Book - A Laboratory Manual of Vertebrate Embryology (1947) Chicken

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Adamstone FB. and Shumway W. A Laboratory Manual of Vertebrate Embryology. (1947) John Wiley & Sons, London.

   A Laboratory Manual of Vertebrate Embryology 1947: Frog | Chicken | Pig
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A Laboratory Manual of Vertebrate Embryology

Part II Anatomy of Chick Embryos

The Germ Cells And Fertilization

The Ovum

The egg cell or ovum of the hen is the portion of the egg usually referred to as the yolk. It consists of a minute island of cytoplasm, the germ spot or germinal disc, floating on the yolk, which is a large ball of stored-up food material. The whole egg cell is enclosed in a very thin, delicate vitelline membrane. In addition to the egg cell proper, the egg, as we ordinarily know it, is made up of several layers of albumen surrounding the yolk; and these in turn are enclosed in tough shell membranes lining the hard calcai^ous shell (Fig. 16).

Fig. le. Diagrammatic vertical section of a fresh hen’s egg. From "Poultry Production,” courtesy I'- E. Card and the pubUthen,

Lea and Febiger.

The yolk is laid down in concentric alternating light and dark layers around a central mass of white yolk called the latebra. This extends upward to the surface as a stalk of white yolk, where it spreads out as the isthmus of Pander, beneath the germinal disc. The innermost layers of albumen become twisted into coiled strands, the chalazae, which extend out from the yolk toward the ends of the egg. The parchmentlike shell membranes are separated from one another at the blunt end of the egg, thus forming the air chamber.

The albumen, shell membranes, and shell are membranes added to the egg during its passage down the oviduct. The albumen is a source of protein food material for the growing embryo, and the shell membranes and shell have a protective function.

It is int^sting to note that a great deal of the stored-up yolk material is still present at the end of the period of incubation, when it is resorbed; and hence it is available as nutrient for the newly hatdted chick.

Eggs vary greatly in size and in coloration, but the yolk is about 40 mm, in diameter and the germinal disc about 4 mm.

The male germ cell or spermatozoon, in contrast to the female reproductive cell, is a v&y minute body of microscopic size. It consists of an elongate head and a long, slender, filamentous tail which merges almost imperceptibly with the head (Fig. 17). The head, which is made up largely of nuclear materials, is surmounted by a slightly recurved acrosome.

1 Fertilization

ft-Head Tjjg ovary of the laying hen contains numerous ova in various stages of development, i . The rate of growth of the vast majority of them is very slow and they are very small in con B I sequence. In succession, certain ova, a few at a time, begin to increase rapidly in uze, so

I / that in the last 5 to 8 days of development fully 99% of the yolk material is added to each

I 1 egg. Then, when the maturing egg has completed its growth, it is set free from the follicle

/ V-Taii ovary in which it developed and is picked up by the funnel of the oviduct. Imme / y diately after it is liberated into the oviduct it is surrounded by fluids which contain myriads

/ /0\ of spermatozoa received from a male in a prior copulation. Some of these enter the egg cell,

C/ / ) and one fertilizes it. The remainder migrate to the periphery of the germ spot where they

/ J ultimately disintegrate.


Pio. 17. Sper- The processes of cleavage and gastrulation in the hen’s egg take place during the ”*he*fowl passage of the ovum down the oviduct. Cleavage, from its earliest stages, is quite irregular and, since it involves only the tiny iidand of cytoplasm lying on the yolk, a blastula is soon formed consisting of a thin, discoidal sheet of cells separated from the yolk by a fluid-filled cavity, the blastocoel. The process of gastrulation follows immediately after cleavage and results in the formation of a two-layered embryo. The exact manner in which this is brought about is very difficult to demonstrate. Until recently it was considered to be a pr;oce8s of involution whereby the cells along a particular portion of the margin of the blastodisc (the dorsal lip of tbe blastopore) rolled under the original layer. Lately it has been shown that the process involves polyinvagination, that is, numerous small grooves appear on the surface of the blastodisc and at these points cells insinuate themselves beneath the surface layer, From these first migrants, strands of cells are produced, which spread out and coalesce to form a continuous sheet of endoderm beneath the original surface layer, which is now composed of presumptive ectoderm and mesoderm (Fig. 18).

a 6 c d '

Fn. 18. Cleavage and endoderm formation in the chick, a, h, early cleavage; e, endoderm formation by pdyinvagitution: email aolid lines represent furrows along which future endoderm invaginates and spreads out (stippled) to form the completed sheet , of underlying endoderm, d. *

1 A. M. Dalcq, "Form and Causality in Early Development,” Cambridge Umvetoty Preee,. 1938.

2 Uncertainty stUl exists regarding the true explanation of gastrulation in the dack. Paateels, who was quoted 1^

has returned to view that a form of delamination prevails, resulting in the eepuratioa of smi^ groups of endodenga edlli. which then coalesce to fonn a sheet. Jeui Pasteels, Biological Bevieu), Vd. 13, 1940.


When development is resumed during incubation, the blastoderm begins to grow out rapidly over the yolk and segregation of the three germ layers, from the two-layered gasirula, begins. This involves an intermediate step, namely, the formation of the primitive streak. This structure has the appearance of a thickened line on the blastoderm. Until recently it was considered to be due to concrescence, a process involving the forcing together of the two halves of the dorsal lip of the blastopore. Actually it appears to be formed by convergence of surface material toward the future primitive streak accompanied by complicated streaming movements of cells in adjacent surface areas. ^ The whole effect, therefore, is to force together cells from the right and left sides of the blastodisc as a linear thickening, and to allocate certain cell groups to special regions of the blastoderm where they are destined to give rise to particular parts of the embryo. Some of the cells which move to the primitive streak invaginate and spread out between the endoderm and surface layer, thus forming an intermediate layer of mesoderm (Fig. 19). The surface layer which is left behind by this process forms the ectoderm. The mesoderm spreads out on each side of the primitive streak as a broad winglike sheet extending laterally and forward. At the same time a rod of cells which forms the notochord also grows forward from the anterior end of the priniitive streak in the ffs-me manner as the lateral sheets.

Fig. 19. Formation of the primitive streak and mesoderm in the chick, a, the eastmla; b, convergence of surface cells toward future primitive streak; c, d, lateral mesoderm and notochord extending outward from the primitive streak. Mesoderm heaviljr stippled, pninilivc streak solid black.

Stages of Chick Embryos

The traditional stages in the development of the chick {Gallus domesticus) studied in laboratory practice are those at the end of each of the first three days of incubation. In addition, the 18- and 33-hour stages are often included because of certain important developments. It is a well-known fact, however, that embryos of the same age often attain varying degrees of development during these first few days of incubation, and, although the length of the embryo has been proposed as a mark of identification, the flexures of the body make this standard impracticable. The remaining alternative is to select some particular structure as a basis of classification. For this purpose the number of somites, suggested by Lillie, is the mast practical. Even here it must still be remembered that, on account of the effect of temperature upon the rate of development, the number of somites is not correlated exactly with the number of hours of incubation. Hence, the stages of different authorities vary somewhat as may be seen in the following table.






About 24 hours

24 hours 6S

24 hours 7-8S

25 hours 78

27 hours 88

AtMUt 33 houTB

38 hours 16S

32 hours 6S

33 hours 12S

33 hours

About 48 hoiUB

48 boors 27-28S

52 hours 27S

48 hours 278

55 hours 298

ikboirt 72 hours

68 hours 378

67 hours 35-378

72 hours .358

72 houre 368

' 04t«% and Oiaistlil^ ia S^rly DevdojKnent,’' Cambridge Univereity Press, 1938.


After 18 hours of incubation the chick embryo begins to take definite form. The early part of this period is occupied by the process of gastrulation and the formation of the third germ layer, or mesoderm. Consequently, at this stage the blastoderm has the form of a small round plaque of tissue about 1 cm. in diameter. In fixed and stained blastoderms there can be seen a goieral division into a small, dear, inner area, the area pellucida, and a broad, outer, heavily stained margin, the area opaca.

