Book - The development of the chick (1919) 5

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Lillie FR. The development of the chick. (1919) Henry Holt And Company New York, New York.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Part I The Early Development to the End of the Third Day

Chapter V Head-Fold to Twelve Somites

(From about the twenty-first to the thirty-third hour of incubation)

I. Origin of the Head-fold

At the end of the period described in Chapter IV, the embryo is represented by a central differentiated area of the blastoderm, lying within the area pellucida, distinguished anteriorly by the medullary plate and head-process, and posteriorly by the primitive streak. The layers of the embryonic area are everywhere continuous with the corresponding layers of the extra-embryonic blastoderm, with no clear line of division between the two. In the course of the second and third days the embryo becomes clearly defined by its own growth, and by the formation of bounding folds.

The delimitation of the embryo from the blastoderm begins immediately after the formation of the head-process by the formation of a fold at the anterior end of medullary plate known as the head-fold (Fig. 42). Seen from the surface, this fold has a semicircular outline, the concavity of which is directed posteriorly (Fig. 44). It involves both the ectoderm and entoderm. A later stage is shown in sagittal section in Figs. 46 and 47: the ectoderm and entoderm immediately in front of the medullary plate make a sharp bend downwards and backwards, and then turn forward again. The head-fold thus produces an internal bay in the entoderm, the beginning of the fore-gut. There is similarly an external bay, the posterior angle of which is the head-fold proper, lying beneath the projecting head. These bays are of course turned in opposite directions, the internal one opening into the subgerminal cavity posteriorly, and the external one opening anteriorly on the surface of the blastoderm.

The transition from the ectoderm of the medullary plate into that of the under surface of the head and the proamnion is a gradual one. The difference is, however, very strongly marked (Fig. 47). The formation of the head-fold is due to the more rapid growth of the medullary plate, which causes the latter to extend forward above the thinner and more pliable membrane in front. The entoderm is attached to the inner surface of the anterior end of the medullary plate (Fig. 47), and is apparently carried forw^ard with the latter to form the anterior portion of the fore-gut. The actual form of the fold depends upon the mechanical properties of the membranes concerned, especially the unequal thickness of their parts produced by unequal growth.

Although the head-fold thus appears to be a single fold involving the two primary layers, it is convenient, for purposes of description, to consider it as two separate folds, ectodermal and entodermal. The deepening of these folds takes place at the same rate up to the time when four somites are formed (Fig. 49). At about this time the paired primordia of the parietal cavity (amnio-cardiac vesicles), which appear in the mesoblast in the lateral extensions of the head-fold (Fig. 50), push in towards the middle line so as to separate the ectodermal and entodermal limbs (Figs. 52 and 58). When six somites are formed, these cavities fuse in the middle line, thus effecting a complete separation of the two limbs. The further progression of the head-fold, after this union, takes place separately in the two limbs.

II. Formation of the Fore-gut

The extension of the amnio-cardiac vesicles between the ectodermal and entodermal layers of the head-fold introduces a section of the body-cavity (pericardium) between these layers and at the same time converts the ectodermal liml) into a portion of the somatopleure, and the entodermal limb into a portion of the splanchnopleure. (See p. 115.) The splanchnopleuric head-fold extends posteriorly very rapidly after the invasion of the body-cavity, while the somatopleuric fold apparently remains fixed for some time, though the head-fold appears to become deeper, owing to the forward extension of the head above the blastoderm. The posterior extension of the splanchnopleuric head-fold lengthens the floor of the fore-gut; it is caused by the median growth and concrescence of folds of the splanchnopleure (Fig. 53). Along with this process is involved the development of the heart described farther on. The growth in length of the fore-gut may be realized by a comparison of Figs. 50, 52, 62, etc.

Fig. 47. — Head-fold region of Fig. 46 highly magnified. For abbreviations see Fig. 46.

Thus by the 12 s stage a considerable section of the fore-gut is already established (Fig. 63); this is the pharyngeal division; from the first it is extremely broad, and lunate in cross-section (Fig. 54), the floor being composed of columnar cells, and the roof of very flat cells. The lateral extensions may be regarded as diverticula; subsequently these grow more rapidly at four places along their length, and come in contact with the ectoderm. Thus four pouches are established on each side as described in detail

Fig. 48. — Stage of first intersomitic groove drawn from an entire mount in balsam by transmitted light, a. c. v., Amnio-cardiac vesicle, a. o., Inner margin of Area opaca. Ect., Ectoderm. Ent., Entoderm H. F., Head -fold. i. s.f.l., First intersomitic furrow, med. pi., Medullary plate. Mes., Mesoderm, n. gr., Neural groove, pr. gr., Primitive groove. Pr'a, proamnion.

in the next chapter. At the 12 s stage one such place of contact is already formed, lying a short distance in front of the thickened ectoderm destined to form the auditory pit.

Another place of fusion between the fore-gut and the ectoderm is the so-called oral plate (pharyngeal membrane), which occupies a mid-ventral position at the extreme anterior end. The parietal cavities meet posterior to the oral plate (Figs. 67 and 75). Transverse sections show the oral plate to be depressed beneath the level of the ventral surface of the head at the stage of 10 somites (Fig. 55), a condition that increases, as development

Fig. 49. — Median sagittal section of the head at the stage of 4 s.

a. i. p., Anterior intestinal portal. F. G., Fore-gut. Ect., Ectoderm. Ent., Entoderm. H. F., head-fold. Mes., Mesoblast. n. F., Neural fold, or. pi., Oral plate.

proceeds, by the formation of the cranial fiexture, and by the upgrowth of the tissues behind and at its sides; thus will be established a deep depression lined by ectoderm, the floor of which is formed by the oral plate, and which is destined to form a large part of the mouth. The depression is known as the stomodseum.

III. Origin of the Neural Tube The Medullary Plate

The medullary plate is the primordium of the central nervous system. At the time of formation of the head-fold it is broad in front and narrower posteriorly, ending opposite the posterior end of the primitive streak. Its central portion is not a separate plate of cells in the region of the primitive streak, but this part becomes distinct as the i^rimitive streak splits into its derivatives. It is therefore only when the latter is entirely used up that the entire length of the medullary plate is established. However, long before this time the greater portion has become converted by folding into the neural tube, a process that proceeds in general from in front backwards. Thus successive stages may be studied in serial sections of the same embryo; an anterior section, for instance, showing the completed tube, one farther back, the folded medullary plate, and yet more posteriorly the central part of the medullary plate disappears in the undifferentiated mass of the primitive streak. These conditions must be born in mind in the following description.

