Book - Developmental Anatomy 1924-16

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Part VI. A Laboratory Manual Of Embryology

Chapter XVI The Study Of Chick Embryos

The Unincubated Ovum And Embryos Of The First Day

The Unincubated Egg

The - yolk - of the hen - s egg is a single ovum, enormously expanded with stored food material. AVhen this egg cell is expelled from the ovary, at the time of ovulation, it is enveloped by the vitelline membrane, secreted by the cytoplasm, and by the delicated zona pellucida, a product of the follicle cells (Fig. 312). By the time the liberated ovum passes into the oviduct, the process of maturation has progressed to the point where one polar cell is given off (cf. Fig. 15 A). Fertilization by mature, waiting spermatozoa now ensues, and, coincidently, the second polar cell is extruded to complete maturation (cf. Figs. 15 B and 17). As the egg passes down the oviduct, the albumen, shell membrane, and shell are added as accessory investments. The ovum is ready to be laid one day after its discharge from the ovary; at this time, the appearance is as indicated in Fig. 312. The cytoplasmic area, already started toward the formation of an embryo, is the familiar whitish disc, technically designated the blastoderm.

  • A majority of the illustrations for this section and the skeleton of many descriptions have been adapted from the manual published by Professor C. W. Prentiss.

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Fig. 312. Diagrammatic longitudinal section of a hen's egg before incubation (Thomson in Heisler).

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Fig. 314. - Gastrulation in the pigeon, as shown by a longitudinal section of the blastoderm (redrawn after Patterson). X 50.

Cleavage, Blastula and Gastrula

Fertilization promptly initiates a series of cell divisions which divide the blastoderm into a cellular disc, separated from the yolk by a cleftlike space (Fig. 313). Such mitoses comprise the process of cleavage, and the resulting, asymmetrical, hollow sphere is the blastula. During the period of gastrulation which follows, the blastoderm becomes two-layered. This is accomplished by the rolling under of cells at the future caudal margin of the blastoderm (Fig. 314). Such proliferation and undertucking gives rise to an inner layer, the entoderm; the original surface layer is the ectoderm.

Primitive Streak and Mesoderm

The first conspicuous structure on the blastoderm is an opaque band which is named the primitive streak (Fig. 315). It appears after 16 hours incubation, lying somewhat caudad in the future midline. The primitive streak is interpreted as the margin of the germinal disc where entoderm formation just occurred; the changed appearance and direction are due to the swinging together and fusion of its two lateral halves (Fig. 30). Directly following the earliest appearance of the streak, a primitive groove courses lengthwise along its surface. Cephalad, this ends in the deeper primitive pit, while at the extreme cranial end is an area not indented by the groove, known as the primitive knot (of Hensen).

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Fig. 315. Chick blastoderm at the stage of the primitive streak (16 hours). X 20.

Transverse sections across the primitive streak prove that it is a thickening of the ectoderm and the site of origin for the middle germ layer - -as it was for the entoderm at an earlier stage (Fig. 316). When the mesoderm cells first arise, they are sparse, migratory elements which soon associate into distinct plates extending laterad and caudad. Later, the mesoderm invades the region in front of the streak. At the primitive knot all three germ layers fuse intimately (Fig. 316 A)\ in the caudal half of the streak the entoderm is free (Fig. 316 B). But at both levels the mesoderm represents lateral proliferations from the primitive streak. It appears that the primitive groove is the mechanical result of this rapid growth, or invagination, of mesoderm. From the three germ layers, thus formed, all the tissues and organs will develop, as listed on p. 6.

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Fig. 316. Transverse sections of a chick embryo at the stage of the primitive streak. X 165., 4 , Through the primitive knot; B, through the primitive groove.

Head Process and Head Fold

Embryos of about 19 hours - incubation show an axial strand of cells extending forward from the primitive knot (Fig. 317). This is the so-called head process; it is also termed the notochordal plate because it becomes the cylindrical notochord which serves as the primitive axis about which the embryo differentiates. Although the head process is often described as an outgrowth from the primitive knot, it more probably represents a later stage of the cephalic end of the primitive streak, and grows progressively at the expense of the primitive streak, as the latter, still maintaining its regional characteristics, recedes caudad. A longitudinal section shows the relation of head process to primitive knot (P^ig. 318); a transverse section demonstrates it as a median, thicker mass, continuous laterally with mesoderm which has grown into this region (Fig. 319). Both sections illustrate the independence of the head process from the ectoderm above, and the temporary fusion with the entoderm below.

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Fig. 317. Chick blastoderm and embryo at the stage of the head process and head fold ( 19 hours). X 19.

After the head process is established, a curved fold appears cephalad to it (Fig. 317). This is the head fold which at first involves ectoderm and entoderm alone (Fig. 318). The future development of this important structure will establish the gut internally and definitely delimit the upper body externally.

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Fig. 318. Median sagittal section of a chick embryo at the stage of the head process and head fold (19 hours). X 100.

Neural Groove and Mesodermal Segments

Even embryos of the previous stage exhibit a broad zone of thickening in the ectoderm overlying the head process. This region constitutes the neural plate (Fig. 319). In an embryo of 21 hours, the plate folds lengthwise to form a gutter-like trough, called the neural groove, which shortly will become rolled into the tubular brain and spinal cord (Fig. 320). The notochord now shows through the ectoderm at the bottom of the groove; laterally, the groove is flanked by elevated ridges, the neural folds.

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Fig. 319. Transverse section through the head process of a 19 hour chick embryo. X 165.

The wings of mesoderm which grew from the sides of the primitive streak have spread cephalad to the extent indicated by the darker shading in Fig. 32c. Next the notochord, the mesoderm is thick, and in it have appeared two pairs of vertical clefts;- these separate the mesoderm into successive masses (the first incomplete cranially), which will be seen better in older stages. The}^ are mesodermal segments.

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Fig. 320 Chick embryo at the stage of the neural groove and first two pairs of segments (21 hours) (after Duval). X 13. The head process is visible through the bottom of the neural groove.

Embryo of Five Segments - 23 hours

It is evident that an embryonic and an extra-embryonic region of the blastoderm are becoming more sharply defined (Fig. 321). Of the extraembryonic territory, that nearest the embryo comprises the clearer area pellucida; peripherad lies the area opaca, darker because of its adherence to the yolk beneath. In the more proximal zone of the opaque area are mottled masses, the blood isla>ids, already observed in younger stages but now fusing into an incomplete network. This mesh is best developed caudally; it is the area vascidosa.

At this period the head is growing rapidly. It rises above the blastoderm and projects ccphalad as a somewhat cylindrical part of the embryo, which, at its cephalic end, is entirely free (Fig. 321). In accomplishing this result, the shallow head fold of earlier stages appears to have grown j caudad and liberated the head by undercutting (Fig. 322) ; the real factor, however, is a true forward growth on the part of the head itself. Simul- ;i taneously with the extension of the head, the entodermal component of the original head fold is elongated into an internal tubular pocket of | roughly corresponding shape; this is the primitive j ore-gut. Cranially, j it is a blind sac; caudally, it opens out onto the yolk through an arched 1 aperture termed the intestinal portal. In Fig. 321 the lateral limits of the darker fore-gut and its relation to the crescentic intestinal portal are shown plainly; Fig. 322 illustrates how the entoderm is reflected into the fore-gut at the level of the portal.

