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McEwen RS. Vertebrate Embryology. (1949) IBH Publishing Co., New Delhi.

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Part IV The Development of the Chick

The Chick: The Adult Reproductive Organs, And The Development Of The Egg Previous To Gastrulation

The Chick has long been an object of ernbryological interest, and the study of its development has been connected with such classical names as Malpighi (1672), Wolff (1759), and Von Baer (1828). In the more modern era of science, moreover, workers in this ﬁeld have continued to study it, until at the present time probably more details regarding its development are known than in the case of any other animal. As will appear, however, certain points concerning the very early stages are even yet in doubt, and are still under investigation.

Some of the reasons for the importance of this form and the study which has been given it may be brieﬂy indicated. In the first place the material is usually easy to obtain and observe throughout most of the developmental stages. Furthermore, unlike the Frog or Fish, the Chick embryo, in common with those of other Birds as well as with those of Reptiles, possesses certain very signiﬁcant extra-embryonic membranes and appendages. The signiﬁcance of these structures lies not only in their character and functions in the groups just cited, but also in the fact that the same appendages and membranes occur also in the Mammals, though in a somewhat modiﬁed condition. Lastly, aside from the features already indicated, the general development of the Chick is more nearly mammalian than that of any of the forms previously considered.

In the following account we shall begin with a brief description of the reproductive organs of the adult Bird.

REPRODUCTIVE ORGANS OF THE ADULT, OCDGENESIS AND OVULATION

THE MALE

The male Bird, or Cock, possesses a pair of testes, each of which is an ellipsoidal body about two inches long and one inch in diameter. THE FEMALE 231

It is made up of seminiferous tubules and supporting tissue, and, as in the case of the Frog, is rather closely attached to the dorsal wall of the coelom by a fold of coelomic epithelium, the mesorchium. By way of 'the vase e ﬂerentia, each testis discharges its products into its respective

Fig. 151. — Section of an ovarian ovum of the Pigeon, drawn from a preparation of

Mr. J. T. Patterson. From Lillie (Development of the Chick) . The actual dimensions of the ovum are 1.44 x 1.25 mm.

f.s. Stalk of follicle. G.V. Germinal vesicle. Gr. Granulosa. L. Latebra. p.P. Pe ripheral protoplasm. pr.f. Primordial follicles. T h.ex. Theca externa. T hint. Theca inter-na. Y.Y. Yellow yolk. Z.r. Zdna radiate.

vas deferens. The latter duct then leads to the cloaca, where its entrance is marked by a papilla. There is some evidence that the sperm attain their motility and functional capacity by the action of a testis hormone during their passage through the vasa eﬂerentia (Munro, ’38).

THE FEMALE

The Ovary.—In the embryo Chick two ovaries are present, but only the left develops. In the adult Fowl this is suspended from the 282 THE CHICK

Fig. 152.—-Reproductive organs of the Hen. (After Duval, based on a ﬁgure by Coste). From Lillie (Development of the Chick). The ﬁgure is diagrammatic in one respect, namely, that two ova are shown in the oviduct at different levels; normally but one ovum is found in the oviduct at a time.

1. Ovary; region of young follicles. 2 and 3. Successively larger follicles. 4. Stigmata (cicatrices), or non-vascular areas, along which the rupture of the follicles takes place. 5. Empty follicle. 6. Cephalic lip of ostium. 7. Funnel of oviduct

.(ostium tubae abdominale‘ 5?. 0"-ﬁn in the upper part of the oviduct. 9. The mag num, where most, if not all, the albumen is actually secreted. 10. Albumen surrounding an ovum. 11. Ovum in portion of duct laid open to show it. 12. Germinal disc. 13. The isthmus where the shell membrane is secreted, and possibly some thin albumen. 14. The uterus where shell is secreted, and both layers of thin albumen separated from remainder, producing thick albumen and chalazae (see text). 15. Rectum. 16. Reﬂected wall of abdomen. 17. Anus, or external opening of cloaca. THE FEMALE ‘ 233

body wall by the mesovarium in about the same position as the left testis in the male. It consists of the usual vascular connective tissue elements, or stroma, within which are imbedded ova in various stages of growth. Each ovum is surrounded by a layer of follicle or granulosa cells, and these in turn are encased in a sheath of the stroma called the theca. It is sometimes customary to refer to such stages together with their coverings as simply follicles (Fig. 151) . Normally only one ovum matures at a time, though there may be several not many hours apart.

The Genital Tract.—-As in the case of the ovary, only the left genital tract develops. This fact is apparently correlated with the production by Birds of fragile shelled eggs, such that the coming together

~ of two at the cloaca would be disastrous. In this connection it is of

some interest to note that although in certain species of Hawks there are two fully developed ovaries, there is still only one genital tract (Stanley and Witschi, ’4«0) . As regards this tract, we ﬁnd that it opens anteriorly adjacent to the ovary and posteriorly into the cloaca just dorsal to the anus. Also it is suspended as usual from the dorsal body wall by a mresentery-like fold of peritoneum, and in the Birds it may be divided into three main parts as follows:

I . The Oviduct Proper. This is the anterior part and is itself divisible into three sections:

(a) The Infundibulum or Ostium. This is a thin-walled muscular funnel, the inner surface of which is lined by ciliated epithelium. It is in the immediate neighborhood of the ovary, but does not directly connect with it. A '

(17) The Magnum. This is sometimes called the “ glandular portion,” but since other parts are also glandular this is not a very good designation. The part in question is a long much convoluted tube following immediately after the ostium. It leads into:

(c) The Isthmus. This is a shorter tube also glandular whose pos

terior end marks the termination of the oviduct proper.

II. The Uterus. This is a relatively short, dilated portion whose walls are also glandular. It immediately follows the isthmus and leads into the third and last main division: ‘

II I . The Vagina. This region is likewise short, but thin-walled, and opens into the cloaca (Fig. 152).

The Oﬁgonia. ———The origin of the primordial germ cells and their multiplication as oiigonia occur during the embryonic life of the Chick. Thisearly history will therefore be dealt with later in connection with the development of the gonadswkt the time of hatching, however, the 284 THE CHICK

oéigonia are said to have ceased to divide, and each is becoming surrounded by follicle cells preparatory to growth (Fig. 153). They may now, therefore, be called oéicytes, or young ova, whose history from this point onward will be taken up in more detail.

The Growth Period.

The Vitelline Membrane or Zona Radiata. — There now appears surrounding each ovum or oiicyte a membrane which is called the vitelline membrane. Whether it is a true vitelline membrane arising entirely from the surface of the egg itself, or whether it is secreted by the follicle cells and is therefore chorionic in character, is somewhat uncertain. As this membrane thickens slightly, it becomes pierced by minute canals; for this reason it is also referred to sometimes as the zona radiata. Throughout these canals by way of the follicle cells the egg receives nourishment from the surrounding theca.

The Germinal Disc. —At ﬁrst the nucleus occupies the center of the oﬁcyte, and the yolk granules are deposited in the cytoplasm around it. This presently results in the existence of yolk-free cytoplasm only around the periphery of the egg. This cytoplasm, however, is thicker upon the side where the theca of the ovum is attached to the ovary; this thickening is called the germinal disc (blastodisc) . Meanwhile the ovum has been growing, and by the time it has become .6 mm. in diameter, the nucleus has migrated into this disc (Fig. 151).

The Deposition of Yolk. -——The growth of the ovum is largely due to the deposits of yolk, which it appears occur in the following manner: The nucleus, as noted, occupies at ﬁrst a central position around which the yolk begins to be formed. This‘ yolk is of a lightish color termed white yolk, and the central mass of it which is thus deposited is known as the latebra. Following this the peripheral layer of the protoplasm starts to deposit around the latebra a darker colored substance, the yellow yolk. As the egg is thus enlarged, the nucleus, as indicated, leaves its central location and takes a peripheral position, which it maintains during subsequent growth. The result is that the yellow layer is everywhere interrupted along the path which the nucleus has taken. Along this path there is thus left a continuous deposit of white yolk extending from the latebra almost to the surface. It is known as the neck of the latebra, and just beneath the blastodisc it spreads slightly to form a plate, the nucleus of Pander (Fig. 154, B).

It should be noted that in some instances the deposit of yellow yolk is interrupted by intermittent, usuallkthinner, layers of more white THE FEMALE

E

Fig. 153. ——Growth stages in the oiigenesis of the Hen’s egg. From Kellicott (Chordute Development). After Sonnenbrodt. A. Oiicyte measuring 0.012 x 0.016 mm., the nucleus of which is 0.006 mm. in diameter. B. Oiicyte measuring 0.018 x 0.028 mm., the nucleus of which is 0.0105 x 0.014 mm., enclosed in follicle. C. Oiicyte measuring 0.040 x 0.045 mm., the nucleus of which is 0.020 x 0.022 mm. D. The nucleus only, of an oiicyte measuring 5.84 x 6.16 mm., the nucleus itself measuring 0.214 x 0.238 mm. Total view showing the small chromosomes in the midst of a collection of chromatin nucleoli. E. Vertical section of the nucleus only, of an oiicyte, the follicle of which measured 37 mm. in diameter. The nucleus itself is 0.455 mm. in diameter and 0.072 mm. in greatest thickness.

c. Chromosomes. cr. Extra nuclear chromosome-like bodies. f. Follicle. m. Nuclear membrane. mf. Folds in nuclear membrane. 11. Nucleus. nu. Chromatin nucleolus. ps. Pseudo-chromosomes. .9. Centrosome. 1;. Yolk nucleus or vitellogenous body.

285 285 _ THE CHICK

Fig. 154. —— Semidiagrammatic illustration of the I-Ien’s egg at the time of laying. From Kellicott (Chordate Development). A. Entire “egg.” Modiﬁed from Mar» shall. B. Vertical section through the vitellus or ovum proper, showing the concentric layers of white and yellow yolk. Actually there are seldom, if ever, as many layers as this under normal conditions.

a. Air chamber. ac. Chalaziferous layer of albumen. ad. Dense layer of albumen. af. Fluid layer of albumen. b. Blastoderm. c. Chalaza. l. Latebra. nl. Neck of latebra. p. Nucleus f Ponder. pv. Perivitelline space. s. Shell. smi. Innerelayer of shell membrane. Smo. Outer layer of shell membrane. 1:. Vitellus or “yolk.” om.

Vitelline membrane. wy. Layers of white yolk. yy. Layers of yellow yolk.

yolk. This alternation was once thought to be universal, and to result from the fact that yellow yolk was deposited during daylight and white yolk at night (Fig. 154-, B). As indicated, however, many eggs can be found in which no such alternation of layers exists, all the yolk aside from the latehra and its neck being yellow. Experiment has now shown that the diﬂerences in color of the layers, when they occur, are due entirely to alternating differences in the character of the food. The deeper yellow is produced by xanthophyl, and appears in the yolk when grass . , <3... .....

FERTILIZATION AND MEIOSIS 237

or yellow corn occurs in the diet; If this is fed periodically, it results in an alternation of darker and lighter layers. Thus by proper feeding thick or thin, few or numerous, layers can be produced at will. The white yolk of the latebra and its neck, however, always occurs, and is evidently of a differentcharacter. It apparently‘ results from some inﬂuence of the nucleus, but its cause is unknown (Conrad and Warren, ’39) .

_ OVULATION, MEIOSIS, AND FERTILIZATION

During these processes the nucleus has greatly enlarged and as usual in its enlarged form it is known as the germinal izesicle. The first maturation division is initiated about 4% hours previous to ovulation, and is completed in about 2% hours, after which the spindle for the second division is formed (Olsen, ’42, ’50) . At this point the large ovum still in the ovary is grasped by the funnel shaped infundibulum or ostium. The theca and follicle then rupture along a non-vascular line, the cicatrix, and the egg is received into the oviduct. ’

Finally it may be noted that occasionally two eggs may mature and be released together, in which case they are enclosed in a single shell and form a “ double yolk egg.” While this is apparently the most usual cause of this condition it is not the only one. Such eggs may also result either from the premature or the late ovulation of one of the “ yolks ” (eggs), or from the picking up by the infundibulum of an extra egg which has previously fallen into the body cavity.

FERTILIZATION AND MEIOSIS

When the egg is taken into the ostium, it is at once surrounded by sperm which have been received from the male at a period from 24 hours to two weeks previous to the ovulation of the ovum in question. Several sperm enter the egg presumably, as in the Pigeon, in the neighborhood of the hlastodisc, following which the second polar body is given off and the egg pronucleus fuses with that of one of the sperm. Many of the remaining sperm nuclei then degenerate, while others (supernamerary ‘nuclei or merocytes) persist for a time and produce certain phenomena to be described later in connection with segmentation.

THE HISTORY OF THE OVUM FROM FERTILIZATION THROUGH GASTRULATION

The stages now to be described have not all been completely worked out for the Chick. It is presumed, however, that they are somewhat 283 THE CHICK

similar to the corresponding stages in the Pigeon which have been fully described by Patterson and Blount. Data concerning doubtful stages in the Hen’s egg have therefore been partially supplied from the facts re: garding the Pigeon. The points where this has been done will be noted

in passing.

v

THE APPLICATION or ALBUMEN, SHELL MEMBRANES AND SHELL

Strictly speaking, the formation of the ovum proper is completed at the time of ovulation, and it thus appears that what is ordinarily spoken of as the “yolk” of the Hen’s egg is really the entire egg. Nevertheless, in the case of the Bird, it is common usage to include under the term egg not only the ovum proper (i.e., the “ yolk ”) but also all its tertiary membranes, and this usage will be adhered to in the following account:

As the yolk passes down the oviduct it takes a position such that a line passing through the blastodisc and the center of the vegetal pole is at right angles to the longitudinal axis of the duct at any particular point. It then revolves slowly about the latter axis, and while so doing receives its respective coverings from certain portions of the duct. In the completed product these coverings of the egg or “ yolk ” are as follows:

Closely applied to the yolk comes a dense layer of albummous substance ﬁlled with fine mucin-like threads. This layer forms a thin but firm covering, the chalaziferous membrane. At each side of the yolk opposite each end of the shell this membrane is twisted into cords, the chalazae. Immediately outside of this chalaziferous membrane there is said to occur a very narrow layer of thin watery albumen (Conrad and Scott, ’38). There then comes a clear but relatively dense and wide layer of albumen called simply dense albumen. Its density is apparently also due to the presence of mucin. This layer in turn is surrounded by a fairly wide layer of thin watery albumen called thin albumen which is bounded externally by the so-called shell membrrme. The latter is a very real and deﬁnite membrane in immediate contact with the outermost coverings of all, the calcareous shell. The chalazae and the wide layers of dense and of thin albumen are easily demonstrated by carefully breaking an uncooked egg into a ﬁnger bowl. The innermost narrow layer of thin albumen next to the chalaziferous membrane, however, is not usually seen except by the use of more reﬁned methods. The shell membrane is readily detectable sticking to the inside of the shell. In a hard-boiled egg the albumen can be more or less unwound in spiral EGG MEMBRANES AND SHELL 289

sheets, apparently a result of the revolving of the egg in the duct during its application. (Fig. 154, A).

The question now arises as to what parts of the genital tract listed above are responsible for the different layers and membranes just indicated. This has been investigated by various workers, Asmundsen and Burrnester (’36), Burmester C40), Cole (’38), Conrad and Phillips (’38), Scott and Wai-Lan Huang (’4~l) and others. These men have attacked the problem by removing parts of the duct to see what layers were reduced or lacking, by studying the histology of parts of the tract and in other ways. While the results of their investigations are not in entire agreement on some details the following conclusions taken largely from the discussion of Conrad and Scott (’38) are probably very near to the truth.

Products of the Magnum. — The egg having taken about 18 minutes to pass the infundibulurn enters the magnum which it goes through in a little short of three hours.‘ This latter region secretes all of the thick or dense dlbumen which owes its character to numerous mucin threads. Some (Asmundsen) claim that a little thin albumen (that of the narrow layer?) is also secreted by the anterior part of the magnum, but this seems to be one of the points on which there is disagreement (see below).

Products of the Isthmus.———The egg passes through this part of the duct in about 74 minutes, and receives here the shell membrane. There may also be a little thin albumen secreted by this part of the duct, though Conrad and Scott claim that almost all, if not all, of this is produced, i.e., differentiated from other materials, while the egg, is in the uterus. As will presently appear, however, not all the constituents of this albumen are actually secreted in the latter organ.

Products of the Uterus.—-—The egg remains longest of all in this region, about 20% hours, and as just suggested it is while the egg is here that practically all of the thin albumen is -differentiated as such. As noted, however, all of the material for this layer does not actually originate in this part of the tract. Instead that portion of it which does arise here consists largely of thin non-albuminous fluid and soluble salts. This solution of salts then passes by osmosis through the already existent shell membrane which is thereby distended. When the ﬂuid in question thus comes next to the dense albumen some of the protein in the latter, other than the mucin, soon diffuses into the ﬂuid. In this

1 Average time spent in Various parts of the duct was kindly furnished by Dr. D. C. Warren. 290 THE CHICK

way the latter becomes albuininous, though still thin because it lacks

mucin threads.

While the egg is in the uterus there are also produced the chalazae, chalaziferous membrane and the narrow layer of thin albumen. In this case, however, none of the materials concerned are secreted here. The substances for these structures are already present in the dense albumen produced in the magnum. What happens is this: The muein ﬁbers in the part of the thick albumen immediately adjacent to the yolk are withdrawn from this albumen, and are concentrated against the yolk to form the chalaziferous membrane. This concentration leaves the albumen next to the membrane without any ﬁbers, and hence it becomes thin, thus forming the very narrow thin layer noted as occurring in this region. The chalazae are simply extensions of the concentration at the two sides of the yolk. They are twisted apparently because the egg was rotating at the time the albumen from which they are derived was laid down, and possibly because rotation is still going on. The cause of the separation of the mucin from the albumen is believed to ‘be mechanical, but the process is not entirely clear. I

Finally the shell is entirely secreted by the uterus, and is known to be substantially advanced, though not completed, after 8-10 hours within that part of the genital tract. The source of the cuticle of the shell is uncertain, but it may be denatured protein. _

The Vagina. —- The egg probably remains only a few seconds in the vagina before it is laid, and there is nothing added to it here.

THE PERIODICITY OF LAYING

The periodicity in the laying of eggs has been a subject of considerable investigation. Most chickens have an annual laying period of eight or nine months, the commonest interval of rest being during the late summer months. During the active period the Bird lays more or less continuously at the rate of about an egg a day, if the eggs are constantly removed. Otherwise when a suﬂicient number have been accumulated the impulse to “ set ” may assert itself, and the laying ceases while a brood is hatched and raised. From this it might be inferred that the impulse to set is dependent merely upon the accumulation of a certain number of eggs, but the word “ may ” in the previous sentence is used advisedly. Not every hen will set when enough eggs are accumulated. On the other hand, the setting impulse, i.e., “ broodiness,” sometimes asserts itself whether there are eggs or not. This is most likely to happen in the spring and early summer, i.e., during the time of year which PERIODICITY OF LAYING 291

is the breeding season of many birds in temperate latitudes. Thus the impulse to set is evidently due to more than the single factor of egg accumulation. It is probably, like so many aspects of reproduction, partly controlled by some of the endocrine glands, particularly the pituitary, and this in turn may well be inﬂuenced by the length of day, the temperature, or both. This irregularity in the advent of broodiness in domestic hens is very likely the result of long selection with a View to increasing the laying period. Even if the eggs are removed, however, and the hen does not become broody, she does not lay one every day for an indeﬁnite period. Instead she lays a series of eggs on successive days, and then skips a day, such an uninterrupted series being known as a clutch. The eggs of a clutch, moreover, are not laid at the same time each day. Rather the first one will be laid fairly early in the morning of the first day, and each succeeding one about two hours later than its predecessor on each of the following days. This continues until the last egg of the clutch is laid around the middle of the afternoon, seldom later. This means that after a_maximum of five or six eggs has been laid, a day will ensue in which none is’ laid, and the hen will then begin again in the morning of the day following.