Fig. 50. — Embryo of 3 s from above, drawn in balsam with transmitted light.

a. c. v., Amnio-cardiac vesicle, a. o., inner margin of Area opaca. F. G., Fore-gut. N'ch., Notochord. n. F., Neural fold. pr. gr., Primitive groove, s. l,s. 2, s. 3, First, second and third somites.

The Neural Groove and Folds

Shortly after the formation of the head-fold the center of the medullary plate becomes sunk in the form of a deep groove beginning a short distance behind the anterior end of the plate (Fig. 48) (the neural groove) ; the margins of the anterior portion of the medullary plate then become elevated somewhat above the surrounding blastoderm, forming the neural folds (Figs. 51 and 56). The latter rise very rapidly,, thus deepening the neural groove, and bend in towards the middle line (Figs. 53, 54, etc.,) meeting, by the time four or five somites are formed, a short distance back of the anterior end of the medullary plate (Figs. 50 and 51). The posterior ends of the neural folds do not, at this time, reach the region of the first somite. The region where the neural folds first come in contact corresponds approximately with the region of the future mid-brain, or anterior part of the hind-brain.

Fig. 5L — Embryo of 4 s from above, drawn in alcohol by reflected light.

a. c. v., Amnio-cardiac vesicle, a. p., Area pellucida. a. v. i., Internal vitelline area. med. pi., Medullary plate, n. F., Neural fold. Pr'a., Proamnion, pr. str., Primitive streak, s. 1, s. 3, First and third somites.

Fig. 52. — The same embryo from beneath, a. c. v., Amnio-cardiac vesicle, a. i. p., Anterior intestinal portal. H. F., Head-fold. Pr'a., Proamnion.

Fig. 52 A. — Median longitudinal section of the head, stage of 4 s. The section passes through the length of one of the neural folds just behind the anterior end. (Cf. Fig. 5L) a. i. p., Anterior intestinal portal. Ect., Ectoderm. Ent., Entoderm. F. G., Fore-gut. H. F., Head-fold. Mes., Mesoderm. Mes. H. C, Mesoblastic head cavity, n. F., Neural fold. or. pi., Oral plate.

The process of closure itself is essentially the same in all regions of the neural tube. Each neural fold has two limbs: an inner thick limb, belonging to the medullar}' plate, and an outer, thin limb, continuous with the general ectoderm (cf. Fig. 68 B). When the folds of opposite sides come in contact, the inner limbs of the two sides become continuous with one another, and also the outer limbs, the ectoderm then passing continuously over a closed neural tube.

Certain cells in the suture and in the walls of the tube next to the ectoderm are destined to form the neural crest, a structure of great significance, inasmuch as the series of cranial and spinal ganglia is derived from it. (See following chapter.)

Fig. 54. — Transverse section through the same embryo a short distance in front of the anterior intestinal portal. For explanation of letters see preceding figure; in addition: Ph., Pharynx. Som'pl., Somatopleure. Spl'pl., Splanchnopleure. v. M., Ventral Mesentery.

Fig. 54 a. — Transverse section through the head of a 10 s embryo. The region of the section is near the center of the hind brain.

Ao., Aorta. End'c, Endocardium. End'c. S., Endocardial septum. H. B., Hind brain. My'c, Myocardium, p. C, Parietal cavity. Ph., pharynx. So'pl., Somatopleure. Spl'pl., Splanchnopleure. v. M., Ventral mesentery.

The Neuropore

From the place where the neural folds first meet, the elevation and fusion proceed both forwards and backwards in a continuous fashion (cf. Figs. 59, 61, 65, etc.). Although the open anterior stretch of the neural tube is very short in comparison to the posterior open part, it is not until about the 12 s stage that the former closes completely (cf. Fig. 64). The final point of closure at the anterior end, known as the neuropore, is supposed by some to be a point of great morphological significance, and to mark the extreme anterior end of the original neural axis. It is identified by these writers with the permanent neuropore of Amphioxus. However, this is open to question. Posteriorly the closure of the neural tube proceeds much more rapidly, though, of course, it is not fully completed untd after the disappearance of the primitive streak.

Fig. 55. — Transverse section through the head immediately behind the optic vesicles; stage, 10 s.

Ao., Aorta, ax. Mes., Axial mesoblast. Ect., Ectoderm. Ent., Entoderm. ]\I. A., Mandibular arch. M. B., Mid-biain. ]Mes., Mesoderm, or. pi., Oral plate, p'a. c, Periaxial cord. p. C, Parietal cavity. Pr'a., Proamnion. Ph., Pharynx, v. Ao., Ventral aorta.

Fig. 56. — Early stage of the neural folds. Transverse section through a 4-5 s embryo between the last somite and the anterior end of the primitive streak.

Ect., Ectoderm. Ent., Entoderm, n. F., Neural fold. N'ch., Notochord. med. pi., Medullary plate. Mes., Mesoderm.

The question as to the position of the anterior end of the original neural axis is one of great morphological significance. Accompanying the closure of the neural tube in this region the anterior end rapidly grows forward beyond the anterior end of the fore-gut. The floor of the neural tube does not, however, take part in this extension, the consequence being that the summits of the neural folds form arching knees extending in front of the original anterior end of the medullary plate (Figs. 51 and 52). The extreme anterior end of the neural tube formed in this way has a ventral as well as a dorsal defect, and when it closes there is a ventral as well as a dorsal suture. The end of the ventral suture marks the original anterior end of the medullary plate, and this lies at the stage of 10 somites a short distance in front of the anterior end of the oral plate in the region of the future recessus opticus (Fig. 62). (Goronowitsch calls the anterior fissure, sutura cerehralis anterior; His divided it into two parts, sutura neurochordalis sen ventralis and sutura termi7ialis anterior.)

Fig. 57. — Later stage of the neural folds. Section through the head of an embryo of 2-3 s; corresponding to about the future mid-brain region.

Coel., Coelome. g. C, Germinal cells, med. pi., Medullary plate. Mes., Mesoblast. n. F., Neural fold. n. Cr., Neural crest. N'ch., Notochord. som. Mes., Somatic layer of mesoblast. spl. Mes., Splanchnic layer of mesoblast.