The following parts of the embryo can be distinguidied:

Head fold. A heavily staining crescentic ridge of blastoderm near one end of the area pellucida. Primitive streak. A heavily staining thickened streak tailing off away from the head fold toward the opposite side of the area pellucida. The primitive ridges constitute the lateral margins of the streak, and between these is seen the sunken primitive groove. The anterior end of the streak is marked by the nodelike primitive knot.

Notochord. A rodlike structure composed of mesoderm extending forward from the primitive knot toward the head fold can be seen in favorable specimens.

Laboratory Directions

Study a stained total mount of an 18-hour embryo and identify the structures described above. Make a drawing of this embryo 2 inches in diameter and label all parts.

Total Mount of ui 18-Hour QiicJc Embryo.

24-HOUR EMBRYO Body Form

In the 6-hour interval of incubation which has elapsed since the 18-hour stage, considerable change has occurred in the embryo:

The head fold has become much more prominent and appears as a flattened finger-shaped structure protruding forwani from the blastoderm, beginning at the level of the anterior intestinal portal (Fig. 20). The body is well marked out, and a number of paired blocklike somites have been formed on each side of the mid-line in the space between the head fold and the primitive knot.

Fio. 20. Diagrammatic lateral view of a 24-hour chick embryo represented as having the overlying ectoderm removed to show relative position of brain, notochord, and gut. The side wall is cut away from the mid-gut so as to show the anterior intestinal portal, which is indicated by an arrow.

The blastoderm surrounding the chick shows several characteristic developments:

The area immediately surrounding the embryo, which is somewhat slipper shaped in general outline, is lighter and clearer in appearance and forms the area pellucida. Beyond the area pellucida the blastoderm is more opaque and hence is called the area opaca. In this region, particularly where it adjoins the area pellucida, the blastoderm is splotchy in appearance owing to the development of heavily staining scattered aggregates of cells. These are blood islands, and the whole region is known as the area vasculosa. It is terminated laterally by a definite margin, which later forms a blood vessel, the sinus terminalis. Beyond the sinus terminalis the blastoderm continues as the area opaca externa.

Ectodermal Derivatives

The ectoderm which forms the outer covering of the dorsal side of the head fold has already begun to differentiate into the beginnings of the neurid tube. This is brought about by the development of a furrow extending longitudinally down the dorsal side of the body. This furrow is bounded laterally by the neural ridges, and the depression between them is the neural groove. As the depression deepens the neural ridges ultimately meet in the mid-dorsal line, thus forming a closed tube, the neural tube. This process begins in the anterior end of the head fold and gradually works backward, so that at 24 hours the neural tube is usually closed as far back as the posterior end of the head fold. Behind this point the neural ridges gradually open out so as to form the boundaries of the original neural groove. This process, which involves the sinking-in and closing-off of the tissues in a certain area, such as the surface ectoderm here, is known as invagination.

It is to be noted that at this stage the anterior end of the neural tube still retains a small opening to the outside which is known as the anterior neuropore. Some differentiation may already be apparent in that portion of the neural tube within the head fold to the extent that the anterior end of the tube, the IpretHam, has prtnninent lateral bid^ indicating fonnation of the optic vesicles.


Endoobbmal Dbbivativiib

Tlie endoderm, which lies next to the yolk, protrudes up into the head fold in the form of a flattened fihgerlike pouch constituting the fore-gut. This lies beneath the brain, which is developing as described above. The fore-gut is a tubular structure communicating with the mid-gut by way of the anterior intestinal portal (Fig. 20). The mid-gut is open ventraUy over the yolk, and, by contrast with the foregut, lacks a floor.

Mesodermal Derivatives

The notochord extends forward from the anterior end of the primitive streak, beneath the neural tube, and up into the head fold in the form of a solid, cylindrical rod. Its anterior end frequently appears to merge with the neural tube. The lateral sheets of mesoderm which extend alongside the notochord do not extend into the head proper but instead proliferate a delicate netlike tissue, the mesenchyme, which grows up into the head and occupies the space between the developing organs already present. The lateral sheets do not meet in front beneath the head fold, and consequently there is no mesoderm in this region, which is known as the proanmion. In the region of the body proper, the portions of the lateral sheets adjacent to the notochord become divided off transversely into paired blocks known as somites. Little differentiation in the individual somites is evident at this stage beyond the fact that a cavity, the myocoel, is present in the center of each. Six pairs of somites are usually present at this stage. Lateral to the somites the mesoderm is separated tangentially into two sheets of cells; the upper in contact with ectoderin is the somatic mesoderm, and the lower in contact with endoderm is the splanchnic mesoderm. The cavity between these layers is the exocoel — a lateral extension of the coelom. In the splanchnic mesoderm, cell aggregates forming the blood islands are to be found.

Laboratory Directions

A. Study a stained total mount of the 24-hour chick and make a drawing of the embryo showing detailed structure in the body proper and in a narrow strip of the blastoderm. The body should be drawn approximately 4 inches long. Label all parts.

B. Study through the transverse serial sections supplied and make a set of drawings of sections corresponding to the following levels: (1) forebrain, (2) anterior intestinal portal, (3) a somite, (4) primitive streak. Label all parts.

33-HOUR EMBRYO Body Form

After 33 hours of incubation the embryo has increased in size to a little over 4 mm. The head fold is considerably longer and bends downward slightly over the anterior end of the notochord. The body ^proper has also increased in length, and at this stage about twelve pairs of somites are present. Thickened ridges, extending out laterally just in front of the anterior intestinal portal, mark the beginnings of the vitdline veins. In the area vasculosa the blood islands are very prominent, and there is some indication that they are beginning to coalesce to form the blood vessels of the yolk sac.

Ectodermal Derivatives

Hie developing neural tube has closed over throughout almost its entire length. The anterior neuropore has usually been obliterated, but at the extreme posterior end the neural folds still remain broadly open, producing the large rhomboidal sinus. The brain shows three prominent vesicles at the anterior e^, the pros^cephalon, mesencephalon, and rhombencephalon, named in order posteriorly. Behind the !dhom]()ei9^»])halmi a varied number of small constrictions are apparent. The sides of the forebrain or

(1) Section through the forebrain.

(2) Section through the anterior intestinal portal.

(3) Section through a somite.

(4) Section through the primitive streak.

Transverse Sections of a 24-Hour Chick.

prosencephalon show prominent lateral bulges, which are the optic vesicles. On the sides of the head fold alongside the rhombencephalon the auditory placodes form thickened ectodermal areas which later invaginate to become the otic (auditory) vesicles.

Endodermal Derivatives

Little change has taken place in the endodermal derivatives except that the fore-gut, extending up into the head fold, has grown in length along with the head fold.

Mesodermal Derivatives

The notochord is still present as a median rodlike structure beneath the neural tube. The lateral sheets of mesoderm are blocked off into twelve pairs of somites alongside the notochord. Mesenchyme is present in the head as at 24 hours.

In the region of the anterior intestinal portal the splanchnic mesoderm lying alongside the lateral walls of the intestinal portal bulges out laterally into the body cavity so as to leave a narrow space between it and the endoderm. This space, on each side, is known as the amniocardiac vesicle. It is occupied by a delicate tube formed from cells proliferated off the adjacent splanchnic mesoderm. In front of the anterior intestinal portal these two tubes fuse, and ultimately the dividing wall between them breaks down. In this manner the inner lining or endocardium of a single-chambered, tubular heart is formed. In a similar manner the thick, overlying bulges of splanchnic mesoderm also meet and fuse above and below this tubular structure so as to form the thicker epimyocardium of the heart and the supporting dorsal and ventral mesocardia. It will be noted, therefore, that the heart arises from two fused primordia, and consequently the vessels which grow forward from the heart are also paired.