It was formerly believed that this interruption in laying was due to a delay in the act of laying itself. The theory was that if an egg was not ready to be laid until late in the afternoon, the Bird would not lay it then, but would retain it over night. Thus a day would pass with no egg laid and the one laid the following morning would be a so-called “ held egg.” This idea was made reasonable by the fact that there is some difference in the degree of development of eggs, and this assumed opportunity for prelaying incubation was supposed to account for it. Further study, however, has rendered this theory untenable. In the first place careful tracing of the history of eggs in the genital tract proves, according to Scott and Warren (’36) that there are no held eggs. Instead it has been found that all eggs spend approximately 25 hours in the genital tract with some minor variations. It is thought that these minor variations are sufficient to account for such differences in embryonic development as are known to occur. Correlated with this near equality of time spent in the tract is the fact that each egg in a clutch is ovulated within a few minutesiof the laying of the previous one of that clutch. These considerations would suggest that the explanation for’ the omitted day must lie either in delay of ovulation of completely formed

eggs, or in a delay in the later growth stages of certain eggs in the ovary. 292 THE CHICK‘

An effort to find which of the latter suppositions is true, and to deter‘mine the cause for whatever delay may occur, has been, made by subiccting the hens to variations in illumination. It has thus been found that artiﬁcially reversing the time of illumination within the 24-hour period will cause a corresponding reversal in the time of laying, but this effect is delayed for about sixty hours. Also constant illumination will cause the hens to distribute their laying more or less regularly throughout the 24-hour period, and will make them lay more eggs to a clutch. Clutches, however, do still occur, i.e.,. the laying is not continu» ous. This and other data ‘led Warren and Scott (’36) to conclude that il ilumination is responsible for normal periodicity in laying. Furthermore

since there are no held eggs the inﬂuence of the light could not be upon the laying itself. It must be upon earlier stages in the entire process.

Finally because of the time lag before changed conditions produced results these authors decided that the inﬂuence was also not upon ovulation, but, as intimated above, upon late stages in the growth of the oocyte. Be this as it may, still later investigations by F raps, Neher and Rothechild (’4-7) have shown that light is not the only environmental factor involved. By giving or withholding food during continuous illumination it was clearly shown that this item and the accompanying activity of obtaining it very deﬁnitely stimulate some step in the reproductive process, apparently ovulation. Also as was so thoroughly demonstrated in the Frog, pituitary secretion seems to be the immediate internal agent through which the external factors act.

SEGMENTATION

While the egg has been passing down the oviduct and receiving its outer coverings, segmentation has been practically completed. As in the Teleost and Gymnophiona eggs, this process involves only the germinal disc (blastodisc) , which at the time of the first cleavage is about 3 mm. in diameter and 0.5 mm. thick. It takes place in the following manner and in the parts of the duct indicated:

The First C1eavage.~—The first cleavage furrow forms in about the middle of the blastodisc, and extends only part way across it and part way through it. It is completed during the passage of the magnum (Fig. 155, A) .

The Second and Third Cleavagesp and the Accessory Cleavage. ——— As the egg enters the isthmus the second cleavage furrow begins to form in the two existing cells; it is approximately perpendicular to the middle of the first furrow, and is of about the same depth. There Fig. 155.——Cleavage in the Hen’s egg. Surface views of the hlastoderm and the inner part of the marginal periblast only. From Patterson. The anterior margin of the blastodisc is toward the top of the page. A. Two cell stage about three hour: after fertilization. B. Four cells, about three and one-fourth hours after fertilization. C. Eight cells, about four hours after fertilization. D. Thirty-four cells, about four and three-fourths hours after fertilization. E. One hundred and ﬁfty-four cells upon the surface; the blastoderm averages about three cells in thickness at this stage (about seven hours after fertilization).

ac. Accessory cleavage furrows. m. Radial furrows. p. Inner part of marginal peniblaet. sac. Small cell formed by the accessory cleavage furrnra. 294 THE CHICK H N.

thus arise four cells, in each of which the furrow of the third cleavage soon appears. These third cleavage furrows may be parallel with the first, but their direction is quite frequently irregular. In this manner

inc. m _ york sc. " CC.

, gggggnnp - uunnm: aaaaw

mp. mc. unuounn-noun using.» me. mp. yolk sc. cc.

.-ago. an

2Z!’.......-. .=-.. ....-.-on-...-uu.

Fig. 156.—Diagrams of vertical sections through the hlastoderm of a Hen’s egg during cleavage stages. A. A section through an approximate 32 cell stage. B. A section through a slightly later stage where marginal cells are being added to the original central cells. C. A section through a still later stage in which the marginal cells have all been used up, and the extra (periblast) nuclei from some of them are invading the periblast to form the germ wall. D. A stage just as gastrulation is about to begin. The zones of junction and overgrowth are well marked, and the germ wall is beginning to add cells to the original marginal cells.

ap. Approximate extent of the area pellucida, not yet marked, however, by the thinning of the blastodermal roof. bld. Blastoderm. cc. Central cells. cp. Central periblast. gw. Germ wall. j. Zone of junction. nmp. New marginal periblast. me. Marginal cells. mp. Marginal periblast. acc. Original central cell region. omc.

Original marginal cell region. amp. Original marginal periblast region. sc. Seg mentation cavity. sub. c. Subgerminal cavity. 9:. Line of separation between the inner portion of the germ wall and the underlyingyolk. zo. Zone of overgrowth.

eight cells are formed, none of which are at first separated from the deeper protoplasm of the disc or from that at the margin.

Before continuing the account of the regular cleavages it is now necessary to pause a moment to note certain so-called accessory cleavages. These cleavages, which are extremely slight and transitory in the Hen’s egg, seem to result from a few divisions of some of the supernumerary sperm nuclei indicated above. They appear at about the four-cell stage as faint radial furrows around the edge of the blastodisc, but by the SEGMENTATION 295

time ten cells have formed they have completely vanished. Scattered and degenerating sperm nuclei are sometimes observable as late as the thirty-two-cell stage; these also, however, are presently lost sight of, and apparently exercise no influence upon the ovum (Fig. 155) .

Fig. 157.—Vertical sections through the Chick blastoderm during the ' process of cleavage. From Kellicott (Chordate Development). After Pat- ‘ terson. A. Section through the two cell stage. B. Median section through

the thirty-two cell stage. C. Part of a longitudinal section through th sixty-four cell stage. b. Blastocoel or segmentation cavity. c. Central cells. i. Inner cell cut oil by horizontal cleavage. 1. Neck of latebra. m. Marginal cells. mp. Marginal periblast. n. Nucleus. p. First cleavage. v. Vitelline membrane.

The Central and Marginal Cells.—Subsequent to the eight-cell condition, following the third cleavage, further furrows soon appear, which result in the production of approximately sixteen cells. Some of these furrows, moreover, are such as deﬁnitely to bound the outer edges of those cells, whose protoplasm has heretofore been -continuous with that which lay further out. Hence, there is thus created a central seg296 THE CHICK

mented area completely delineated from the unsegmented prqtoplasm about it; the cells of this area are termed the central cells.

Cleavage then continues about the rim of this central area, producing new cells here which because of their position are called marginal cells. These cells are for the time being unseparated both from the yolk ﬁlled cytoplasm beneath, and from that lying still further toward the periphery. This condition is characteristic of what is later known as the zone of junction (see below) . As the process of cleavage goes on these marginal cells are constantly being cut oil and added to the central cells; meanwhile beyond them more marginal cells arise. In this manner the central segmented area is continually increasing in diameter (Fig. 156, A; Fig. 157).

The Segmentation Cavity. —Furthermore, at the same time -that the central cells are being defined as such by the furrows at their margins, horizontal cleavages are also taking place. These cleavages intersect the furrows which are visible from the surface, and thus cut off a single superﬁcial layer of the central cells from the protoplasm beneath them. Fluid then begins to collect between this layer of cells and the protoplasm, thus establishing a shallow space, the rudiment of the-segmenzatirm cavity.

As the egg leaves the isthmus, there have been formed in this manner approximately thirty-two cells; 9 it next enters the uterus, in which cleavage is completed and gastrulation begun.

The Periblast and Its Segmentation.-—Further division both horizontal and otherwise now takes place, so that the layer of central cells, at first only one cell thick, soon acquires a thickness of several cells; the area covered by the central and marginal cells has likewise been increased. All the cleavage thus far indicated, however, has taken place within the central region of the blastodisc (Fig. 156, B ) . About the margin of this area, there remains a ring of the disc slightly darker in color than the central portion, and about .5 mm. wide. It is still entirely unsegmented and is known as the pcriblast.

The Germ Wall and Subgerminal Cavity.” —— Presently the formation of marginal cells about the edge of the central region reaches to the inner margin of the ring, defined as periblast. At this point, although the nuclei of the marginal cells continue to divide, the cytoplasmic

3 There are, according to Kiilliker, about forty-four cells in the blastoderm of the Chick at this stage.

3 The ensuing description of the organization of the periblast and other later phases of segmentation are from the accounts of Blount and Patterson, of homologous processes in the Pigeon. SEGMENTATION 297

cleavages do not lceep pace with them. The extra nuclei (periblast nuclei) thus produced then wander out into the region of the periblast and convert. it into a syncytium. Some of these nuclei even move centrally for a short distance into the unsegmented protoplasm beneath the rim of the segmentation cavity. The latter region of protoplasm thus occupied by the extra nuclei is usually known as the central or subgenninal pcriblast (see below), to distinguish it from the strictly marginal periblast, the two regions, however, being.perfcctly continuous. Following the above-mentioned penetration by the periblast nuclei, what was periblast both central and marginal, is known as germ wall, the peripheral non-nucleated cytoplasm in turn becoming periblast (Fig. 156, D). Meanwhile, the last of the original marginal cells have been cut off from the outlying periblast (now germ wall), and have become continuous with, and similar in character to, the cells originally deﬁned as central. Vllithin the syncytial germ wall, cytoplasmic cleavage next begins to take place, and the cells which are thus produced are added to the former marginal cells. Thus, partly by the multiplication of the cells already in existence, and partly by the peripheral addition of new cells arising within the wall, the central area of completely deﬁned cells spreads outward over the surface of the yolk‘ Upon this basis it might be imagined that the germ wall would soon he used up, and as regards the portion of it deﬁned as central periblast this appears to be true. The marginal part of the wall, however, is never exhausted during this process of overgrowth. This is due to the fact that as fast as its inner margin becomes nucleated and then converted into cells, a new germ wall is created by the peripheral movement of more periblast nuclei into the new periblast region which lies continually further out. Meanwhile, as the cellular area is thus extended, the original segmentation cavity likewise enlarges beneath it. This augmented central space is then often referred to as the subgerminal cavity,‘ whose outward extension as such ceases about the time gastrulation is completed.

The Zone of Junction and the Zone of Overgrowth.-——Beyond the extent of the subgerminal cavity, however, the cellular area continues to spread over the yolk. Although the actual cavity as such ceases to expand subsequent to gastrulation, this outgrowth of the cellular region is accompanied by an ever-widening zone, in which the newly formed cells are nevertheless distinctly separated from the underlying yolk. The separation is then continuous at its inner margin with the subger 4 The above distinction between segmentation cavity and subg-erxninal cavity is frequently not adhered to, the two terms being considered synonymous. 298 THE CHICK

minal cairity. It should further be noted that at its outer edge this zone of separation extends somewhat beyond the region where the germ wall has been entirely organized, within its deeper portions, into cells. in other words at the inner margin of the germ wall, the latter is already slightly separated from the yolk beneath it (Fig. 156, D, x) . In its more peripheral part, on the other hand, the germ wall, as already indicated, is quite continuous with the underlying yolk. Likewise, the cells which, even in this outer zone, now cover the upper surface of the wall as fast as it forms, are unseparated by cytoplasmic cleavage from the unsegmented portion of the wall beneath them. Because of this lack of separation between these superﬁcial cells and the wall beneath them, and also between the wall and the underlying yolk, this outer portion of the germ wall is known as the zone of junction (Fig. 156, D). Lastly, beyond the extreme limit of the zone of junction there exists a narrow superficial rim of cells which extends out over the unsegmented yolk lperihlast}, from which it is quite separate. This is called the zone of 0vergr0lL‘t/I, and, although arising from the outer edge of the zone of junction, it seems to be maintained by the multiplication of its own cells (Fig. 156, D).

The BZastoderm.——— It may now be added that with the appearance of these zones the egg has become a blastula, while the entire cellular and partially cellular area, including the zone of junction and the zone of overgrowth, may henceforth be referred to as the blastoderm (Fig. 156, 1)). its establishment terminates the period of segmentation as distinguished from that of gastrulation. Nevertheless, the outward extension of the blastoderm over the yolk continues for some time after the latter process is completed. This is brought about by the steady out-pushing of the zone of overgrowth and the germ wall, which not only themselves increase somewhat in width (particularly the germ wall ), but leave behind them an ever-widening area of extra-embryonic ectoderm, mesoderm, and endoderm. The exact method by which these cell layers are differentiated within the extra-embryonic blastoderm will be discussed in detail later.

Before proceeding with a description of gastrulation, and the origin of these layers, in the Bird, it is desirable to recall one point discussed in connection with the Fish and Cymnophiona. It may he remembered that in both the latter forms the rim of the blastoderm was homologized with the lip of the blastopore. It was, nevertheless, indicated in the introduction that this homology is denied by some in the case of the Bird because of the method of gastrulation in this form as about to be deTHE BLASTODERM 299

scribed. This problem will be mentioned again in that connection. One point of functional similarity between the rim of the hlastoderm in the Fish and Cymnophiona and that in the Bird is, however, already apparent. The process of overgrowth of the yolk, or epiboly, by the blastedermal rim, call this rim what one will, is the same in all. ASTRULATION ‘ AND DEVELOPMENT THROUGH THE FIRST DAY 2 OF INCUBANON

GASTRULATION

T H E problem of gastrulation in the Chick is one which has received considerable attention both by study of normal total hlastoderms and sections, and more recently by experimental procedures. The latter have involved removing living blastoderms and parts of blastoderms to artiﬁcial locations, cutting them at various levels, and marking them with vital dyes. The object has been to determine exactly what movements are taking place, where the primary layers are derived from, and what parts of the early blastoderm give rise to speciﬁc features of the early embryo. In spite of all this study investigators are still not in entire agreement on the answers to some of the above questions. At the risk of satisfying no one, therefore, the writer is going to attempt to piece together a more or less connected account. in doing so it will beneaessary to select conclusions regarding some of the moot points from different workers on the basis of what seems to us most reasonable and likely. Statements over which there is especial disagreement will be indicated in order that the student may be aware of what is most generally accepted and what is not. It will be noted at once that the accepted items largely concern the existence of successive stages of certain structures. Those matters under controversy, on the other hand, have mainly to do with the interpretation of these structures, i.e., questions of their homologies, of how they arise and what they produce. The investigators whose accounts have been particularly consulted are Chen, Hunt, Rawles, Ruclnick, Woodside, Pa.-steels, Peter and Spratt. The review of the subject by Rudnick (’/-14) is especially valuable as a critical summary of the situation to that date, and the interested student is referred to this and to articles by the other authors cited for further details.

1 Gastrulation is usually only slightly under way when the egg is layed (see below).

1’ The term day as used in connection with the development of the Chick refers to a period of 24- hours. ‘

i 3 FIRST DAY: GASTRULATION 301

.dark area within area pel|ur.Ida=

embryonic shield

area opaca

area pellucida

primitive streak

Fig. 158. -——- Photographic surface views of early Chick blastoderms. After Spratt. A. An unincubatcd blastoderin of the pre-streak stage. The dark area opaca, and the lighter area pellucida with a darker region within it, the embryonic shield, are clearly shown. B. A blastoderm of eight hours incubation showing the primitive streak at an early stage. The darker embryonic shield lateral and anterior to the streak shows clearly but is not labelled in this case.

The Area Pellucida and Area Opaca.—As gastrulation begins the blastoderm above the subgerminal cavity becomes thinned somewhat by the outward movement of its cells. For this reason, the absence of adhering yolk and the existence of the cavity, this central region when viewed from above appears different from the surrounding parts. Thus when observed upon the living egg it appears darker, while in a stained blastoderm mounted upon a slide it is more translucent. Be302 THE CHICK

cause of this it is referred to as the area pellucida. The surrounding parts comprising the zone of junction and zone of overgrowth on the other hand are more whitish in the living egg, and more heavily stained and opaque in preserved material. Therefore this surrounding region is appropriately termed the area opaca (Fig. 158).

The Primordial. Hypoblast.—-The first step in actual gastrulation seems to he the appearance within the subgerminal cavity of a sec A are: pellu: Na r—- eplblut

archenteron yolk

Z°"m W3“ . primordial hypoblasl: ]zg°;,r-'g°ﬁ§',f~, primitive streak B

ectoderm

Fig. 159.—-«Diagrams of sections through the Chick blastoderm showing the origins of the primordial hypohlnst, the deﬁnitive endoderm and the mesoderm. A. A median sagittal section through a very early Chick blastoderm such as is shown in Fig. 158, A, in which the primitive streak has scarcely begun to form. The hypohlast has just been delaminated (and, or, inﬁltrated) from the epiblast. The area option at this stage consists only of the zone of junction and the zone of overgrowth. At this stage the zone of junction is mostly, though not entirely, identical with the germ wall. Thus it will be noted that the latter extends slightly medially beneath the archenteric space. Later only a small part of the outer periphery of the germ wall is thus identical with the zone of junction. B. A cross section of the hlastoderm of a slightly later stage where the streak has formed,

and mesoderm, and perhaps deﬁnitive endoderm, is arising in connection with it in the manner indicated in C.

ond cell layer which may be termed the primordial hypoblast. The space between this layer and the underlying yolk then, as in the case of the Fish, becomes the archenteron. The new layer is designated “ primordial” because it appears doubtful that it represents the ﬁnal or deﬁni. tive hypoblast, or at least that it represents all of it. The method of its origin is one of the disputed questions. It was formerly supposed to originate by involution of marginal cells through a temporary interrupFIRST DAY: GASTRULATION 303

tion in the zone of junction along a small part of the hlastodermal rim. The location of this activity if it occurred would of course represent, as in the Fish, the dorsal blastoporal lip, and hence also as in the Fish the future posterior region of the embryo. It has even been claimed by one observer that an actual invagination occurs here, giving rise to a pocket with both roof and ﬂoor, i.e., a complete archenteron (Jacobson. ’38) . At present, however, the belief in either involution or invagination as deﬁned in this text is no longer entertained in the case of the Chick. Instead Peter (’38) and others seem to think that the process is rather what we have designated as inﬁltration. That is to say, these workers believe that individual cells wander in from the surface and detach themselves within the subgerminal cavity where they eventually become arranged to form a more or less continuous layer. lt should be noted incidentally that the sponsors of this View do not use the term inﬁltration, preferring to call the inwandering of these individual cells “invagination." This, however, seems to the writer a misnomer—and confusing.-At all events regardless of the terminology the activity is said to be as designated.

It must further be stated that those who are agreed on the character of the process as described are not entirely agreed on just where it takes place. According to some (Pasteels, ’45) it occurs more or less all over the pellucid area of the hlastoclerm. Peter, however, seems to think it takes place mainly toward the future posterior side, especially near the margin, with a subsequent forward movement. This would approach more nearly the older idea of an involution from one side.

Finally it may be said that some workers (Spratt, ’46) describe the process of hypoblast origin as one of splitting oil or delamination of cells rather than their inwanclering (Fig. 159, A). Also at least one investigator (Fraser, ’54«) has observed the inﬁltration of cells from the epiblast at the anterior and posterior borders of the area pellucida, suggesting once more a sort of modified involution at these borders, but without interruption of continuity in the epihlast. It is of interest to note here that a similar problem regarding the nature of hypoblast origin occurs in the Mammal where again some form of inﬁltration or delamination seems to occur. This matter will be referred to later in the appropriate connection.

After the formation of the layer of primordial hypohlast it might be assumed that gastrulation, as deﬁned in this text, would be complete. However, as noted, this hypoblast is.~prol3ably only part of the deﬁnitive hypolalast (endoderm) , and in the Bird more than in the Frog and Fish 304 THE CHICK

it is diﬁicult to separate sharply the origin of the deﬁnitive hypoblast from the origins of the mesoderm and notochord. Also the appearance of the primitive streak, a structure previously related primarily to gastrulation, is, as we shall see, probably involved here both in the formation of deﬁnitive hypoblast, and in the origin of mesoderm and notochord. We shall therefore have to continue our discussion of these activities more or less simultaneously as a later aspect of gastrulation.