Fig. 58. — Ventral view of the head region of an embryo of 5 somites, drawn in balsam with transmitted light. X 30.

a. c. v., Amnio-cardiac vesicle, a. i. p.. Anterior intestinal portal. F. G., Fore-gut. My'c, Myocardium. N'ch., Notochord. n. F., Neural fold. s 2, s 4, Second and fourth somites^

Fig. 59. — Embryo of 7 s from above drawn in balsam with transmitted light, x 30.

a. c. s., Anterior cerebral suture, ceph. Mes., Cephalic Mesoblast. F. G., Fore-gut. N'ch., Notochord. n. T., Neural tube. op. Ves., Optic vesicle. Pr'a., Proamnion. i)r. str., Primitive streak, s 2, s 7, Second and seventh somites. V. o. m., Omphalo-mesenteric vein.

The neuropore question resolves itself into this: What part of the sutura cerebralis anterior is to be called neuropore? As the suture extends from near the infundibulum to the pineal region at least, there is a wide range of choice. However, there is a point in the suture near its dorsal end where the separation of the ectoderm from the neural tube takes place later than elsewhere. This may be regarded as the equivalent of the neuropore. The suture is the site of formation of the lamina terminalis (Chap. VIII).

Fig. 60. — The head of the same embryo from below X 30.

a. i. p., Anterior intestinal portal. End'c. s., Endocardial septum. F. G., Fore-f:;ut. Ht., Heart. N'ch. T., Termination of Notochord. op. Ves., Optic vesicle, p. C, Parietal cavity. Pr'a., Proamnion. V. o. m., Omphalo-mesenteric vein.

It will l^e seen that according to this account most of the primary fore-brain includes no part of the original floor of the neural tube.

Primary Divisions of the Neural Tube. The neural tube is the primordium of the brain and spinal cord. Its cavity becomes the ventricles of the brain and the central canal of the cord. There is no clear distinction between l)rain and cord at first, the one passing without any anatomical landmark into the other. Now the brain is the central nervous system of the head, so it is not until one can determine the posterior boundary of the embryonic head that it becomes possible to determine the hind end of the brain. The first clear landmark is given by the mesoblastic somites, because it is known that the four anterior somites are cephalic. All of the neural tube in front of the fifth somite is therefore cranial. What a large proportion of the neural tube this is in early stages may be seen by comparison of figures of embryos in the period covered by the chapter (cf. Fig. 61). Before the appearance of the first somite the entire medullary plate in front of the primitive streak is in fact cranial.

Fig. 61. — Embryo of 9 s from above drawn as a transparent object with transmitted light. X 30.

Abbreviations same as before; in addition: H. B., Hind brain. M. B., Mid brain. n. S.j Neural suture.

Fig. 62. — The head of the same embryo from beneath more highly magnified. In this drawing an attempt is made to show different levels of the embryo superposed: thus the heart is uppermost in the figure, beneath this the fore-gut (F. G.), beneath this the notochord, and at the lowest level, the neural tube, a. c. s., Anterior cerebral suture. Inf., Infundibulum.

M. A., Mandibular arch. p. C, represents the anterior boundary of the parietal cavity, or. pi., Oral plate. Other abbreviations as before.

Origin of the Primary Divisions of the Embryonic Brain

The embryonic brain is divided into three divisions of unequal length, viz., the fore-brain (prosencephalon), mid-brain {mesencephalon), and hind-brain (rhombencephalon). The first division is characterized in the period we are considering by its very considerable lateral expansions, the rudiments of the optic vesicles (Figs. 59, 61, 63, etc.), and also by the fact that there is a suture in the anterior portion of its floor owing to the mode of its origin (Fig. 62). A definite constriction between it and the following division first appears in embryos Avith six or seven somites (Fig. 59). At the stage of 9-10 somites the next division (mid-brain) becomes clearly marked off by a constriction from the hind-brain (Fig. 61). The latter is relatively very long, and its anterior half is characterized in the 12-somite stage by the existence of five divisions (neuromeres) separated by constrictions (Fig. 63).

It will be noted that the first neuromere of the hind-brain appears about twice as large as the succeeding ones ; it really includes two neuromeres according to some authors. Similarly, it is maintained that the mid -brain includes two neuromeres and the fore-brain three.

According to Hill's account the entire brain of the embryo chick is composed of eleven neuromeres or neural segments, which are formed even in the 1 s stage. The first three enter into the composition of the fore-brain; the next two, viz., 4 and 5, form the mid-brain, and the last six the hind-brain.

The three that enter into the composition of the primary fore-brain have the following fate according to Hill: the first forms the telencephalon, the second the anterior division (parencephalon) and the third the posterior division (synencephalon) of the diencephalon. The cerebellum arises from the first neuromere of the hind-brain, sixth of the series. This question is more fully discussed in Chapter VI. (See Fig. 83.)

Fig. 63. — Embryo of 12 s, from above, drawn as a transparent object with transmitted light. X 30. Abbreviations as before.

IV. The Mesoblast

The changes in the mesoblast during this period are of great importance. At the time of appearance of the liead-fold it consists of two great sheets of cells between ectoderm and entoderm beginning on each side of the head-process and primitive streak, and extending laterally and posteriorly to the margin of the vascular area. The lateral margins at this time extend anterior to the embryonic axis, so that the anterior margin of the mesoblast forms a curve with the concavity directed forward.

Fig. 64. — Head of the same embryo from below. X 30. Abbreviations as before.

The mesoblast in the region in front of the primitive streak is known as gastral mesoblast, and in the region of the primitive streak as prostomial mesoblast; the latter is fused with the primitive streak. However, the distinction between the gastral and prostomial mesoblast is not of permanent significance, because the latter is being continually converted into the former as the primitive streak undergoes separation into ectoderm, notochord, and mesoderm.

Confining our account now to the gastral mesoblast: a transverse section across an embryo in which the head-fold is forming shows a sheet of cells lying on each side of the notochord between the ectoderm and entoderm. It is several cells deep near the notochord, and thins gradually peripheralh' (cf. Fig. 56). The thicker portion next the notochord is distinguished as the paraxial mesoblast (vertebral plate) from the more peripheral portion or lateral plate. The mesoblast is sparser, the cells more scattered, and the whole tissue of much looser texture in the more anterior portions of the embryo.

