

14. With each vertebral segment there is associated a pair of ribs, which arise by a process of chondrification in the layers of tissue which separate the muscle-segments (the ligamenta intermuscularis).

15. In Man the various regions of the vertebral column are produced by metamorphosis of the vertebral and costal fundaments.

(1) The thoracic part of the vertebral column (dorsal vertebrae) is characterised by the following peculiarities : the ribs attain to complete development ; a part of them become expanded at their ventral ends, and united to form the two sternal bars, by the fusion of which the unpaired sternum is produced. (Fissura sterni, an arrested formation.) (2) In the cervical and lumbar regions of the column the funda ments of the ribs remain small, and fuse with outgrowths from the vertebrae the transverse processes to form

THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 649 the lateral processes. In the neck-region there is retained, between the transverse process and the rudiment of the rib, the foramen transversarium for the vertebral artery.

(3) Atlas and epistropheus [axis] assume special forms, owing to the fact that the body of the atlas remains separate from the fundaments of its arch, and unites with the body of the axis to form its odontoid process. (Separate centre of ossification in the odontoid process.) (4) The sacrum results from the fusion of five vertebrae and the sacral ribs belonging to them. The latter by their fusion produce the so-called massse laterales, which bear the articular surfaces for the ilium.

B. The Head- Skeleton.

16. The skull, like the vertebral column, passes through three morphological conditions, which are designated as membranous and as cartilaginous primordial cranium and as bony cranial capsule.

17. The membranous primordial cranium consists of (1) The anterior end of the chorda, which extends to the anterior margin of the mid-brain vesicle, and (2) A connective-tissue layer, which surrounds the chorda as skeletogenous layer, and also furnishes a membranous investment around the five brain- vesicles.

18. The cartilaginous primordial cranium arises by a histological metamorphosis of the membranous one.

(1) At the sides of the chorda there are first formed two car tilaginous rods, the two parachordals, which soon grow around the chorda both above and below, and become united into a single cartilaginous plate.

(2) In front of the parachordals RATHKE'S trabeculse cranii make their appearance ; their posterior ends soon unite with the parachordal cartilages, their anterior ends become enlarged and by fusing with each other produce the ethmoid plate ; in the middle they remain for a long time separate and embrace the hypophysis (region of sella turcica).

(3) From the cartilaginous base of the cranium thus produced, the process of chondrification, as in the development of the vertebral column, first extends into the lateral walls, and at last into the roof of the membranous primordial cranium, partly enclosing the higher sensory organs.

19. In the Selachians the cartilaginous primordial cranium is a permanent structure, and possesses rather thick uniform walls ; in Mammals and Man, on the contrary, it is of only short duration, serving as foundation for the bony cranial capsule that takes its place; it is therefore less completely developed than in Selachians, for only the base and lateral parts are in all cases cartilaginous, whereas the roof presents large openings closed by dermal membranes.

20. From its relation to the chorda dorsalis, there are distinguishable in the cartilaginous primordial cranium two chief portions, a vertebral (chorda!) and a non-vertebral (prechordal), or, according to its relations to the sensory organs, it may be divided into four regions ethmoidal, orbital, labyrinthal, and occipital.

21. As the ribs are associated with the vertebral column in the form of ventral arched structures, so also the visceral skeleton is united to the primordial cranium in the head-region.

22. The visceral skeleton is composed of segmented cartilaginous rods, which have arisen by a process of chondrification in the tissue of the membranous visceral arches between the successive visceral clefts.

23. The cartilaginous throat- or visceral arches are well developed only in the lower Vertebrates (permanently in the Selachians), and are distinguished, according to differences of position and form, as jawarch, hyoid arch, and branchial arches, the last being variable in number.

24. The jaw-arch is divided into the cartilaginous upper jaw (palato-quadratum) and the cartilaginous lower jaw (rnandibulare) ; the hyoid arch into the hyomandibulare, the hyoides, and the unpaired copula.

25. In Mammals and Man the cartilaginous visceral skeleton attains only a very rudimentary condition, and is converted into the cartilaginous fundaments of the three auditory ossicles and the hyoid bone.