These tubes .growing forward from the heart are the ventral aortae. They are located beneath the fore-gut, and as they grow forward from the heart swing dorsally around the anterolateral ends of the fore-gut as the first aortic arches. They then course backward, dorsal to the gut, as the paired dorsal aortae. The actual extent to which this development has progressed in 33 hours varies with individual specimens, but at least the heart and vitelline veins are uniformly present.

Laboratory Directions

A. Study a stained total mount of the 33-hour embryo. Make a drawing stressing particularly the form of the brain and also what can be seen of the developing circulatory system.

B. Study through the series and make drawings as follows: (1) through the region just in front of the anterior intestinal portal showing the heart; (2) through the region of the anterior intestinal portal to show the vitelline veins.

48-HOUR EMBRYO External Form

IVom the 33-hour to the 48-hour stage the body of the chick has increased greatly in size and its shape has been altered profoundly. The increase in size is due largely to the growth of the head and the addition of more somites to the body so that its length is now about 7 mm. and the number of somites is approximately twenty-six. The change in form is brought about by the development of several bends or flexures in the head fold and by twisting or torsion in the body proper so that Ihe anterior portion lies mostly on its left dde. As a result of this change in form, the head is more readily accommodated in the space available beneath the ^ell since it lies flat on the yolk. Moreover, the anterior end of the head is directed almost straight backward because of the cranial flexure, developed in the mid-brain at 33 hours, and the newly devdoping cervical flexure in the region of the neck. Twisting or toraion

(1) Anterior end of 33-hour chick showing heart and forebrain.

(2) Section through the heart.

(3) Section through the anterior intestinal portal and vitelline veins.

33-Hour Cbick Embryo.


occurs at the level where the head fold rises from the blastoderm. Hence the posterior end of the body lies in its original position with the ventral side down. At the extreme caudal end the tail fold is being marked but very much in the same manner as that by which the head fold originated.

The head at this stage shows five prominent bulges corresponding to the divisions of the brain. These are the telencephalon at the anterior end, followed by the dicncephalon, the mesencephalon, the metencephalon and the myelencephalon (in the order named). The myclencephalon tapers backward into the spinal cord. The beginnings of the paired sense organs are also present, including the nasal pits on the ventrolateral surface of the head associated with the telencephalon, the eyes on the sides of the head at the level of the dienccphalon, and the otic vesicles alongside the myelencephalon on the dorsolateral body wall. Just posterior to the head on the lateral walls of the head fold three transverse slitlike openings can be seen. They are visceral clefts I, II, and III (in the order named). They mark the pharsmgeal region of the embryo. Protrudmg out veiitrally from the body at about this level is the prominent heart.

Fia. 21. Development of the amnion and chorion in the 48-hour chick.

Amnion and Chorion (Fig. 21)

The anterior part of the body of the chick is covered by a fold of somatopleijrc (ectoderm -f somatic mesoderm), which is thrown up over the body, beginning first in front of the head fold in the region recognized earlier as the proamnion. Mesoderm, however, has grown into this region in the meantime. Since the fold is reflected back out over the yolk sac again, two independent coverings are formed, namely, the amnion, which lies next to the embryo, and the chorion outside the amnion. The amnion contains a cavity, the amniotic cavity, which becomes filled with amniotic fluid. The space between the amnion and chorion represents an outward extension of the coelom calleil the exocoel.

Laboratory Directions

Study carefully a stained total mount of the 48-hour chick, and make a drawing of the embryo blowing detailed structures. The body of the embryo should be drawn approximately 6 inches long. Label all parts.


The circulatory system of the 48-hour chick represents a feature of fundamental importance. It should, therefore, be studied with the greatest care in both injected total mounts and in serial sections. The system is of mesodermal origin. Most of the details described below can be readily identified in injected specim^s, and after they have been worked out ^ould be recorded by a drawing. â–


As pointed out in the study of the 33-hour chick, the heart develops from paired primordia, the jppeic^ €»ids of v^hich reineseat the stumps to which the vitelline veins from the yolk sac will ultimately attach themselves. The tubular structure lying in front of the anterior intestinal portal grows much more rapidly than the space in whidi it is accommodated. It adjusts itself to this utuation by:

(1) Bendmg outward to the right (as at 33 hours) (Fig. 22).

(2) Looping so that the posterior end is pushed up under the anterior end (as at 48 hours) (Fig. 22). Furthermore, a certain amount of differentiation in the character of the walls of the chambers of the

heart has occurred at this stage so that we may recognize:

SiniM Venosus. The thin-walled posterior end situated in the mid-line which serves as a general receiving chamber. Since this region receives the fused vitelline veins, it can be readily recognized by locating these vessels alongside the anterior intestinal portal and tracing them, after they have fused to become the meatus venosus, forward into the heart.

Fra. 22. Stases in the development of the heart of the chick.

Atrium. Thin-walled and receiving blood from the sinus venosus. This chamber lies anterior to the sinus venosus but swings posteriorly to join the ventricle.

Ventricle. Thicker-walled with a developing reticulum of muscle bands, lined by a delicate endothelium and receiving blood from the atrium. The ventricle constitutes the posterior portion of the loop formed by the heart,

Bulbus Arteriosus. Thick-walled, round or oval in cross-section and possessing a delicate endothelium. This chamber runs forward from the ventricle and swings toward the mid-ventral line just below the visceral arches and clefts of the pharyngeal region.

Ventral Aortae. The blood leaves the bulbus arteriosus by way of the ventral aortae, which at this stage have almost ceased to exist, owing to their failure to keep pace with the growth of related vessels.


Aortic Arches. From the ventral aorta the blood was originally carried to the dorsal aorta by means of paired vessels swinging around the front end of the fore-gut and constituting the first aortic arch. These vessels still persist as the paired first aortic arch flowing through the first visceral arch, which has been established by the demarcation of the first visceral cleft. In addition, paired aortic arches have developed in the second and third visceral arches. These are the second and third aortic arches, respectively. They also course upward from the ventral aorta to join the paired dorsal aortae.

External Carotid. A minute vessel located at the point of origin of the first aortic arch. It can sometimes be seen to extend into the base of the first visceral arch where it breaks up into a capillary network.

Internal Carotid Arteries. From the point where the first aortic arch and the dorsal aorta meet, the dorshl aorta attends forward into the head as the internal carotid artery to supply a capillary network to the brain.

Dorsal Aortae. The dorsal aortae are still paired in the region of the pharynx, and each vessel is often spoken of as a "radix” of the dorsal aorta. At id)out the levd of the heart they fuse to form a single dorsal aorta. This opens out again to the paired -condition slightly anterior to the level at which the vitelline arteries flow out to the yolk sac.

Vitelline Arteries. These course out laterally over the yolk sac, from the dorsal aortae, about the level of the twenty-second somite.

Caudal Arteries. The backward prolongations of the dorsal aortae beyond the vitelline arteries form the caudal arteries.

Intersegmcntal Arteries. Since the body is growing rapidly by the lengthening of the head fold and the addition of somites to the trunk, a series of intersegmental arteries develop to supply its nutritional needs. These vessels are paired, arising from the dorsal aorta (or aortae) and course outward and dorsally between the somites.



The blood distributed to the yolk sac by the vitelline arteries is returned to the heart by the vitelline veins. The latter converge toward the region of the anterior intestinal portal, where the two main trunks fuse to form the meatus venosus just before entering the sinus venosus. The vitelline arteries and veins anastomose freely over the yolk sac, but large trunks from both arteries and veins communicate with the sinus terminalis encircling the blastoderm.

B. SOMATIC vessels: the cardinal system

Since a great increase has occurred in the amount of blood supplied to the developing body, a new set of veins, the cardinals, develop to drain blood back to the heart. These are laid down following the general plan of the letter H — ^modified, of course, to the contours of the body.

Anterior Cardinal Veins. These vessels drain the head arising from the capillary plexus of the internal carotids over the brain. They course backward, following the ventrolateral angles of the neural tube until they have just passed the level of the otic vesicles. At this point they begin to swing ventrally but pass lateral to the dorsal aorta to join the common cardinals.