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Fig. 321. Dorsal view of a chick embryo with hve segments (23 hours). X 14- 1

The neural groove is broad and deep (Fig. 321). Midway, its lateral folds are approximated and ready to fuse. Caudally, the folds diverge and become increasingly indistinct.

The mesodermal segments are clearly defined and block-like. The notochord shows through the transparent ectoderm, and the x^rimitive streak is shorter, both relatively and actually. Later, when the body form is further indicated by the formation of the tail fold, the primitive streak will disappear. It is a notable fact that the head not only arises soonest but retains its early advantage over lower levels of the body. The progressive advance of differentiation first reaches the end of the trunk at a considerably later period.

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Fig. 322. Median longitudinal section of a five-segment chick embryo (redrawn from Patten). X 25.

Embryo of Seven Segments - 25 hours

A surface view of a chick embryo at this stage resembles the one last described, but shows certain distinct advances (Fig. 323) ; yet the descriptions that follow will apply in all essentials to embryos having from five to ten primitive segments. The vascular area of the blastoderm is better organized than before and extends far cephalad. In front of the head is a light area, not yet invaded by mesoderm and known by the unsuitable name proamnion. The primitive streak is still prominent caudally and measures about one-fourth the length of the embryo. The notochord may be followed cephalad from the primitive knot until it is lost beneath the neural tube.

Neural Tube

The lips of the neural folds have met throughout the cranial two-thirds of the embryo, but have not fused to any extent. The neural tube, formed thus by the closing of the ectodermal folds, is open at each end; the delayed closure of the cranial extremity leaves a temporary opening to the outside, designated the anterior neuropore. In succeeding stages, the more caudal regions of the present neural groove will be rolled progressively into a tube and added to that already completed. At the head end, the neural tube has begun to expand into the brain vesicles. Of these, only ihe forc-hrain is prominent, and from it the optic vesicles are budding out laterally.

Fore -gut except for an increase in size, the fore-gut is little changed. Near its blind end, the floor of the gut is applied to the ectoderm and the two comprise the temporary pharyngeal membrane (cf. Fig. 335). The fore-gut will ultimately constitute the alimentary canal as far as the middle of the small intestine, the way in which the entoderm is folded up from the blastoderm and forward into the head is shown well in Figs. 322 and-335 Mesoderm and Coelom. - The tissue of the middle germ layer assumes two different forms. Throughout most of the head region it comprises a diffuse meshwork of cells which fills in the spaces between the various epithelial layers. This tissue is mesenchyme (Fig. 331). In the caudal part of the head, and in the remainder of the body, the mesoderm at this stage is organizing more definitely. Nearest the midplane, it is divided by transverse furrows into seven block-like primitive segments, four of which belong to the future head (Figs. 323 and 324). Caudad, between the segments and the primitive streak, there is the undifferentiated mesoderm of the segmental zone, but new pairs of segments will develop progressively throughout this region. Lateral to each segment is a plate of unsegmented mesoderm, termed the intermediate cell mass; it is also called the neplirotome because it will play an important role in the development of the excretory system (Fig. 324). The nephrotome plate serves as a bridge between the segments and the unsegmented lateral mesoderm. The lateral mesoderm, when first formed, aggregates into two solid plates (Fig. 316) each of which splits secondarily into two lamellce, separated by a space (Figs. 324 and 325). The dorsal layer comprises the somatic mesoderm, the ventral layer the splanchnic mesoderm.

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Fig. 323. Dorsal view of a chick embryo with seven segments (25 hours). X 20. numbered lines indicate the levels of the sections. Figs. 325-332.

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Fig. 324. Diagrammatic transverse section through a hen - s egg at an early stage of development (Minot-Prentiss).

The space between the two layers first occurs as isolated clefts, which soon unite to form the body cavity, or coelom (Fig. 324). The originally bilateral coelomic chambers will later become confluent v^entrally, as in the adult (Fig. 324). In the region of the heart, the ccelom is already enlarged locally, anticipating its destiny as the pericardial cavity. Other portions will become the pleural cavities of the thorax, and the peritoneal cavity of the abdomen.

Heart and Blood Vessels

The heart is a simple, straight tube, l3ung in the midplane, ventral to the gut. In a dorsal view of the total embr^m it is inconspicuous because largely concealed by overlying structures.

Caudally, it is continuous with the converging vitelline veins which enter the body by following along the margins of the intestinal portal: the two veins unite as they join the heart (Fig. 323). From the cephalic end of the heart is given off the ventral aorta; dorsal to the gut course paired descending aortae.

Transverse Sections

The first embryo to be studied in serial section is easiest understood if the student begins caudad, where differentiation is least, and works toward the head. Important facts pertaining to the germ layers and the jirinciples underlying the development of the neural tube, gut, heart, and head are then made simple. The following illustrations and descriptions may be used to interpret sections of chick embryos between the stages of five and ten segments. The level of each section may be determined from the numbered lines on Fig. 323.

Sections through the Primitive Streak and Knot. - Conditions are essentially the same , as in the younger embryos already examined (Fig. 316). Section through the Fifth Primitive Segment (Fig. 325). - This general level is characterized l.)y the differentiation of the mesoderm, the approximation of the neural folds, and the presence of two vessels, the descending aortir, one on each side between the mesodermal segments and the entoderm. The neural folds are thick, as is the adjoining ectoderm to a less degree. The jwtochord is a sharply defined oval mass of cells. The mesodermal segments are somewhat triangular in outline and connected by the intermediate cell mass, or j ucplirotomc, with, the lateral mesoderm. The lateral mesoderm is partially divided by irregu- j lar, flattened spaces into two sheets, the dorsal of which is the somatic layer, the ventral the splanchnic layer. Later, the spaces unite to form the coelom, or primitive body cavity, J and the mesodermal lining then becomes mcsothelium.

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Fig. 325. Transverse section through the fifth pair of mesodermal segments of a seven segment chick embryo. X 90.

Through the higher segments in the series the differentiation of mesoderm and ccelom is more advanced (cf. Fig. 343). Caudal to the seventh segment, in the region of the segmental zone, the mesoderm forms solid plates (cf. Fig. 344).

Sections through the Area Vasculosa (Fig. 326). - The illustrations show a little of the extra-embryonic territory, peripheral to the area pellucida. In this region of the area 0 paca , the entoderm is associated intimately with the coarsely granular yolk. The splanchnic mesoderm contains aggregations of cells known as Mood islands, many of which are fusing into the network that characterizes the area vasculosa (Figs. 323 and 326 I). The cellular thickenings of the blood islands undergo differentiation into two cell types : fluidfilled vacuoles appear and expand so as to set free the innermost cells which later separate and float about as primitive Mood corpuscles; the same process flattens the peripheral cells into an endothelium {B, C). The endothelial spaces both coalesce and form new vascular sprouts, and in this way the system of extra-embryonic vessels is extended. All blood vessels at first consist of an endothelial layer only.

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Fig. 326. Transverse sections through the area vasculosa of a seven-segment chick embryo.X 300.

Section Caudal to the Intestinal Portal (Fig. 327). - The section is characterized: (1) by the meeting of the neural folds preparatory to closing the neural tube; (2) by the arching of the entoderm, which, a few sections nearer the head end, forms the fore-gut; (3) by the presence of the vitelline veins laterally between the entoderm and splanchnic mesoderm; (4) by the wide separation of the somatic and splanchnic mesoderm and the consequent increase in the size of the coelom. In this location the coelom later surrounds the heart and is converted into the pericardial cavity.