26. In the membranous jaw-arch arise () The incus, which corresponds to the palato-quadratum of lower Vertebrates ; (b) The malleus, which is the representative of the articular part of the cartilaginous mandibulare ; and (c) The cartilage of MECKEL, which corresponds to the remain ing portion of the mandibulare, but which afterwards completely degenerates.

'27. The membranous hyoid arch furnishes, [beginning with] its uppermost part, (a) The bow of the stapes, whereas its plate is derived from the cranial capsule and is, as it were, cut out to form the fenestra ovalis, (6) The processus styloideus, (c) The ligamentum stylohyoideum, and (d) The lesser horn and body of the hyoid bone.

28. The third membranous visceral arch is chondrified only in its lowest [ventral] part, to form the greater horn of the hyoid bone.

29. At no stage of its development does the primordial cranium exhibit evidence that, like the vertebral column, it is composed of separate segments.

30. The original segmentation of the head is expressed in only three ways in the appearance of several primitive segments (myotomes), in the arrangement of the cranial nerves, and in the fundament of the visceral skeleton.

31. The primordial cranium is therefore an unsegmented skeletal fundament in a region of the body that is segmented in another manner.

32. The ossification of the head-skeleton is a much more complicated process than that of the vertebral column.

33. Whereas in the vertebral column there are developed bones of only one kind, through substitution for cartilage, there are to be distinguished in the ossification of the head-skeleton, according to their formation and source, two different kinds of bone primary and secondary.

34. The primary bones of the head arise in the cartilaginous primordial cranium and visceral skeleton, like the separate bonenuclei in the cartilaginous vertebral column.

35. The secondary bones, covering or membrane-bones, arise outside the primordial skeleton of the head in the connective-tissue foundation of the skin and mucous membrane ; they are therefore dermal and mucous-membrane ossifications, and constitute in lowerVertebrates a portion of a dermal skeleton that covers the surface of the whole body.

36. The covering bones are developed in some instances, which can be regarded as reproductions of the original method, by fusion of the bony bases of numerous denticles which arise in the skin and mucous membrane.

37. Primary and secondary bones sometimes remain separate in later stages, sometimes they fuse with each other to form bonecomplexes, like the temporale and sphenoidale.

38. After the conclusion of the process of ossification only unimportant remnants of the primordial cranium persist as the cartilaginous partition of the nose and as the nasal cartilages.

C. The Skeleton of the Extremities.

39. The skeleton of the limbs, excepting the clavicle, the development of which exhibits many peculiarities, is established in the cartilaginous stage. (Cartilaginous shoulder-girdle, cartilaginous pelvic girdle, cartilages of arm and leg.) 40. The ossification takes place, in the same manner as in the vertebral column and primordial cranium, from centres of ossification by disintegration of cartilaginous tissue and its replacement by osseous tissue.

41. The most of the small cartilages of the wrist and ankle ossify from a single bone-nucleus, but the larger flat cartilages of the shoulder and pelvic girdles from several centres.

42. The cartilaginous fundaments of the tubular [long] bones ossify at first in the middle, which region is designated as diaphysis, whereas their two ends the epiphyses remain for a long time cartilaginous, and are the means of the elongation of the skeletal element.

43. In Man the cartilaginous epiphyses begin to ossify from centres of their own (epiphysial nuclei), some of them in the last month before, others not until after birth.

44. The fusion of the bony diaphysis with the bony epiphyses does not take place until the termination of the growth of the skeleton and body in length, and is accompanied by the removal of the intervening cartilaginous tissue.

45. Before growth is at an end the tubular bones can be divided into a larger middle piece (diaphysis) and two small bony epiphyses.

46. Of the cartilaginous fundament of a tubular bone there is preserved only a small remnant as a cartilaginous covering of the articular ends (articular cartilage).

47. The medullary cavity of the tubular bones is formed by the resorption of the spongy bone-substance that first replaced the cartilage.

48. Whereas the articular ends of bones preformed in cartilage are covered over with hyaline cartilage, the articular surfaces of bones of connective-tissue origin (covering bones) present an investment of fibrous connective substance (articulation of the jaw).