Before proceeding with this it may be remarked that it is at about this stage of development that the egg is usually laid. The diameter of the entire blastoderm is approximately 3.36 mm., and that of the area pellucida about 2.16 mm. (Spratt, ’46). If unincubated it may remain in this condition for some time. If incubation ensues before too long an interval has elapsed further development proceeds as follows:

The Primitive Streak.—The second step in gastrulation is the development of the primitive streak whose history is as follows: Just before the streak begins to form, about three fourths of the area pellucida_ as viewed from the surface, starts to become more darkly stain ing and opaque toward what later proves to be its posterior side. This

is due both to a thickening of the epiblast in this region, and to the presence of the underlying hypoblast. The part so affected is sometimes designated as the embryonic shield, though not entirely homologous with the region similarly named in the Fish as previously described (Fig. 158, A ) . Presently the streak begins to appear at the posterior side of this shield, as a still more darkly staining somewhat triangular structure with its base in Contact with the inner rim of the area opaca (Fig. 158, B). This appearance is produced by a further thickening of the epihlast in the region concerned in a manner to be indicated below. At first the thickened cpiblast reaches only a short distance cephalad, but soon, as its growth is completed, its anterior end occurs at about the middle of the pellucid area. As a result of this increase in length the structure loses its triangular shape, and appears more as a broad band or actual streak with a tapering and rounded anterior end. At the same time sections reveal that from its first appearance the thickened epiblast of this band has been in intimate contact with the underlying hypoblast. A little later the hand (primitive streak) becomes still narrower, and a distinct groove develops down its middle with a little twist or irregularity at its cephalic extremity where the groove terminatcs in a slight pit. The groove is termed the primitive groove,3 and

3 The term primitive streak is sometimes rather carelessly used to refer to both streak and groove. FIRST DAY: GASTRULATION 305

the pit is the primitive pic. The latter together with the surrounding cells is called Hensen’s knot or Hensen’s node, also the primitive node (Figs. 160, 161, 162) . The sides of the groove are sometimes designated as the primitive folds, having nothing to do of course with the later neural folds. So far as the writer is aware no one questions the existence of these structures as described. Again the real problem concerns the homology of the streak or groove, its origin and its functional relation to the parts about it. Since the answer to the first of these queries depends

vnedullary told ya vlrelllnu Intern:

Henna’: knot

= blood islands at the ma vnmlon

Fig. 160.-— Surface of the Chick hlastoderm and early embryo. A. A pre-incubation blastoderm showing the primitive streak, actually the primitive groove. B. An 18 hour blastoderm showing the beginning of the head process (notochord). C. A 24 hour blastoderm with embryo well started and the area vasculosa forming.

largely upon the answers to the last two, we shall take these latter up in order. We shall then be prepared to return to the problem of homology.

The Origin of the Streak.--As a result of numerous marking experiments it appears to be fairly clear that the streak originates by the convergence of epiblast cells from the lateral regions toward the place where the initial short “ streak” is ﬁrst seen (Chen, ’32, Spratt, ’46), (Fig. 163) . This produces an aggregation of material here which constitutes the thickening described as characteristic of this structure. It should also be noted, as Spratt points out, that the cells thus aggregated do not pile up upon the surface of" the blastoderm, but pass inward, as he expresses it by “ invagination.” It is this process which almost at once, as previously indicated, brings them in contact with the underlying hypoblast. After being started in this manner the lengthening of the streak occurs, according to Spratt, by the proliferation of its cells as follows: At its front end these cells are so added as always to be at or near the tip, as in the growing point of a plant. Posteriorly the growth seems to be more by intussusception pushing this end backFig. 161. —— Five transverse sections through the head process and primitive streak of a lCcIhick embryo. The head process is very short. From Lillie (Development of the Chic’ ).

A. Through the head process, now fused to the entoderm. B. Through the primitive knot. C. Through the anterior end of the primitive groove. D. A little behind the center of the primitive streak. E. Through the primitive plate. The total number of sections through the head process and primitive streak of this series is 102. B is 4 sections behind A. C is 12 sections behind A. D is 59 sections behind A. E is 87 sections behind A.

Ect. Ectoderm. Enz. Entoderm. GJV. Germ wall. H.Pr. Head Process. med.pl. Medullary plate. Mes. Mesoblast. pr.f. Primitive fold. pr.g . Primitive groove. pr.Im. Primitive knot. pr.pl. Primitive piste.

306

.-.w 30?

Fig. 162. —— Three transverse sections of a late stage, through the head process and primitive streak of a Chick embryo. From Lillie (Development of the Chick). A. Near the hind end of the head process. B. Through the primitive pit. C. A short distance behind the center of the primitive streak.

BLI. Blood island. coel.Mes. Coelemic mesoblast. Ecl. Ectoderm. Ent. Entoderm. G.W. Germ-wall. med. pl. Medullary plate. Mes. Mesoderm in area pellucida. N’ch. Notochord. pr.]. Primitive iold. pr.gr. Primitive groove. pr.p. Primitive pit. 308 THE CHICK

ward, Accompanying, and perhaps partially caused by this movement, the whole pellucid area changes its shape‘ from that of a circle to a pear with the small end posterior. Finally it may be stated that this growth of the primitive streak appears to be induced by the underlying primordial hypoblast. This is concluded from the fact that this hypoblast is at

Fig. 163.——A diagram to illustrate the movements occurring on a Chick blastedcrm during gastrulation and primitive streak formation. After Spratt. The movements are indicated by changes in the positions of carbon particles placed on the hlastoderm at the start of the process. Horizontal rows A, B and C illustrate three different plans of placing the particles. Vertical rows I, II and III indicate the positions of the particles in each plan during successive stages in gastrulation. The short horizontal lines outside the blastoderms are points of reference. Note the general tendency of convergence toward the forming streak.

first chieﬂy toward the posterior of the bl-astoderm, and as it spreads anteriorly the growth of the primitive streak follows it. There are also other facts which support this hypothesis (Fig. 164) .

Functional Relations of the Primitive Streak. Diﬂerentiation of Mesoderm, Endoderm and Ectoderm. —It is now rather generally conceded that not only are materials moved into the FIRST DAY: GASTRULATION 309

streak from the outlying epiblast, but they also pass through it to speciﬁc destinations (Hunt, ’37, Spratt, ’4-6) . One of these is apparently a layer of cells pushing out on either side of the streak between the epiblast and the primordial hypoblast. This layer is the mesoderm. It is also claimed that some of the cells moving through the streak pass into and augment the previously existing primordial hypoblast (Hunt, ’37) , (Fig. 159, B, C). Thus this latter layer is converted into deﬁnitive hypoblast, or as it may now be called endoderm. The question as to just how much of the endoderm owes its origin to this movement of cells through

epiblqsf A _prirnitive streak

pre-head prccess cells

germ wall germ wall 3 primitive node primitive pit primitive streak (groove)

head process (notochord) endoderm 99"“ w°"

Fig. 164.——A diagram of a median sagittal section through the primitive streak, A, and groove, B, and parts anterior to each, showing the origin of the head process inotochord) according to Spratt and Fraser.

the streak, and how much to the spread of the primordial hypoblast is one of the unanswered questions. As usual after the origin of these layers the remaining epiblast may be called ectoderm.

Lastly, it may be noted that the process just indicated in connection with the origin of the mesoderm and endoderm is again what we should term a kind of inﬁltration. Nevertheless, as will be pointed out subsequently, it does bear some resemblance to the passage of cells around a blastoporal lip, i.e., involution, and might help to account for the development of the groove. Also, as in the case of the inwandering of cells from the surface into the primordial hypoblast, it has been referred to, ambiguously the writer thinks, as “ invagination.”

The Head Process (Notochord). ——This leaves the origin of the notochord still to be accounted for. Accompanying the above-mentioned activities there also appears in front of the primitive streak or groove another somewhat narrower line temporarily termed the head process (Fig. 160, B). It begins at Hensen’s knot with which it maintains constant contact, and extends anteriorly. Sections reveal that it consists of a line of cells somewhat like the streak, but in this case they have no definite connection with the epihlast, now ectoderm, save at Hensen’s 310 THE CHICK

knot (Figs. 161, A; 162, .4). This head process rapidly increases in length, and eventually undergoes histological changes to become the notoclzord. Concerning the above statements there is no question. The problem again arises, however, as to where the head process (notcchord) originates from, and by what method it develops. It has been claimed that it arises by a splitting off of streak material from the epiblast in a posterior direction. Thus as the head process grows at its back end the streak would shorten proportionally at the front end (Lillie, ’19). The streak does indeed shorten, but not proportionally. Hence it has been claimed by others that the head process grows from cells budded off from the anterior end of the streak, and pushed forward. Finally according to Spratt, ’47, and Fraser, ’54, the following occurs: At first the streak, as noted, is quite short. As its substance grows anteriorly beneath the epiblast, the cells of the latter, originally just in front of the streak, come to lie posterior to its anterior tip, i.e., somewhat behind the primitive node and pit. Some of these cells then pass into the substance of the streak and forward within it to a point under the node. Here they form a mass from which the head process is budded, almost entirely posteriorly (Fig. 164, A). This means that the primitive streak is forced to recede before it. However, according to Spratt’s evidence it does not shorten at its anterior end in the region where it is in contact with the head process. Instead the substance of the streak is “ pushed ” back, or at least it migrates backward. But though the streak does not shorten at the front end, it does shorten at the back end. It does this simply by “ dissolution ” into the ectoderm and mesoderm of this region. As indicated in connection with one of the other theories, however, this shortening is not quite at the same rate (i.e., proportional to) the lengthening of the head process. Therefore Spratt suggests that there must be some condensation of material in the shortened streak. Eventually, nevertheless, the latter does entirely disappear, except in so far as its remains may constitute the “ end bud ” (posterior tip) of the embryo. Figures 164 and 165 illustrate diagrammatically the processes supposed to be involved. This theory of head process (notochord) origin is supported by extremely careful studies based on a somewhat new technique. Instead of the dyes previously used for marking points on the living blastoderm, carbon particles were introduced into it, thereby eliminating the spreading of the marks by mere dilfusion. Their movements were then kept track of in relation to certain ﬁxed points outside the area where the critical changes were occurring. The results seem

conclusive, but will of course have to be conﬁrmed by other workers. FIRST DAY: GASTRULATION 311'.

Distribution of Formative Materials in the Streak and Prestreak Blastoderrn.———In our consideration of gastrulation in the Frog emphasis was laid on experiments indicating the distribution of germ layer materials previous to the gastrulation process. The question naturally arises therefore as to whether it has been 1: ssible to make comparable pre-gastrular maps in the case of the Bird. The answer is

REEKREDK.

SNORTINIHG Of STRZAK REGRE3$|DN

POST. BORDER 0? PSLLUCID AREA 0

n.————-———-n 00 GSIIK

Fig. 165. —- A diagram to illustrate the movements occurring in the primitive groove (“ streak”) and parts connected with it during head process (“ chorda") formation. After Spratt. Three cells in the groove were marked by carbon particles just before the head process started to appear as shown by the dots on the streak at the left. As the head process forms, the location of the particles and the changes in the parts are seen in successive stages as one passes to the right. Note what happens to the groove as the head process lengthens.

that if one considers the existence of the primordial hypoblast as denoting the completion of gastrulation, such maps have not been made. This is not surprising since this stage is reached prior to the laying of the egg. However, in so far as the formation of the primitive streak is regarded as part of gastrulation, the answer is quite otherwise. Many studies have been made of the potentialities of the various regions of the blastoderm beginning with the late pre-streak stage, and extending on to that of the head process. Wetzel, ’29, Rawles, ’36, Pasteels, ”37, Rudnick, ’38, most recently Spratt, ’42, and others have worked on this problem largely by two techniques. (1) They have vitally stained or otherwise marked the various regions of the hlastoderm in situ, and noted the subsequent movements of the stained parts. (2) They have isolated pieces of the blastoderm on various culture media, and observed what each piece is able to produce. Naturally, the later in de312 THE CHICK

velopment the experiments were performed, the more precise have been the results, but also of course the further they are removed from the pregastrular situation. It is not feasible to go very deeply into this topic,

cartilage I bonemuscles» mcaortephroa

Fig. 166. —-A diagram showing the sections into which a primitive groove and head process stage of a Chick blastoderm was cut, and the tissues and structures derived from the mesoderm of each isolated piece. After Rawles.

but we may present as an example of the conclusions of some of the work on later stages one of the maps by Rawles (Fig. 166) . With reference to this map it should be stated that the results upon which it is based were all obtained by the isolation method, and it must be admitted that this method has one weakness. Since the isolate is in a new environment the potentialities which it exhibits are not necessarily those FIRST DAY: HOMOLOGY OF STREAK 313

it would have realized had it been left intact. In fact they are apt to be greater, due perhaps to the removal of inhibition by neighboring parts, or to lack of speciﬁc induction by those parts. It should be understood that though the map selected is for mesoderm only this does not mean that this was the only layer studied, or that the layers were transplanted separately. The results for the different layers were merely recorded separately as a matter of convenience, and our choice of the map of this particular layer has no special signiﬁcance. As regards the conclusions, in view of the results on earlier stages to be indicated presently, it is perhaps noteworthy that for all layers the regions capable of producing therhost structures were those near the center of the blastoderm, i.e., about Hensen’s node. It is of further interest that the left side showed more potentialities than the right.

An example of a study of very early stages (early streak and late pre-streak) is that of Spratt’s isolation work (742). Stated very brieﬂy his conclusions are essentially as follows: He ﬁnds, in substantial agreement with most others, that prospective neural plate material lies near the center of the area pellucida. Notochord, on the other hand, is formed from the region just behind this in about the third quarter of the pellucid area. Potential mesoderm, including heart forming material, appears to be somewhat more widely diffused both anteriorly and posteriorly. From this we see that although it has not been possible to map prospective germ layer and organ-forming regions quite as early or as accurately as in the case of the Amphibians, some progress has been made. Thus it is at least evident that the materials for the nervous system, the mesoderm and notochord exist independently in more or less separate, though overlapping, localities at the pre-streak stage, and that they are subsequently moved into their deﬁnitive positions as the streak develops. Whether the separation of these substances occurs still earlier, perhaps even in the unsegmented egg, as in Amphioxus and the Arnphibia, we do not yet know.

THE HOMOLOGY OF THE PRIMITIVE STREAK

It will be recalled that the term primitive streak was used in connection with the Frog, Fish and Cymnophiona to denote the line formed by the closed blastopore. The question now is whether the primitive streak of the Chick is really homologous with this line, and hence represents a closed blastopore. ‘ 314 THE CHICK

REASONS AGAINST HOMOLOGY OF PRIMITIVE STREAK AND CLOSED BLASTOPORE

(1) The streak is not at any time an opening into the archenteron, as a real blastopore is supposed to he.

(2) The origin of the primordial hypoblast at least is not related to it, nor to its “ lips ” (sides of the groove) .

REASONS FAVORING HOMOLOGY or PRIMITIVE STREAK AND CLOSED BIASTOPORE

(1) In the Frog and Fish it was shown that there is a convergence of materials on the outside of the hlastula toward the forming blastopore. Various marking experiments on the epiblast of the Chick blastoderm show similar movements of material in its postero-lateral regions toward the forming primitive streak.

(2) In Amphioxus, the Frog, and Fish there was shown to be an involution of the materials just mentioned over the dorsal lip into the roof and sides of the archentcron. In the Chick there is, strictly speaking, no blastopore in the region of the streak, and hence no blastoporal lip. The streak, however, does have contact with the primordial hypohlast, and it does develop along either side of it, ridges which would correspond to the lateral lips of a blastopore. Most important of all it has been shown that there is a movement of material through these ridges into the forming mesoderm, and possibly into the endoderm. In other words as previously suggested there is a kind of “involution,” in which the presumed homologues of the blastoporal lips are intimately involved.

(3) In Amphioxus, the Frog, and Fish the notochord arises from material involuted at the dorsal lip of the blastopore, and budded forward from that region. In the Chick we have seen that the notochord originates from cells passing inward not, to be sure, through the pit, whose anterior rim is the homologue of the dorsal blastoporal lip, but posterior to it. Yet even here such movement is suggestive, even though the material grows backward instead of forward to form the notochord.

(4) In Amphioxus and the Frog we have found the neurenteric canal originating by the uniting of the neural folds over the anterior part of the closing blastopore (primitive streak), while in the Fish Kupffer’s vesicle, the homologue of that canal, occurs at the same location. Now in the Chick, to be sure, there is no neurenteric canal at the anterior end of the primitive streak. There is, however, a pit at this point which is FIRST DAY: THE AREA OPACA 315

eventually covered by the neural folds, and in some Birds (Duck, Goose and others) this pit does ﬁnally open to the archenteron. Thus in these cases a neurenteric canal, incipient or actual, is formed in the proper place if the streak be regarded as a closed blastopore.

( 5) In the Frog, certainly, and probably in the Fish, the anus forms at the end of the closed blastopore opposite from the neurentcric canal, the line between the two being designated as the primitive streak. We have just seen that at least in some Birds what amounts to a neurenteric canal forms at the anterior end of the streak. On this basis the anus should arise at the posterior end of this structure, and apparently it does so (Lillie, ’l9). V

( 6) In the Frog the material in and about the lip of the early blastepore is known to have remarkable inductive powers. In the Chick the primitive streak is said by some (Woodside, ’37) to have similar powers when transplanted beneath the epiblast of a very early primitive streak host.

EXTENSION OF THE GERM LAYERS AND FORMATION OF THE AREAS VASCULOSA AND VITELUNA

Up to this point the processes of gastrulation and germ layer formation have been considered only in relation to the area pellucida. It now remains to consider what is happening in these connections in the area opaca.

ORIGIN OF ENDODERM IN THE AREA OPACA

In connection with the origin of the primordial hypoblast before the advent of the primitive streak, it was noted that this hypoblast arose by the inwandering (inﬁltration) of cells from the surface of the blastederm, or by delamination from its under-surface. It was also said that this probably occurs mostly about the posterior half of the blastoderm, perhaps more especially around its margins. This hypoblast was then supposed to be later augmented to form endoderm by inﬁltration of cells through the streak. Upon this basis it is not surprising therefore to learn that according to some accounts the endoderm of the area opaca is derived as follows:

It is said that the nuclei from the zone of junction keep moving in toward the area pellucida. As they do so, the cytoplasm about each nucleus engulfs yolk granules, and becomes cut off from that about it to form a definite cell. Thus the lower part of the germ wall becomes or316 THE CHICK

ganized so that toward its inner margin (the edge of the area pellucida) , it begins to form a cell layer. This layer is endoderm which becomes continuous with the deﬁnitive endoderm of the area pellucida. If this account be correct it would seem that a process which is essentially inﬁltration, in this case from the margins of the blastoderm, is still giving rise to some of the endoderm, i.e., that of the area opaca. It may now be stated that because of its subsequent history the endoderm of this area is

often referred to as yolk-sac endoderm.

THE BLOOD ISLANDS AND THETMESODERM IN THE AREA OPACA

The Blood Islands.—Though the origin of the endoderm of the area opaca has been described first, it actually follows slightly, both in

time and peripheral location, the formation of the mesoderm which comes about somewhat indirectly as follows: It appears that cells from the

postero-lateral margins of the mesoderm in the area pellucida wander into the upper part of the germ wall of the area opaca, where they also engulf yolk granules. These cells become aggregated into small masses in this region, and these masses presently anastomose to form a network. Throughout this network spaces or lacunae are then developed which contain little groups of cells. Presently the walls of the lacunae become differentiated into the ﬂat endothelial cells characteristic of the inner lining of blood vessels, while the cells within the lacunae be come blood corpuscles. Because of the manner of their formation these corpuscles are at first necessarily aggregated into groups, which appear from the surface as darker splotches. These splotches of corpuscles, or forming corpuscles and their surrounding endothelium, are known as blood islands. Obviously they arise somewhat previous to the main parts of the circulatory system with which they presently become connected (see below).

The Mesoderm of the Area Opaca.—Coming now to the mesoderm of this region we ﬁnd that it is produced by the budding off of cells from the surface of the developing blood islands, between the islands and the overlying ectoderm. At its inner margin this mesoderm like the endoderm becomes continuous with that occurring in the area pellucida ( Fig. 162, C).

It remains to state that because of the indirect method of production of this mesoderm its source as just described has been questioned by some. Thus it has -been claimed that the blood islands, and hence the mesoderm, come from cells originating in the zone of junction in the FIRST DAY: THE AREA OPACA 317

same manner as the endoderm of this area. The account as we have previously given it, however, is afforded strong support by the following fact: Patterson (’O9) has shown that where the mesoderm of the pellucid area fails to reach the germ wall no blood islands and no mesoderm develop in the area opaca. It may ﬁnally be noted that if the mesoderm of this area does arise from that in the area pellucida, as seems most probable, then like the latter it also, though somewhat indirectly, has its ultimate source in the primitive streak.