The paraxial mesoblast increases rapidly in thickness and thus becomes clearly distinguishable from the lateral plate. Shortly after the formation of the head-fold a transverse cleft appears in the paraxial mesoblast a short distance in front of the anterior end of the primitive streak (Fig. 48). This is soon followed by a second cleft, a very short distance behind the first, and thus a complete mesohlastic somite is established. The division is accomplished rather by segregation of the cells than by an actual folding. The mesoblast cells immediately in front of the first cleft aggregate so as to form a somite continuous anteriorly with the mesoblast of the head, thus lacking an anterior boundary; this is the first somite, and the one formed between the first two clefts in the mesoblast is the second.

The first somite established is first, not only in point of time, but also in position, all the remainder forming in succession behind this (cf. Figs. 48, 50, 51, 59, 61, etc.). As this is a point of considerable importance for understanding the topography of the embryo, and as previous text-books have a different account of it, it is worth while to give the evidence for this position in some detail. It has been believed up to a very recent time that from two to four somites were formed in front of the first one. This belief was due very largely to a misconception of the nature of the primitive streak, which was believed by some to be extraeml^ryonic, that is to lie behind the embryo and not to be a part of the embryo itself. The first somite lies so near to the anterior end of the primitive streak that it was difficult to believe that room could be made by growth between it and the primitive streak with sufficient rapidity to accommodate the rapidly forming somites. In the entire absence of differentiated organs it was impossible to find landmarks by which to distinguish the first somite among the first five or six; hence it was natural to suppose that a certain number of somites arose in front of the first, especially as it Avas not known how much of the anterior portion of the embryonic axis represented the head. However, in the absence of natural landmarks identifying the first somite formed, it is quite possible to create artificial ones, and in this way to identify it in later stages. This has been done by Miss Marion Hubbard and by Patterson in the following manner: The position of the first somite was marked immediately after the appearance of the first cleft with a delicate electrolytic needle which left a permanent scar. The eggs thus operated on were closed up and permitted to develop to a stage of from 10 to 25 somites; and then the mark was found to coincide with the first somite of the series. In the next place it was possible by similar means to mark out the topography of the embryonic head in the stage of one or two somites. Thus it was determined that a mark made immediately in front of the first somite formed appeared later in the region of the otocyst; but this arises normally at the stage of 12-14 somites, a very short distance in front of the first somite of the series, which is thus shown to have the same position as the first somite formed. On the other hand, if one assumed that the first somite formed became the third or fourth of the series, it is clear that one would have to make a mark some distance in front of the first somite formed, to strike the place of origin of the otocyst. Marks made on this theory were always found a considerable distance in front of the otocyst. Altogether a large number of experiments was made, the concurrent testimony of which was perfectly conclusive.

Fig. 65. — Embryo of 12 s, from above, drawn in alcohol with reflected Hght. au. ep.. Auditory epitheUum. Other abbreviations as before.

Fig. 66. — The same embryo from beneath, drawn in alcohol with reflected light. Abbreviations as before.

The somite formed in front of the first cleft is thus the first in position of the definitive series and the remainder arise in succession behind it. The formation of the somites therefore follows the usual law of antero-posterior differentiation.

There is always a stretch of unsegmented paraxial mesoblast between the last somite and the anterior end of the primitive streak.

The first four somites belong to the head, and enter into the composition of the occipital region. The more anterior part of the mesoblast of the head never becomes segmented in the chick. In the anamniote vertebrates, segmentation of the mesoblast extends farther forward, and there is a greater number of cephalic somites. This may be taken as evidence that a large part, at least, of the head was primitively segmented like the trunk. As we shall see later, the primitive metamerism of the head is also expressed in other ways: neuromeres, branchiomeres, etc.

The segmentation of the mesoblast finally extends to the hind end of the tail, new segments being continually cut off from the anterior end of the paraxial mesoblast until it is all used up. This is not complete until the fifth day. The number of somites thus formed is perfectly constant, as is also the fate of the individual somites.

Primary Structure of the Somites

Each somite is primarily a block of cells arranged in the form of an epithelium around a small central lumen, towards which the inner ends of all the cells converge (Fig. 68 B). The central cavity (myocoele) is, however, filled with an irregularly arranged group of cells, and, though the cavity must be regarded as part of the primitive body-cavity, or ccelome, it has no open communication with it. After the somites are formed they rapidly become thicker so that their lateral boundary becomes very sharply marked; this is not due to a longitudinal constriction external to the paraxial mesoblast, as usually stated. Each somite has six sides, of which five are free, viz., dorsal, ventral, anterior, posterior, and median. The sixth or lateral side is continuous with the nephrotome.

The Nephrotome, or Intermediate Cell-mass (Middle Plate)

The somites and the lateral plate are not in immediate contact but are separated by a short stretch of cells continuous with both, known as the nephrotome or intermediate cell-mass or middle plate. The intersegmental furrows do not extend into the intermediate cell-mass, and the latter therefore remains unsegmented like the lateral plate. It consists fundamentally of two layers of cells, dorsal and ventral, of which the former is continuous with the dorsal wall of the somite and the somatic layer of the lateral plate, and the latter with the ventral wall of the somite and the splanchnic layer of the lateral plate (Fig. 68 B). Thus if the two layers of the intermediate cell-mass were separated the space between them would be continuous with the coelome that arises secondarily in the lateral j^late. This condition actually exists in some of the Anamnia (Selachii, for instance) in which the intermediate cell-mass is also segmented.

The Lateral Plate

This name is given to the lateral mesoblast within which the body-cavity arises. It is separated from the somite by the nephrotome and its lateral extension coincides with the margin of the vascular area.

Development of the Body-cavity or Coelome

The coelome or body-cavity arises within the lateral plate as a series of separated small cavities, distributed throughout its whole extent, which appear first in the anterior portion (1-3 s stage). By successive fusion of these cavities and their extension centrally and laterally, there arises a continuous cavity, the coelome, which extends from the nephrotome to the margin of the vascular area (Fig. 68), and which becomes the pleuro peritoneal and pericardial cavities in the embryo, and the extra-embryonic bodycavity beyond the boundaries of the embryo.