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Fig. 327. Transverse section caudal to the intestinal portal of a seven-segment chick embryo.X 90.

The neural tube at this level is transforming into the third brain vesicle, or hind-brain. The neural folds have not yet fused, and at their dorsal angles are the neural crests, the anlages of the spinal ganglia. Mesodermal segments never develop as far cephalad as this region; instead, diffuse masses of mesenchyme occupy comparable positions adjacent to the neural tube. On the left of the section, an asterisk marks the junction of somatic and splanchnic mesoderm.

Section through the Intestinal Portal (Fig. 328). - This section passes through a vertical fold of entoderm at the exact point where the latter is reflected into the head as the forc-gut (cf. Figs. 322 and 335). The entoderm forms a continuous bridge of tissue between the vitelline veins, thereby closing the fore-gut ventrally. The splanchnic mesoderm is differentiated into a thick-walled pouch on each side, lateral to the endothelial layer of the veins.

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Fig. 328. Transverse section through the intestinal portal of a seven-segment chick embryo.X 90.

A few sections cephalad, the gut separates from the general entoderm; this will allow first the endothelial heart tubes to meet, and then the flanking folds of splanchnic mesoderm.

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Fig. 329. Transverse section through the heart of a seven-segment chick embryo. X 90.

Section through the Heart (Fig. 320). - Passing cephalad in the series, the vitelline veins open into the heart just cranial to the intestinal portal. The entoderm in the head fold now lines the crescentic pharynx of the fore-gut, and is separated by the heart, ccelom, and splanchnic mesoderm from the entoderm of the germinal disc. The descending aortae are larger, making conspicuous spaces between the neural tube {hind-brain) and the pharynx. The heart, as will be seen, results from the union of two endothelial tubes, continuous with those constituting the vitelline veins in the preceding sections. The median walls of these tubes disappear at a slightly later stage and thereby establish a single tube, the endocardium. Thickened layers of splanchnic mesoderm, which, in the preceding section, invested the vitelline veins laterally, now form the mesothelial wall of the heart; this tissue will become the later myocardium and epicardium. In the median ventral plane, the layers of splanchnic mesoderm of each side have fused and separated from the splanchnic mesoderm of the germinal disc; thus, the two pericardial cavities are put in communication. Dorsally, the splanchnic mesoderm, as the dorsal mesocarduim, suspends the heart, while still more dorsally it is continuous with the somatic mesoderm at the point where the mesenchyme of the head extends to the coelom.

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Fig. 330. Transverse section through the head fold of a seven-segment chick embryo. X 90.

Origin of the Heart and Embryonic Vessels

From the two sections last described, it is seen that the heart arises as a pair of endothelial tubes lying in folds of the splanchnic mesoderm. Later, the endothelial tubes fuse and the mesodermal folds are also brought together. The heart then consists of a single endothelial tube within a thick-walled investment of mesoderm. The endothelial cells of the heart often appear to arise from the entoderm but this is perhaps a deception, for elsewhere endothelium is mesodermal in origin. The vascular system is primitively a paired system, the heart arising as a double tube with two veins entering and two arteries leaving it (cf. Figs. 180 and 181). The blood vessels of the body are delicate endothelial channels which originate as clefts in the mesenchyme. Coalescence and budding produce a plexus from which definite vessels are selected (Fig. 179).

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Fig. 331. Transverse section through the pharyngeal membrane of a seven-segment chick embryo. X 90.

Section through the Head Fold

(Fig. 330). - It will be remembered that a crescentic ectodermal fold lies both beneath the head and lateral to it, and that the portion of the body cephalad to this head (old is free from the blastoderm (Figs. 322 and 323). The present section, from a level just cephalad of the heart, is located at the critical region where these folds meet. The inspection of a few sections in each direction will demonstrate how the embryonic and extra-embryonic territories are related and how they become separate. The coelom does not extend into the head. Midway of the blastoderm is a space which lacks mesoderm; it is the proamnion. Ventral to the pharynx, the ventral aorta are becoming separate vessels as they leave the heart.

Section through the Pharyngeal Membrane (Fig. 331). - This section shows the head free from the underlying blastoderm (cf. Fig. 322). The ectoderm surrounds the head, and near the midventral line it is bent dorsad, is somewhat thickened, and comes in contact with the thick entoderm of the pharynx. The area of contact between ectoderm and pharyngeal entoderm constitutes the pharyngeal membrane. Later, this plate breaks.

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Fig. 332. Transverse section through the fore-brain and optic vesicles of a seven-segment chick embryo. X 90.

Through and establishes the oral cavity, which, accordingly, is partly ectodermal. The expanded neural tube is closed and forms the middle brain vesicle, or mid-brain; the |i superficial ectoderm is entirely separate from it. The descending aortse appear as small vessels dorsal to the lateral folds of the pharynx. The blastoderm in the region beneath the head is composed of ectoderm and entoderm only; this is the proamnion. Laterad may be seen the layers of the mesoderm.

Section through the Fore -brain and Optic Vesicle (Fig. 332). - The neural tube is open here and constitutes the first brain vesicle, or fore-brain. The opening is the temporary anterior neuroporc. The ectoderm is composed of two or three layers of nuclei and is continuous at the neuropore with the much thicker wall of the fore-brain. The two ectodermal layers are in contact with each other except in the midventral region, where the mesenchyme is beginning to penetrate and separate them. The lateral expansions of the : fore-brain are the optic vesicles, which eventually give rise to the retina of the eye.

Embryo of Seventeen Segments - 38 hours

The stage selected as a type for illustrating the significant advances j since the seven-segment embryo is a chick of about 38 hours incubation which possesses 17 primitive segments. At this time, the segments are j| developing rapidly and the descriptions that follow will apply satisfactorily to embryos between 33 hours (12 segments) and 40 hours (18 segments).

The long axis of the embryo is still nearly straight, but specimens of full 17 segments should show a flexing of the head ventrad (Fig. 335) and a slight turning of the tip of the head on its left side. Fig. 333 does not illustrate this feature. The area pellucida is dumb-bell shaped and is developing a vascular network. The extra-embryonic vessels of the area opaca are well differentiated, and the vascular area is bordered by a terminal sinus. Opposite the caudal end of the heart, the vascular networks converge and become continuous with the stems of the vitelline veins. Connections have been established also between the descending aortae and the vascular area at the level of the lowest segments, but as yet the vitelline arteries have not appeared as distinct trunks (Fig. 334). The tubular heart is bent to the embryo - s right; the head is more prominent and the three primary vesicles of the brain are evident; the proamniotic area is reduced to a small region in front of the head ; the primitive streak is inconspicuous.

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Fig. 333. Dorsal view of a chick embryo with seventeen segments (38 hours). X 20.

Central Nervous System and Sense Organs

The tardy closure of the anterior neuropore has occurred, and the neural tube is complete save at its caudal end where the divergent neural folds form the so-called rhomboidal sinus (Fig. 333)- In the head, the neural tube is differentiated into three brain vesicles, marked off from each other by constrictions. The fore-brain (prosencephalon) is characterized by the outgrowing optic vesicles. The mid-brain (mesencephalon) is a simple dilatation. The elongate hind-brain (rhombencephalon) gradually merges with the spinal cord; it shows a number of secondary constrictions, the neuromeres ectoderm of the ventral surface of the head and the entoderm caudal to the intestinal portal have been removed. Numbered lines indicate the levels of Figs. 337 - 344.