Though beginning in the postero-lateral regions as indicated the processes thus described are gradually working forward upon each side of the area opaca, the proliferated mesoderm of the area pellucida keeping pace with that which arises from the blood islands further out. Finally, as the level of the anterior end of the head process is reached, the mesoderm of the pellucid area ceases to form. That in the area opaca, however, continues upon either side as a pair of anteriorly projecting wings, which after proceeding somewhat beyond the future head region begin to turn toward one another so that they eventually meet (see second day). In the area pellucida, however, immediately in front of and slightly to the sides of the head region, no mesoderm forms for some time, the zone thus marked out being termed the proamnion (Fig. 160, C). Following the advent of the blood islands it soon becomes possible to subdivide the blastoderm into further parts as follows:

The Area Vasculosa. ——-The blood vessels, having once become formed in the area opaca, are not conﬁned there. Very soon, especially postero-laterally, they begin to extend into the, area pellucida, where they unite with other vessels which have arisen in situ from the mesoderm; the entire region thus covered by them is then termed the area vasculosa. Presently, around the outer edge of this area, its boundary begins to be clearly deﬁned by an encircling blood vessel, the sinus ter minalis (Fig. 160, C).

The Area Vitellina. — The remainder of the blastoderm peripheral to the area vasculosa is termed the area vitellina, and is in turn subdivided as follows: The part at and near the blastodermal rim continues to consist of the relatively narrow zone of overgrowth and zone of junction, and is known as the area vitellina externa. Between this area and the area vasculosa there is then a. region which, with continued expansion of the blastoderm, soon becomes ‘rather extensive. Within it, although the germ wall is becoming occupied with yolk ﬁlled cells, these cells have not yet become deﬁnitely organized into endoderm or blood islands. Nevertheless this part of the wall is clearly separated from the 318 THE CHICK

epiblast above it, and is beginning to be more or less delimited from the non-cellular yolk beneath it. The relatively broad region thus characterized is called the area vitellina interna (Figs. 167, 170, A, E ).

As has already been suggested, all of these areas, while retaining the same relative position as regards each other, are constantly moving outward over the surface of the yolk by a process of epiboly (Fig. 167).

Fig. 167. - A. Hen’s egg at about the twenty-sixth hour of incubation, to show the zones of the blastoderm and the orientation of the embryo with reference to the axis of the shell. B. Yolk of hen’s egg incubated about 50 hours to show the extent of overgrowth of the blastoderm. From Lillie (Development of the Chick). After Duval.

a.c. Air chamber. a.p. Area pellucida. a.v. Area vasculosa. a.v.e. Area vitellina externa. a.v.i. Area vitellina interna. Y. Uncovered portion of yolk; i.e., the “yolk blastopore" or yolk-sac umbilicus (see below, and page 362).

FURTHER HOMOLOGIES

The Margin of the Blastoderrn.——It was stated in connection with the Fish that the margin of the blastoderm, or germ ring in that form was entirely homologous with the blastoporal lips, and that it finally closed to form a primitive streak. It was then indicated that in the Gymnophiona the margin of the blastopore is again the homologue of the blastoporal lips. In this instance, however, these lips (germ ring) become divided into two parts by the early contact of points on the lateral lips a short distance from the dorsal lip. In this manner a small true blastopore (later a primitive streak) is formed immediately in front of which the embryonic axis proceeds to develop. The remainder of the blastodermal rim is then employed in covering the yolk. As it completes this process there appears what amounts to a second or yolksac blastopore, with the closure of which the yolk is entirely enveloped.

>41! ._.......-,.-a.« W .. . .. ‘J FIRST DAY: FURTHER HOMOLOGIES 319

The question now to be answered is what if any homologies exist between the avian primitive streak and blastodermal rim, and the blastopores of the Fish and Gymnophiona. We have already given reasons for homologizing the primitive streak of the Chick with the streak of less advanced forms such as the Fish and Frog in which this structure represents the entire closed blastopore. What then of the remaining blastodermal rim in the Bird?

In answering this let us ﬁrst consider the character, and then the behavior of this rim. From what has been said it is clear that according to present views there is no real involution at the blastodermal rim of the Chick. Hence the epiblast and primordial hypoblast do not actually unite along this line as at the typical lip of a blastopore. This is most clearly true in the very early stages when the inﬁltration or the delamination of primordial hypoblast cells is said to occur more or less all over the blastoderm. Even at this time, however, there is some evidence that this process is more active about the postero-lateral margins. Later, moreover, when the area vitellina externa has been established it has been indicated that the origin of the cells for the endoderm of the yolk sac, according to many, is mainly dependent upon, nuclei migrating from the zone of junction. Thus it can be said that a kind of modiﬁed involution is after all occurring at essentially the margin of the blastoderm, and that ectoderm and endoderm are ultimately in contact in that region. So much for the character of the margin. As to its behavior. it has already been said that the blastoderm spreads over the yolk by the usual process of epiboly, and this continues until ﬁnally the yolk is completely enveloped. By virtue of its method of formation the covering thus developed consists of all three germ layers, and is called the yolk-sac.

Upon the basis of both structure and function, therefore, it is evident that the hlastodermal rim of the Chick bears a striking resemblance to the blastoporal lips or germ ring of the Fish, and even more to that of the Gymnophiona. Indeed there are only two essential differences between the rim of the blastoderm in the latter and that in the Bird. One is the fact that in the Gymnophiona there is deﬁnite involution at one point on the margin, while in the Bird there is not. The second difference is that in the Gymnophiona the blastoporal lips (blastodermal rim) immediately adjacent to the region of involution soon fuse to form a primitive streak. In the Bird, on the other hand, the primitive streak is apparently formed by a convergence of material in the posterior part of the blastoderm, but not from material actually in the blastodermal 320 THE CHICK

rim. In both cases the remainder of the yolk beyond the blastoderm is temporarily uncovered, constituting the so-called yolk-sac blastopore (Fig. 168). This is later enclosed by a yolk-sac in the Bird, and by what virtually amounts to that in the Gymnophiona. In the Fish, of course, the blastodermal rim is not thus divided into two parts, and hence there is no question about the homology of all of it with a blasteporal lip. In the Fish, however, there is no endoderm in the yolk-sac.

Summary of Gastrulation Processes and Homologies in the Chick.—We may conclude the discussion of gastrulation by summarizing the processes involved in the Chick as follows: According to

Fig. 168. —Median sagittal section. Stage of the first intersomitic groove. (Cf. Fig. 169). Owing to the bending of the primitive streak the section passes to one side of the middle line posteriorly. From Lillie (Development of the Chick).

Ect. Ectoderm. F.G. Fore-gut. CJV. Germ-wall. H.F. Head~fold. med.pl. Anterior end of medullary plate. Mes. Mesoderm. N’ch-l-Ent. Notochord and entoderm. Pr’a. Proamnion. pr.kn. Primitive knot. pr.p. Primitive pit. pr.str. Primitive streak. Y.S. Ent. Yolk-sac entoderm. '

the definitions adopted in this book they would include inﬁltration (i.e., a modiﬁed kind of involution), or (and) delamination, convergence and epiboly. ‘

As to homologies, the primitive streak of the Bird is probably homologous with all other primitive streaks, including those in the Frog, Fish, Gymnophiona, and, as we shall see, the Mammal. Furthermore, there is good reason to homologize the blastodermal rim plus the primitive streak of the Bird with the whole blastodermal rim of the Fish, though the latter contains no endoderm. Likewise we may equally well homologize the rim of the blastoderm of the Bird minus the primitive streak with the rim minus the streak in the Gymnophiona.

DETERMINATION OF THE EMBRYONIC AXIS

It is of course obvious that whatever fixes the position of the primitive streak determines the embryonic axis; The question therefore is what ﬁxes the position of the streak. We must immediately answer that, as in the case of the Fish, we do not certainly know. However, there are some reasonable hypotheses up to a certain point.

If a hen’s egg is allowed to rest on its side for a short time it' will he FIRST DAY: THE EMBRYONIC AXIS

found upon opening it that the yolk (ovum proper) has turned so that the blastederm is uppermost. Furthermore, if the egg is fertile, and has been incubated, the long axis of the primitive streak, and hence of the embryo, is sometimes exactly, but more often roughly, at right angles to that of the egg shell. Lastly, it will also be true that if the small end of the shell is to the right of the observer, the anterior end of the streak, and hence later the head end of the embryo, will usually be away from him (Fig. 167). These facts have long been known, but in themselves only raise further questions, to wit: Why is the embryo transverse to the length of the shell? Why is the head end away from the observer and why are there exceptions? These are the crucial points. It may be stated to begin with that, granted one initial

321 u. r.

H. F.

\./'

Fig. 169.—Stage of first intersomitic groove drawn from an entire mount in balsam by transmitted light. From Lillie (Development of the Chick).

a.c.v. Amnio-cardiac vesicle. a.o. Inner mar gin of Area opaca. Ect. Ectoderm. Ent. Ento-_ ,

derm. H. F. Head-fold. i.s.f.l. First intersomitic furrow. med.pl. Anterior end of medullary plate. Mes. Mesoderm. n.g.r. Neural groove. pr.gr. Primitive groove. Pr’a. Proamnion.

assumption, one group of known facts might account for the transverse position, the direction of the head and the exceptions. The unproved assumption and the facts are as follows:

The assumption is that the egg passes from the ovary into the oviduct in such a position that the blastoderm will rest against the wall of the duct, not toward its lumen. It has been suggested by T. H. Morgan (’27) that this might occur if the ovum is regularly more compressible in any axis at right angles to the one vertical to the blastoderm. Granted this initial assumption, it is then known that the blastoderm retains its position against the side of the duct as the ovum passes along it, revolving Ent. spl. Mes. Coel. Nch. C09’ Somp.

$pl’p|.

Fig. 170.-—A. Transverse section across the axis of the embryo and the entire blastoderm of one side. The section passes through the sixth somite of a 10s embryo, and is intended to show the topography of the blastoderm. The regions B, C, D, E are represented under higher magniﬁcation in the Figs. B, C, D, E. From Lillie (Development o/ the Chick).

A0. Dorsal aorta. a.u.e. Area vitellina exrerna. a.v.i. Area vitellina interna. Bl.i. Blood island. Bl.v. Blood vessel. Cael. Coelom. GJV. Germ wall. M.0. Margin of overgrowth. Nch. Notochord. N.F. Neural fold. Nph. Nephrotome. S. Somite. Somp. Sammopleure. Spl’pl. Splanchnopleure. Som.Mes. Somatic layer of mesoblast. spl.

Mes. Splanchnic layer of the mesoblast. S.T. Sinus terminalis. Y.S.Em. Yolk-sac cntoderm. ZJ. Zone of junction.

322

I FIRST DAY: THE EMBRYONIC AXIS 323

as it goes. This means that the blastoderm traces an imaginary spiral path around the wall of the duct. It is also known that the small end of the shell is usually found at the leading end. Under such circumstances Morgan further points out that the following conditions might then ensue. As the egg revolves, the two sides of the blastoderm might be under unequal pressure. This might then determine the transverse position

Fig. 171.—Median longitudinal section of the head, stage of 4 s. The section

passes through the length of one of the neural folds just behind the anterior end. From Lillie (Development of the Chick).

a.i.p. Anterior intestinal portal. Ect. Ectoderm. Ent. Entoderm. F’ .0. Fore-gut.

H.F. Head-fold. Mes. Mesoderm. Mes.H.C. Mesohlastic head cavity. n.F. Neural fold. or.pl. Oral plate.

of the primitive streak, its long axis lying parallel to the direction of pressure. Furthermore, the pressure might presumably be greater on the side toward which the egg was revolving. If so, and if the egg always revolves in the same direction, this might determine that the anterior end of the streak and embryo would always be on a certain side. Bartelmez (’18) has added the notion that the primitive streak axis is determined before the egg leaves the ovary. Then, if as suggested, it always passes into the duct in a certain way this might result in making the primitive streak axis always transverse to the duct and shell. The assumption of Bartelmez may be true, but there is no adequate proof for it, and it seems only to push the ultimate solution further back. ll V 5: it

324 THE CHICK

Morgan’s theory involves fewer unproved premises, and, due to slight differences in direction of pressure, may account for the variations.

THE HEAD FOLD

A short distance in front of the anterior end of the head process, there develops shortly a slight depression, and immediately posterior to this depression a crescentic fold appears, involving both ectoderm and endoderm (Figs. 168, 169, 171). Its ends extend almost from one side of the area pellucida to the other. The crest of this fold is not raised perpendicularly to the surface, but extends forward so that it overhangs the depression indicated above. It is the head fold, and its anterior edge marks the anterior end of the embryo. The lateral and posterior limits of the embryo are not distinguishable until much later. \

THE FORE—GUT

From the method of its formation, the cavity within the head fold is necessarily lined by endoderm which is co-extensive with the endoderm of the archenteric cavity posterior to it. It is the anterior portion of the future fore-gut, the portion which may be said to represent the pharyngeal region. It is a broad, ﬂattened cavity, and as suggested, opens posteriorly into the extensive archenteric space over~lying the yolk. The region of this wide opening is known as the anterior intestinal portal. The endoderm on the antero-ventral side of the fore-gut soon fuses with the ectoderm below it in a limited region to form the oral plate (Fig. 171) ; elsewhere between the ectoderm and endoderm of this vicinity, there are scattered mesoderm cells, i.e., mesenchyme.

DIFFERENTIATION DF THE EMBRYONIC MESODERM IN THE AREA PELLUCIDA

THE SOMITES AND LATERAL PLATES

The lateral sheets of mesoderm of the area pellucida now become thickened along either side of the head process and primitive streak The ridges thus formed are known as the vertebral or segmental plates,

while the remaining lateral portions of the sheets are called the lateral ’

plates. Just in front of the anterior end of the primitive streak a transverse ﬁssure now appears in each of the vertebral plates. The region of the plates immediately anterior to these ﬁssures then constitutes the first pair of.sorr_zitcs; they remain continuous anteriorly with the mesoderm FIRST DAY: SOMITES, LATERAL PLATES 325

of the head region (Fig. 172) . Slightly behind the first pair of ﬁssures a second pair develops, and the part of the vertebral plates between the first and second pairs of ﬁssures is the second pair of somites. The exact number of somites, and correlated development, varies consider €. 0.

F. G.

Fig. 172.—Chick embryo with three pairs of somites (about 23 hours). Dorsal view. From Lillie (Development of the Chick).

zz.c.v. Amnio-cardiac vesicle. a.a. Inner margin of area opaca. F .G. Fore-gut. N’ch. Notochord. n.F. Neural fold. pr.gr. Primitive groove. 31, .92, 3;. First, second, and third somites. .

ably, especially in the early stages, due to the breed of hen, the condition of the egg at laying, the precise temperature and other factors. At the end of 24- hours, however, there are usually from three to six of them——often about four——lying anterior to the primitive streak and hence upon either side of the head process, i.e., the rudiment of the notochord. The first four pairs of these somites later disappear, being included in the posterior part of the head. 326 THE CHICK

The Nephrotome. —A narrow strip of each lateral plate immediately adjacent to the somites serves, as it were, to unite them to the main part of the plate. It is known as the nephrotome, and later gives rise to the excretory organs.

THE COELOM

Within the lateral sheets, which for a time remain connected with the somites by means of the nephrotomes, horizontal splits now develop. They occur first in the anterior portions and gradually spread elsewhere. Of the two sheets thus formed, the one next to the ectoderm is the somatic or parietal mesoderm (somazopleure) , and that next to the endoderm the splanchnic or visceral mesoderm (splanchnopleure) . The space between them is the coelom (Fig. 170).

THE RUDIMENT OF THE PERICARDIAL CAVITY

In the region of the head fold, the coelomic spaces on each side push toward each other. By so doing, they ﬁnally work their way in between the ectoderm and endoderm just at the bend where these two layers pass up from the depressed area under the fold on to its ventral surface. At the end of 24 hours, the walls of the opposite spaces have met each other and fused, so that the spaces themselves are separated only by a thin layer of mesoderm. This process tends to separate the ectoderm and the endoderm by pushing.the latter further back, and thus increasing the length of the fore-gut. These in-pushing portions of the coelom are

the amnio-cardiac vesicles, and they represent the rudiment of the pericartlial cavity (Figs. 172, 183).

THE NERVOUS SYSTEM

Among the most conspicuous features of the early embryo is the rudiment of the central nervous system. This system first appears in the following manner:

THE MEDULLARY OR NEURAL PLATE

Beginning almost at the anterior limit of the head fold the ectoderm above and along each side of the head process is thickened somewhat; this thickening is the medullary plate. Posteriorly, the lateral portions of the plate extend also along each side of the primitive streak (groove). while the central portion merges with the ectoderm of the groove. FIRST DAY: THE NEURAL TUBE 327

THE MEDULLARY GROOVE AND MEDULLARY FOLDS

Presently a depression appears running down the middle of the medullary plate above the head process, and on each side of this depression, the lateral portions of the plate rise up as two parallel ridges. The depression is, of course, the medullary or neural groove, while the ridges are the medullary or neural folds (Fig. 172). Approximately at the anterior end of the plate, the ends of the folds meet one another. However, because of the fact that they are already quite close together, this meeting does not form an extensive transverse ridge as in the Frog. Posteriorly, the folds do not at first reach quite to the region of the first somite, but before the end of the day they have extended backward to about the anterior end of the shortened primitive streak.

THE NEURAL TUBE

The parallel medullary folds now bend toward one another until their crests meet and fuse a little distance posterior to the anterior limit of the head fold, in the region of the future rnid-brain. As in the case of the Frog, a continuation of this fusion results in the formation of a thick-walled tube, whose roof, sides, and ﬂoor are derived from the inner walls of the medullary folds and from the groove; it is the neural tube and its cavity of course is the neural canal. As in the Frog, also, there occurs shortly after the fusion of the folds, a separation between their inner (neural) and outer walls, the latter reconstituting above the tube a continuous layer of ectoderm.

These processes continue both anteriorly and posteriorly until the tube is entirely closed in. During the closure, however, the usual anterior and posterior openings into the neural canal persist. The former is the neuropore, corresponding to the structure of that name in the forms previously studied; this opening is closed during the first day. It should also be noted that because of the protrusion of the folds in this region, they extend forward slightly beyond the anterior limit of the fore-gut (Fig. 172). Later, as growth proceeds, this region is actually carried over the anterior end of the embryo on to the ventral side (see below under ﬂexures). Posteriorly fusion takes place more rapidly, keeping pace with the extension of the medullary folds. Because of the greater distance to be traversed, however, the process in this direction is not completed until some time later. The completion at this end is marked by the disappearance of the primitive streak (Fig. 173). 328

THE CHICK

8.0.8.

op. Ves.

ceph. Mes. F. G.

V. o. m.

s. 2.

n. T.

s. T.

N'ch.

Fi . 173.—-Chick embryo with seven pairs of somites (alxaout 26-27 hours). Dorsal view. From Lillie (Development of the Chick).

a.c.s. Anterigr cerebral suture; i.e., line of fusion of neural folds ‘here. ceph.Mes. Cephalic mesoderm. F.G. Fore-gut. N’ch. Notochord. n.T. Neural tube. op.Ves. Op ..tic vesicle. Pr’-a. Proamnion. pr.str. Primitive streak.

3.2,.-r.7. Second and seventh somitee. V.a.m. 0mphaIomesenteric (vitelline) vein. Fig. 174.—Transverse section Am.I". Amniotic fold. A0. Aorta. Coel. Coelom. 362). My. Myotome. My’c. Myocoel. N’ch. Nolochord. somite. Scler. Sclerotome. V.c.p. Posterior cardinal vein.

W

n

D

through the twentieth somite of a 29 s embryo.

Derm. Dermalome. Gn. N.Cr. Neural crest. Nep/z.T

Ganglion.

Wolﬁian duct.

1

L5.

Nephrogenous tissue. 3.20.

From Lillie (Development of the Chick).

Lateral limiting sulcus (see page Twentieth

FIRST DAY:

THE NEURAL TUBE 329 330 THE CHICK.

THE NEURAL CRESTS

At the same time that fusion of the folds is occurring, cells are proliferated between the outer and inner layers of each fold, just in the region of its crest. Thus, as fusion takes place, these cells form a band along either side of the dorsal part of the neural tube between it and the surface ectoderm. These bands are the neural crests, which at this time are united with one another across the dorsal surface of the tube

(Fig. 174). THE OPTIC VESICLES

Anterior to the ﬁrst point of fusion, the neural tube is broadened somewhat. This is the region of the future optic vesicles.