Common Cardinal Veins (Ducts of Cuvier). These vessels (also paired) receive blood from the anterior and posterior cardinals and empty into the dorsolateral angles of the sinus venosus.

Posterior Cardinals. These vessels drain the somites of the posterior end of the body coursing lateral to the dorsal aortae and in close association with the developing mesonephroi, or embryonic kidneys.

Intersegmental Veins. The blood distributed by the intersegmental arteries is returned to the cardinals by way of the intersegmental veins which lie between somites and anastomose with the intersegmental arterioles.

NOTE: One of fhe most oatstandinc features of tfae dereloping drcnlatofy system is tibe fact that aU ^

origboal system are paired and fliat practically aU new additions t6 tite aystem are also paired. Futnra detdopmant dapenda on elimination of certain parts cS fhe system and general simplification.

Laboratory Directions

Using injected 4fl-hour total mounts, study the circulatory system and make a diawh^ showiDg fhe vessels described above. Label all parts.

Refrbsentative Sections of the 48-Houb Embryo *

A. Section through the upper part of head (Fig. 23)

This section was taken through the head at a level just below the floor of the brain in the region of the cephalic flexure. For this reason it shows the mesencephalon (9) as a thick-walled rounded structure quite separate from the thin-roofed myelencephalon (1). In the mid-line between the two is a amftll portion of the notochord (4). Alongside the left ventrolateral margin of the myelencephalon is a heavy condensation in the mesenchyme (2). This is the semilunar ganglion of the trigeminal nerve (V). The nerve proper (3) can be seen leaving the lower side of the ganglion. The other trigeminal

PiQ. 23. Section through the head of a 48-hour chick at the level of the mesencephalon.

nerve (11) has been cut so that it appears as a small band of fibers in the mesenchyme on the opposite side. The anterior cardinal veins (12) also lie along the ventrolateral margin of the myelencephalon, and the one shown on the right side (10) extends forward toward the mesencephalon. Small branches of this vein (13) are located alongside the myelencephalon. Closely applied to the surface of the mesencephalon are a number of small blood vessels (5, 6) which represent portions of the capillary plexus of the internal carotid artery supplying the brain. Surrounding the head are: the amnion (15), which f(»ms a closed sac; the chorion (14), which lies outside the amnion; and the yolk sac (16), also covered by the chorion and containing numerous blood vessels (17). The amniotic cavity (8) and the exocoel (7) are also shown.

B. Section through the upper end of aortic arch I at the point of origin (rf internal carotid arteries (Fig. 24)

In this section the head is cut twice, owing to the cranial flexure, and the postericH* end is almost completely separated from the anterior end. In the posterior end note the thin-roofed myelencephalon (1), alongside which lie the otic vesicles (2, 20). Owing to the tilt in the plane of section, the left (2) appears to be just invaginating but the right (20) appears as a closed vesicle. Associated with the left otic vesicle is the acoustic ganglion (3) of the auditory nerve VIII. The anterior cardmals (19) lie at the ventrolateral angles of the myelencephalon; the notochord (4) is ventral to it. Immediatdly above or dfsrsal to the gut are the paired dcmal atnrtae (18)

All aeetions are shown exactly as they appear in the mierosoope. DeseripticmB i^idy coly to the Modoas Slostiated in tbemannal. The student wiU probably enoonnter soine variations in the seetioai studied hy him.

In the anterior portion of the head the brain is represented by the thick-walled, oval-shaped diencephalon (12) which is constricted ventrally to form the infundibulum (13). Beneath the infundibulum is Rathke’s pocket (9), which represents an evagination of the stomodeum, and, in association with the infundibulum, forms the hypophysis or pituitary body. Extending alongside the fore-gut (8) is a pair

Fia. 24. Section through the head of a 48-hour chick at the level of the eye and ear.

of elongate blood vessels. These are the internal carotids (16) and the first aortic arch (17) combined. Small branches of the carotids (11) lie close to the brain. Branches of the anterior cardinals (10) are evident in the lateral areas. The right optic cup (14) is cut so that it shows as a hollow structure containing the solid lens primordium (15). The pharynx shows two lateral outpocketings which form the first visceral pouches (6). Anterior to the first pouch on the left side, the head shows a distinct bulge which represents the first visceral arch (7), in which the first aortic arch flows. The first visceral groove (5) is to be seen on the side of the head.

Fio. 26. Section through the heart of a 48>hour chick.

C. Section through the body proper and heart (Fig. 25)

At thk lev^, and throuffhout the rest of the series, four typical structures are to be seen in the midline airanged fhim deseal to ventral, as follows: spinal cord (1), notochord (2), dorsal aorta (3), gut

(4). Lateral to the spinal cord the mesoderm of tlie somites is differentiated into the sclerotomes (15) and the thicker dermatomes ( 14) . Lateral to the gut are. two cavities which represent forward extensions of the coelom (13). The left, in this section, communicates directly with the exocoel. In Hie ventrolateral extensions of the body wall are large paired blood vessels, the common cardinals (12), and of these the right communicates with the sinus venosus of the heart. The parts of the heart are arranged in order forming a loop and marked off by slight constrictions, as follows: sinus venosus (5), atrium (6), ventiide (7), bulbus arteriosus (10). Note the thin endothelial lining (9) of the bulbus and ventricle contrasting with the thicker outer epimyocardium (8).

D> Section through the mid-body at the level of origin of vitelline arteries (Pig. 26)

At this level the thick-walled neural tube (10) which lies below the surface ectoderm is somewhat elliptical in outline. The notochord lies beneath the neural tube. The dorsal aorta (3), paired at this

Fia. 26. Section through the body of a 4S-hour chick at the point of origin of the vitelline arteries.

level, lie beneath the notochord but slightly lateral in position. The vitelline arteries (1) course outward laterally from them. The endoderm of the wide-open mid-gut (2) forms the basal layer. Lateral to the neural tube are the paired somites. In these we may distinguish a heavy platelike dermatome (11) closely associated with the surface ectoderm, the poorly defined sclerotome (8) lying next the spinal cord and notochord, and the myotome (9), or somite proper, forming an inner hook on the dermatome. The nephrotbme has already differentiated into the nephric duct (5) and tubules (4) ; the duct being thick-walled and circular in outline while the tubule is ventromedial to it and irregular in section. The lateral sheets of mesoderm are divided into the upper somatic layer, which is associated with the ectoderm to form the somatopleure (7), and the lower splanchnic layer associated with endoderm to form the splanchnopleure (6). The exocoel (12) lies between these layers.

Fig. 27. Section through the tail fold of a dS-hour chick.

£. Section through base of tail fold (Fig. 27)

The tail fold has begun to protrude above the surface of the blastodmn in a manner mmllar to that of the head fold. Into it a blind extension of the endodermal^tube) the him^EUt (3), wi!^ and at this level the postmor intestinal portal, whidi repremnts the point of mid-gut, is to be seen. The bulk of the tail fold is made up of a solid mass of ectodermal (5) and mesodermal cells (4). Ectoderm (6), somatic mesoderm (7), splanchnic mesoderm (1), and endoderm (2) are clearly recognizable.

Laboratory Directions

Study the serial sections provided and find, draw, and label sections illustrating the following levels;

(1) Section through the head showing developing eye or ear (Fig. 24).

(2) Section through the heart (Fig. 25).

(3) Section through mid-body region showing the amniotic raphe (which is the level at which the lateral folds of the amnion mfeet above the embryo).

(4) Section through the tail fold (Fig. 27).

Study of Serial Sections

Methods. It should be quite obvious, from the work already done in drawing and identifying structures in a few representative sections of an embryo, that a thorough knowledge of its anatomy and the relations of various parts can never be obtained by such a procedure. set of serial sections consists of a series of slices taken in order through the embryo from the tip of the head to the end of the tail. Consequently every structure in the embryo will be represented in a certain group of sections in some particular part of the series. As a result, it is possible to build up a mental picture of any particular structure and its relation to surrounding structures by tracing it, forward and backward in the series, and studying it independently of, or in relation to, associated structures.