49. The form of the articular surfaces is determined at a time when an influence on the part of the musculature is not to be considered.

Literature

Development of the Diaphragm and Pericardium.

Cadiat, M. Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragma, des pleures, du pericarde, du pharynx et de 1'oesophage. Jour, de VAnat. et de la Physiol. T. XIV. 1878. Faber. Ueber den angeborenen Mangel des Herzbeutels in anatomischer, entwicklungsgeschichtlicber und klinischer Beziebung. Yirchow's Archiv.

Bd. LXXIV. 1878, p. 173. His, W. Mittheilungen zur Embryologie der Saugethiere und des Menscben.

Arcbiv f. Anat. u. Physiol. Anat, Abth. 1881. Lockwood. The Early Development of the Pericardium, Diaphragm and Great Veins. Philos. Trans. Roy. Soc. London, 1888. Vol. CLXXIX. P..

1889, p. 365. And Proceed. Pioy. Soc. London. Vol. XLIII. 1888, p. 273. Ravn. Bildung der Scheidewand zwischen Brust- und Bauchhohle in Sauge thier-Embryonen. Biol. Centralblatt, Bd. VII. 1887. Ravn. Ueber die Bildung der Scheidewand zwischen Brust- und Bauchhohle in Saugethier-Embryonen. Archiv f. Anat. u. Physiol. Anat. Abth. 1889. Ravn. Untersuchungen liber die Entvvicklung des Diaphragmas und der benachbarten Organe bei den Wirbelthieren. Archiv f. Anat. u. Physiol.

Anat. Abth. 1889. Suppl.-Band. Uskow, W. Ueber die Entwicklung des Zwerchfells, des Pericardiums und des Coeloms. Archiv f. mikr. Anat. Bd. XXII. 1883. Waldeyer. Ueber die Beziehungen der Hernia diaphragmatica congenita zur Entwicklungsweise des Zwerchfells. Deutsche medic. Wochenschrift.

No. 14. 1884.

Development of the Heart and Blood-vessels.

Bernays, A. C. Entwicklungsgeschichte der Atrioventricularklappen. Mor phol. Jahrb. Bd. II. 1876. Born, G. Beitrage zur Entwicklungsgeschichte des Saugethierherzens.

Archiv f. mikr. Anat. Bd. XXXIII. 1889. Brenner, A. Ueber das Verhaltniss des N. laryngeus inf. vagi zu einigen Aortenvarietaten des Menschen und zu dem Aortensystem der durch Lungen athmenden Wirbelthiere iiberhaupt. Archiv f. Anat. u. Physiol.

Anat. Abth. 1883. Gasser. Ueber die Entstehung des Herzens bei Vogelembryonen. Archiv f.

mikr. Anat. Bd. XIV. 1877. Hasse, C. Die Ursachen des rechtzeitigen Eintritts der Geburtsthatigkeit bcim Menschen. Zeitschr. f. Geburtshilfe u. Gynakologie. Bd. VI.

1881, pp. 1-9.

Hochstetter, F. Ucber die Bildung der hinteren Hohlvene bei den Saugethieren. Anat. Anzeiger. Jahrg. II. No. 16, 1S87, ]). 517.

Hochstetter, F. Ueberden Einfluss der Entwicklung der bleibenden Nieren auf die Laare des Urnierenabschnittes der binteren Cardinalvenen. Anat.

o Anzeiger. Jabrg. III. 1888. Hochstetter, F. Beitrage zur vergleichenden Anatomic und Entwicklungs geschicbte des Venensystems der Amphibien und Fische. Morphol. Jabrb.

Bd. XIII. 1888. Hochstetter, F. Ueber das Gekrb'se der hinteren Hohlvene. Anat. Anzeiger.

Jahrg. III. 188.S. Hochstetter, F. Beitrage zur Entwicklungsgeschichte des Venensystems der Anmioten. Morphol. Jahrb. Bd. XIII. 1888.