SUMMARY OF THE CONDITION AT THE END OF THE FIRST DAY OF INCUBATION ‘

I. THE MESODERMAL STRUCTURES

About four pairs of somites are present, lying in front of the primitive knot and connected with the mesoderm of the respective lateral plates by the longitudinal nephrotomal bands.

The lateral mesoderm extends throughout the area pellucida except in the region of the proamnion, and together with the endoderm is being differentiated in the area opaca. In the latter area, the formation of this layer has progressed anteriorly until a pair of wing-like extensions are level with the tip of the head fold. Also in the area pellucida this mesoderm has been split into two sheets, the somatopleure and splendinopleure, with the coelomic space between them, and this process is spreading into the area opaca. Beneath the fore-gut, the walls of the amnio-cardiac portions of the coelorn have just met each other, and the rudiment of the pericardial cavity is thus indicated in this region.

ln connection with the formation of the mesoderm, blood vessels and corpuscles have started to appear in the area opaca and area pellucida, transforming both into the area vasculosa. The latter is beginning to be bounded by the sinus terminalis.

4 Degree of development, including somite number, as noted, varies considerably, especially through 48 hours of incubation. and the hour or stage conditions designated in this text do not exactly agree with the carefully obtained results of Hamburger and Hamilton, 51. However, they are believed to correspond well with those indicated on the slides sold by most of the Biological Supply companies. FIRST DAY: SUMMARY 331

Outside the area vasculosa is an area consisting only of partially differentiated germ wall, the zone of junction, and the zone of over-xmwth the area vitellina. 5 ’

II. THE HEAD FOLD AND THE FORE—GUT

The head fold has formed and in the process has given rise to the an. terior or pharyngeal portion of the fore-gut.

III. THE RUDIMENTS OF THE NERVOUS SYSTEM

The medullary folds have appeared in the region in front of the primitive knot and have fused for a short space at their anterior ends_ The neural crests have begun to appear, and the rudiments of the optic vesicles are also indicated. 10

HE CHICK: DEVELOPMENT DURING THE SECOND DAY OF INCUBATION GENERAL APPEARANCE

TH E embryos of the higher vertebrates, including Reptiles, Birds and Mammals, all develop in a more or less confined space, i.e., either within an egg shell or within the uterus. Also, in the early embryonic life, almost the anterior half of the organism in these forms is occupied by the brain which is growing very rapidly. Not only is this true, but the dorsal part of the mid-brain is growing with disproportionate rapidity, and this, combined with the conﬁning space, causes a very marked bending of the entire anterior region. This bending presently leads also to a turning of the head end (torsion), and ﬁnally of the whole embryo, upon its side, as described below. Thus though the bending and turning are basically due to changes in the brain, and will be described in terms of that

structure, it is convenient to do it under the heading of general external features.

FLEXURES AND TORSION

The Cranial Flexure. ——The ﬁrst bend, and one previously noted in connection with the brain of the Frog, is the cranial ﬂexure. In the latter animal it was the only marked ﬂexure of the brain, and had nothing to do with development in a confined space. Indeed the curve of this region of the brain was rather in part the remains of a portion of the original curvature of the egg. In the Chick and other higher animals the cranial ﬂexure does not have this origin, but it does involve exactly the same regions of the brain, and the front of the embryo; i.e. it involves the fore-brain region which is bent down anterior to the notochord. This ﬂexure begins at about thirty hours, and by the end of the day the bending is so great that the morphologically dorsal side of the midbrain is actually the most anterior part of the embryo. The morphologically anterior side of the fore-brain, on the other hand, faces posteriorly so that this part of the embryo almost touches the heart (Figs. 175, 176). Finally, it should be noted that, as in the Frog, this ﬂexure, in so SECOND DAY: LIMB BUDS

far as it concerns the brain, is permanent, and is the only one of those indicated at this time which is so.

The Cervical F1exure.——By the end of the day another broad curvature is evident, extending through the region of the hindbrain and back into the trunk. This is the cervical flexure, and has no counterpart in the Amphibian.

The Lateral Rotation or Torsion. —— Finally as a result of both these ﬂexures the front of the embryo would be thrust deep into the yolk were it not for a lateral twist which begins at the anterior end. By 48 hours it has progressed posteriorly about as far as the back end of the cervical ﬂexure, i.e., approximately to the thirteenth somite. It is called the lateral ratation or torsion, and eventually results in turning the entire embryo over so that it lies upon its left side (Fig. 176) .1 It should be clearly understood in this connection that the terms dorsal, ventral and lateral in the present and following descriptions are used in their morphological sense. Thus dorsal will always refer to the side of the embryo upon which

Fig. 175. —Chicl: embryo with twenty pairs of somites (about 4-3 hours). Dorsal view. From Lillie (Development of the Chick).

A.o.m. Vitelline artery. au.P. Auditory pit. Cr.Fl. Cranial ﬂexure. D.C. Ductus Cuvieri. Dienc. Diencephalon. M esenc. Mesencephalon. M etenc. Metencephalon. Myelenc. I and 2. Anterior and posterior divisions of the myelencephalon. 0p.Ves. Optic Vesicle. Ph. Pharynx. pr.str. Primitive streak. s.2.s.5., etc. Second, ﬁfth, etc., somites. Telenc. Telencephalon. Vel.tr. Velum transversum. I/en. Ventricle.

the nerve cord and notochord occur, and ventral will refer to the opposite side regardless of how the embryo lies.

LIMB BUDS

No limb buds are ordinarily visible at 48 hours. Nevertheless, if tissue from the locations where they would later appear is transplanted to

1 Occasional embryos are found lying upon the right side. Apparently this does not prevent subsequent normal development. 334 THE CHICK

Fig. 176. ——Chick embryo with twentyseven pairs of somites (about 48 hours). From Kellicott (Chardatc Developmerm. After Lillie.

a. Auricle. am. Posterior margin of amniotic folds. c. Carotid loop. cf. Cranial ﬂexure (cervical ﬂexure also shown, see p. 333). d. Diencephalon. dC. Ductus Cuvieri. g1, g2, g3. Visceral clefts. i. Isthmus. 1. Lens. ma. Mandibular arch. ms. Mesencephalon. mt. Metencephalon. a. Otocyst; to right of otocyst is ganglion of VII and VIII cranial nerves. r. Retinal layer. S2, 510, 520. Second, tenth, and twentieth somites. L. Tail-bud. 1;. Vemricle. va. Vitelline artery. vv. Vite]line vein. 1, 2, 3. First, second, third aortic arches. V. Ganglion V cranial nerve.

other locations it will produce there either a wing or a hind limb depending upon its source. Furthermore, the dorso-ventral and antero-posterior axes of these transplanted tissues will not have been altered, i.e., such potential limb tissue (anlage) transplanted in an inverted position will produce an inverted limb. Thus it appears that the destiny of this tissue has already been rather completely determined. It will not only form a limb, but a limb of a particular type which retains all

its original axes (Hamburger, ’38).

THE SOMITES

When last mentioned, the somites were described as masses of mesoderm connected with the lateral plates by means of the nephrotomes. During the second 24 hours the connection between nephrotome and somite is obliterated throughout the greater part of the embryo; the number of pairs of the latter increases to 27, and beginning at the anterior end the development of each of the sornites proceeds in the following manner:

THE MYOTOMES AND THE CUTIS PLATES

Each somite is at first composed of an outer layer of comparatively dense cells surrounding an inner

mass of mesenchyme, the latter SECOND DAY: THE FORE-GUT 335

comparable to a myocoel, so far as one exists (Fig. 170, B). Presently, however, the denser layer of cells on the side of the somite next to the nerve cord and notochord largely disappears, leaving the latter structures in direct contact with the mesenchymatous mass indicated above. At the same time the dense layer upon the dorsal and outer side of the somite becomes thicker. The dorsal portion of this outer layer is the rudiment of the myotome, while the more lateral and ventral portion is the cutis plate or dermatome. Before the second day has passed, the dorsal or myotomal portion of the above plate of cells begins to turn sharply upon itself and grow downward between the mesenchyme and the cutis plate. Thus a double layer of cells begins to be fonned consisting of the cutis plate on the outside and the myotomal plate on the inside (Fig.

1 74) . THE SCLEROTOME

The mesenchyme which now begins gradually to surround the notochord and the ventro-lateral region of the nerve cord is the rudiment of the sol otome.

THE ALIMENTARY TRACT

THE F ORE—GUT

The Stomodaeum.—--During the first day it was noted that the antero-ventral end of the fore-gut came in contact with the ectoderm at a point on the ventral side of the head fold to form the oral plate. Now, as the result of the downward flexure of the head and also of the pushing forward of the mandibular arches (see below), the central region of the plate becomes relatively depressed to form a pit lined by ectoderm. It is the beginning of the stomodaeum, and by a continuation of the above process it presently acquires a considerable depth.

Rathke’s Pocket. — From the antero-dorsal wall of the stomodaeum a small diverticulum now appears growing anteriorly along the morphologically ventral side of the posterior portion of the fore-brain which has been bent down in front of it. It is called Rathke’s pocket, and is destined to become the anterior part of the hypophysis or pituitary. (See the footnote on this under the Frog.)

The Visceral Pouches and Arches.

The Pouches. —— ln the anterior or pharyngeal portion of the fore-gut, a series of vertical folds of the endodermal wall begin to push outtaward the ectoderm on each side of the head. These are the visceral 336 THE CHICK

pouches, and they develop in regular order, the most anterior pair appearing ﬁrst. The first pair are known as the first visceral or hyomandibular pouches, and the remaining pairs, of which there are three, as the second, third, and fourth visceral (“ branchial ”) pouches. They decrease in size posteriorly, the last pair being relatively small. The ﬁrst pair of pouches, i.e., the hyomandibulars, fuse with the corresponding ectodermal invaginations (visceral furrows) only at their dorsal ends, while the second and third pairs fuse with their respective furrows throughout their lengths, except for a slight interruption in their upperhalves. The point of fusion of the ﬁrst pouch now becomes perforated as the first or spiracular cleft. The fusion of the fourth pair of pouches and furrows, and the perforation at the points of fusion of the second and third pairs to form actual visceral clefts, occurs later (Figs. 176 and 194).?

The Arches. -—~ Anterior and posterior to each pouch the mesenchyme becomes thickened to form the visceral arches. The arch in front of the first or hyomandibular pouch is the ﬁrst visceral or mandibular arch, and the one between it and the second pouch is the second visceral_or hyoid arch. The remainder are simply the third, fourth, and fifth visceral (“ branchial ”) arches, and they appear in the same order as the pouches; the fifth and last arch is hardly more than a transitory vestige. Presently, blood vessels and nerves pass into the arches, as will be indicated later.

It should be noted in passing, that though these pouches and arches correspond to the similarly developed structures in the Frog, in this case no gills ever appear in connection with any of them. The term visceral rather than branchial is therefore more aptly applied to them all.

The Thyroid. -—-This begins to develop near the end of the second day as a small thickening in the middle of the ﬂoor of the pharynx, between the ventral ends of the second pair of visceral arches. Before the end of the day it has become slightly evaginated so as to form a shallow depression in the pharyngeal ﬂoor (Fig. 184) .

2 According to a recent investigator (Dudley, ‘42) there are actually six visceral pouches in the Chick embryo, but the last two are very early merged with the

fourth to form what this author calls the “fourth visceral complex,” the “sixth pouch ” component later forming the post-branchial body (see below). As will he

noted later, others have regarded the primordial lung outgrowths as ﬁfth visceral_

pouches. It appears to the present writer that these are all somewhat forced

attempts to make the situation in the Bird square more nearly with that in some.

of the lower Chordates. Whether either the lung outgrowths or the rudimentary structures referred to by Dudley really represent any visceral pouches or not, is, the writer believes, still open to considerable question. SECOND DAY: THE HIND—GUT 337

The Respiratory Systems--Late the second day a longitudinal groove, with a pair of slight posterior expansions, appears in the floor of the pharynx caudal to the visceral pouches. lt is the beginning of the larynx, the trachea, and the lungs, and thus represents the start of the entire respiratory system. In this connection it may be recalled that according to one View the lung primordia of the Frog are to be homologized with a hypothetical seventh pair of gill pouches. It is therefore of interest to ﬁnd that in this case the above expansions which later develop into the lung primordia of the Chick are similarly homologized by some with a ﬁfth pair of visceral pouches. (See, however, preceding footnote.) ”

The Liver. —Just at the posterior limit of the fore-gut behind the pharyngeal region, there appear at this time two slight antero-ventrally directed evaginations of the endoderm whose development is said to depend on Contact with the veins ( cardiac primordial in this region {W illier, and Rawles, ’3] T). The diverticula are not of course suspended in space. but pushed forward into the mass of splanchnic mesoclerm (ventral nzesentery) which unites the gut and the ductus venosus in this vicinity. One of the diverticula is a little in advance of the other both in position and in time of appearance. lt presently pushes forward so as to lie just dorsal to the point of union of the vitelline veins (see below), while the other, at this period, is barely distinguishable. These two diverticula represent the rudiments of the liver.

THE MID—GUT

There is little indication of any real mid-gut during the secondday, but rather merely a wide enteric space overlying the yolk. The beginning of folds along the sides of the embryo continuous with the lateral margins of the head fold suggests, however, the manner in which this portion of the gut will be formed.

THE HIND—GUT

The Posterior Intestinal Portal and Anal Plate. —— At the close of the second day the hind-gut begins to develop in connection with a tail fold very similar to the head fold. There is thus formed a posteriorly directed cavity lined by endoderm, and lying beneath the remains of the primitive streak. It is the hind-gut, and opens anteriorly into the wide enteric space overlying the yolk (rudiment of the mid-gut). As in the case of the fore-gut, the region of this opening is termed an intestinal portal—in this instance, the posterior intestinal portal. There is li338 THE CHICK

nally one further resemblance between fore- and hind-guts in that at the end of the latter the endoderm comes in contact with the ectoderm and fuses with it. This point of fusion at the posterior end of the primitive streak. and marks the location of the future anus. lt is termed the anal plate or eloacal membrane. Besides these points of resemblance, there are now to he noticed Certain important differences as follows

(Fig. 177):

T.B. tf

Eat. all. t.

Fig. 177.~—Median longitudinal section through the hind end of an embryo of about 21 s. From Lillie (Development of the Chick}. an.p[. Anal plate. an.!. Anal tube I’l1ixirl-grill. Ect. Ectoderm. Ent. Endoderm. files. Mesoderm. p.IT.p. Posterior intestinal portal. T.B. Tail-bud. t.f.So’pl. Tail iold.in the zomatopleurc and ectoderm. t.f.Sp‘pl. Tail fold in the splanchnopleure and endoem.

The Ventral Mesentery.—lt has been stated that the hind-gut is formed in connection with a tail fold, just as the fore-gut is formed in connection with the head fold, and in a general way this is true. In the case of the tail fold, however, there is this difference. The endoderm is folded in to form the hind-gut and the intestinal portal, but in this case the ectoderm follows this infolding much more slowly than it did in the case of the head fold. Thus it happens that the hind-gut arises before there is any very marked indication of a tail fold on the surface of the blastoderm. For this reason the anal plate, unlike the oral plate, remains dorsal for some time, and is only gradually carried around onto the ventral surface (Fig. 177) . ‘

Furthermore, this lagging behind of the ectodermal portion of the fold necessarily means that there is a gap between the two cell layers; this gap in the case of the tail fold is ﬁlled by mesoderm. Presently lateral extensions of the embryonic coelom press back into this region upon either side, but for a time they do not meet one another. Thus there is left a median mesodermal mass extending from the ventral side

l SECOND DAY: THE HEART 339

of the hind-gut backward and upward to the underside of the lagging ectoderm. That portion in contact Wiitll the gut may be referred to as splanchnic, and that in contact with the ectoderm as 50ma;gc_ The two

portions are continuous, however, and together are known as the ventral mesenteri-' of the hind-gut.

B

Fig. 178.—Ventral views of the head ends of Chick embryos. From Lillie (Development of the Chick). A. Embryo with five pairs of somitcs (about 23 hours). B. Embryo with seven pairs of somites (about 25 hours).

a.c.v. Amnio-cardiac vesicle. a.i.p. Anterior intestinal portal. End’c.s. Endocardial septum. F .0. Fore-gut. Ht. Heart. M }*’C. Myocardium. N’ch. Notochord. N’ch.T. Anterior tip of nomchord. n.F'. Neural fold. op.Ves. Optic vesicle. p.C. Pericardial cav ity (amnio-cardiac vesicles). Pr’a. Proamnion. 32.54. Second and fourth mesodermal somites. V .o.m. Omphalomesenteric vein.

THE CIRCULATORY SYSTEM THE HEART

The Origin and the Formation of the Enclothelial Lining. — While blood vessels and corpuscles have been developing from the germ wall in the area opaca, vessels have also begun to form in the area pellucida. These latter vessels, which are in direct continuity with ‘chose already formed, also arise from blood islands, though these islands are slightly different from those of the area opaca. They are merely aggregations of cells, apparently detached from the splanchnic mesoderm, and the vessels into which they develop are temporarily entirely devoid 340 THE CHICK

of corpuscles. Erythrocytes, however, are soon supplied from the area opaca, and also by cells buddecl from the posterior ends of the dorsal aortae (Danchakoii, ’07). Thus from the cell aggregates, as indicated, rudiments of two large vessels (the omphalomesenzeric or vitelline veins)

Fig. 179. ——~ Sections through Chick embryos showing particularly the formation of the heart. pericardial cavity. and pharynx. From Kellieott (Chordate Developrmnzt). After Lillie. A. Just posterior to the anterior intestinal portal of a Chick with seven pairs of somitcs (about 25 hours). B. Section just anterior to A. C. Through the heart of an embryo with ten pairs of somites (about 29 hours).

am. Axial mesodermal thickening. (:0. Lateral dorsal aorta. ebc. Exocoelom. cc. Ectoderm. en. Endoderru. hb. Hind-brain. 17. Blood islands. 17]). Anterior intestinal portal. my. Mayocardium (muscular layer of heart). n. Notochord. nc. Nerve cord.

p. Pharynx. pc. Pericardial cavity (atnnio-cardiac vesicles). 3. Endothelial septum. so. Somatic mesorlerm. sp. Splanchnic mesoderm. th. Cardiac entlothelium. 11. Area vasculosa. um. Ventral mesocardium. w. Germ wall. y. yolk~sac endodcrm.

soon appear in the area pellucida (Fig. 178) : Each rudiment rests upon one of the ventro-lateral walls of the fore-gut, between it and the median-lateral wall of the respective amnio-cardiac vesicle from which it has arisen (Fig. 179, A) .3 The anterior portions of these rudiments then form the ehdothelial lining of the heart in the following manner:

It is to be recalled thatthe amnio-cardiac vesicles have already become fused beneath the fore—gut, just in front of the endodermal wall

3 The evidence of this ﬁgure would seem to indicate that the vessels are derived from the walls of the gut rather than from those of the vesicles, and some authorities hold this to he the case. In view, however, of the origin of the other blood

vessels of this area from the mesoclerm, it seems more likely that the latter derivation is the true one.

i

a—u—an.m..._..,.. SECOND DAY: THE HEART 341

which marks its posterior limit (Fig. 178, A). The fusion now progresses posteriorly, as it does so pushing back and closing in the ventralateral gut walls against which the veins indicated in the preceding paragraph are resting. Thus as these walls come together the anterior ends of the above mentioned vessels are likewise brought together side by side beneath the newly formed gut, and as this occurs they fuse with one another to form a single vessel with a median partition. This partition soon disappears, and the single median tube which remains is t: e endothelial lining of the rudimentary heart (Figs. 178, B and 179, B, C).

The Myocardium of the Heart. — The median walls of the amniocardiac vesicles which now lie against each side of the endothelial tube presently press in above and below it, and fuse with each other. Thus the tube is completely surrounded by mesoderm which forms the myocardium or muscular element of the heart, and its covering the visceral pericardium.

The Mesocardia.———The above fusion leaves the endothelial tube and its myocardium suspended from the mesodermal covering of the ventral wall of the fore-gut, or pharynx, by a double layered sheet of mesoderm (ventral mesentery) here termed the dorsal mesocardium. Ventrally also a similar sheet attaches the tube to the underlying splanchnic mesoderm. The latter quickly disappears, and the former does so later, except at the anterior and posterior ends of the heart (Fig. 179, C).