It is highly desirable, therefore, that the student sliould now attempt to substitute this method of study for the time-honored process of making drawings, except, for example, in the case of the total amount. The drawings already made have served a useful purpose in making the student familiar with the general appearance and structural plan of such organs as the neural tube, gut, and blood vessels.

Orientation of the Sections (.Fig. 28). Before beginning to study a series of sections it is dways advisable to try to get some conception of the plane of section ABCD in relation to the longitudinal axis of the body x-y. It should be realized that no two series are absolutely identical because the plane of section may be tilted in many ways from left to right or from front to back. It is possible, for example, to cut the eye on one side of the head and miss it altogether on the other. The use of a ruler laid across a total mount drawing will often enable the student to appreciate the plane of sectioning.

Records. As the study progresses a record should be kept of the various structures encountered. The most convenient method is to list the location of the structure as to slide, row, and section alongside the name of the structure in the manual, thus:

1/6/17: Thyroid gland.

Since most structures persist through many sections, it is not necessary to state the exact sections in -whidi they occur but <^y the slide and row.

It is also desirable to make small sketches of the structures identified as well as their immediate ^rtoandmgs, ^ese drawinp wre not intended to be elaborate but should be placed in the manual in provided wherever an asterisk occurs a^ should include enough of the adjacent structures

Fio. 28. Diagram illuatrating the relation of the plane of section ABCD to the longitudinal axis of the body x~v

(1) Section through the head.

(2) Section throu^ the heart.

48«Hour Chick Embryo.


(3) Section through mid-body region.

(4) Section throng the tail fold.

48-Hour Chidr Embryo.


In beginning the study of a set of sections of the 48-hour chick, the amnion, chorion, and yolk sac show to advantage in the first few sections through the head. These structures may always be recognized by the following characteristics.

Amnion. A thin membrane composed of ectoderm (inner layer) and somatic mesoderm (outer layer). It lies next to, and completely surrounds, the body but is separated from it by a space, the ammotic cavity.

Chorion. A thin membrane also composed of ectoderm (outer layer) and somatic mesoderm (inner layer) forming a second covering layer outside the amnion. The space between amnion and chorion is the exocoel.

Yolk Sac. A thin membrane composed of endoderm and splanch nic mesoderm lying outside the amnion. It may be distinguished from the chorion by the fact that its mesodermal layer contains vitelline blood vessels, whereas none are present in the chorion.


BPmERMis. A single layer of ectoderm cells forming the outer covering of the body.

BRAIN AND SPINAL CORD. In Working down into the series the first prominent structure encountered is the brain. ^ Learn to recognize its parts by the following diagnostic features:

MyelencephcAon. Very thin roof. Associated with otic vesicle.

Metencephalon. Located in front of the myelencephalon but behind or posterior to the narrow isthmus which separates mesencephalon and metencephalon.

Mesencephalon. Located in front of the isthmus. In cross-section it is thick-walled and almost always round.

Diencephalon. Associated with the eye.

Telencephalon. Associated with the nose. Located at the anterior end of the neural tube.

Spinal Cord. The thick-walled, narrow, tubular extension of the neural tube posterior to the myelencephalon .


Otic Vesicle. Situated alongside the myelencephalon. Has the form of a thick-walled pit of ectoderm widely open to the outside of the head.

1 It is well to remonber at this point that as you get deeper into the series the front end of the neural tube wiO eventhaUy separate from the hind end, owing to the flexing of the head. This also applies to other strueturee such as the notoehoid and tlw digestive tube. Hence it may be necessary to follow oertain structures into the front end of the bea d first and then i^ieat the study, tracing them posteriorly.

> The organs of special sense follow a general plan in eariy development. They axe first set aside as rhiiaawied -i wfa^ M tl plates or placodes which undogo invagination. For the optic cop, however, the proeess is complicated by the fact tjyff Ihe sense {decode is first taken in with the invapnatii^ bnin and is later evagbated as the bpfie vm^. fWibr eutor surface of the optic vesicle is invaginated to form the oprio cup.


Optic Cup and LeP&. Situated lateral to the diencephalou. The developing eye is made up of two primordia, as follows :

Optic Cup. A double-walled, cup-shaped structure attached to the diencephalon by the narrow optic stalk.

Lens. A thick-walled, pitlike invagination of surface ectoderm whidi will ultimately occupy the mouth of the optic cup.

Olfactory (Nasal) Placode. Situated on the ventrolateral surfaces of the head and associated with the telencephalon. A thickened plate of surface ectoderm just beginning to invaginate.

CRANIAL NERVES. At this stagc of development certain of the cranial nerves have already appeared, namely, V, VIII, and IX. Each of these possesses a ganglion, which, as shown in Fig. 23. has the appearance of a heavy ectodermal cellular condensation in the mesenchyme situated alongside the brain. Using the otic vesicle as a landmark, identify the following nerves associated with the myelencephalon:

• A. Anterior to otic vesicle.

(VIII) Acoustic ganglion of the auditory nerve. Located immediately in front of, or anterior to, the otic vesicle and also slightly ventral to it.

(T) Semilunar ganglion of the trigeminal nerve. This ganglion is quite large and lies in front of VIII near the anterior end of the myelencephalon.

B. Posterior to otic vesicle.

(/X) Superius ganglion of the glossopharyngeal nerve. A heavy mass of nerve cells lying imme^ately posterior to the otic vesicle and slightly dorsal to it. If present, this ganglion is usually quite small.



In its original form the fore-gut extended into the head fold of the 24-hour chick as a flattened tubular structure ending blindly just behind the forebrain. At this stage the anterior end has acquired an opening to the outside, the mouth. This is developed by the junction of the fore-gut and stomodeum.

Fio. 29. Diagrunmatio aectioa of the head of the 4S-hour chick showing relation of the stomodeum to the developing mouth and pituitary body.

The Stomodeum is formed as an invagination of ectoderm meeting the ventral side of the fore-gut but not quite at the end. Thus a short, blind pocket of the fore-gut protrudes anteriorly as the preoral gut or SesseU’s pocket. The closing plate of ectoderm and endoderm formed by the meeting of the stomodeum and fore-gut soon ruptures, thus producing the oral cavity. The mouth, therefore, is lined partly with ectoderm and partly with endoderm.

Hypophysis. From the stomodeal portion of the mouth cavity a flattened tubular evagination arises on the dorsal surface and grows upward to make contact with a similar evagination from the floor of the brain. These structures are known as Rathke’s pocket and the infundibulum, respectively. They eventually join mul form the beginnings of the hypophysis or pituitary body.


Visceral Clefts. Just posterior to the mouth a series of paired visceral clefts are developed on the side walls of the fore-gut in the region known as the pharynx. Each of these visceral clefts is produced by the outward evagination of an endodermal visceral pouch from the side wall of the gut to meet an inward invagination or branchial groove formed from surface ectoderm. The process is similar to that by which the mouth is formed. The rupture of the ectendodermal closing plate produces a visceral cleft or gill slit.^ At 48 hours three pairs of these are present.

Thyroid Gland. In the floor of the pharynx opposite the second branchial wch the thyroid gland has begun to form as a rounded ventral diverticulum which communicates with the pharynx by means of the widely open thyroglossal duct. The cells constituting the wall of the thyroid diverticulum are characteristically tall and columnar at this stage.

B. mOEBTIVB TUBE POSTERIOR TO PHARYNX Fore-gut. A thick-walled tube of small bore posterior to the pharynx. Circular in cross-section.

Liver. Just in front of the anterior intestinal portal there is a narrow ventral diverticulum from the floor of the gut. This projects downward toward the meatus venosus and by cellular proliferation invades the walls of the vitelline veins which are prominent at this level, thus producing irregular masses of cells which represent the primordium of the liver.