The ectoderm is thickened laterally over the optic vesicles to form the lens placode of the eye (Fig. 337). The optic vesicle flattens at this point and will soon invaginate to produce the optic cup. Dorso-laterally, in the hind-brain region, the ectoderm is thickened and indented as the auditory placodes (Fig. 334). Each placode will become an otocyst, or otic vesicle, from which differentiates the sensory epithelium of the internal ear (membranous labyrinth).

Fore -gut. - The entoderm is still flattened over the surface of the yolk, caudal to the intestinal portal. In Fig 334, the greater part of the entoderm is cut away. The broad fore-gut, folded inward at the portal, shows indications of three lateral diverticula, the pharyngeal pouches. Cephalad, the pharynx is closed ventrally by the pharyngeal membrane, and the ectodermal depression external to it is the stomodeum (Fig. 335).

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Fig. 335. Median sagittal section of the head end of a seventeen-segment chick embryo..X about 50.

Heart and Blood Vessels

After receiving the vitelline veins just cephalad of the intestinal portal, the venous end of the heart tube dilates into the ventricle which bends ventrad and to the embryo - s right (Fig. 334). It then is flexed dorsad and to the median plane, and narrows to form the bulbns, and its continuation, the ventral aorta. The ventral aorta lies beneath the pharynx and divides into two divisions. These diverge and course dorsad around the pharynx as the first pair of aortic arches. Before reaching the optic vesicles they bend sharply caudad, and, as the paired descending aorta:, may be traced to a point opposite the last primitive segments. In the region of the intestinal portal they lie close together and have fused to form a single vessel, the dorsal aorta. Below this level they separate again, and, opposite the last primitive segments, connect by numerous capillaries with the vascular network. In this region, the trunks of the paired vitelline arteries presently will be differentiated. The heart beats spasmodically at this stage; the blood flows from the vascular area by way of the vitelline veins to the heart, thence by the aortse and vitelline arteries back again. This constitutes the vitelline circulation, and through it the embryo receives nutriment from the yolk for its future development.

Heretofore, the body of the embryo has been without definite veins, but now two pairs of vessels are developing for the purpose of returning lilood to the heart. The anterior cardinal veins drain blood from the head region; the posterior cardinals, just appearing at this stage, will perform a similar function in the lower body (cf. Fig. 348). The two vessels unite on each side into a common cardinal (duct of Cuvier) which enters the venous end of the heart.

Differentiation of Mesoderm

The production of early mesodermal segments, and the addition of new ones by a progressive furrowing of the segmental zone, has been observed in previous stages. The segments thus formed are block-like with rounded corners when viewed dorsally, the ectoderm is removed from the dorsal surface.

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Fig. 336. Reconstruction through the lower mesodermal segments of a two-day chick embryo.

Triangular in transverse section (Fig. 336). In higher vertebrates, the segments contain at most only indications of a cavity; in the chick there is a minute central space, representing a portion of the coelom, which is filled with a cellular core, while the other cells of the segment form a thick, radially-arranged layer about it (Fig. 343). The ventral wall and a portion of the median wall of each primitive segment break down into a mass of mesenchyme termed the sclerotome (Fig. 2 1 1 ) ; these later surround the notochord and neural tube, and transform into the axial skeleton. The remaining portions of the segment constitute the dernio-myotome (Figs. 212 and 340). The cells of the dorso-mesial wall of the plate, the myotome, eventually give rise to the skeletal musculature of the body. The lateral plate is the dermatome which is destined to furnish the deeper layers of the integument.

The bridge of cells connecting a primitive segment with the lateral mesodermal layer constitutes the intermediate cell mass, or nephrotome (Fig. 336). In the chick, the nephrotomes of the fifth to sixteenth segments give rise to segmental pairs of bud-like sprouts which extend dorsad (Fig. 343). These are the prone phric kidney tubules. Although rudimentary, their ends unite to form a tube, known as the pronephric duct, which grows to the cloaca (Fig. 128). More caudal nephrotomes will soon form the embryonic kidney, or mesonephros, whose tubules open into the pronephric duct, then called the mesonephric duct (Fig. 336). Later still, the permanent kidney develops partly from the pronephric duct and partly from nephrotome tissue. Accordingly, the intermediate cell masses may be regarded as the anlages of the urogenital glands and ducts - all mesodermal in origin.

In the embryo of seven primitive segments, the lateral mesoderm was observed to split into two layers, the dorsal somatic and the ventral splanchnic mesoderm (Fig. 336). These layers persist, the somatic mesoderm giving rise to the pericardium, parietal pleura, and peritoneum, while the splanchnic layer forms the epi-myocardium, the visceral pleura, and the mesenteries and mesodermal layers of the gut. The somatic mesoderm and ectoderm are closely associated in development and together are designated the somatopleure (Fig. 324) ; it forms the body wall. Similarly, the splanchnic mesoderm and entoderm are jointly termed the splanchnoplcure. Both the mesodermal segments and the unsegmented mesodermal layers contribute the mesenchymal cells which play such an important part in development.

Transverse Sections

In studying serial sections of an embryo it is not sufficient merely to identify the structures seen. The student should determine also the exact level of each significant section with respect to the illustrations of the total embryo, as indicated for this series along the margins of Fig. 334, and trace the organs from section to section in the series. He is then ready to reconstruct mentally the complete picture of a part and to interpret its origin and relations.

The following sections are drawn, viewed from the cephalic surface; hence, the right side of the embryo is at the reader - s left. These illustrations and descriptions may be used for the stud}" of chick embryos between 33 hours (12 segments) and 40 hours (18 segments).

Section through the Fore-brain and Optic Vesicles (Fig. 337). - The optic stalks connect the optic vesicles laterally with the ventral portion of the fore-brain. Dorsally. the section passes through the mid-brain, due to the somewhat ventrally flexed head (cf. fig- 335)- The lens placodes are thickenings of the surface ectoderm over the optic vesicles. Note that there is now a considerable amount of mesenchyme between the ectoderm and the neural tube; the small spaces are terminal branches of the anterior cardinal veins. Layers of mesoderm are present in the underlying blastoderm.

Section through the Pharyngeal Membrane and Mid-brain (Fig. 338). - In the midventral line, the thickened ectoderm bends up into contact with the entoderm of the rounded pharynx of the fore-gut. The resulting ectodermal pit is the stomodeum, and the two apposed layers represent the pharyngeal membrane. At this point the oral opening will break through. ( )n either side of the pharynx a pair of large vessels is seen; the ventral pair are the ventral aorta. Two sections cephalad, their cavities open into those of the dorsal pair, the descending aorta. The section is thus just caudad of the first aortic arches. The caudal end of the mesencephalon is the portion of the neural tube showing; it is thick walled, with an oval cavity. Note the large amount of undifferentiated mesenchyme throughout the section. The structure of the blastoderm is complicated by the presence of collapsed blood vessels.

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Fig. 337. Transverse section through the fore-brain of a seventeen-segment chick embryo.X 75 .

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Fig. 338. Transverse section through the pharyngeal membrane of a seventeen-segment chick embryo. X 7,3.