Of the two lavers of the lateral mesoblast thus established, the external is known as the somatic and the internal as the spla7icknic layer. In the course of development the somatic laver becomes closelv bound to the ectoderm, thus constituting the somatopleure, and the splanchnic layer becomes similarly united to the entoderm, thus establishing the splanchnopleure. The somatopleure is destined to form the body-wall and the extra-embryonic membranes known as the amnion and chorion; from the splanchnopleure is derived the alimentary canal with all its appendages, and the yolk-sac. As described in detail in the next chapter, this splitting of the mesoblast progresses with the overgrowth of the yolk until it extends completely around the latter

Returning now to the first stages in the formation of the coelome. In the 3 s stage it undergoes a precocious expansion in the region lateral to the head of the embryo (Figs. 51, 52, etc.), forming a pair of large cavities known as the amnio-cardiac vesicles, because they participate in the formation of the amnion and pericardium. These cavities extend in rapidly towards the middle line, and enter the head-fold in the 4-5 s stage (Figs. 52, 58). At the stage of 6-7 s they meet in the floor of the fore-gut immediately behind the oral plate and fuse together, thus dividing the head-fold into somatic and splanchnic limbs, as previously described. A median undivided portion of the body-cavity known as the parietal cavity (forerunner of the pericardium) is thus established beneath the fore-gut; and it extends backward with the elongation of the fore-gut in the manner already described. A pair of blind prolongations of this cavity extends a short distance forward at the sides of the oral plate at the 10-12 s stage (cf. Fig. 62), lying lateral and ventral to the ventral aortse.

The median angle of the body-cavity, where the somatic and splanchnic layers meet, is a point of fundamental morphological importance. In the region of the somites the nephrotome is attached here, and in the head the wdng of cells leading to the axial mesoblast (cf. Figs. 68 B, 53, and 54). In an embryo with ten somites this angle may be traced forward to near the hinder end of the oral plate, lying beneath the lateral angles of the pharynx.

Mesoblast of the Head

Mesoblast exists in two forms in the embryo: (1) in the form of epithelial layers or membranes (mesothelium), and (2) in the form of migrating cells which usually unite secondarily to form a syncytium in the form of a network, the meshes of which are filled with fluid; the nuclei lie in the thickened nodes. This form of the mesoblast is known as mesench3'me. It is always derived from a pre-existing epithelial layer, usually, but not necessarily, mesothelium, for, as we shall see, parts of it are derived from ectoderm and entoderm; on the other hand, mesenchyme may secondarily take on an epithelial arrangement (endothelium). The terms mesothelium and mesenchyme have therefore merely descriptive significance in the early embryonic stages. The mesenchyme has no single embryonic significance either as to origin or fate, but is to be regarded as a mixed tissue.

The mesoblast of the head is derived from several sources: (1) from a continuation forward of the paraxial mesoblast; (2) by proliferation from the fore-gut; and (3) from proliferations of ectoderm.

  1. The axial mesoblast of the head is an anterior continuation of that of the trunk; it terminates at the anterior end of the fore-gut with which it is continuous from the stage of the headprocess up to about the 6 s stage (Figs. 43 and 49). In the anterior part of the head it is mesenchymal in its general structure, grading posteriorly into the mesothelial paraxial mesoblast of the hinder part of the head and trunk. It is continuous at first with the lateral mesoblast in which the amnio-cardiac vesicles are forming; but this connection is lost in the anterior part of the head that projects forward above the blastoderm; that is, in front of the head-fold.
  2. The anterior end of the fore-gut proliferates mesenchyme from the time of its first formation to about the 6 s stage (Fig. 49). The proliferation is so rapid that it may give rise to the appearance of diverticula. The extreme anterior end of the floor forms a sac which lies just in front of the oral plate at the 4 s stage (Fig. 52 A), but soon after breaks up into mesenchyme. There is a considerable mass of mesenchyme formed from this source in the space bounded by the anterior end of the fore-gut, the neural tube and the ectoderm ; at the 4 s stage this appears fused with the floor of the neural tube and the surface ectoderm, and probably receives cells from both; the anterior end of the notochord also disappears in this mass (cf. Fig. 67).
  3. Ectodermal proliferations forming mesenchyme in the head. (This subject is discussed in the next chapter.)

Vascular System

The origin of the blood-islands in the opaque area was described in the preceding chapter. They lie between the coelomic mesoblast and the yolk-sac entoderm derived from the germ-wall. When the somatopleure and splanchnopleure are formed the blood-islands lie between the two layers of the latter, and the somatopleure is entirely bloodless. About the stage of 1 somite a vascular network continuous with the original network of the opaque area begins to appear in the pellucid area, at first at the margin of the opaque area, but by degrees nearer and nearer to the embryo, until, by the 7 or 8 s stage, blood-vessels begin to appear in the embryo itself. It is important to note that the order of appearance of the vascular primordia is first in the area opaca in the order previously described, then in the pellucid area and finally in the embryo itself. Moreover, the parts appearing later are, usually at least, in continuity with those first formed.

Before discussing the way in which the blood-vessels arise in the pellucid area and in the embryo, we should consider the first differentiation within the original, or peripheral, bloodislands. Between the 3 and 5 s stage it may be noticed in sections that vacuoles are forming within the peripheral bloodislands near the entodermal surface. The expansion of these vacuoles carries the peripheral layer of cells away from the main mass of cells composing the blood-islands, and by degrees the process is carried completely around the blood-island, so that the peripheral layer becomes entirely separated from the central mass and encloses it (See Fig, 68 C). The enclosing cells become flattened during this process to form an endothelium; inasmuch as the blood-islands are not separate, but anastomose to form a network, the process results in the formation of a network of endothelial tubes enclosing cell-masses. Thus arise the first blood-vessels. The enclosed masses of cells rapidly acquire hsemoglobin, become separated from one another, and form blood-cells.

There is a great difference in the relative amounts of bloodcells formed in different regions. Thus in the anterior part of the opaque area and in the pellucid area the original bloodislands are relatively small (Figs. 44 and 45), and furnish material sufficient only for the formation of the blood-vessels. On the other hand, in the peripheral part of the vascular area, especially towards its posterior end, the largest masses of blood-cells are found; and these conditions grade into one another. In other words, the formation of blood-cells is restricted at this time to the opaque area, and is most abundant posteriorly. In the pellucid area only empty blood-vessels are formed. Similarly the blood-vessels of the embryo itself are at first empty; they become filled secondarily from the opaque area when circulation begins.