Anterior Intestinal Portal. Posterior to the liver diverticulum the gut continues as a tube for a short distance and then its floor opens out ventrally. This marks the anterior intestinal portal.

Mid-gut. From the anterior intestinal portal the gut continues backward as the floorless mid-gut untfl it reaches the posterior intestinal portal which marks the beginning of the tail fold.

Hind-gut. At the level of the posterior intestinal portal the gut again assumes a flattened tubular appearance and extends back into the tail fold as a blind tube, the hind-gut.

' 1 It Aonld be noted st this point that the wsceral clefts are separated by pillars of tissue, wfaioh are known as visceral or bnuMhial lurebes. These are important at this stage as the territory through which certain blood vessels, the aortic arches, flow. They are identified numerically as follows: viscenl arch I lies in front of visoeral cleft 1; visceral arch II lies in hrat of viiei^ t^t II, etc. (See table <m page 8, Figs. 10 and SI.)


Mesenchyme. Loose reticular embryonic connective tissue. Constitutes the general padding material between organs and surface ectoderm. Examine under high power of the microscope.

Notochord. Cylindrical, rodlike (sometimes sinuous in outline). Situated direxitly beneath neural tube. Represents the embryonic forerunner of the skeletal system and curves to follow the contour of the floor of the brain. Examine jmder high power.



CARDINAL VEINS. Have the general form of the letter H, of which: the upper limb represents the anterior cardinals, the lower limb represents the posterior cardinals, and the cross bar represents the common cardinals.

Anterior Cardinals. Drain the head. Arise from a capillary plexus over the brain.' The main trunks lie at the ventrolateral margins of the neural tube. They follow the neural tube closely until the level of the heart is reached. At this level they work ventrally but lateral to the dorsal aorta to join the common cardinals.

Common Cardinals. Receive blood from both anterior and posterior cardinals. Drain into the dorsolateral angles of the sinus venosus.

Posterior Cardinals. Drain the posterior end of the body. Situa’ted dorsal to the mesonephric tubules. Join the common cardinal veins.

VITELLINE VEINS. Return blood to the heart from the yolk sac.



In a Bcction such as that diown in Fig. 24 we encounter the upper end of the first aortic arch near the point where it joins the dorsal aorta and where the dorsal aorta is prolonged forward into the head as the internal carotid artery. Trace Uiese vess^ deeper into the series and also identify the second and third aortic arches, etc., as follows:

• Internal Carotid. Runs forward into the head to form a capillary

plexus closely associated with the brain. ' T

First Aortic Arch. Separates from the dorsal aorta, Runsthsoug^ the firat brandiial arch. Eventually swings into the xoid-lihe : ' at its point of oripn from the.heart. T' '

Ilorsdl Aortae. Situated beneath notochord lahsral to the line and just dorsal to the gut. They receive, in order posteriorly, t&e first, second, and third aortic arches. At the level of the heart they fuse into a single vessel. Finally divide again to form the two caudal arteries.

Second Aortic Arch. Runs through second visceral arch lateral to pharynx.

Third Aortic Arch. Runs through third visceral arch and hence is found a few sections posterior to the second arch. Note association with thyroid gland.

Vitelline Arteries. Arise from dorsal aorta just posterior to level at which the vessel splits to form the caudal arteries.

Intersegmental Arteries and Veins. Small paired vessels which supply the somites. Situated between somites. Arteries arise from the dorsal aorta. Veins join the anterior or posterior cardinal veins, depending on the level of section.


Bulbus Arterumis. Usually circular in cross-section. Its anterior end lies in the mid-line beneath the aortic arches. Its posterior end swings laterally to the right and so comes to lie near the chorion. Ventricle. Usually oval in section. Its long axis lies across the section. It is farthest out ventrally from the body proper. It has a delicate endothelial lining. Its wall is thickened by the presence of a reticulum of developing muscle cells.

Atrium. Thin-walled. Situated ventral to sinus venosus.

Sinus Venostis. Thin-walled. Situated in mid-line beneath gut. Receives the common cardinal veins at its dorsolateral angles.

Meatus Venosus. Although not a part of the heart, it is a single tube entering the sinus venosus. It represents the fused vitelline veins.

differentiation of the sohttes

It is usual to consider the lateral sheet of mesoderm as being made up of three regions which are dearly distinguidiable at this stage (Fig. 30).

(a) Somite proper. Hie raeso(Jerm lying alongside the notochord is divided by transverse constrictioDB into small paired blocks or somites from v«y early stages of development. The cavity in the smidte is the myoeod. Each somite undergoes a typical differentiation, which is clearly shown in any M tWiPr occurring about mid-way back in the body. It will be seen by reference to Pig. 30 that three be identified. Of these the deeply staining layer lying just below the surface ectoderm is well out wd fotrns the dermatome. The hook of tissue continuous with the dermatome and adjacent to the epinal cord is not very clearly marked out but in later development gives rise to skeletal muscles, and hence is known as the myotome. The remainder of the mesodermal block made up of the bulk of the somite is the sclerotome, and it will give rise to skeletal structures.

(b) Nephrotome. A smaller portion of mesoderm lateral to the somite but not blocked oS (in

Fia. ao. Differentiation of a typical aomite as illustrated by a section throufd^ mid-body of a 4S-bour chick.

the chick) by the transverse constrictions which separate the somites. The cavity in the nephrotome is the nephrocoel.

(c) Lateral mesoderm. The broad lateral sheet of mesoderm not mariced off by transverse divisions. It is split tangentially into two layers, the somatic, or outer layer lying next to the ectoderm, and the splanchnic, or inner layer, lying next to the endoderm. The cavity between these layers is the coelom.


At this stage little progress has been made toward closing in the ventral side of the body by lateral undercutting. Consequently the coelom is in wide-open communication with the exocoel. There is, however, a blind poudilike extension of the coelom forward but lateral to the fore-gut in the region of the heart (Fig. 25). The pericardial cavity is continuous with the exocoel.


PBONBPHROS. The excretory system of the chick is first developed as a pronephros or head kidney. This structure is composed of a series of pronephric tubules which grow backward and unite so as to form the pronephric duct. At 48 hours a number of these tubules usually persist about the level of the fifteenth somite. Anterior to this they are degenerate. Identify where possible;

Pronephric Tvbules. Contorted thick-walled tubules. Equipped with a nephrostome or opening to the coelom.

Pronephric Duet. Thick-walled, circular in cross-section. Formed by union of backward extensions of pronephric tubules.

MESONBPHBOS. The excretory S3^tem is represented in the 48-hour chick in the main by the mes onephros, a series of individual meson^;fimc tubules whidi doveiop from the nephrotomes posterior to the pronendmw and communicate with the mesonephric duct. In the case of the chick the nephcfrtome is tucked up under tiie somite proper, and hence the mmsmephrie tubules appear to lie at the lateral an|^ of the somite. A 'Hose association with the dorsal amta and posterk^r cardinal vein is evidait. Identify:

iiesmephric Tubule. Thick-walled. Contorted and irregular in shape.

Mesonephric Duct. Thick-walled. Circular in cross-section. Lateral to the mesonephric tubules which empty into the duct. Represents a backward prolongation of the pronephric duct.

72-HOUR EMBRYO General Body Form

The 72-hour chick embryo is much larger than the 48-hour embryo, but the curvature of the body is so great, owing to the presence of cranial, cervical, and caudal flexures, that its total length is only about 7 mm. In general, the body may be divided into head, trunk, and tail regions, and, in addition,' the beginnings of the appendages are apparent in the form of anterior limb buds at the level of the seventeenth to nineteenth somites and hind limb buds at the level of the twenty-sixth to thirty-second somites.

The head, which is bent backward beneath the body, shows a series of prominent bulges corresponding to the divisions of the brain. The cerebral hemi^heres of the telencephalon cause prominent lateral bulges in the extreme front end. Diencephalon and mesencephalon are clearly marked out, and, posterior to the isthmus, the metencephalon and thin-roofed mylencephalon. Associated with the extreme ventrolateral surfaces of the head are the groovelike nasal pits. The optic cup and lens are clearly apparent alongside the diencephalon, and the otic vesicle ctm be seen as a small saclike structure alongside the myelencephalon.