Section through the Hind-brain and Auditory Placodes (Fig. 339). - This section is characterized by: (1) the auditory placodes, which represent the anlages of the internal ear; (2) the large hind-brain, somewhat thin and flattened dorsad; (3) the broad pharynx, cut through the second pair of pharyngeal pouches, above which on each side lie the descending aortae; (4) the presence of the bulbar and ventricular portions of the heart. The bulbus is suspended dorsally by the mesoderm, which here forms the dorsal mesocardium. The ventricle lies on the right side of the embryo; a few sections caudad in the series it is continuous with the bulbus (cf. Fig. 334). Between the somatic and splanchnic mesoderm is the large pericardial cavity, surrounding the heart. Ventro-lateral to the brain are the anterior cardinal veins, which return blood from the head region.

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Fig. 339. Transverse section through the hind-brain and auditory placodes of a seventeen segment chick embryo. X 75.

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Fig. 340. Transverse section through the caudal end of the heart of a seventeen-segment chick embryo. X 73.

Section through the Caudal End of the Heart (Fig. 340). - The section still includes the hind-brain. The descending aortce are separated only by a thin septum which is ruptured at this level. The anterior cardinal veins are cut where they bend ventrad to enter the heart. The mesothelial wall of the heart is continuous through the dorsal mesocarduim with the splanchnic mesoderm. On the right side of the section there is fusion between the epi-myocardium of the heart and the somatic mesoderm. iMesodermal segments were not observed at higher levels, but now they appear lateral to the hind-brain. The ventro-mesial part of the segment is breaking down into the sclerotome; the dorsomesial wall represents the myotome, and the lateral plate the dermatome.

Section through the Intestinal Portal

(Fig. 341). - The descending aortre now form a single vessel, the dorsal aorta, the medium septum having disappeared. The section passes through the entoderm at the point where it is folded dorsad and cephalad into the head as the fore-gut (cf. Fig. 335). Two sections caudad is found the opening {intestinal portal) where the fore-gut communicates with the flattened open gut between the entoderm and the yolk. On each side of the fore-gut are the large vitelline veins, sectioned obliquely. The splanchnic mesoderm overlying these veins is pressed by them against the somatic mesoderm, and the cavity of the ccelom is thus interrupted on each side. The section is close to the level where the common cardinal veins open into the venous end of the heart.

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Section Caudal to the Intestinal Portal (Fig. 342). - In general, this section resembles the preceding save that the primitive gut is without a ventral wall, and, therefore, may be called mid-gut. The right vitelline vein is still large. Lateral to the enclosed coelom, on each side, are spaces which represent the posterior cardinal veins, just differentiating.

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Fig, 342. Transverse section caudal to the intestinal portal of a seventeen-segment chick embryo. X 90.

Section through the Fourteenth Pair of Primitive Segments

(Fig. 343). - The body of the embryo is now flattened on the surface of the yolk. Here the descending aorta: are again separate and occupy arched spaces under the primitive segments. The section is characterized by the notochord and the differentiated mesoderm which forms typical primitive segments, nephrotomes, and somatic and splanchnic mesoderm. Arising from the nephrotomes are sprout -like pronephric tubules. The tips of these hollow out and unite to I form the primary excretory, or pronephric duct.

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Fig. 343. Transverse section through the fourteenth pair of mesodermal segments of a seventeen-segment chick embryo. X 90.

Section through the Segmental Zone

(Fig. 344). - The section is at the level of the segmental zone, where mesodermal segments have not formed as yet. The mesodermal ' plates are splitting laterally into layers, but the ccelomic cavities are mere slits. Between I the splanchnic mesoderm and the entoderm, blood vessels may be seen. The open neural 1 groove is called the rhomboidal sinus. The ectoderm is characterized by the columnar form of its cells. At the point where the ectoderm joins the neural fold, a ridge of cells projects ventrally on either side. These projecting cells constitute the neural crests, and from them the spinal ganglia are formed in older embryos.

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Fig. 344. Transverse section through the segmental zone of a seventeen-segment chick embryo. X 90.

Section through the Primitive (Hensen's) Knot

(Fig. 345). - The three germ layers fuse inseparably at the 'knot - into a mass of undifferentiated tissue. The lateral mesoderm is split into somatic and splanchnic layers; the latter contains numerous small blood vessels of the vascular network.

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Fig. 345. Transverse section through the primitive knot of a seventeen-segment chick embryo. X 90.

Section through the Primitive Streak (Fig. 346). - In the mid-dorsal line is the primitive groove. The germ layers may be seen taking their origin from the undifferentiated tissue of the primitive streak, beneath. Laterad, between the splanchnic mesoderm and entoderm, lilood vesSels are present as in the preceding sections.

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Fig. 346. Transverse section through the primitive streak of a seventeen-segment chick embryo. X 90. h'

Embryo of Twenty-Seven Segments - 2 days

Although a chick embryo with 27 segments is chosen as the norm, the descriptions which follow are applicable to stages between 45 hours (23 segments) and 60 hours (32 segments).

During the latter half of the second day a remarkable change occurs in the appearance of the embryo and in its positional relation to the blastoderm (Fig. 56). The bending of the head, already begun in the stage last studied, has continued until the fore- and hind-brains are nearly parallel. This marked cephalic flexure occurs at the region of the mid-brain. As long as the embryo retained its original prone position with respect to the yolk, it is manifest that the head could not bend greatly ventrad, so, in order that the flexion might proceed to completion, the upper body has twisted about its long axis until the left side lies squarely next the yolk. In a dorsal view, therefore, one sees the right side of the head but the dorsal side of the lower body. The actual zone of torsion, now half way down the trunk, will advance progressively until the whole embryo is turned, after which additional curvatures will make it assume the shape of the letter C (Fig. 363). One of these flexures is already appearing opposite the lower end of the heart, at the junction of head and trunk; for this reason it bears the name cervical flexure.

Most of the body is rather sharply delimited from the blastoderm; the head is free; much of the midbody is bounded by deep lateral folds; caudally, the tail bud represents the future hind end of the body and is bordered by a tail fold. The further combined activities of head-, lateral-, and tail folds will constrict the embryo from the extra-embryonic blastoderm.

The head is now covered by a double fold of the somatopleure, the head fold of the amnion; it envelops the upper half of the body like a veil. The heart bends in the form of a letter S, and the extra-embryonic vascular ple.xus is profuse. Three ectodermal furrows form branchial grooves on the sides of the neck. Eye and ear anlages are prominent. Primitive segments extend far down the former segmental zone.

Central Nervous System and Sense Organs

The brain region of the neural tube is separated by constrictions into five vesicles. The first subdivision of the primitive fore-brain is the telencephalon; the rest constitutes the diencephalon. The mesencephalon remains undivided but is bent at its middle by the cephalic flexure. The hind-brain shows two indistinct regions of differentiation; a short section with a thick roof adjoining the mid-brain is the metencephalon, the thin-roofed remainder is the myelencephalon. The spinal cord is now closed to its extreme end and consequently the rhomboidal sinus no longer exists.

The lens anlage has assumed the form of a lens vesicle; coincident with its invagination the outer wall of the optic vesicle also folds inward, thereby making a double-walled structure, the optic cup. The latter is not a complete cup, for on one side a segment of the wall is missing; this chorioid fissure gives the cup a horse-shoe outline in surface view (Fig. 347). The auditory placode of earlier stages has become a sac, the otocyst or otic vesicle, which, however, retains a connection with the body ectoderm.