Lindes. Ein Beitrag zur Entwicklungsgeschichte des Herzens. Inauguraldissert. Dorpat 18G5. Marshall, J. On the Development of the Great Anterior Veins in Man and Mammalia. Philos. Trans. Roy. Soc. London. 1850. Masius. Quelques notes sur le developpement du coeur chez le poulet.

Archives de Biologic. T. IX. 1889. Oellacher. Ueber die erste Entwicklung des Herzens und der Pericardial oder Herzhohle bei Bufo cinereus. Archiv f. mikr. Anat. Bd. VII. 1871, p. 157. Peremeschko. Ueber die Entwicklung der Milz. Sitzungsb. d. k. Akacl. d.

Wissensch. Wien. Math.-naturw. Cl. Bd. LVI. Abth. 2. 1867, p. 31. Rabl, Carl. Ueber die Bildung des Herzens der Amphibien. Morphol.

Jahrb. Band XII. 1887, p. 252. Rathke, H. Ueber die Bildung der Pfortader und der Lebervenen bei Sauge tbieren. Meckel's Archiv. 1830. Rathke, H. Ueber den Bau und die Entwicklung des Venensystems der Wirbelthiere. Bericht u'ber das naturhist. Seminar der Universitiit Kouigs berg. 1838. Rathke, H. Ueber die Entwicklung der Arterien, welche bei den Sauge thieren von dem Bogen der Aorta ausgehen. Archiv f. Anat. u. Physiol.

Jahrg. 1843. Rose, C. Zur Entwicklungsgeschichte des Saugethierherzens. Morphol.

Jahrb. Bd. XV. 1889, p. 436. Sabatier. Observations sur les transformations du systeme aortique dans la serie des Vertebres. Ann. d. Sci. Nat. Ser. 5. T. XIX. 1874. Schmidt, F. J. Bidrag til Kundskaben om Hjertets Udviklingshistorie.

Nordiskt medicinskt Arkiv. Bd. II. 1870. Sertoli. Ueber die Entwicklung der Lymphdriisen. Sitzungsb. d. k. Akad.

d. Wissensch. Wien. Math.-naturw. Cl. Bd. LIV. Abth. 2. 1866. Strahl, H., und Carius. Beitrage zur Entwicklungsgeschichte des Herzens und der Korperhohlen. Archiv f. Anat. u. Physiol. Anat. Abth. 1889. Tiirstig. Mittheilung iiber die Entwicklung der primitiven Aorten nach Untersuchungen an Huhnerembryonen. Dissertation. Dorpat 1886. Wertheimer, E. Recherches sur la veine ombilicale. Jour, de 1'Anat. et de la Physiol. T. XXII. 1886, pp. 1-17.

Development of the Skeleton.

Ahlborn, Fr. Ueber die Segmentation des Wirbeltbierkorpers. Zeitschr. f. wiss. Zoologie. Bd. XL. 1884, p. 309.

Albrecht, P. Sur la valeur morphologique rle 1'articulation rnandibulaire, du cartilage de Meckel et des osselets de 1'ouie, etc. Bruxelles 1883. Balfour. On the Development of the Skeleton of the Paired Fins of Elasmo branchii considered in Relation to its Bearings on the Nature of the Limbs of the Vertebrata. Proceed. Zool. Soc. London. 1881. Bardeleben, K. Das Os intermedium tarsi der Saugethiere. Zool. Anzeis;er.

Jahrg. VI. 1883. Bardeleben, K. Ueber neue Bestandtheile der Hand- und Fusswurzel der Saugethiere, etc. Jena. Zeitschr. Bd. XIX. Suppl.-Heft III. 1886 (?) Baumiiller. Ueber die letzten Veranderungen des Meckei'schen Knorpels.

Zeitschr. f. wiss. Zoologie. Bd. XXXII. 1879. Bernays, A. Die Entwicklungsgeschichte des Kniegelenks des Menschen mit Bemerkungen liber die Gelenke im Allgemeinen. Morphol. Jahrb.