The pharyngeal region is marked by the presence of the first three visceral clefts. The fourth, although indicated, has not broken through. Anterior to the first visceral cleft the first visceral arch is prominently developed, and the maxillary process, primordium of the upper jaw, has grown forward to become attached to the head proper. The mandibular process, which is the primordium of the lower jaw, constitutes the main body of the arch. In total mount preparations the maxillary and mandibular processes have the appearance of a heavy, bilobed tissue mass lying just above the heart.

The heart is attached prominently to the ventral side of the body just below the pharynx. It is still a looped structure (Fig. 22), and consequently the bulbus arteriosus as well as the more posteriorly located ventricle form the uppermost limb of the heart. It will be noted that the anterior end of the bulbus swings to the mid-line ventral to the gill slits. The atrial region is beneath the ventricular limb but situated well forward. It communicates with the Sinus venosus, which is located in the median line posterior to the atrium. The vitelline veins from the yolk sac converge toward the posterior end of the sinus venosus.

In the body proper the neural tube swings backward in the mid-line and paired somites lie alongside it. The somites, of which there are thirty-five pairs, begin just posterior to the otic vesicle and extend back into the tail. The vitelline arteries leave the dorsal aorta at approximately the mid-level of the body.

The tail fold is lifted above the blastoderm and curves slightly forward. At this level the ftllantnis protrudes ventrally as a prominent vesicle.

Covering the whole body are the delicate amnion and chorion. The fusion of the head and tail folds of these membranes is complete except for a small opening in the mid-dorsal line.

Laboratorg Directions

Study carefully a total mount of a 72-hour embryo, and make a drawing showing the structures identified.

72-Hour Chick Embryo.


Directions fob Study of Transverse Sebul Sections

For convenience in working out and identifying the structures of the 72-hour chick we shall study ectodermal, endoderniaJ, and mesodermal derivatives as individual units after first considering the embryonic membranes. A record should be made as in the foregoing study of the 48-hour chick in the form of drawings of the various structures and the listing of their location in the series.

embryonic membranes

Just as in the 48-hour chick series, the embryonic membranes are prominent and easy to identify in the first few sections through the head as follows:

Amnion. L)dng next the embryo and forming a complete sac around the head. Composed of an inner layer of ectoderm and an outer layer of somatic mesoderm.

Chorion. Lying outside the amnion and forming a single sheet. Composed of an outer layer of ectoderm and an inner layer of somatic mesoderm.

Yolk Sac. Lying outside the amnion and forming a single sheet. Composed of endoderm and splanchnic mesoderm. Contains vitelline blood vessels in the mesodermal layer.

ectodermal derivatives

EPiDiaiMis. The outer covering of the body composed of a single layer of flattened cells.


Myelencephalon. Thin-roofed. Associated with otic vesicles. Side walls marked by a series of indentations, the neuromeres.

Metencephalon. Anterior to myelencephalon and not clearly marked off from it. Thick-walled and tapering inward toward isthmus.

Isthmus. Constriction between metencephalon and mesencephalon.

Mesencephalon. Anterior to metencephalon. Characteristically thick-walled and almost round in cross-section.

Diencephalon. Associated with eye.

Epiphysis. A small but prominent rounded bulge on the dorsal side of the diencephalon. Situated in the mid-line.

Infundibulum. A flattened diverticulum arising from the floor or ventral side of the diencephalon and directed toward the stomodeum.

Telencephcdm. Associated with the nasal pits.

Cerebral hemispheres. Prominent rounded lateral bulges of the telencephalon.

SPINAL COBD, The extension of the neural tube posterior to the myelencephalon. A narrow thick-walled tube. Situated just beneath the dorsal surface of the body.


Otic Veside. Thick-walled, hollow oval vesicle. Lateral to myelencephalon. Frequently retains connection to the outside of the head by the endolymphatic duct.

• Optic Cup and Lens. The developing eye is made up of two

primordia situated lateral to the diencephalon, as follows:

Optic Cup. A double-walled, cup-shaped structure. Attached to diencephalon by the optic stalk on the under side of which is a groove, the choroid groove. Its thick inner layer is the retinal layer. Its thin, dark, outer layer constitutes the pigmented layer.

Lens. A rounded, ball-like structure occupying the mouth of the optic cup.

Olfactory Pit. Situated on the extreme ventrolateral surfaces of the head adjacent to the telencephalon. Thick-walled.

External orifice much more constricted than at 48 hours.




A. Anterior to otic vesicle and listed in order anteriorly. lyill) Acoustic ganglion of the auditory nerve.

(VII) Geniculate ganglion of the facial nerve. Closely associated with VIII.

(V) SemUuTMr ganglion of the trigeminal nerve. (Becomes tripartite.)

B. Posterior to otic vesicle and listed in order posteriorly. (IX) Superius ganglion of the glossopharyngeal nerve.

(X) Jugular ganglion of the vagus nerve. (Seldom present.)

(XI) Spinal accessory. The external branch is best developed at this stage and appears as a long band of fibers adjacent to the myelencephalon.

(XII) Hypoglossal nerve. When present the roots of this nerve appear as a row of small dots ventrolateral to the floor of the brain at the level where it is just separating from the spinal cord.


(Ill)Oculomotor. This nerve arises from the floor of the mesenceph^on. Hence it is found beneath the brain and its basal portion is directed almost straight away from the brain.


. * {I)Olfactory. This nerve runs laterally from the wall of the

telencephalon to the nasal pit.

SPINAL NERVES. The primordia of the segmentally arranged dorsal root-ganglia are located dorsolateral to the neural tube as paired condensations of neural crest tissue. They are usually somewhat difficult to recognize (Fig. 30).


Oral Aperture. The external opening of the mouth cavity formed by invagination of the ectodermal stomodeum. Situated beneath or ventral to head. Bounded by:

Maxillary process of first visceral arch which has grown forward and become prominently attached to the head proper.

Mandibular process of the first visceral arch which constitutes the main body of the arch.

Mouth Cavity. The anterior chamber of the digestive tube. Originates from stomodeum and fore-gut.

Pituitary Body. Consists of two closely associated ectodermal primordia: (1) Rathkes’ pocket, a dorsal diverticulum of the stomodeum.

(2) Infundibuliun, a ventral diverticulum of the diencephalon.

Pharynx. The broad anterior end of the original fore-gut posterior to mouth cavity. Flattened dorsoventrally. Marked by presence of visceral clefts. 'â–

Vieceral Clefts. Paired lateral openings extending from the sides of the head into the pharynx. Formed by the junction of:

(1) Visceral pouches, which represent lateral outpocketings of the gut.

(2) Visceral grooves, which represent inpocketings of the surface ectoderm. Hiree. pairs are present — a fourth usually so.

At this time it is highly desirable to learn to identify the visceral grooves and the tdseerfd arches


which lie between them. This is important because of their relation to the aortic arches. (See page 57.) In such a section as that in Fig. 31 the arches and grooves are clearly indicated. Learn to identify:

Fia. 31. Section through the head of a 72-hour chick showing relation between visceral arches and grooves.

(la) Maxillary process of arch I attached to the head proper.

(l b) Mandibular process of arch I attached to the body proper.

(1) Hyomandibular cleft between arches I and II.

(II) Hyoid arch next dorsal to Ib.

(2) Visceral cleft II between arches II and III.

(III) The third arch.

(3) Visceral cleft III between arches III and IV.

(IV) The fourth arch.

• Thyroid Gland. A rounded median ventral diverticulum from the floor of the pharynx.

Situated opposite the second arch.

Laryngotracheal Groove. Posterior to the fourth pharyngeal pouch the gut elongates ventrally. The dorsal portion represents the oesophagus, the ventral, the beginnings of the trachea.