Digestive System

The entodermal canal shows three regional divisions. Of these, the fore-gut is best differentiated and will be referred to again. In Fig. 348 most of the entoderm has been removed, so that the open mid-gut scarcely shows; it extends from the intestinal portal to the tail bud, and, without a ventral wall, overlies the yolk. Caudad, a small portal leads into the hind-gut which is just beginning to invaginate into the tail fold; in development and relations it duplicates the fore- gut. The pharyngeal membrane now lies at the bottom of a deep pit, the stomodeum, formed by depressed ectoderm. A median ectodermal sac, just in front of the pharyngeal membrane and lying next the brain wall, is Rathke's pouch; it is the anlage of the epithelial portions of the hypophysis. The entodermal pharynx bears three pairs of lateral out-pocketings known as the pharyngeal pouches. They occur opposite the three external branchial grooves, and here ectoderm and entoderm are in contact, forming closing plates. At about this stage the first pair of plates ruptures, thereby making a free opening, or branchial cleft, into the pharynx. These transitory apertures correspond to the gill clefts in lower aquatic vertebrates. Between the successive pouches lie solid, bar-like portions of the body wall, the branchial arches; in animals with aquatic respiration the arches bear gills, and even in higher embryos, like the chick, an aortic arch courses through each (cf. Fig. 184). At the level of the second pair of pouches, a broadly open pocket grows from the median floor of the pharynx; it is the anlage of the thyroid gland (Fig. 353). Toward the intestinal portal the fore-gut is flattened from side to side, and before it opens out into the mid-gut there is budded off a bilobed liver diverticulum (Figs. 348 and 355). It lies between the vitelline veins which later break up into the sinusoidal spaces of the liver.

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Fig. 347. Dorsal view of a chick embryo with twenty-seven segments (two days). X 14.

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Fig. 348. Ventral reconstruction of a twenty-seven segment chick enbryo. X 18. The ectoderm of the upper body and the entoderm of the lower body have been mostly removed. Numbered lines indicate the levels of Figs. 350-361.

Vascular System

The disappearance of the dorsal mesocardium leaves the huge, tubular heart attached only at its two ends. Since the heart tube is growing faster than the surrounding body, it of necessity bends like the letter S, when seen from the ventral side (Fig. 348). Four regions may be distinguished: (i) the sinus venosus, into which the veins ; open; (2) a dilated dorsal chamber, the atrium; (3) a tubular ventral portion, bent in the form of a U, of which the left limb is the ventricle, the right limb (4) the bulbus cordis. From the bulbus is given off the ventral ;! aorta. There are now three pairs of aortic arches which open into the paired descending aorta - . The first aortic arch extends through the first branchial arch, cranial to the first pharyngeal pouch, and is the primitive connecting vessel seen in the thirty-eight-hour embryo (cf. Fig. 184). The second and third aortic arches course in the second and third branchial arches on either side of the second pharyngeal pouch. They are developed by the enlargement of channels in primitive capillary networks between ventral and descending aortre. Opposite the sinus venosus, the paired aortic trunks fuse to form the single dorsal aorta which extends as far back as the fifteenth pair of primitive segments. At this point the aortae again separate, and, o])posite the twentieth segments, each connects with the trunk of a vitelline artery which conveys blood to the vascular area (Fig. 348). Caudal to the vitelline arteries, the aortas decrease rapidly in size and soon end.

As in the previous stage, the blood is returned from the vascular area to the heart by the vitelline veins, now two large trunks (Fig. 348).

In the body of the embryo, the anterior cardinal veins course ventrolateral to the brain and already are of large size. The smaller posterior cardinal veins are developing caudal to the atrium. They lie in the mesenchyme of the somatopleure, laterad in position ( Fig. 355). Opposite the sinus venosus, the anterior and posterior cardinal veins of each side unite and form the common cardinal veins (ducts of Cuvier) which open into the dorsal wall of the sinus venosus (Fig. 348). The primitive veins are thus paired like the arteries, and like them develop by the enlargement of channels in a network of capillaries.

Differentiation of Mesoderm

The formation of new mesodermal segments and the progressive differentiation of older ones into sclerotome, myotome, and dermatome continue as described for the preceding embryo (p. 330).

The nephrotome region shows the beginning of additional features. The prone phric duct has continued beyond its original site of formation, and as a blind, growing cord extends tailward (Fig. 336). A set of new mesonephric tubules is now starting to differentiate, caudal to the pronephric group, between the thirteenth and thirtieth segments. They arise from the intermediate cell masses as vesicles that will become tubules and join the pronephric (hereafter called mesonephric) duct. They constitute the embryonic, but not the definitive kidney. Additional information concerning the pro- and mesonephroi may be found on pp. 135-139

The splanchnopleure is chiefly involved in gut formation. The somatopleure is deeply folded into the lateral body folds whose union will progressively close the ventral body wall (Fig. 356). The establishment of a complete body wall, at any level, of necessity separates embryonic from extra-embryonic coelom.

Amnion and Chorion

At the end of the second day, two extra-embryonic, protective membranes have become prominent. They are theawnion, which will form a membranous, fluid-filled sac about the embryo itself, and the chorion which eventually encloses both embryo and all extra-embryonic structures (Fig. 37). The two membranes arise simultaneously from the extra-embryonic somatopleure by a single process of folding (Fig. 349). In front of the embryo a fold of the somatopleure is thrown up, followed later by others lateral and caudal to the embryo (A). These hood-like, arching folds close in from all sides until they meet and fuse over the embryo (B-D). The inner layer of somatopleure is the amnion; the remainder constitutes the chorion, of little importance to the chick. It should be noted that the folding brings the mesodermal components of these membranes facing each other, but separated by the extra-embryonic coelom.

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Fig. 349. - Diagrams illustrating the development of the amnion, chorion and allantois in longitudinal section (after Gegenbauer in McMurrich). Ectoderm, mesoderm and entoderm are represented by heavy, light and dotted lines respectively. .4/., Allantois; Am., amniotic cavity; lA., yolk sac.

The head fold of the amnion had begun in the chick of the previous stage (Fig- 335) : at the end of the second day it is continuous along a crescentic margin with the lateral folds, and envelops the upper half of the body (Figs. 347 and 356). As yet the tail fold has scarcely started.

Transverse Sections

The following series of transverse sections from a two-day chick shows the fundamentally important structures; they are equally applicable to the study of embryos between 45 hours (23 segments) and 60 hours (32 Segments). The sections are drawn from the caudal surface; hence, the left side of the embryo is at the reader - s left. The precise level of each significant section should be ascertained with respect to Figs. 347 and 348, as has been indicated for this series along the margins of Fig. 348. Since the head is bending rajudly during the last hours of the second day, minor variations in the appearance of different series of sections are unavoidable; this, however, is chiefly a question of what particular structures happen to appear together in the fore-brain and hind-brain portions of a section.

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Fig. 350. Transverse section through the eyes and first aortic arches of a twenty-sevensegment chick embryo. X 50.

Sections through the Cephalic Flexure

Due to the flexed brain, the first sections encountered pass through the mesencephalon, but soon the hind-hrain and then the diencephalon are included. The blood vessels seen are the anterior cardinals. Presently the brain becomes cut twice in each section; the myelencephalon may be recognized always by its thin roof and by its close association with the notochord. Note that in these sections through the bent head, progress is caudad down the hind-brain half of the section, but cephalad toward the tip of the fore-brain.