The appearance of blood-vessels within the pellucid area and the embryo has been interpreted in two principal ways: (1) that they are an ingrowth from the original vascular primordium of the opaque area; and (2) that they arise by differentiation in situ. The first view was originally stated by His, and has been supported by Eolliker and others. The second is supported by Riickert, P. Mayer and others. The observations, on which the ingrowth theory of His were based, were made originally on whole blastoderms of the chick, and concerned primarily the order of origin of the blood-vessels, which is centripetal and continuous. But it is obvious that such observations do not in themselves demonstrate the existence of an independent ingrowing primordium; they are not altogether inconsistent with the view that the blood-vessels differentiate from cells in situ. Within the embryo itself parts of certain vessels appear in sections to arise separately, and form secondary connections with the vessels formed at an earlier time; this is the case for instance with the dorsal aorta in the region of the head. But such appearances seen in sections may be deceptive, as Evans has shown by injections of the ingrowing vascular system of early chick embryos. The entire system appears in such injections to be continuous from the first and there was found no evidence of independently formed parts.

Origin of the Heart

The embryonic heart possesses two layers: an internal delicate endothelium, the endocardium, and an external strong muscular layer, the myocardium. The endocardium arises in continuity with the blood-vessels of the pellucid area, and is in no wise different from them; the myocardium, on the other hand, arises from the splanchnic mesoblast. The heart is thus to be regarded as a portion of the embryonic vascular system, specially provided with a muscular wall for the propulsion of the blood. The first incUcation of the heart is a thickening of the splanchnopleure of the amniocardiac vesicles, which forms the primordium of the myocardium. This is situated a short distance lateral to the hind-brain region of the embryo, and makes its appearance between the stage of 3 and 5 somites.

The endocardium soon appears between the thickened entoderm and the myocardium, in the form of a delicate endothelial vessel on each side, continuous with the extra-embryonic bloodvessels. This is, indeed, the place where the blood-vessels first reach the embryo. The myocardium then becomes arched towards the body-cavity and includes the endocardium in its concavity (Fig. 53). The heart thus comes to consist of two parts on each side: a myocardial gutter semicircular in cross section, open towards the entoderm, and an endothelial tube lying in the gutter, and in contact with the entoderm. At this time the lateral limiting sulci appear in the splanchnopleure just central to the endocardium on each side, and, as the foregut closes from in front backwards, the following changes take place (Figs. 54 and 54 A): (1) the entoderm withdraws completely from the fused apices of the lateral folds in the splanchnopleure, and thus a wide separation is made between the floor of the pharynx and the splanchnopleure below; (2) the right and left endocardial tubes come into immediate contact in the floor of the pharynx; (3) the two myocardial gutters coming together form a single tube around the endocardium, suspended by a double mesodermal membrane {mesocafdium or dorsal mesentery of the heart) to the floor of the pharynx, and attached by a similar mesentery {ventral mesentery of the heart) to the splanchnopleure beneath (Fig. 54). The latter connection is ruptured almost as soon as formed, so that the floor of the myocardium becomes complete (Fig. 54 A). Soon after the completion of the floor of the pharynx the two endocardial tubes press together until the common wall becomes reduced to a vertical partition, which then ruptures; and finally (10-12 s) all traces of the original duplicity of the heart disappear (Figs. 60, 62, 64).

The heart thus arises from two lateral halves which fuse secondarily to form a single tube. This fusion takes place from in front backwards, hence the anterior end of the heart is formed first. Indeed, the full length of the cardiac tube is not formed in the period covered by this chapter; the definitive hindermost division is established by concrescence after the 12 s stage. But the actual hind end is always continuous with the extra-embryonic network of blood-vessels and this connection develops into the main splanchnic veins.

As a rare abnormality the lateral primordia of the heart may meet and fuse dorsal to the embryo, instead of in the floor of the pharynx. This condition is known as omphalocephaly; in other rare cases the lateral halves may fail to unite, and two hearts may be formed.

There are three views concerning the origin of the endocardium:

(1) that it is an ingrowth of the extra-embryonic vessels, (2) that it arises from the mesoblast in situ, (3) that it arises from the entoderm in situ. Appearances such as that shown in Fig. 53 favor the last view.

The heart is then a double-wallecl tube attached to the floor of the pharynx. The posterior end rests squarely against the anterior intestinal portal and is continuous with the rudiments of the splanchnic veins running in the diverging folds of the portal; the anterior end of the heart is continued as a simple endothelial tube (ventral aorta) as far forward as the oral plate, where it is divided in two (Figs. 62, 64, etc.).

This primitive simplicity of the cardiac tube continues throughout the period considered in this chapter without substantial alteration. The heart increases in length wdth considerable rapidity, but being attached at its anterior and posterior ends by the aortic and venous roots respectively, it is forced to bend, nearly always to the right, so that a convexity of the heart appears to the right of the embryonic head, at about the 11-12 s stage (Figs. 63, 64). About this time the mesocardium (dorsal mesentery of the heart) disappears except at the posterior end, and the cardiac tube thus becomes free except at its two ends.

The Embryonic Blood-vessels

The dorsal aorta arises from the median edge of the vascular network, which extends across the pellucid area in the splanchnopleure. At the stage of 7-9 somites, it has reached the nephrotomic level. The marginal meshes gradually straighten themselves out into a longitudinal vessel, continuous with the net-work at the sides and behind. Onh^ the trunk part has been shown to arise in this manner. The cephalic part may arise by a forward growth of the trunk part or from mesenchyme in situ. A connection is formed around the anterior end of the fore-gut with the ventral aortse (Fig. 55), and an arterial pathway is thus established from the heart by way of the ventral and dorsal aortae to the vascular network of the splanchnopleure.

The arterial system consists at thirty-three hours (12 s stage) of the following parts: (1) ventral aorta; (2) first visceral or mandibular arteries connecting 1 and 3 ; (3) dorsal aortae ; (4) segmental branches of the dorsal aortae. The ventral aorta is, as we have seen, the anterior prolongation of the endocardium extending between the extreme anterior end of the heart proper and the oral plate. At the oral plate it divides into two branches, right and left mandibular arteries or arches, that surround the anterior end of the fore-gut, and arch over to be continued into the two dorsal aortse. The tissue in which these arches run is destined to form the mandibular arch or lower jaw. The two dorsal aortse are very large vessels running above the roof of the pharynx near its lateral angles. They give off no branches in the head. In the trunk they pass backwards in the splanchnopleure beneath the somites (Fig. 68 B), and are connected at intervals with the extra-embryonic blood-vessels. These connections are more important in the region of the primitive streak (Fig. 63) where the dorsal aortse disappear in the general extraembr3'onic network. Slight diverticula of the dorsal aortse ascend in the interspaces between successive somites (segmental arteries).