The Pericardial Cavity and Parietal Pericardium. —With the fusion and disappearance of the median walls of the amnio-cardiac vesicles, it is clear that their cavities have become a single space which surrounds the heart. This space is the pericardial cavity, and its walls constitute the rudiments of the greater part of the parietal pericardium. Postero-laterally, however, the pericardium is still incomplete, and hence the above cavity continues to communicate in this direction with the general coelom.

The Rudiments of the Atria, Ventricles, Bulbus and Truncus Arteriosus. — In connection with the description of the development of the Frog heart it was noted that the development of all Vertebrate hearts is essentially similar. This similarity has already become apparent as between the Frog and Chick in that the hearts of both start with the fusion of two vessels to form a tube. Further similarities will now reveal themselves in the transformations of this tube in the Chick to form the adult organ. 342 THE CHICK

As in the Frog, the straight tuhe ﬁrst increases in length, and, its

‘ends being ﬁxed, its middle hows laterally to the right (Figs. 180 and

181). The broad apex of the how is then drawn ventrally, and usually slightly posteriorly, while the whole tube is at the same time thrown into a loop. (These terms of direction it should here be recalled are being used in the morphological sense regardless of the rotation of the embryo onto its side.) Again as in the Frog, the loop which has been produced in the originally straight tube lies to the right of the median line. This means that the posterior limb of the loop extends ventrally,

op. Ves.

VII -VIII an. F.

V. o. rn. 5.4.

a. i. p.

Fig. 180. —— Ventral view of the anterior end of ti Chick embryo with sixteen pairs of somites (about 38 hours). From Lillie (Development of the Chick). (I.i.p. Anterior intestinal portal. au.P. Auditory pit. B.a. Bulbus arteriosum F.B. Fore-brain. Inf. lnfundihulum. op.Ves. Optic vesicle. 0r.p1. Oral plate. Pr’am. Proamnion. 3.4-. Fourth somite. Tr.a. Truncus arteriosus. v.Ao. Ventral aorta. Ven. Ven- ' tricle. V.o.m. Omphalomesenteric (vitelline) ‘vein. V II—-VIII. Acustico-facialis ganglion. and as suggested, usually slightly posteriorly. The middle part then curves laterally toward the right, where it passes into the ascending limb which extends dorsally, anteriorly and medially back into the median plane (Figs. 108, 176) . It now remains to indicate the parts of the future heart which the various regions of this loop are destined to form. Beginning at the posterior end the region where the posterior limb starts to descend will comprise the atria. The apex of the loop and a small portion of the descending and ascending limbs will become the ventricles. The larger part of the anterior ascending limb will become the bulbu: and truncus arteriosua. SECOND DAY: BLOOD VESSELS 343

As regards the functioning of the Chick heart, the first indications of it have been found to occur about the twenty-ninth hour of incubation, and as in the Frog, long before any innervation. T he contractions begin along the right side of the heart tube in the future ventricular region, and then spread to the left. As the atrial region forms behind the ventricular, the contractions also extend to it, and ﬁnally to the sinus venosus. As in the case of the Frog, experimental transections of the heart tube show that the inherent rate of contraction increases as one passes posteriorly. Also the most posterior region at any given stage acts as the pacemaker, while the older anterior regions gradually lose the power of automatic contraction. Thus the rate for the whole heart is slowly stepped up and is ﬁnally set by the sinus, which is ultimately incorporated into the right atrium ( Patten and Kramer, ’33, Barry, ’42). Later on following innervation the rate of heat is of course partially under nervous control.

THE BLOOD VESSELS OF THE EMBRYO

The Arteries.

The Dorsal Alarms and Their Branches. Along each side of the embryo, just at the inner margin of the pellucid area, two vessels now develop. These are the dorsal aortae (Fig. 181, A). Anteriorly each is continued into a vessel differentiated in the mesenchynie on either side of the head.l’osteriorly they give elf branches between the somites (segmental arteries) , and ﬁnally leave the sides of the embryo at about the level of the seventeenth somite to pass out into the general vascular network as the vitellinc arteries. Near the end of the second day the two dorsal aortae fuse with one another in the region above the heart, forming for a short distance a single dorsal vessel.

Development of the Aortic Arches. -——-The truncus arteriosus at first runs anteriorly a short distance, this short relatively horizontal extension being called a ventral aorta. It is. however, merely a continuation of the truncus, and is presently so incorporated with it that there is no distinction. At its anterior end this short extension of the truncus divides into two vessels which extend still further forward in the pharyngeal ﬂoor. They also are frequently called ventral aortae. As will presently appear, however, their proximal portions really constitute the proximal parts of the ﬁrst pair of aortic arches (Figs. 180. 176). Somewhat anterior to the oral plate each of these vessels bends sharply upward to join the respective dorsal aorta, this bend being termed the 344 THE CHICK

Fig. 181.~—Chick embryo with 12 pairs of somites (about 33 hours). From Lillie

(Development of the (Jhiclr). A. Dorsal view of entire embryo. B. Ventral view of anterior end.

A.C.S. Anterior cerebral suture. a.z'.p. Anterior intestinal portal. A0. Dorsal aorta. F.C. Fore-gut. H.B. Hind-brain. Ht. Heart. M.B. Mid-brain. op.Ves. Optic vesicle

or.pl. Oral plate. pr.slr. Primitive streak. 82 S12. Second and twelfth somites. v.Ao. Ventral aorta. V.o.m.. Omphalomesenteric vein.

carotid loop. Meanwhile, as previously indicated, the visceral pouches and arches have been forming, and in the arches certain blood vessels have been developing on each side as follows:

In the ﬁrst place the single or common ventral aorta has, as pre-'

dicted, become incorporated into the truncus whose wide dorsal end now terminates directly beneath the visceral arches. While this has been SECOND DAY: BLOOD VESSELS 345

occurring each first or mandibular arch has pushed ventrad. As a result of this the proximal part of each of the separate ventral aortae comes to lie within about the ventral four-ﬁfths of the respective mandibular arch. Thus, as suggested above, this part of each ventral aorta comes to form the proximal portion of each ﬁrst aortic arch. The more distal fifth of each first aortic arch which will lie within the corresponding distal ﬁfth of the mandibular arch, remains for the time being incomplete. The proximal four-ﬁfths of this vessel is, however, still connected with the dorsal aorta by way of the remaining anterior tip of the respective ventral aorta and carotid loop as previously indicated (Fig. 176). The actual completion of the distal portion of the first aortic arch so that this artery lies entirely within the‘ mandibular arch apparently does not occur until the third day, and wiil be described when that stage is reached. The development of the remaining aortic arches is more straightforward. The second aortic arches develop in the second visceral or hyoid arches, and the third aortic arches develop in the third visceral arches. These last pairs arise as buds from the dorsal aortae which grow almost directly ventrad through the arches to -join the dorsal end of the truncus.

The Veins and the Lateral Mesocardia. —— As has been indicated above, the endothelial portion of the heart is formed by the growing together of two large vessels (omphalomescnteric veins) . It now remains to state that this union continues for a short distance posterior to the atrial rudiments. The most anterior part of this continuation is somewhat dilated and is known as the sinus venosus, while slightly further back it receives the name of ductus venosus. The most anterior portion of the sinus venosus is sometimes regarded as part of the heart proper, because later it is involved in the development of the right atrium. At this stage, however, it may best be considered as a part of the venous system.

During the second day there develops in the mesenchyme on each ventro-lateral side of the brain a vessel which runs posteriorly as far as the level of the heart. These are the anterior cardinal veins. Meantime there has occurred on each side of the embryo a fusion of the lateral body wall with the posterior part of the sinus venosus. Thus a pair of septa have been formed each of which passes somewhat diagonally laterally and dorsally from the posterior part of the sinus to the respective body wall. These are called the lateral mesocarclia, and within each of them develops a rather large vein, the ductus Cuvicri (Figs. 176; 182, C). Each ductus Cuvieri connects ventrally with the sinus venosus and dorFig. 182.——Diagrams of the circulation in the Chick embryo and area vasculosa. From Kellicott (Chordate Development). The vascular network of the area vasculosa is omitted for the most part. A. Anterior and central parts of the embryo and vascular area at about thirty-eight hours (sixteen pairs of somites). Viewed from beneath. After Popoﬁ. B. Median and anterior parts of vascular area and embryo at about seventy-two hours. (twenty-seven pairs of somizes; the number is usually nearer to 36 at. this age). Viewed from beneath. After Popoff. C. The main vascular trunks of the fourth day. After Lillie (modiﬁed).

a. Dorsal aorta. aa. Aortic arches (first and second in A, second, third, and fourth in C) . ac. Anterior cardinal vein. al. Allantois. au. Atrium. b. Bulbus arteriosus. 11G. Ductua Cuvieri. dv. Ductus venosus. cc. External carotid artery. 1:. Heart. ic. Internal carotid artery. la. Lateral dorsal aorta. It). Left anterior vitelline vein. p. Anterior intestinal portal. pc. Posterior cardinal vein. u. Posterior vitelline vein. ru. Right anterior vitalline vein. t. Sinus terminalis. tr. enoue trunks oi the area vasculosa.

v. Venn-icle. va. Vitelline artery. w. Vitelline or omphalomesentcric vein (in this region really lateral vitellinc vein) . . 546 SECOND DAY: CIRCULATION 347

sally with the posterior end of the respective anterior cardinal vein. From this point of union still another Vein grows posteriorly along each side of the body. These veins are known as the posterior cardz'.vzals tl7i;:.

182, C ). THE‘ EXTRA:-EMBRYONIC BLOOD VESSELS

Extension of the Area Vasculosa and the Mesoderm. —~By about the end of the second day the two anterior wings of the area vasculosa, and the extra-embryonic mesoderm and entoderm which accompany them, have bent toward one another and have fused in front of the proaxnnion; The area vasculosa, therefore, now entirely surrounds the latter region, and is itself completely encircled by the sinus terrninalis, which has been referred to above (Fig. 182, A, B ) . Meanwhile, certain veins and arteries have extended from the embryo into the vascular area, as follows:

At the posterior end of the ductus venosus, the union of the vessels which form it terminates, and each passes outward into the area pellucida. At this point the y are known as the vitelline or omphalomesenteric veins. Upon coming into this region each of the veins turns e.nteriorly and runs past the head around the inner boundaries of the approaching wings of the area vasculosa. Hence these extensions are known as the right and left anterior vizelline veins. First by a system of capillaries, but presently directly, each of these veins then becomes connected with the anterior extremities of the sinus terrninalis. It thus happens that as the vascular wings meet one another, the sinus terminalis not only be comes complete, but the ends of the two anterior vitelline veins also

' meet and form one vessel (Fig. 182). At the proximal ends of these

veins each gives rise during this period to a slight lateral outgrowth the beginnings of the lateral vizelline veins.

The vitelline arteries, already referred to, extend out into the lateral portions of the area vasculosa some distance back of the vitelline veins, i.e., by the end of the day at about the twentieth somite.

THE CIRCULATION AS ESTABLISHED ON SECOND DAY

It will now be seen that with the establishment of the capillary network within the area vasculosa, and the formation of the arches connecting the ventral and dorsal aortae within the embryo, a complete system of circulation has been made possible. The further development of this system will be described as it occurs. 348 THE CHECK

THE NERVOUS SYSTEM THE MAIN DIVISIONS OF THE EARLY BRAIN

Early on the second day of incubation a slight constriction appears just back of the optic vesicles, marking the posterior boundary of the fore-brain or prosencephalon. Presently this is followed somewhat further back by another constriction which marks the posterior limit of

Fig. 183. ——-Median sagittal section through the head end of a Chick with 18 pairs of somites labout 4-0 hours). From Lillie (Development of the Chick).

tt.i.p. Anterior intestinal portal. Aa. Dorsal aorta. At. Atrium. E.E.B.C. Exocoelom (extra-ernliryonic body cavity). F.B. Fore-brain. H.B. Hinv.l~l)rain. H .F.Am. Head~fold of amnion. Inf. lnfundibulum. Isth. Isthmus. M.B. Mid-brain. N’r:h. Notochord. or.pl. Oral plate (oral membrane) . P.C. Pericardial cavity. Ph. Pharynx. Pr’a. Proamnion. pr’n.g. Preoral gut. Retzopt. Optic recess. S.V. Sinus venosus. Tr.a. Trnncus arte riosus. Vcn. Ventricle.

the mid-brain or mesencephalon. The part posterior to this is the hindbmin or rhombencephalon which passes insensibly into the region of the spinal cord. The posterior limit of the hind-brain, however, may be ﬁxed in a general way at this time by the position of the fourth somite (Figs. 181, 183). lt should again be noted that the cranial and cervical flexurcs are especially concerned with the brain. As suggested, however, because that organ occupies so large a part of the anterior of the embryo at this stage these ﬂexures affect the whole organism in this region and were therefore described under general appearances.

THE FORE—BRAIN OR PROSENCEPHALON

Its Extent.—-—0n the posterior wall, i.e., on the floor of that part of the brain where the cranial flexure is most pronounced, at the end of the slightly bent notochord, is an invagination. It is directed antero ..‘,,t-.-yw SECOND DAY: THE‘ FORE—BRAIN 34.9

ventrally into the cavity of the brain, and is called the tuberculium. posterius (F ig. 184). On the opposite or anterior wall of the brain a little below the level of this evagination is the slight, but broad, constriction referred to above as marking the posterior boundary of the fore-brain. This boundary may now be more accurately deﬁned as a plane passing from the tuberculum posterius on the posterior wall to the mid-point of the broad constriction on the anterior wall. This mid-point marks also the position of the future posterior commissure (see fourth day).

Fig. 184.——— Optical sagittal section of the head of an embryo of 22-23 s. The heart is represented entire. From Lillie (Development of the Chick).

Atr. Atrium. B.a. Bulbus arteriosus. Cr.Fl. Cranial ﬁexure. Dienc. Diencephalon. Hyp. Rathlce’s pocket, rudiment of anterior hypophysis. Inf. lnfundilmlum. Md. Mantlilmlar arch. Melenc. Metencephalon. Myelenc. Myelencephalon. or.pl. Oral plate. Pr’o.C. Preoral gut. Th. First indication of thyroid. Rec.opt. Optic recess. Telcnc. Telencephalon. T.p. Tuherculum posterius. V.tr. Velum transversum.

Parts of the Fore—brain. The Infundibulum.——Just ventral to the tuberculum posterius, a small posteriorly directed evagination now appears lying slightly be neath the anterior end of the notochord. It is the beginnirg of the in- frmdibulum, the future posterior part of the pituitary (Fig. 184).

The Region of the Optic V esicles.——Ventral to the infundibulum, but still on the posterior wall, is a thickening, the rudiment of the future optic chiasma (not noticeable in Fig. 184) , while immediately ventral to this thickening is a small evagination, the optic recess. From this recess the hollow optic vesicles have grown out on either side, and as they have grown their proximal parts have been constricted, as in the case of the 350 THE CHICK

Frog, to form the optic stalks. Below the optic recess, the posterior wall begins to curve anteriorly onto the present ventral surface. This region is relatively thin and is known as the lamina. zermirzaiis. Within it the torus transversus is scarcely visible as yet.

The Cerebral Hem.ispheres.—Near the end of the second day the sides of the fore-brain just dorsal to the lamina terminalis begin to push out as the future cerebral hemispheres. Their cavities will be theylateral ventricles opening into the cavity of the fore-brain or third ventricle, through the foramina of Monro.

The Velum Transversum and Region of E piphysis. —— Beyond the region of the lamina terminalis on the antero—ventral side of the forehrain, we come to a portion of the wall which is slightly depressed. it is known as the velum trcrnsversum. Further dorsal to this point on approx imatcly the anterior surface may he found, also, the suggestion of an outpushing; it marks the general region from which the epiphysis (pineal gland) later (fourth day) arises. This brings us to the slight but broad constriction mentioned above as indicating the location of the future. posterior commissure, and the limit of the fore-brain.

The Divisions of the Fore—brain. ———As in the case of the Frog, it is customary to divide the fore-brain into two parts, which with the aid of the above lanclmarks may now be easily deﬁned. That part of the fore-brain which lies vcntro-anterior to a plane passing from the pos ,terior wall just ventral to the optic recess to the anterior wall slightly

anterior to the middle of the velum transversum is the telenceplzalon. The remaining portion of the fore-brain, whose posterior limit is defined above, is then the diencephalon. The cerebral hemispheres arise from the former.

THE MID—BRAIN OR MESENCEPHALON

The anterior boundary of the rnesencephalon ‘coincides with the posterior boundary of the diencephalon, marked by the broad constriction previously referred to. The posterior boundary may be deﬁned as a transverse plane passing from the postero-ventral wall or ﬂoor just above and behind the tuberculum posterius, upward to about the middle of another rather broad constriction on the antero-dorsal wall (Fig. 184). The roof of the mid-brain, moreover, is growing so rapidly in connection with the cranial ﬂexure, that it soon arches outward as the most anterior region of the embryo. Other parts of the mesencephalon

have not appeared, and will, therefore, be described later as they arise. SECOND DAY: SPINAL CORD, NEURAL CRESTS 351

THE HIND-BRAIN OR RI-IOMBENCEPHALON

Its Extent. —The hind-brain lies entirely dorsal to the notochord, and extends from the constriction marking the boundary of the midhrein posteriorly into the spinal cord. Its posterior boundary, as stated above, can be deﬁned only as that part opposite the fourth somite_ A5 in the case of the mid-brain, the parts of the hind-brain are not yet discernible, and will be indicated when they appear.

The Divisions of the Hind-brain.——The divisions of the hindbrain are also difficult to deﬁne at this early stage. We may say, however, that the anterior division is relatively short, and is known as the nzretencephalon. The remainder of the brain constitutes the posterior di~ vision known as the myelencephalon. The cavity which extends through both is called the fourth ventricle.

THE SPINAL CORD AND ETS NEIURAL CRESTS,

The Cord. —~As fast as the neural tube is formed by the fusion of the neural folds, its central canal tends to become compressed laterally and elongated dorso-ventrally. Its lateral walls also gradually thicken, and at the end of the second day these walls consist chiefly of two sorts of cells. First, there are elongated cells extending from the central canal out to its outer walls. These are the cells originally lining the canal, now known as ependymal cells, and their function is that of support. Secondly, among the ependymal cells and near the central canal are numerous rounded cells known as germinal cells. They later give rise to neuroblasts or primitive nerve cells, and also probably to more supporting elements termed glia cells. It has recently been claimed (Barron, ’46) that some of the germinal (indifferent) cells are stimulated to become neuroblasts by contact with growing dendrites of other neuroblasts already partially differentiated.

The Neural Crests and Rudimentary Spinal Ganglia. — As indicated in the /previous chapter, the neural crests when first formed are simply bands of cells which extend along the dorso-lateral walls of the neural tube, on either side between it and the ectoderm. As was also stated, these bands or crests are at first fused with one another dorsally. By the end of the second day, however, in the older (i.e., anterior) portion of the tube, this dorsal fusion has been obliterated. In this region there have also appeared in the crests successive enlargements, which presently become separated from one another to form a series of rudimentary spindl ganglia. There is one of these ganglia for each somite, 352 THE CHICK

except for those of the head region, opposite whose somites the crests disappear. The spinal ganglia at this time contain both neuroblasts and indifferent cells.

THE CRANIAL GANGLIA

The neural crests of the head region anterior to the somites do not disappear. but also form enlargements which separate and take part in the formation of certain of the cranial ganglia. Parts of these ganglia, however, are placodal in origin, and surprisingly, according to some authors some of them even contain endodermal elements as indicated below. By the end of the second day the ganglionic rudiments are visible, beginning at the anterior end, in the following positions:

The V Nerve Gang1ion.—~The ganglion for the V or trigeminal nerve is somewhat anterior to the dorsal end of the first or mandibular Ztl‘t".l1. At the end of the second day it usually appears merely as at Clark patch in this region (Fig. 176), but later (see third day) it acquires distimxtly the form of an inverted Y. Apparently most, or all, of this ganglion is derived from crest material tYntema, 314-) .

The VII and VIII Nerve Ganglia. —-The ganglia for these nerves form a single mass, the acustico-faciallis ganglion. It lies at this time just antero-ventral to the auditory sac (see below) ; i.e._, it is above and slightly in front of the dorsal end of the second or hyoid an-h. Though unlabeled, it is shown in Figure 176 in the position indicated. Jones, ’-42 has claimed that part of the VII ganglion is derived from the dorsal wall of the first visceral pouch, an unusual source of nerve tissue since the pouch is of course endoderm. Later study (Yntema, 7&4‘), however, seems to show that the origin is, as might be expected, partly crest and partly placode. The geniculate portion is thought to come from the placode, which, though closely associated with a pouch, is definitely not part of it, while the remainder of the facial nerve ganglionic complex is from the crest. The VIII ganglion appears to be entirely placodal.