Section through the Eyes and First Aortic Arches

(Fig. 350). - The section passes cephalad of the optic stalks, consequently the optic vesicles appear unconnected with the fore-brain. The adjacent, thickened ectoderm is invaguiated to form the anlages of the lens vesicles. The thicker wall of the optic vesicle, next the lens anlage, will give rise to the nervous layer of the retina; the thinner outer wall becomes the pigment layer of the retina. Ventrad in the section are the wall and cavity of the fore-brain, dorsad the myelencephalon of the hind-brain with its thin, dorsal ependymal layer. Between the brain vesicles, on either side, are longitudinal sections of the first aortic arches, and lateral to the hind-brain are the smaller, paired anterior cardinal veins, which convey the blood from the head to the heart. The splanchnopleure (that is, the yolk side of the blastoderm) is characterized in this and subsequent sections by the presence of blood vessels in its mesodermal layer. The entire head is enveloped by the amnion; the chorion passes along the right side of the head, and, continuing, surrounds both embryo and yolk. In these membranes the mesodermal components face each other across the extra-embryonic ccelom.

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Fig. 351. Transverse section through the optic stalks and hypophysis of a twenty-seven segment chick embryo. X 50.

Section through the Optic Stalks and Hypophysis

(Fig. 351). - The section passes just caudal to the lens. The optic vesicles are connected with the wall of the fore-brain by the optic stalks, which later form the path through which the fibers of the optic nerve grow from the retina to the brain; sections cut in this plane do not show the chorioid fissure. Both the ventral and the descending aortae are seen about the cephalic end of the pharynx. Between the ventral wall of the fore-brain and the pharynx is an invagination of the ectoderm. Rathke's pouch, which will become the epithelial hypophysis; a few sections farther along it o])ens externally into the stomodcum, close to the pharyngeal membrane.

Passing caudad in the series a short distance, the fore-brain region of the bent head liecomes isolated from the body and soon the tip of the head is reached.

Section through the Otocysts and Second Aortic Arch

(Fig. 352). - The otic vesicles arc sectioned caudal to their apertures, and so appear as cloSed sacs, lateral to the wall of the hind-biain. The cavity of the pharynx is somewhat triangular and its dorsal wall is thin. The anterior cardinal veins pass between the otocysts and the wall of the hind-brain.

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Fig. 352. Transverse section through the otocysts and second aortic arches of a twenty-seven segment chick embryo. X50.

Ventral to the pharynx, the bulbus cordis is sectioned obliquely where it leaves the heart, and at this level gives off the second pair of aortic arches which connect dorsad with the descending aorta. Surrounding the bulbus cordis is the large pericardial cavity, not yet enclosed by the body wall. The student should note that in the sections so far studied, the mesenchyme of the head is undifferentiated, the tissues peculiar to the adult not yet having formed. The amnion attached to the right side of the embryo is folded. This is because the primitive amnion folds fuse over the original dorsal line, regardless of the turning of the embryo; consequently, on the right there is - slack. - .

Section through the Second Pharyngeal Pouches and Thyroid Anlage (Fig. 353). - As this section is taken at a level between the second and third aortic arches, the descending aortae and heart are unconnected. Tangential shavings have been cut from the walls of the otocysts. Extending laterally from the pharynx are the second pair of pharyngeal pouches which have already come in contact with the ectoderm to form closi)ig plates. A pocket-like depression in the midventral floor of the pharynx represents the thyroid anlage; later, it becomes saccular and loses its connection with the pharyngeal entoderm. The splanchnic mesodermal wall of the heart is destined to give rise later to the epi- and myocardium. A short distance caudad in the series, the large, looped ventricle is met; it is not attached by the former dorsal mesocardium.

Section through the Sinus Venosus and Common Cardinal Veins (Fig. 354). - At this level, the common cardinal trunk, formed by the union of anterior and posterior cardinal veins, opens into the thin-walled sinus venosus. The sinus receives all of the blood passing to the heart and is separated from the larger atrium by a slight constriction only.

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Fig. 353. Transverse section through the second pharyngeal pouches and thyroid anlage of twenty-seven-segment chick embryo. X50.

The descending aortae have united to form the single dorsal aorta. On either side of the pharynx are subdivisions of the coelom which will form the pleural cavities when the lung buds appear. These cavities are separated from the pericardial cavity by the septum transversum (anlage of diaphragm) in which the common cardinal veins cross to the sinus venosus. Since the last section, the myelencephalon has merged into the spinal cord, and the dermo-myotomes of the first mesodermal segments are seen. The mesodermal components of the amnion folds are not fused at this or subsequent levels.

Section through the Liver Anlage

(Fig. 355). - In this section, the fore-gut is flattened from side to side and its cavity is narrow. Ventrad, there are evaginated from the entoderm two diverticula which constitute the earliest anlage of the liver. On either side are sections of the vitelline veins (the left swinging in from the blastoderm), on their way to the sinus venosus at a higher level in the series. This primitive liver anlage does not always appear bilobed; at a slightly later stage it is found ventral to the united vitelline veins and a second anlage, more cephalad in origin, lies dorsal to the vein. Note the intimate relation between the entodermal epithelium of the liver and the endothelium of the vitelline veins. In later stages, as the liver anlages branch, there is a mutual intergrowth between the entodermal cells constituting the liver and of the vascular endothelium of the vitelline veins. Thus are formed the hepatic sinusoids of the portal system, which surround the cords of hepatic cells.

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Fig. 354. Transverse section through the sinus venosus and common cardinal veins of a twenty-seven-segment chick embryo. X 50.

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Fig. 355. Transverse section through the liver anlage of a twenty-seven-segment chick embryo. X 50.

The septum transversum is still present at this level, and lateral to the fore-gut are small body cavities. Lateral to the body cavities appear branches of the posterior cardinal veins.

Section through the Open Gut and Amnion Folds

(Fig. 356). - The intestine is now open ventrad, its splanchnopleure passing directly over to that of the vascular area. The dorsal aorta is again divided by a septum into its primitive components, the right and left descending aortae. Lateral to the aortae are the small posterior cardinal veins. The coelom is in communication with the extra-embryonic body cavity. Deep lateral body folds of somatopleure indicate how, by their ventral union, the body becomes established free from the blastoderm. The folds of the amnion have not joined, thus leaving the amniotic cavity open; (some variation may be found in the exact level where this condition occurs). In such a section, the somatopleuric components of the amnion and chorion are easily traced, and, a few sections cephalad, the manner of union of the two folds is illustrated.

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Fig. 356. Transverse section through the open gut and amnion folds of a twenty-seven segment chick embryo. X 50.

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Fig. 357. Transverse section through the seventeenth pair of mesodermal segments of a twenty-seven-segment chick embryo. X 50.

Section through the Seventeenth Pair of Mesodermal Segments (Fig. 357). - The body of the embryo is no longer rotated. On the left side of the figure, the mesodermal segment shows a der mo -myotome plate. The median and ventral portion of the segment is being converted into sclerotomic mesenchyme. On the right side, near the upper angle of the coelom, appears a section of the proncpliric (mesonephric) duct. The open space above it is the posterior cardinal vein; some sections show the median nephrotome tissue organizing into mesonephric tubule anlages. The embryonic somatoplcure is arched and will form the future ventro-lateral body wall of the embryo. The lateral infoldings of the somatouleure give indication of the later approximation of the ventral body walls, by which the embryo it separated from the underlying layers of the blastoderm.