Bd. IV. 1878. Brock. Ueber die Entwicklung des Unterkiefers der Saugethiere. Zeitschr.

f. wiss. Zoologie. Bd. XXVII. 1876, p. 287. Carius. Ueber die Entwicklung der Chorda und der primitiven Eachenhaut bei Meerschweinchen und Kaninchen. In.-Diss. Marburg 1888. Decker. Ueber den Primordialschadel einiger Saugethiere. Zeitschr. f.

wiss. Zoologie. Bd. XXXVIII. 1883. Dohrn, A. Studien zur Urgeschichte des Wirbelthierkb'rpers : IV. Die Entwicklung und Differenzirung der Kiemenbogen der Selachier. V. Zur Entstehung und Differenzirung der Visceralbogen bei Petromy zon Planeri. VI. Die paarigen und unpaaren Flossen der Selachier.

Mitth. a. d. Zool. Station Xeapel. Bd. V. 1884, p. 102.

Duges. Recherches sur 1'osteologie et la myologie des Batraciens a leurs differents ages. Paris 1834.

Dursy, E. Zur Entwicklungsgeschichte des Kopfes des Menschen und der hoheren Wirbelthiere. Tubingen 1869.

Ebner, von. Urwirbel und Neugliederung der Wirbelsaule. Sitzungsb. d. k. Akad. d. Wissensch. Wien. Math.-naturw. Cl. Bd. XCVII. Abth. 3. 1889, p. 194.

Fraser. On the Development of the Ossicula Auditus in the Higher Mammalia. Proceed. Roy. Soc. London. Vol. XXXIII. 1882, pp. 446-7.

Frenkel, F. Beitrag zur anatomischen Kenntuiss des Kreuzbeines der Saugethiere. Jena. Zeitschr. Bd. VIII. 1873.

Froriep, August. Zur Entwicklungsgeschichte der Wirbelsaule. insbesondere des Atlas und Epistropheus und der Occipitalregion.

I. Beobachtung an Hiihnerembryonen. Archiv f. Anat. u. Physiol.

Anat. Abth. 1883.

II. Beobachtung an Saugethierembryonen. Archiv f . Anat. u. Physiol. Anat. Abth. 1886.

Froriep, August. Ueber ein Ganglion des Hypoglossus und Wirbelanlagen in der Occipitalregion. Archiv f. Anat. u. Physiol. Anat. Abth. 1882.

Gadow. On the Modifications of the First and Second Visceral Arches, with especial Reference to the Hornologies of the Auditory Ossicles. Philos. Trans. Roy. Soc. London. 1888. Vol. CLXXIX. B. 1889, pp. 451-87.

Gegenbaur. Ueber die Entwicklung der Clavicula. Jena. Zeitschr. Bd. I. 1864, pp. 1-16.

Gegenbaur. Zur Morphologic der Gliedmaasson dcr Wirbelthierc. Morphol.

Jahrb. Bd. II. 1876. Gegenbaur.

(1) Ueber die Kopfncrven von Hexanchus und ihr Verhaltniss zur Wirbeltheorie des SchJidels. Jena. Zeitschr. Bd. VI. 1871, p. 497.

(2) Das Kopfskelet der Selachier, ein Beitrag zur Erkcnntniss der Genese des Kopfskelets der Wirbelthiere. Leipzig 1872.

(3) Ueber das Archipterygium. Jena. Zeitschr. Bd. VII. 1873, p. 131.

(4) Die Metamerie des Kopfes und die Wirbeltheorie des Kopfskelets. Morphol. Jahrb. Bd. XIII. 1887.

Gotte, A. Beitrage zur vergleichenden Morphologic des Skeletsystems der Wirbelthiere (Brustbein und Schultergurtel). Archiv f. mikr. Anat.

Bd. XIV. 1877. Gradenigo, G. Die embryonal e Anlage des Mittelohres: die morphologische Bedeutung der Gehb'rknochelchen. Mitth. a. d. embryol. Inst. d. Univ.