The IX and X Nerve Gang1ia.—The IX and X nerve ganglia arise together, but at the end of the second day they begin to become separated. The former, or glossopharyngeal ganglion, is then situated above the dorsal end of the third visceral arch while the latter, or vagus ganglion, lies above the ends of the fourth and ﬁfth visceral arches. These ganglia are not visible in Figure 176. As to their sources, it appears that both contain some crest material, while it has again been claimed by both Winiwarter, ’39 and Jones, ’42 that material for the

L SECOND DAY: THE EYE 353

petrosal portion of IX and the jugulare part of X are from't'he second and third visceral pouches respectively. It seems most probable, however, that, as in the case of the VII nerve ganglion, difficulty in separating the ectodermal and endodermal elements has led to error and that only “ adjacent ectoderm,” i.e., placode, is involved. A diagram of the location and form of the cranial gan- ' glia viewed from above early on the second day is given in Figure 185.

ORGANS OF SPECIAL SENSE

THE EYE

The Optic Stalks, the Uptic Cup and the Choroid Fissure. —The optic vesicles, it will be recalled, are hollow out-pushings from the forehrain with which they remain connected by constricted regions known as the optic stalks (Fig. 186). These stalks are the so-called “ optic nerves,” though as will appear, the real optic nerves develop later. It is to be noted that the above constriction has occurred in such a manner that each stalk connects with its vesicle near the ventral side of the latter, rather than at its center. Invagination of the outer wall of the vesicle now occurs, oblit Fig. 185. — Diagram of the cephal_ . ic neural crest of a chick of about crating its original cavity, and con» 12 somiles From Lillie (Develop ment of the Chick). After Wilhelm

vetting it into the two-layered optic His at Auditory Sm 3. 50mm,"

cup, with the optic stalk attached to

its ventral edge. The walls of the cup on either side of the point where the stalk is attached now grow outward, i.e., toward the ectoderm, but their ventral edges do not quite meet one another. Thus a ﬁssure is left in the ventral side of the cup extending from its edge inward to the optic stalk. This, as in the Frog, is the choroid ﬁssure. Meanwhile the rim of the cup bounding its aperture, the pupil,’ becomes slightly constricted. The invaginated or outer wall of the vesicle has now necessarily become 354 THE CHICK

the inner wall of the cup, and will, therefore, be referred to as the inner wall in future discussion. It is the rudiment of the nervous layer of the retina (see Chapter ll).

The Development of the Lens. —Before the above invagination of each optic vesicle occurred, the vesicle had pushed out far enough to ‘touch the surface ectoderm. When this happened, the ectoderm at the point of contact began to thicken, and when the invagination of the vesicle took place, this thickened ectodermal wall also invaginated. Thus a hollow thick-walled sac was formed resting just within the rim of the

Fig. 186. — Diagrams of sections through the eye of the Chick embryo at the end of the second day. From Kellicott (Chordate Development). After Lillie. The dorsal margin is toward the top of the page in A and B. A. Eye as viewed directly. B. Vertical section through the line x—cf, in A. C. Horizontal section through the line y—y in A.

cf. Choroid ﬁssure. co. Cavity of primary optic vesicle. ec. Superﬁcial ectoderm of head. i. Inner or nervous layer of the retina. l. Lens. 0. Outer or pigmented layer of optic cup. 01. Opening of lens sac from surface of head. pc. Posterior (vitreous) chamber of eye.-s. Optic stalk, continuous with the ﬂoor and lateral wall of the

diencephalon. optic cup. This is, of course, the rudiment of the lens; at the end of the second day it has not quite detached itself from the outer ectoderm. As in the case of the Amphibian, it has been shown that the optic cup has the power to induce lens formation in ectoderm which would not otherwise form it. Thus optic vesicles or cups from embryos up to the 4.0-somite stage (fourth day) will induce lenses when transplanted to young hosts (primitive streak to’ eight somites). In a host older than four somites, however, the transplant will produce positive results only when implanted as far anterior as the potential head or neck region. In any case actual contact of the cup with the ectoderm seems necessary to

effect induction. Also as in the Amphibian, the new lens may come from '

cells of the optic cup itself as well as from the host ectoderm (Alexander, ’37) , and the inductive process is a gradual one (McKeehan, ’54).

3 3 l 5. l E

.oa.......« ~ e .-

SECOND DAY: THE EXCRETORY SYSTEM 355

THE EAR

The sensory part of the ear begins as a thickening of the ectoderm on the side of the head above and slightly posterior to the dorsal end of the hyoid arch. This thickening presently starts to invaginate, thus forming a depression -— the auditory pit. During the second day the process of invagination continues, and is soon accompanied by an approximation of the anterior and posterior lips of the pit. Near the end of the second day the ventral lip also takes part in the closure by moving dorsally, and thus the pit is transformed into a small mouthed sac. It is the auditory sac or otocyst (Fig. 176).

THE URINOGENITAL SYSTEM

Because of their close connection in the adult, the excretory and reproductive systems are, as usual, considered under a common heading. Their development, however, is largely separate, and must, therefore, he so treated. Of the two systems, only certain parts of the excretory appear during the second day.

THE EXCRETORY SYSTEM

The excretory system of the Chick in common v",'.Li that of other Amniota consists of three separate parts, the pronephros, mesonephros, and metanephros. These parts develop in the order named, and the first two have largely disappeared by the close of embryonic life; only the last remains functional as the permanent excretory organ of the adult. During the second day the pronephros develops, and near its close the mesonephros has just begun to appear.

The Pronephros.——The pronephros is vestigial in character, and only appears typically from the tenth to the fifteenth somites. Rudiments of it, however, are sometimes found as far forward as the ﬁfth somite. In the more posterior region indicated, its development is as follows:

The Pronephric Tubules. —— In the dorso-lateral portion of the nephrotome opposite the posterior end of each somite a thickening occurs, and from it a cord of cells grows outward and upward for a short distance (Fig. 187, pr’n. 1). At the same time the nephrotome becomes detached from the somite. These lateral outgrowths are termed the pronephric tubules, though they usually do not acquire any lumen. Some356 THE CHICK

times, however, a slight lumen is present in the proximal part of the tubule (Fig. 187, pr’n. 2), and it opens into the coelom as a rudimentary nephrostome. It is also said that degenerate glonwrztli (or more properly glomi) sometimes develop later on the coelomic wall opposite the nephrostomal mouths (Lillie).

The Pronephric and W olﬁian Ducts. — The distal part of each of the above cell cards or “ tubules ” presently bends posteriorly and grows in

Fig. 187.-——A. Transverse section through the twelfth somite of a 16s embryo. From Lillie (Development of the Chick). B. Three sections behind A to show the nephmstoine of the same pronephric tubule.

A0. Aorta. CC. Central canal. Coel. Coelom. E.E.B.C'. Extra-emllryonic coelom Iexocoelnm 1. .lIs'c/L. Mesenchyme. N’c}1. Notoclmrd. n.Cr. Neural crest. .’V’.st Nephrostome. n.T. Neural tube. pr'n. 1,2. Distal and proximal divisions of pronephric tubule.

$.12. Twelfth sornite. Sa’pl. Somatopleure. Spl’pl. Splanchnopleure. V.c.p. Posterior cardinal vein.

this direction until it comes in contact with the tubule following it. In this manner, a continuous backwardly directed cord of cells is formed which connects with each successive tubule. Finally, the bent portion of the last cell cord continues to grow posteriorly between the nephrotomal mass and the body wall. As will appear subsequently, the anterior end of this backward growing rod of cells is the rudiment of the pronephric duct. and its more posterior portion, the rudiment of the mesonephric or Wolﬂian duct. Before the end of the second day, indeed, the

anterior or pronephric section of the rod has acquired a lumen, thus becoming a real duct. SECOND DAY: THE AMNION 357

The Mesonephros. —— The mesonephros corresponds to the organ of the same name which functions as the permanent excretory organ of the Frog. In the Chick, however, as indicated above, this excretory function continues only during a part of embryonic life. The antericr end of the inesonephros slightly overlaps the posterior end_of the pronephric region, but its development here is rudimentary, the organ acquiring its typical form only from the twentieth to the thirtieth somites. During the close of the second day it begins to appear in the following manner, development progressing posteriorly.

The Primary Mesonephric Tu‘ou1es.——The nephrotome in the region indicated becomes separated both from the somites and the lateral plate. It then lies just ventro-medially to the rod of cells which is to become the Woliiian duct. Above this duct the posterior cardinal vein presently appears, while between the nephrotome and the median line of the embryo runs the dorsal aorta. The nephrotorne is thus between the aorta and the future Woliiian duct (Fig. 174«). Presently in the neighborhood of each somite, there appear in this nephrotomal band two or more spherical condensations. Then beginning at the anterior end of the band each of these condensed spheres starts to acquire a cavity, each vesicle thus formed being the rudiment of a mesonephric tubule and a Malpighian. body. The more ventral spheres in each somite are the first thus to become vesicular, and they are the rudiments of the socalled primary mesonephric tubules as distinguished from the others. (See next chapter, Fig. 207.)

T AMNION AND OTHER EXTRA-EMBRYONIC STRUCTURES

From the embryological point of view all Vertebrates belong to one of two classes; i.e., the Anamniota or the Amniota. The former group includes Amphibians and Fishes, while the latter includes Reptiles, Birds, and Mammals. The Amniota. as the name implies. are those which possess an amnion, while the Anamniota are those which lack it. Amphioxus, the Frog, and Fish have been studied as representatives of the latter class, and we are now studying the Chick as an example of the former or Amniote group. The amnion begins to form on the second day of the Chick’s incubation, but is not completed until about the fourth day. In order to make the structure of this organ more clear, however, it seems best to describe its entire development, together withthat of certain other extra-embryonic organs and membranes. 358 . THE CHICK

THE AMNION IN PROCESS OF DEVELOPMENT

Development during the Second Day. —- During the second day a fold in the blastoderm occurs just in front of the head of the embryo in the region of the proamnion. Since there is as yet no mesoderm in this region, the fold at first contains only ectoderm and endoderm. Presently, however, the mesoderm extends into this vicinity, and here, as elsewhere, is split into the extra-embryonic extensions of the somatic and splanchnic layers with the extra-embryonic coelomic space between them; both these layers then become involved in the fold. The splanchnic layer together with the endoderm, however, is soon withdrawn to the surface of the yolk, while

t:,§t;§it:,2;?§;8t:;::‘:t.. ;;:a:::‘; the some layer me

the yolk (,yolk-stilk uénbilicxislk in a ChicFlc of extra-embryonic ectoderm 3 -' ' ‘r. . ' . .

ﬁ‘2‘;:,;:,:"zm;:,:;:e:,mi: 9.32:: winch -r a Dorsal aorta. c. Coelom. ebcxexocoelom. ig. In- the two permanent layers

testinal groove. la. Lateral folds of amnion. ‘UYJ.

Vitemne vein of the amniotic head fold.

The embryo has now begun to sink somewhat into the surface of the yolk, and as it does so the amniotic fold gradually grows back over it. This backward growth is also accompanied by the development of lateral amniotic folds extending posteriorly on either side. By the end of the second day the embryo has been covered over in this manner almost as far back as the vitelline arteries (Figs. 176 and 188). The latter ﬁgure shows a cross section through a region where the folds have not yet quite covered the embryo.

Development during the Third Day. —— About the end of the second day, or the beginning of the third, another fold appears at the posterior end of the embryo, and grows forward toward the head fold. This is the amniotic tail folcl, which soon becomes coextensive upon either side with the posterior ends of the lateral amniotic folds. It is similar to the corresponding head fold except that from the ﬁrst it contains only ectoderm and somatic mesoderm. Since the anterior portion of the amnion starts earlier and grows rapidly, the point at which the converg

l l ~ SECOND DAY: THE COMPLETED AMNION 359

e is quite near the posterior end of the

ing folds ﬁnally meet and {us sting above the Chick previous to the

animal. The oval opening exi is the amniotic umbilicus. Fourth Day. —The end of the third, or beginning of the fourth day, marks the meeting and fusion of the am niotic folds at the center of the amniotic umbilicus. The embryo has by D

' ‘this time turned upon its left side throughout the greater part of its

closure Development during the

ith 35 pairs of somites (about -third somite. From Kellicott

ransverse section of Chick embryo w hrough the region of the twenty

(Chordate Developm . . Dorsal aorta. c. Embryonic coelom. ch. Chorion.

Fig. 189. —-T 72 hours), passing t

a. Amnion. ac. Amniotic cavity. ao. d. Derrnatome. ebc. exocoelom. g. Rudiment of spinal ganglion. m. Mesonephric

tubule. my. Myotome. p. Posterior cardinal vein. 5. Sclerotome. sa. Sero-amniotic connection. so. Subcardinal vein. so. Somatic mesoderm. sp. Splanchnic mesoderm.

12. Vitelline artery. W. Wolﬂian duct. ds do not turn with it, the closure occurs

not above itsback, but above its right side. It also follows from this, that the fold of the left side covers the hack of the embryo as well as a

part of the right side. The amnion may now be said to be complete.

_ THE COMPLETED AMNION AND RELATED PARTS The Amnion and Amniotic Cavity. ——- It is obvious that the amniotic folds, like any other folds, must be composed of two main parts, ther at the crest of the fold. It is

each part being continuous with the 0 also obvious that one of these parts, i.e., the inner or lower one, lies everywhere next to the embryo. When fusion occurs, therefore, this inner

length, and inasmuch as the fol 360 THE CHICK .

part will become continuous, completely bounding a new cavity which surrounds the embryo at every point except for a restricted region on its ventral side (see below under somatic umbilicus). This continuous inner membrane is the amnion, and the cavity thus formed is the amniotic cavity. Moreover, inasmuch as the folds involve both ectoderm and mesoderm, the inner membrane or amnion must likewise consist of ectoderm and mesoderm, the former lining the amniotic cavity and the latter‘ ' forming a coat outside the lining (Figs. 189 and 190).

The Chorion.— At the fusion of the folds the outer part, like the inner, necessarily becomes continuous. Likewise, it too consists of both ectoderm and mesoderm, but in this case, the ectoderm will lie outside and the mesoderm inside, i.e., toward the amnion. The outer membrane thus formed is called the chorion, serosa or false amnion. Between it and the inner membrane or true amnion, there is naturally the same space which separated the inner and outer parts of the amniotic folds, i.e., the extra-embryonic coelom or exocoelom. This relationship will be made clear by reference to Figure 190. it may be mentioned incidentally in this connection that this exocoelomic space eventually becomes ﬁlled by an important sac-like organ (allantois) whose origin and structure will be described below.

The Sero-Amniotic Connection.———It has been implied that the extra-embryonic coelom, with whatever may occupy it, everywhere separates the amniotic membrane from the chorionic membrane. This is true except at one point. At the point of ﬁnal fusion of the amniotic folds, i.e., the amniotic umbilicus, the coelomic space is interrupted by a small area of mesoderm which persists, and serves to unite the above membranes. It is called the sero-amniotic connection. (Figs. 189 and 190).

The Amniotic F luid.——Shortly after the completion of the amniotic cavity, ﬂuid begins to accumulate within it. Thus the embryo is soon practically surrounded by a liquid cushion which protects it from pressure by its membranes and rigid shell. This is the amniotic Presently, about the ﬁfth day, muscle ﬁbers develop in the mesoderm of the amnion and begin to send waves of contraction over it. This causes a gentle rocking of the embryo, and is apparently instrumental in preventing its adhesion to the various embryonic membranes. It may also help to obviate the stagnation of blood in the vessels, a condition

which might tend to occur on account of the pressure from the growing organs. SECOND DAY: THE COMPLETED AMNION 361

All. Am. char. 5. am. 7

Figs. 190, 191, 192.——Diagrams of the" relations of the extra~embryonic membranes in the Chick. Figures and description from Lillie (Development of the Chick). The ectoderm and endoderm are represented by plain lines; the mesoderm by a cross-hatched line or band. The yolk-sac is represented by broken parallel lines. In Fig. 190 the allantois is represented as a sac. In Figs. 191 and 192, where it is supposed Ito be seen in section. its cavity is represented by unbroken parallel lines. The stalk of the allantois is exaggerated in all the diagrams to bring out its connection with the embryo.

Fig. 190. —Fourth day of incubation. The embryo is surrounded by the amnion which arises from the somatic umbilicus, Umb., in front and behind: the seroamniotic connection, S.am., is represented above the tail of the embryo; it consists at this time of a fusion of the ectoderm of the amnion and chorion. The allantois, AIL, is represented as a sac, the stalk of which enters the umbilicus behind the yolk-stalk; the allantois lies in the extra-embryonic body-cavity (exocoelom) , and its mesodermal layer is fused with the corresponding layer "of the chorion above the embryo. The septa of the yolk-sac, Y.S.S., are represented at an early stage. The splitting of the mesoderm has progressed beyond the equator of the yolk-sac, and the undivided portion is slightly thickened to form the beginning of the connective-tissue ring that'surrounds the yolk-sac umbilicus. The ectoderm and endoderm meet in the zone of junction, beyond which the ectoderm is continued a short distance. The vitelline membrane, V.M., is ruptured, but still covers the yolk in the neighborhood of the yolk-sac umbilicus. The albumen is not represented in this ﬁgure. (For complete explanation of lettering see Fig. 192.) 362 THE CHICK

THE SOMATIC UMBILICUS, THE YOLK—STALK, AND THE YOLK——SAC

Though they are not a part of the amnion, it seems best to include in connection with its description an account of these structures which, to some extent, develop with it. 4

The Somatic Umbilicus. ——During the formation of the amnion, the gradual separation of the embryo from the yolk has been progressing. This has been accomplished by the steady in-pushing of the ventral portions of the head, tail, and lateral folds (limiting sulci} beneath the body of the growing Chick. The result is that by the time the amnion is completed, these folds have approached one another quite closely, though without coming into contact. In this manner they give rise to a short, thick, hollow stalk which connects the embryo with the yolk-sac and its extra-embryonic membranes. The outermost wall of this stalk is continuous with that of the amnion, and is, therefore, composed of ectoderm and somatic mesoderm: for this reason, this outer wall is referred to as the somatic umbilicus (Fig. 190).

The Yolk-Stalk. —— Within this wall and surrounding the inner wall of the stalk, is a space continuous externally with the extra-embryonic coelom and internally with the coelom of the embryo itself. Finally, the inner wall of the stalk consists of splanchnic mesoderm and endoderm. It is known as the yolk-stalk, but is really merely an inner tube of the somatic umbilicus separated from it by coelomic space.

The Yolk-Sac.——The wall of the yolk-stalk is coextensive within the embryo with the wall of the gut, and externally with the layer of endoderm and the splanchnic mesoderm which overlies the yolk. This layer is continually growing out around the yolk, and at its outermost border, i.e., the region of the zone of junction, the endodermal portion of it becomes continuous with the chorion which overlies it. Thus by means of the extension of these layers the yolk is gradually enclosed in a covering, whose inner layer of splanchnic mesoderm and endoderm constitutes the yolk-sac, attached to the embryo by means of the yolkstalk. Upon the ninth day of incubation this sac has become virtually complete, save at a point on the side of the yolk postero-ventral to the body of the Chick, where an opening remains, known as the yolk-sac umbilicus. This opening, however, is ﬁnally closed about the seventeenth day by a solid mass of tissue. It may be recalled in this connection that the rim of the blastoderm, which has thus overgrown the yolk, was previously homologized with the lip of a very extended blastopore,

4 .. .a_...m.,,.,,,_.,.j, SECOND DAY: THE ALLANTOIS 353

the true blastopore (primil".Ve Streak) haVing been separated from the remainder of the rim during gastrulation. Hence upon this basis it is possible to consider the uncovered yolk mass as a sort of very large secondary, or yolk-Lla.stopare, the latter term being really only another name for the yolk-sac umbilicus. A somewhat similar separate blastepore, it may be noted, also occurs in the development of the Elasmobranchs (i.e., the cartilaginous or non-bony ﬁshes) in which the term yolk-blastopore is regularly applied to it.