Section through the Origin of the Vitelline Arteries (Fig. 358). - At this level, the embryo is more flattened and simple in Structure, as at higher levels in earlier embryos.

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Fig. 358. Transverse section through the origin of the vitelline arteries of a twenty-seven segment chick embryo. X 50.

Mesodermal segments, nephrotomes, and lateral layers of somatic and splanchnic mesoderm are little differentiated. The amniotic folds have not appeared. On the left side of thehgure, the vitelline artery leaves the aorta; on the right side, the connection of the vitelline artery with the aorta does not show, as the section is cut somewhat obliquely. The posterior cardinal vein is present just laterad of the right mesonephric duct. The small clusters of cells dorso-lateral to the spinal cord are the neural crests which will differentiate into spinal ganglia.

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Fig. 359. Transverse section through the segmental zone of a twenty-seven-segment chick embryo. X 50.

Section through the Segmental Zone

(Fig. 359). - The mesodermal segments are replaced by the segmental zone, a somewhat triangular mass of undifferentiated mesoderm from which later are formed the segments and nephrotomes. The notochord is larger the aorta smaller, and a few sections caudad they disappear. Laterally, the somatoplcure and splanchnopleure are straight and separated by the slit-like ccelom.

Section through the Tail Bud, Cranial to the Hind-gut (Fig. 360). - With the exception of the ectoderm, the structures near the median plane are merged into an undifferentiated mass of dense tissue, the notochordal plate. The cavity of the closed neural tube and its dorsal outline may, however, still be seen. Laterally, the segmental zone and the various layers are differentiated.

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Fig. 360. Transverse section through the tail-bud of a twenty-seven-segment chick embryo.X 50 .

Section through the Hind-gut and Primitive Streak (Fig. 361). - In this embryo, the caudal evagination to form the hind-gut has just begun. The section shows the small cavity of the hind-gut in the midplane. Its wall is composed of columnar entodermal cells and it is an outgrowth of the entodermal layer. A few sections cephalad in the series, the hind-gut opens by its own posterior intestinal portal. Dorsal to the hind-gut may be seen undifferentiated cells of the primitive streak, continuous dorsad with the ectoderm, ventrad with the entoderm of the hind-gut, and laterally with the mesoderm.

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Fig. 361. Transverse section through the hind-gut of a twenty-seven-segment chick embryo.X 50

Embryos of Three to Four Days

During the third and fourth days of incubation the chick attains a stage of development corresponding to the younger of the pig embryos customarily studied. It is advisable, therefore, to describe only such essential features of developmental advance in these older chick embryos as are necessary for introducing the detailed pig studies which follow.

External Form

The whole body shows the effect of torsion, and the embryo now lies on its left side (Fig. 362). The former flexures, especially the cervical, are pronounced, and new dorsal and caudal flexures have appeared; as a result, the embryo becomes so curved that its head and tail approach. The final number of 42 primitive segments is present and the body ends in a distinct tail. Up]ier and lower limb buds extend from the body wall, and the saccular allantois projects through the unclosed lower abdomen. Four branchial clefts show, separated by prominent branchial arches. The continued undercutting of the body folds, especially the more recent tail fold, has reduced the area of attachment with the yolk sac to a relatively narrow yolk stalk (Fig. 363).

Central Nervous System and Sense Organs

The five secondary divisions of the brain are easily identified; the telencephalon bears lateral hemispheres and the distinction between metencephalon and myelencephalon is now plain. Most of the cranial nerves and ganglia have begun j to appear (Fig. 362). From the roof of the diencephalon is the evagination of the epiphysis, in its floor the anlage of the neural lobe of the hypophysis (Fig. 363). The eye is a prominent organ, with its lens freed from the ectoderm if but with the narrowed chorioid fissure still showing (Fig. 362). The otic vesicle is a detached, closed sac from which the tubular endolymph duct is growing. Olfactory anlages, not seen hitherto, have appeared as ectodermal placodes on the ventro-lateral sides of the head; they are now depressed as olfactory pits.

Digestive and Respiratory Systems

The fore-gut and hind-gut are complete tubes, and the open mid-gut is a relatively short segment connected by the yolk stalk with the yolk sac (Fig. 363). As the pharyngeal membrane has ruptured, the stomodeum becomes an integral part of the mouth cavity. Y over pharyngeal pouches are prominent; in all but the fourth, the closing plates perforate and form temporary branchial clefts. At this time, the median, thyroid diverticulum loses connection with the floor of the pharynx. The trachea has arisen from a midventral groove which separates from the caudal end of the pharynx and bifurcates into two lung buds. The esophagus is a short, narrowed segment and the stomach a slightly spindle-shaped dilatation. Both liver anlages have fused into a branching mass and at the same level the pancreas is appearing.

Except for the attachment of the yolk sac. there are no additional features of interest above the caudal end of the hind-gut. Here the gut is separated from an ectodermal pit, the proctodeum, by a thin cloacal membranc which later perforates (Fig. 363). The mesonephric ducts join the hind-gut, and a stalked vesicle, the allantois, grows from its ventral floor. This common chamber, which receives the contents of the intestine and the secretions of the urinary and reproductive glands, is the cloaca.

Urinary System

The pronephric tubules disappear on the fourth day. Mesonephric tubules are still developing and consist of elongate, | coiled tubules associated with a glomerulus at one end and with the mesonephric duct at the other. The metanephros, or permanent kidney anlage, is just appearing; its collecting tubules and ureter arise as a bud from the mesonephric duct near the cloaca; the secretory tubules will develop from caudal nephrotome tissue.

Vascular System

The ventricular loop has moved caudad and the atrial region cephalad, thus reversing the original positional relations of these parts (Fig. 362). Both atrium and ventricle show external indications of a beginning division into right and left chambers, and the myocardial wall is assuming the characteristics of muscle cells. As a whole, the heart has sunk caudad considerably from its early cephalic position.

Below the heart, the primitive aortae are fused throughout their lengths. Since the second day, a fourth, a rudimentary fifth, and a . sixth pair of arches have developed; of the full set, only the third (carotid), fourth (aortic), and sixth (pulmonary) arches remain. The cardinal veins are well developed, and the paired vitelline arteries and veins have both , fused inside the body into single vessels. New umbilical arteries pass to the allantois, and umbilical veins return the blood by way of the lateral body wall to the heart. These will become still more important in the mammal.

Extra-embryonic Membranes

During the third day the tail-fold of the amnion develops, and soon the embryo becomes enclosed by the fluidfilled amnion sac which is protective in function (Figs. 37 and 349); the chorion, formed by the same process, but of little significance, ultimately surrounds the embryo and all extra-embryonic structures. Much of the yolk sac is covered by advancing splanchnopleure which is continuous over a relatively narrow yolk stalk with that of the gut (Fig. 363). As the embryo elongates, the yolk stalk appears relatively narrower. Through the vitelline vessels the yolk supplies all the food material for embryonic growth. The allantois arises late in the third day as a diverticulum of the splanchnopleuric floor of the hind-gut (Figs. 349 and 363). It later becomes a large, stalked sac occupying the space beneath the shell. Umbilical vessels ramify in its walls and the allantois serves as the principal organ of respiration and excretion.

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   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Cite this page: Hill, M.A. (2024, February 28) Embryology Book - Developmental Anatomy 1924-16. Retrieved from

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