Wien. Heft 1887, p. 85. Hannover, A. Primordialbrusken og dens Forbening i det menneskelige Kranium for fodselen. Danske Videnskabernes Selskabs Skrif ter. K^joben harn. Ser. 5, Bd. XI. p. 349. 1880. Hannover, A. Primordialbrusken og dens Forbening i Truncus og Ex tremiteter hos Mennesket for Fodselen. (Table des matieres et Extrait en francais.) Kjobenliavn. Ser. 6, Bd. IV; p. 2G5. 1887. Hasse, C. Die Entwicklung des Atlas und Epistropheus des Menschen und der Saugethiere. Anatomische Studien. Bd. I. Leipzig 1872. Henke und Reyher. Studien iiber die Entwickelung der Extremitaten des Menschen, insbesondere der Gelenkflachen. Sitzungsb. d. k. Akad. d.

Wissensch. Wien. Bd. LXX. 1875. Hertwig, Oscar. Ueber das Zahnsystem der Amphibien und seine Bedeutung fiir die Genese des Skelets der Mundhohle. Eine vergleichend anato mische, entwicklungsgeschichtliche Untersuchung. Archiv f. mikr. Anat.

Bd. XI. Snpplementheft. 1874. Hoffmann, C. K. Beitrage zur vergleichenden Anatomic der Wirbelthiere.

Xieder. Archiv f. Zool. Bd. V. 1875).

Huxley. Lectures on the Elements of Comparative Anatomy. London 1864. Jacobson. Abstract by Hannover in Jahresbericht, p. 36, Archiv f. Anat.

u. Physiol. Jahrg. 1844. Julin, Charles. Kecherches sur 1'ossification du maxillaire inferieur chez le foetus de la balaenoptera. Archives de Biologic. T. I. 1880. Kann. Das vordere Chordaende. Inauguraldissert. Erlangen 1888. Keibel. Zur Entwicklungsgeschichte der Chorda bei Saugern. Archiv f.

Anat. u. Physiol. Anat. Abth. 18s<). K6 Hiker, A. Allgemeine Betrachtungen iiber die Entstehung des kncicher nen Schadels der Wirbelthiere. Berichte von der kb'nigl. zoot. Anstalt.

Wiirzburg. Leipzig 1849. Kolliker, Theodor. Ueber das Os intermaxillare des Menschen und die Anatomic der Hasenscharte und des Wolfsrachens. Nova acta Acad.

Leop.-Carol. Bd. XLIII. 1882. Leboucq., H. Kecherches sur le mode de disparition de la corcie dorsale chez les vertebres superieurs. Archives de Biologic. Vol. I. 1880. Magitot et Robin. Memoire sur un orgaue transitoire de la vie foetale

LITERATURE. 657 clesigne sous le noni de cartilage de Meckel. Ann. des. tSci. Nat. T. XVIII.

1862. Masquelin. Ilecherches sur le developpement du maxillaire infcrieur de 1'homme. Bull, de 1'Acad. roy. de Belgique. 2e serie. T. XLV. 1*7*. Oken. Ueber die Bedeutung der Schadelknochen. Jena 1807. Parker, W. K., and Bettany. The Morphology of the Skull. London 1877. German translation by Vetter. 1*79. Perenyi. Ent wicklung der Chorda dorsalisbei Torpedo niarrnorata. Math. u.

Naturw. Berichte aus Ungarn. Budapest. Bd. IV. p. 214. u. V. p. 218.

1886, 1887. Rabl, Carl. Ueber das Gebiet des Xervus facialis. Anat. Anzeiger. Jahrg.

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u. Physiol. 1837. Rosenberg, E. Untersuchungen iiber die Occipitalregion des Cranium und den proximalen Theil der Wirbelsiiule einiger Selachier. Dorpat ISsl. Rosenberg, E. Ueber die Entwicklung der Wirbelsaule und das Centrale carpi des Menschen. Morphol. Jahrb. Bd. I. 1875. Ruge. Untersuchungen iiber Entwicklungsvorgiinge am Brustbein und an der 8ternoclavicularverbindang des Menschen. Morphol. Jahrb. Bd. VI.

1880. Salensky, W. Beitrlige zur Entwicklungsgeschichte der knorpeligen Gehbr knb'chelchen bei Siiugethieren. Morphol. Jahrb. Bd. VI. 1880. Schwegel. Die Entwicklungsgescbichte der Knochen des Stanimes und der Extremitiiten mit Klicksicht auf Chirurgie, Geburtskunde und gerichtliche Medicin. 8itzungsb. d. k. Akad. d. Wissensch. Wien. Math.-naturw. Cl.