Concerning the veins in the period under consideration there is nothing additional to be said.

V. Description of an Embryo with 10 Somites

It will now be in place to describe rather fully the anatomy of the stage at which we have arrived; this will serve as a point of departure for the next chapter.

The blastoderm is a circular membrane covering a considerable portion of the yolk (cf. Fig. 32 A). The embryo appears to the naked eye as a whitish streak in the central pear-shaped pellucid area. The surface views and the two views of the embryo viewed as a transparent object show the topography of the various parts of the embryo (Figs. 63-66).

A section across the entire blastoderm at the stage of 10 s, through the sixth somite (Fig. 68), shows the following parts:

The ectoderm bounds the section above; it is thickened in the angle between the neural tube and the somites, and becomes thinner as it is traced peripherally; at the extreme periphery of the blastoderm it merges into a mass of cells that interpenetrate the yolk. Ventrally the boundary of the section is formed by the entoderm which is slightly arched upwards in the middle line.

In the region of the area pellucida the entoderm is very thin; at its boundary it passes rather abruptly into the large rounded vesicular cells of the yolk-sac entoderm, which becomes continuous at the margin of the vascular area with the germ-wall; the latter continues to the periphery where it merges in the undifferentiated cell-mass (zone of junction) (Figs. 68 A-68 E). The large neural tube is not vet closed. Beneath the neural tube is a section of the solid rod-like iiotochord.

Fig. 67. — Median longitudinal section of the head of an embryo of l.'i s.

Ectam., Ectamnion. F. B., Fore-brain. H. B., Hind-brain. Inf., Infundibulum. M. B., Mid-brain, pr'c. pi., Precardial plate. T. p., Tuberculum posterius. Other abbreviations as before.

The mesoderm (Fig. 68 A, B, C) lies between the parts already named; it consists on each side of the middle line of the following parts: (1) the mesohlastic somite, a block of cells that radiate from a central cavity filled with irregularly disposed cells; (2) the intermediate cell-mass or nephrotome, forming a narrow connecting bridge between the somite and the lateral plate; (3) the lateral 'plate, split into two layers, external, known as the somatic layer, and internal or splanchnic layer. The cavity between the two layers is the coelome or hody-cavity; it is very narrow next the nephrotome, but widens as it extends laterally to the margin of the vascular area, and is divided by various strands of cells extending from somatic to splanchnic layers, thus indicating its origin by fusion of coelomic vesicles.

The ectoderm plus the somatic layer constitute the somatopleure, from which the body-wall, amnion, and chorion are derived, and the entoderm plus the splanchnic layer form the splanchnopleure, from which arises the intestine and all its appendages, including the allantois and the yolk-sac. Blood-vessels lie between the splanchnic mesoblast and the entoderm. The large vessels beneath the somite and nephrotome are the dorsal aortce; small vessels are present in the area pellucida, and there are many large ones in the area vasculosa. The walls of the vessels are constituted of a single layer of flat endothelial cells bulging in the region of the nuclei; in the vascular area are true bloodislands with eml:)ryonic blood-cells more or less fully filling the cavity.

In a median sagittal section (Fig. 67) the following points should be noticed: (1) the neural tube is enlarged in the region of the head to form the brain, more fully described below; (2) the entoderm forms a tube in the head known as the pharynx or cephalic enteron (cephalic part of the fore-gut), opening behind the heart into the space between the entoderm and yolk. The floor of the anterior end of the fore-gut is fused to the ectoderm in the middle line forming the oral plate. The entoderm forming the floor of the fore-gut turns forward around the hind end of the heart, and beneath the anterior part of the head forms part of the proamnion or mesoderm-free region of the pellucid area; (3) the large pericardial (parietal) cavity lies beneath the floor of the fore-gut. Attached to the posterior wall of the pericardium one sees the hind end of the heart with its two walls, the endocardium and the myocardium a fold of the mesoblastic lining of the pericardium. Between the anterior end of the pericardium and the oral plate is seen the endothelial ventral aorta; (4) the notochord lies between the fore-gut and neural tube and ends anteriorly in a mass of mesenchyme lying between the infundibulum and fore-gut.

The Nervous System

The neural tube is closed at the 12 s stage (Figs. 63 and 65) to a point a little behind the last mesoblastic somite; beyond this the medullary folds diverge and are lost to view towards the hind end of the primitive streak. We may distinguish a cephalic portion {brain or encephalon) and a trunk portion (spinal cord or myelon) of the neural tube; the boundary lies between the fourth and fifth somites, for the first four somites enter into the composition of the head. The brain is thus at this time about as long as the portion of the cord formed or indicated by the medullary folds. For description, see p. 108.

Fig. 68. — A. Transverse section across the axis of the embryo and the entire blastoderm of one side. The section passes through the sixth somite of a 10 s embryo, and is intended to show the topography of the blastoderm. The regions B, C, D, E are represented under higher magnification in the Figs. B, C, D, E. a. V. e., Area vitellina externa, a. v. i.,Area vitellina interna. Bl. i., Blood island. Bl. v., Blood vessel. Coel, Coelome. G. W., Germ-\yall. M. O., Margin of overgrowth. N'ph., Nephrotome. S., Somite. Som'pl. Somatopleure. Sprpl., Splanchnopleure. Som. Mes., Somatic layer of mesoblast. spl. Mes., splanchnic layer of the mesoblast. S. T., Sinus terminalis. Y. S. Ent., Yolk-sac entoderm. Z. J., Zone of junction.

Alimentary Canal

The alimentary canal and its appendages exist only potentially in this embryo in the form of the splanchnopleure, except in the head. The cephalic enteron of this stage corresponds to a large part of the pharynx. The oral plate has already been described in connection with the sagittal section (Fig. 67). In transverse section the extreme anterior end of the fore-gut is quite narrow, elsewhere it is very wide laterally, and in one place its lateral expansions come in contact with the ectoderm on each side and fuse to it, thus indicating the hyomandihular cleft. The floor and lateral walls of the pharynx are composed of columnar cells, the roof of flattened squamous cells (Fig. 54).