Oesophagus. The oesophagus continues beyond this level as a thick-walled oval tube.

Primary Bronchi The posterior end of the tracheal portion of the laryngotracheal groove bifurcates into two lateral budlike prominences, the primary bronchi. Note the close association of the pulmonary vein with the bronchi. This vessel flows directly into the left side of the atrium of the heart.

Stomach and Duodenum. These follow the oesophagus but as yet they are not clearly defined except for the fact that the posterior boundary of the duodenum is marked by the development of the liver diverticulum.

Liver. A ventral diverticulum from the gut. Its tissues spread laterally and invade the walls of the vitelline veins and meatus venosus.

Anterior Intestinal Portal. Just posterior to the level at which the liver arises the gut elongates ventrally once more to open out on the yolk sac. This marks the anterior intestinal portal.

Pancreas. At the level at which the ventral elongation of the gut begins, a thickening of the dorsolateral wall of the gut on the right side can be seen. This is the beginning of the pancreatic diverticulum.

Mid-gut. Posterior to the anterior intestinal portaL Has no floor. Lateral walls thick. Bounded anteriorly by the anterior intestinal portal and posteriorly by the posterior intestinal portaL Hind-gut. Traced posteriorly the mid-gut closes in again on the ventral side at the levk of the p<»terior intestinal portal, thus forming a tube extending into the tail fold.

Cloaca. The gut becomes narrowed from side to side at the levd the future doaca. Recdves tine mesonephric ducts at its dorsolateral angles.


Allantois. A small, rounded, bladderlike, ventral diverticulum from the hind-gut growing out into the ezocoel.

Postcloacal Gut. The extension of the hind-gut into the tail fold posterior to the cloaca.


Mesenchyme. Loose, reticular embryonic connective tissue filling spaces between organs in the head.

Notochord. A solid rodlike structure representing the embryonic forerunner of the skeletal system.

Located beneath the brain and spinal cord. Curves to follow contours of the floor of the brain.

Circular in cross-section — sometimes elongate and sinuous.



CARDINAL VEINS. These vessels still have the general form of the letter H but conform to the contours of the body and are still paired throughout.

Anterior Cardinals. Drain the head.

Common Cardinals. Receive anterior and posterior cardinals. Drain into dorsolateral angles of sinus venosus.

Posterior Cardinals. Drain posterior part of body.

Vitelline Veins. Large paired vessels which drtun the yolk sac and converge to form the meatus venosus before entering the sinus venosus.

Allantoic Veins. These vessels are not yet well developed but are represented by a capillary plexus in the ventrolateral body wall posterior to the level of the sinus venosus.

Pulmonary Veins. Closely associated with the primary bronchi. Located ventral to the bronchi. Drain into the left side of the atrium.


AORTIC ARCHES AND RELATED VESSELS. Since the aortic arches flow through the corresponding visceral arches they can be easily identified when once the visceral arches have been worked out as described above (pages 66 and 67). It is well to begin the study of the aortic arches with a section such as Fig. 31, working forward in the series to their union with the radices of the dorsal aorta and then backward to their origin from the bulbus arteriosus. Remembering that the aortic arches flow through the corresponding visceral arches, identify aortic arches II, III, IV, and V-VI (usually combined).

Internal Carotid Arteries. Anterior extension of dorsal aortae to form capiUary plexus over brain.

Dorsal Aorta. Paired radices dorsal to pharynx. These vessels may be confused with the anterior cardinals at this level but can be readily distinguished because of the fact that they are situated more toward the mid-line. Traced posteriorly they fuse to form a single vessel.

• External Carotid Arteries. Minute paired vessels arising from the base of aortic arch II. Extending ventrolaterally into the mandibular process of arch I.

Pulmonary Arteries. Small paired vessels arising from aortic arch VI and directed posteriorly toward the lung buds.

Intersegmental Arteries {and Veins). Snaall paired vessels ramifying between the somites. First encountered posterior to the otic vesicle. The arteries anastomose with the intersegmental veins which in turn join the anterior or posterior cardinals.

Mesenteric Arteries. The dorsal aorta can be followed posteriorly for a considerable distance, giving off small mesenteric arteries ventrally to the gut and small paired dorsolateral intersegmentals between the somites.

Vitelline Arteries. A pair of large vessels which are given off from the dorsal aorta ventrally, and course out over the yolk sac at about the level of somite twenty-two.

Caudal Arteries. Extensions of dorsal aorta into the tail fold. At the level of the posterior limb buds these usually give off lateral capillaries, which are the forerunners of the Uiac arteries, and which in turn give off small vessels to the allantois, the allantoic arteries.


The heart of the 72-hour chick is greatly increased in size but it still has the general form of a looped tube. It is attached to the body proper at two points: (1) In the region beneath the visceral clefts by the bulbus arteriosus, and (2) at a more posterior level by the sinus venosus. The looped tube is appended ventrally. Hence, while it is usually possible to find a section where the regions of the heart can be easily identified, it is always necessary to trace the parts of the loop very carefully.

Sinus Venosus. Attached to ventral side of body. Receives right and left common cardinals at its dorsolateral angles. Re- ceives the meatus venosus representing the fused vitelline veins

at its posteriOT end. Shifts to the ri^t side of heart.

Atrium. Ventral to the sinus venosus. Lies nearest tbe yolk sac. It is usually delimited from the sinus venosus and ventricle by slight constrictions.

Ventricle. Ventral to atrium. Represents tiie outer curved portion of the loop and hence lies transversely across the secUmi.

Bulbus Arteriosus. Lies lateral to the other parts of tbe heart and nearest the chorion. Communicates with vimtiiGie |und

• also with the aortic arches. It usuaUy appears diarply

as circxdar or oval in mxSwsections.


wffbbiiktution of the bomitbs. The pwts of a typical somite are essentially the same as those already encountered in the 48-hour embryo,, as follows:

SOMITE FBOFBB. Lateral to neural tube and consisting of: Dermatome. Thick, dorsolateral plate. Priraordium of the dermis.

Myotome. The dorsal ‘'hook” of mesoderm continuous with the dermatome but nearest the neural tube. Primordium of the axial musculature.

Sclerotome. The main body of the somite consisting of loose mesodermal cells.

Nephrotome. Ventrolateral to the somite proper. Primordium of the excretory tubules.

Lateral Mesoderm. The broad sheets of the mesoderm extending out laterally from the body proper between ectoderm and endoderm. Somatic mesoderm adjacent to the ectoderm is separated from splanchnic mesoderm by the coelom.


At the 72-hour stage the lung buds have begun to develop. They extend out laterally into the anterior coelomic pouches which now become part of the pleural cavity. They are also being undercut by medial extensions from this cavity so as to form the pleural grooves.

At the posterior end of the body the tail fold has developed and in it the cloaca is taking form. A pair of backward extensions of the coelom extend alongside the cloaca into the tail fold.

In the trunk the lateral undercutting is more extensive and consequently the coelom within the body is more clearly delimited from the exocoel.

The pericardial cavity is still continuous with the exocoel.

Excretory System

Mesonephros. This system of tubules already encountered in the 48-hour embryo is still present at 72 hours. A close association with the dorsal aorta and postcardinal vein is apparent. Mesonephric Tubules. Thick-walled. Contorted shapes. Mesonephric Duct. Thick-walled. Circular in cross-section. Lateral to mesonephric tubule. Empties into cloaca.

Metanephros. The mesonephros is replaced in later development by the metanephros, which is also made up of a series of tubules. These, however, have a very different origin. The metanephros is not always present at 72 hours, but when beginning to develop the following parts may be identified:

Ureter. A dorsal evagination from the mesonephric duct at the point where it joins the cloaca. It may show several bifurcations.

Nephrogenous Tissue. A heavily staining, caplike condensation of nephrogenous mesoderm over the end of the ureter.

   A Laboratory Manual of Vertebrate Embryology 1947: Frog | Chicken | Pig
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