Bd. XXX. 1858, p. 337. Spondli, H. Ueber den rrimordialschadel der Saugethiere und des Menschen.

Inaugural-Dissertation. Zurich 1846.

Stohr. Zur Entwicklungsgeschichte des Kopfskelets der Teleostier. Festschrift d. medicin. Facultiit Wiirzburg. Leipzig 1882. Stohr. Zur Entwicklungsgeschichte des Urodelenschadels. Zeitschr. f. wiss.

Zoologle. Bd. XXXIII. 1879. Stohr. Zur Entwicklungsgeschichte des Anurenschadels. Zeitschr. f. wiss.

Zoologie. Bd. XXXVI. 1881. Stohr. Ueber Wirbeltheorie des Schadels. Sitzungsb. d. physik.-med.

Gesellsch. Wiirzburg. 1881. Wiedersheim. Ueber die Entwicklung des Schulter- und Beckengiirtels.

Anat. Anzeiger. Jahrg. IV. 1889 u. Jahrg. V. 1890.

658 EMBRYOLOGY.

Besides the writings treating of the development of the separate systems of organs, the following larger monographic works should be cited : Embryology of Man.

Coste. Histoire generate et particuliere du developpement des corps organises.

18471859.

Ecker. Icones physiologicae. Leipzig 1851 185$. Erdl. Die Entwicklung cles Menschen und HUhnchens im Eie. Leipzig 1845. His. Anatomie menschlicher Embryonen.

Heft I. Embryonen des ersten Monats. Leipzig 1880.

Heft II. Gestalt und Grossenentwicklung bis zum Schluss des zweiten Monats. Leipzig 1882. Heft III. Zur Geschichte der Organe. Leipzig 1885.

Embryology of Mammals.

Baer, C. E. von. Ueber Entwickhmgsgeschichte der Thiere. Beobacbtung und Reflexion. Konigsberg 1828 u. 1887.

Balfour. A Monograph on the Development of Elasmobranch Fishes. London 1878.

BischofF. Entwicklungsgescbicbte des Kaninchens. Braunschweig 1842.

Bischoff. Entwicklungsgeschicbte des Hundeeies. 1845.

BischofF. Entwicklungsgeschichte des Meerschweinchens. 1852.

Bischoif. Entwicklungsgeschichte des Eehes. 1 854.

Bonnet. Beitriige zur Embryologie der Wiederkauer, gewonnen am Schaf ei. Archiv f. Anat. u. Physiol. Anat. Abth. 1884 u. 1889.

Duval. Atlas d'embryologie. Paris 1889.

Gotte. Entwicklungsgeschichte der Unke. Leipzig 1875.

Hatschek, B. Studien liber Entwicklung des Amphioxus. Arbeiten a. d. zool. Inst. d. Universitiit Wien. 1882.

Hensen. Beobachtungen Uber die Befruchtung und Entwicklung des Kaninchens und Meerschweinchens. Zeitschr. f. Anat. u. Entwicklungsg. Bd. I. 1870.

His, W. Untersuclmngen iiber die erste Anlage des Wirbeltbierleibes. Die erste Entwicklung des Hiihnchens im Ei. Leipzig 1868.

Hubrecht. Studies in Mammalian Embryology. Placentation of Erinaceus, etc. Quart, Jour. Mic. Sci. Vol. XXX. 1890, p. 283.

Rathke. Entwicklungsgeschichte der Xatter. Konigsberg 1839.

Remak. Untersuphungen Uber die Entwicklung der Wirbelthierc. Berlin 1855.

Riickert. Ueber die; Entstehung der Excretionsorgane bei Selachiern. Archiv f. Anat. u. Physiol. Anat. Abth. 1888.

Schultze, M. Die Entwicklungsgeschicbte von Petromyzon Planeri. 185B.

Selenka. Studien Uber Entwicklungsgeschicbte der Thiere. Wiesbaden 1886, etc.