Vascular System

The heart lies in the parietal cavity beneath the pharynx; it is bent near its middle to the right. It is an undivided double-walled tube, the internal wall or endocardium being a continuation of the blood-vessels, and the external wall, myocardium or muscular heart, being a duplication of the wall of the pericardium. It has not yet reached the stage of regular contraction, though it may be observed to twitch from time to time. The chambers of the heart are indicated, but not clearly defined at this time; one can only say that the posterior end is the venous end from which the sinus and auricles are to form, and the anterior two thirds, the arterial end, destined to form the ventricles and bulbus.

The endocardium is continued anteriorly into the ventral aorta, which divides on each side of the oral plate (Fig. 64), to form the mandibular arches that describe a loop around the anterior end of the fore-gut and are continued posteriorly as the dorsal aortce, which run above the roof of the pharynx, lateral to the notochord, into the trunk, where they lie ventral to the nephrotome, and send off short blind branches (segmental arteries) between the somites. Near the primitive streak they disappear by merging in the vascular network of the blastoderm.

The posterior end of the endocardium divides in two branches that pass out along the postero-lateral margins of the fore-gut into the general vascular network of the blastoderm (Fig. 64j. This connection constitutes the beginning of the vitelline veins through which the blood from the yolk-sac enters the posterior end of the heart.

General. The elongated form of the entire embryo and the preponderance of the head are marked features of this stage. The latter condition is largely due to the order of origin of parts: the anterior parts preceding the more posterior in their appearance. The head is really, therefore, in a more advanced stage of development than the trunk, hence larger. The elongated condition of the head and the arrangement of all its organs in longitudinal sequence, however, are probably conditions of phylogenetic significance, and point towards an ancestral condition. The topographical values of the cUvisions of the embryonic head are very different from those of the adult, to attain which certain regions develop to a relatively enormous extent, and others comparatively little.

A number of features in the anatomy of the 12 s stage are purposely omitted from this description, as they represent the primordia of structures described more fully beyond; such, for instance, are the neural crest, the pronephros, etc.

Zones of the Blastoderm

The following zones may be recognized in the blastoderm : (1) the pellucid area surrounding the embryo; (2) the vascular zone of the opaque area; (3) area vitellina interna; (4) area vitellina externa. The pellucid area is readily defined by its transparency and by the existence of the subgerminal cavity beneath it. The vascular zone is most readily defined by the extension of the blood tissue which has a very definite margin, coincident with the extension of the mesoblast. The area vitellina includes all of the blastoderm peripheral to the vascular area, and it is characterized by the presence of two layers only, ectoderm and entoderm (germ-wall). It is again divided into two concentric zones, internal and external. The internal is much the wider (Fig. 32 A), and is characterized by the existence of a perilecithal space, i.e., a slight fluid-filled cavity between the entoderm and yolk continuing the subgerminal cavity peripherally. The external vitelline area is relatively narrow, and consists (1) of the zone of junction adjoining the internal vitelline area, and (2) a free margin separate from the 3^olk (margin of overgrowth). The zone of junction is the persistent embryonic or formative part of the blastoderm from which the extra-embryonic ectoderm and entoderm arises. Thus as it spreads peripherally over the surface of the yolk, it leaves on its central margin the differentiated extra-embryonic ectoderm and entoderm; in other words, the zone of junction is the youngest part of the blastoderm, and the concentric zones that may be drawn within it represent successively older stages in a centripetal direction. Therefore in a transverse section through the entire blastoderm successive stages of differentiation of the ectoderm and particularly of the entoderm are met as one passes from the zone of junction towards the center.

The free margin arises from the zone of junction in the manner already described in Chapter II. It may be considered as a part of the ectoderm and it terminates in a row of enlarged cells that often exhibit amoeboid prominences on their margins. It would appear that these cells have the function of a marginal wedge that separates the vitelline membrane and yolk.

The germ-wall has been the subject of many extended researches, but a definitive solution of its origin and function has not hitherto been obtained, mainly on account of the incomplete knowledge of its early histor}^ The ground here taken is in some respects different from that of the various authors, but it is based on a study of its early history given in ChajDter II. There is no deviation from the mode of formation of the zone of junction in the stage under consideration from what was found in earlier stages, and there is no reason to believe that its subsequent history varies in any important respect. It appears to be produced by continuous proliferation of the cells in the 3'olk as in earlier stages (see Fig. 68 E). These cells actively engulf the large yolk granules, and the histological structure becomes in consequence difficult of analysis. The cells lose their individuality and constitute an extended syncytium, the protoplasm of which is packed with yolk-granules. In removing the blastoderm from the egg in saltsolution one finds always, after removing the yolk that may be washed off, a narrow submarginal zone of adherent yolk that is not readily removed, and this is the site of the zone of junction.

Centrally to the zone of junction we have the differentiated ectoderm and germ-wall sharply separated from the yolk by the perilecithal space. The ectoderm of the inner zone of the vitelline area requires no extended notice ; it consists at this time of a single layer of flattened cells. The germ-wall next to the zone of junction consists of two or three layers of large, more or less rounded, cells with definite boundaries, each of which contains one or more yolk-spheres and smaller yolk-granules (Fig. 68 E). We may say roughly that whereas in the zone of junction we have cells in the yolk, in the vitelline area we have yolk in the cells. This mav indicate sufficientlv the wav in which a several layered epithelium becomes differentiated from the zone of junction. As this epithelium is traced centrally we find usually a short distance from the zone of junction a thinner area (Fig. 68 D), and beyond this again the several layers of cells even more laden with yolk-spheres and granules than previously; so that it would appear that these cells may actively engulf yolkgranules. At the margin of the vascular area the entoderm becomes one-layered, and is composed of columnar cells with swollen free margins turned towards the yolk and still containing some yolk-granules and spheres (Fig. 68 C). At the margin of the pellucid area there is a rather sudden transition to the flat entodermal epithelium characteristic of this area.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix

Cite this page: Hill, M.A. (2019, October 16) Embryology Book - The development of the chick (1919) 5. Retrieved from

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