On the basis of this description, it is clear that beyond the boundaries of the amnion the chorion is really nothing more than the uppermost layer of the blastoderm. It is to be noted, however, that this upper layer consisting of ectoderm and somatic mesoderrn is soon separated from the lower layer composed of splanchnic mesoderm and endoderm by the extra-embryonic coelom. Furthermore, this space presently becomes occupied by another extra-embryonic organ (allantois) , to be described below. Finally it must also be mentioned that early in its development, the lower layer, just indicated, ie, the real yolk-sac layer, consisting of endoderm and splanchnic mesoderm, becomes covered internally with deep folds, the yolk-sac septa, which gradually press downward into the yolk. These septa in common with the remainder of the yolk-sac endoderm in the area vasculosa, contain glandular and absorbing cells which digest the yolk in situ before passing it into the blood vessels. Thus though a slight lumen exists in the yolk-stalk connecting the inside of the yolk-sac with the enteric canal, no yolk appears to pass into the embryo through this lumen. Abnorrnally high or low temperatures during incubation, e.g., 39.5° C and 3-15° C, appear

to slow up the process of absorption of both yolk and albumen (Romanoﬂ', ’43) .

THE ALLANTOIS

Another extremely important extra-embryonic organ possessed in some degree by all Amniota is the allantois, and it will be found convenient to consider its entire history also at this time.

Its Early Development. ——The allantois starts in the form of an out-pushing from the ventral wall of the hind-gut (Fig. 193). This is scarcely visible before the beginning of the third day, and was, therefore, not referred to in the foregoing description of the alimentary tract. This out-pushing naturally involves the endoderm and the mesodermal

ventral mesentery which occurs in this region. Thus the sac which is '

presently formed possesses an inner endodermal and an outer mesoder364 THE CHICK

mal layer. By the fourth day the allantois has pushed out through the coelomic space between the somatic umbilicus and the yolk-stalk, and is beginning to spread out in the extra-embryonic coelom (Fig. 190}. The narrow neck of the organ which then connects the outer sac-like

Fig. 191.—Ninth day of incubation. The yolk-sac um , bilicus has become much narrowed; it is surrounded by the mesodermal connective-tissue ring, C.T.R., and by the free edges of the ectoderm and endoderm. The vitelline membrarie still covers the yolk-sac umbilicus and is folded into the albumen. The allantois has expanded around the amnion and yolk-sac and its outer wall is fused with the chorion. It has pushed a fold of the chorion over the sero-amniotic connection, into which the mesoderm has penetrated, and thus forms the upper fold of the albumen-sac. The lower fold of the albumen-sac is likewise formed by a duplication of the chorion and allantois; it must be understood that lateral folds are forming also. so that the albumen is being surrounded from all sides. The stalk or neck of the allantois is exaggerated so as to show its connection with the embryo; it is supposed to pass over the amnion, and not. of course, through the cavity of the latter. (For explanation of lettering see Fig. 192.)

portion with the gut is known as the allantoic stalk or neck. Along this stalk pass the two allantoic arteries (later only one), and the single allantoic vein, ‘to end in abundant ramiﬁcations over the surface of the sac. The allantois now grows rapidly, and within a couple of days has entirely covered the amnion, occupying the space between that organ and the chorion. Presently the amniotic and chorionic mesoderm fuse, forming the chorio-allantoic membrane (Figs. 191 and 192). In this

3 i SECOND DAY: THE ALLANTOIS T 365

manner, the above ramiﬁcations of the blood vessels are brought very near to the shell, through which an exchange of gases is possible. Thus the allantois serves as an organ of respiration for the Chick during embryonic life. Its cavity also acts as a receptacle for the waste products of

gm. 3. Am.

All. 5. En: 5-"W AILC. Chor. »(

Am.

Fig. 192.—Twelfth day of incubation. The conditions are more advanced than those represented in Fig. 191. The albumen-sac is closing; its connection with the cavity of the amnion by way of the sero-amniotic connection will be obvious. The inner wall of the allantois has fused extensively with the amnion. The umbilicus of the yolksac is much reduced, and some yolk protrudes into the albumen (sac of the yolk-sac umbilicus, transitory structure soon drawn into the yoll-:-sac proper). Alb. Albumen. Alb.S. Albumen-sac. .411. Allantois. AIL]. Inner wall of allantois. /1ll.C. Allantoic cavity. AZLS. Allantoic stalk or neck. All. + Am. Fusion of allantois and amnion. Am. Amnion. Am.C. Amniotic cavity. Chor. Chorion. C.T.R. Connective--ti.-rsue ring. Eat. Ectoderm. E.E.B.C. Exococlom (extra-embryonic bodycavity). Ent. Endoderm. Mes. Mesoderm. S.-Am. Sero-amniotic connection. S.Y.S.U. Sac of the yolk-sac umbilicus. Umb. Umbilicus. (somatic). V./ll. Vitelline membrane. Y.S. Yolk-sac. Y.S.S. Septa of yolk-sac.

metabolism, which are conveyed thither through the allantoic stalk from the region of the cloaca. lt is thus to be noted that this organ is homologous not only in method of origin, but also partly in “function with the urinary bladder of the Frog. The latter, however, of course never extends outside of the coelomic cavity, and though it may or may not be endodermal, the allantois is certainly so. 366 THE CHICK

The Later Development of the Allantois and the Formation of the A1bumen-Sac. — Meanwhile the albumen is becoming concentrated on the side of the egg next to the yolk-sac urnbilicus, and by the ninth or tenth day has become very much condensed. Concurrently the real yolk-sac layer, together with the chorion, has grown around the yolk so that the edges of the over-growth have more than kept in con tactﬁwith the receding albumen. They have in fact thrust themselves in

between it and the yolk, so that the albumen is bounded upon its inner side by a layer of chorion. At the same time, save postero-dorsally in the region of the sero-amniotic connection, the allantois has been following this overgrowth of .the yolk-sac layer and chorion; it lies between these two layers in the exocoelom, and its walls are fused respectively with the chorionic layer and that of the yolk-sac. Thus as the latter layers push in between the yolk and the albumen to close the yolk-sac umbilicus, they are accompanied, except postero-dorsally, by the allantois. Ventro-laterally a fold of the chorion presently pushes its way around the outside of the albumen between it and the shell membrane. Here too, moreover, between the two layers of the chorionic fold there follows an outer fold of the allantois. Meanwhile in the postero-dorsal region, as already suggested, the expansion of this organ is obstructed by the seroamniotic connection. At this point, therefore, it pushes up over this connection, carrying the chorion before ‘it. Thus this dorsal fold, consisting of a layer of chorion and allantoic wall, comes down between the albumen and shell membrane to meet the similarly constituted ventrolateral folds already described. Hence, at ten days the albumen at the yolk-sac umbilicus is surrounded by a double layer of fused chorionic and allantoic tissue, the albumen-sac. There is just one region in the

wall of the sac, however, where all of these layers are not present. This ,

is a small area on its internal dorsal side where the allantois could not extend because of the sero-amniotic connection. There, therefore, the wall consists only of chorion, and at one point of the connection itself (Figs.

191,192). A perforation appears in this connection, and on the twelfth ‘

day some albumen enters the amniotic cavity. The remainder of the albumen is absorbed, and the albumen-sac together with the yolk-sac is

drawn within the embryo just previous to hatching. According to Randles A

and Romanoff, ’50, a periodic turning of the egg is necessary if all these events are to be accomplished normally at the times indicated. Hatching is apparently aided by the contraction of the muscular walls of the allantois and by the muscles of the somatic umbilicus (see also Fig. 193).

l i i l l “"

SECOND DAY: SUMMARY . 367

SUMMARY OF THE CONDITION AT THE END OF THE SECOND DAY OF INCUBATION '

I. GENERAL APPEARANCE

The cranial ﬂexure has been initiated, and has brought the fore-brain to a point where it almost touches the heart, and the mid-brain faces anteriorly. The cervical ﬂexure is also evident in the region of hind-brain and trunk. In correlation with these ﬂexures lateral rotation has started so that the embryo lies on its side as far back as the 13th somite.

II. THE SOMITES

There are approximately 27 somites, in which the myotomes and cutis plates have begun to differentiate, together with the mesenchymatous rudiment of the selerotome.

III. THE FORE-GUT

In the fore-gut the stomodaeum is formed, and in connection with it Rathke’s pocket, a part of the future hypophysis, is beginning to appear. Four pairs of visceral pouches and ﬁve pairs of arches have begun to develop, and the ﬁrst pair of pouches have acquired openings to the exterior. The -rudiments of the thyroid, the respiratory system, and the liver are also present.

IV. THE MID—GUT

This is but slightly developed, although the lateral folds are beginning to mark it off from the extra-embryonic archenteron.

V. THE HIND—CUT

The hind-gut has begun to form and its posterior end has fused with the ectoderm to form the anal plate or cloacal membrane. In connection with it there has also arisen the ventral mesentery.

VI. THE CIRCULATORY SYSTEM

The Heart.—-A bent tubular heart has been developed, lined by endothelium and covered with a myocardium. The regions of the atria,

the ventricles, and the bulbus and trztncus arteriosus are indicated, and pulsation has been initiated. 368 THE CHICK

The Arteries. —The dorsal aortae are in evidence. Also the ventral aorta has appeared and become incorporated into the truncus. The first pair of aortic arches are in process of formation, and the second and third aortic arches are completed. The vitelline arteries have appeared.

The Veins.——The anterior and posterior cardinals, the sinus venosus, the (luctus venosus, and the ducts of Cuvier have been developed. In connection with the latter the septa known as the lateral mesocardia

Fig. 193.——Median sagittal section through posterior end of four-day chick. From Kellicott (Chordate Development). After Gasser (Maurer).

al. Allantois. am. Amnion (tail-fold). c. Cloaca. rn. Cloacal membrane. 11. Notochord. r. Rectum. s. Spinal cord. y. Wall of yolk-sac (endoderm and splanchnic mesoderml.

have also been formed. Outside the embryo the anterior vitelline veins have arisen, and with them the rudiments of the lateral vitelline veins. The sinus terminalis has become complete.

VII. THE NERVOUS SYSTEM

The Brain and the Cranial Ganglia.-—As indicated under external appearance the cranial and cervical flexures have become well marked. The fore-brain, mid-brain and hind-brain are now clearly indicated, and within the first main division certain parts are apparent, as follows: The outgrowth of the optic stalks is well advanced, and there

may also be evident the rudiments of the optic chiasma, the optic recess,‘

the cerebral lzernispheres, the in fundibulum, and some other minor struc‘tunes. The roof of the mid-brain is becoming prominently arched. SECOND DAY: SUMMARY 369 The cranial ganglionic rudiments of the V, VII and Vlll, and IX and

X nerves are visible, and the latter pair are beginning to separate.

The Spinal Cord and Ganglia. —— The spinal cord has become thick-walled laterally, and has developed ependymal and germinal cells. The neural crests are segmenting to form the spinal ganglia.

VIII. THE ORGANS OF SPECIALSENSE

The optic vesicles have become invaginated to form the optic cups, and the external ectoderm opposite each cup has invaginated in the process of forming a lens. In connection with the ear, the auditory portion of the ectoderm has become invaginated to form the auditory sac.

IX. THE URINOGENITAL SYSTEM

Only the embryonic parts of the excretory portion of this system appear during the second day. These are the pronephros, including the Wolﬂian duct, and the rudiments of the mesonephros. These rudiments consist of concentrations of nephrogenous tissue, some of which are beginning to become vesicular in the formation of the mesonephric tubules and the Malpighian bodies.

X. THE AMNION

This extra-embryonic organ begins its development on the second day with the appearance of the amniotic head fold, the amniotic lateral folds, and sometimes an indication of the amniotic tail fold.

The complete development of the amnion, the chorion, the allantois, and the yolk-sac is described in this chapter. TI

HE CHICK: DEVELOPMENT DURING THE THIRD DAY OF INCUBATION

GENERAL APPEARANCE FLEXURES AND TORSION

THE embryo has of course increased somewhat in size, but the most obvious changes concern the ﬂexures. The cranial ﬂexure is somewhat more marked, while the cervical ﬂexure has greatly increased, so that the region of the hind-brain, rather than the mid-brain is now the most anterior part of the embryo. By the close of this day also a new curvature has become evident at the posterior end. It involves mainly the tail, and is called the caudal ﬂexure. Between this ﬂexure and the cervical ﬂexure the back of the embryo is temporarily somewhat bent in a ventral direction, i.e., opposite to the other curvatures. This is because of the broad attachment to the yolk which still extends throughout the middle region and tends to draw this part of the embryo ventrad (Fig. 200). Accompanying these increases in ﬂexure the lateral rotation has progressed posteriorly until by the end of the day the embryo is on its side about as far back as the twenty-first somite.

LIMB 'RUDS

The limb buds become clearly visible by the end of the third day, and appear as broad swellings on either side of the embryo. The anterior buds extend from about the fifteenth to the twentieth somite, and

the- posterior buds from about the twenty-seventh to the thirty-third soniite.

THE SOMITES

During the third day the number of pairs of somites increases to about 36. The newer posterior somites when first formed are in the same condition as were those which are now anterior, and are destined to go THIRD DAY: THE FORE—GUT 371

through the same process of development. Meanwhile, the more advanced anterior members of the series do not greatly change except for further modiﬁcations along the lines already indicated on the second day. These modifications are as follows:

Each myotome or muscle plate continues to grow down along the inside ot its respective cutis plate, until in the most mature somites it reaches the ventral end of the cutis plate and fuses with it. In this manner a complete double layer of cells arises. In the inner layer or muscle plate thus formed, the cells or rnyoblasts presently begin to become spindle-shaped, reaching from the anterior to the posterior walls of each myotome. These are mostly rudiments of dorsal voluntary muscles. Somewhat later on the third day the outer or cutis plates of somites which have reached this stage begin to break up into mesenchyme, which wanders outward toward the ectodermal wall. There it eventually gives rise to the dermis of the dorsal region, that of the lateral and ventral parts being derived from the adjacent somatopleurc (Murray, ’28) .

The sclerotomal mesenchyme continues to collect about the notochord and the sides of the nerve cord.

THE ALIMENTARY TRACT

THE FORE—GUT

The Oral Cavity.———During the third day, the oral plate breaks through, placing the stomodaeum in direct communication with the pharynx (Fig. 204-). The region in which the digestive tract opens to the exterior anteriorly is thus partly stomodaeal and partly pharyngeal. It is called the oral cavity.

The Hypophysis or Pituitary Body.——It will be recalled that at 24 hours a hollow diverticulum called Rathke’s pocket was extending forward from the roof of the stomodaeum toward the floor of the dien_cephalo_n in the vicinity of the infundibulum. At about the 30-somite stage it has nearly reached the latter organ (Fig. 204), and shortly its end begins to broaden out and become branched. Finally, near the end of the incubation period, these branches have become a mass of tubular tissue well supplied with blood vessels. This glandular mass then loses all connection with the oral epithelium from which it arose, and be« comesrﬁrmly attached to the infundibulum. In this manner the original Rathl-:e’s pocket comes to constitute the anterior part of the hypophysis or pituitary body, while the infundibulum becomes the posterior part 372 THE CHICK

and stalk of that organ. Experimental work has shown that the out. growth of Rathl<e’s pocket is originally induced by the presence of the infundibulum, and that both structures inﬂuence one another in the normal development of the completed organ (Hillemann, ’4-3). It may be recalled that this same relationship is true in the Frog, except that there the homologue of Rathke’s pocket is merely a strand of cells.

v.C.d.1 v,P...2, v.C.d.2

-"v_’.;,~1_‘ M‘ __« v.P.3 _prfo.G. buss. ‘

tar. - tr. Gr. _ Ls ,

Fig. 194.—Reconstruction of the fore-gut of a Chick of 72 hours. From Lillie (Development of the Chick). After Kastschenko.

Hyp. Rathke’s pocket, rudiment of anterior hypophysis. Iar.-tr.Cr. Laryngotracheal groove._ Lg. Lung. Md.a. Mandibular arch. Oes. Oesopliagus. pr’o.G. Preoral gut. Stom. Stomach. Th. Thyroid. v.C.d. 1, 2. Dorsal division of the first and second visceral clefts. v,C.i:.2. Ventral division of the second visceral cleft. 1.2.1’. 1,2,3,4-. First, second, third, and fourth visceral pouches.

The Visceral Pouches and Arches.

The P0uch.es.———lt will be remembered thatduring the second day four pairs of visceral pouches had appeared; the ﬁrst three had reached the ectoderm, and each member of the ﬁrst pair had acquired a cleft opening ‘(O the outside. During the third day the first pair of pouches retain their openings, while each member of the second pair develops a short dorsal and a long ventral cleft, corresponding to the points of fusion between ectoderm and endoderm described in the preceding chapter. The members of the fourth pair of pouches now acquire connections with the ectoderm at their dorsal ends, but never develop any cleits (Fig. 194).

The Arches. —-—— The visceral arches undergo no special change on the third day, except the development in some of them of the aortic blood vessels (arches) which will be described below.

The Thyroid. ~—~— During the third day, the rudiment of the thyroid which was last described as a slight depression in the ﬂoor of the pharTHIRD DAY: THE FORE—GUT 373

ynx, continues to evaginate. By means of this process, the end of the third day ﬁnds the above depression transformed into a wide-mouthed sac. Figure 195 shows in cross section this and other structures indicated above.

The Laryngotracheal Groove and Lung Prirnordia.——At the end of the second day a shallow longitudinal groove with a pair of

Fig. 195.—Frontal section through the pharynx of a 35 somite embryo. From Lillie (Development of the Chick).

a.a. 1, 2, 3, 4. First, second, third, fourth aortic arches. Hyp. Rathkc’s pocket, ru<iimr:nt of anterior hypophysis. J. J ugular vein. lar.-tr. Gr. Laryngotracheal groove lpost branchial pharynx). or. Oral cavity. Ph. Pharynx. v./1. 1, 2, 3. First, second, third visceral arches. L'.G. 1, First visceral cleft. v.F. 2, 3. Second and third visceral furrows. v.P. 2, 3, 4. Second, third, fourth visceral pouches. III. Third cranial nerve. postero-lateral expansions had appeared in the floor of the pharynx just caudal to the visceral pouches, indicating the beginning of the respiratory system. This groove now becomes much narrower and deeper, and is called the laryngotracheal groove. Also its postero-lateral expansions develop into tubelil-re outgrowths "which, as previously indicated, are then ordinarily termed the lung prirnordia. Strictly speaking, however, they really represent, not only the beginnings of the lungs, but also of the bronchi, i.e., the entire respiratory system.

The Esophagus and the Stomach.—— By the end of thethird day the esophagus is represented by an abrupt narrowing of the fore-gut immediately posterior to the pharynx. The narrowed portion leads into a slightly dilated region just anterior to the liver rudiment, and this di lation is the beginning of the stomach, i.e., the proventriculus and gizzard (see the ﬁfth day). 374 -THE CHICK

The Liver. —— At the end of the second day the liver was represented by two anterior-ly directed diverticula from the region of the anterior intestinal portal; the more anterior of these had extended far enough forward to overlie slightly the point of union of» the vitelline veins. During the third day, these diverticula grow somewhat further forward, the anterior member of the pair along the left dorsal side of the ductus venosus, and the posterior member along its right ventral side. Both

Fig. 195.--Rec-onstructions of the liver diverticula of the Chick. From Lillie (Development of the Chick). After Hammar.

A. On the third day of incubation; from the left side; the diverticnlar arise from the anterior intestinal portal.

B. Beginning of the fourth day; from the left side.

a.z'.p. Anterior intestinal portal. D.V. Indicates position of ductus vcnosus. g.b. Gall bladder. l.d.d'.(cr.). Dorsal or cranial liver diverticulum. l.d.v.(caud.). Ventral or caudal liver diverticulum. pad. Dorsal pancreas. X. Marks the depression in the floor of the duodenum irom which the common bile duct is formed.

now also branch profusely, the branches spreading around the ductus venosus and anastomosing freely with one another. At the same time capillaries from the ductus venosiis begin to develop among the interstices of these anastomosing branches; this is the beginning of the main body of the liver.

The Bile Ducts. ———- In the meantime, the intestinal portal has, of course, moved backward beyond the point of origin of the diverticula. This lengthens the gut and leaves these diverticula attached to its ventral side at their ‘points of origin. The parts of the diverticula between the region of their anastomosis and the points of attachment to the gut are at the nature of short tubes, the rudiments of the future bile ducts. Presently the ﬂoor of the gut comprising the region where these ducts enter it becomes depressed, and then drawn out so as to form a common

. , ,......l......>-yep " THIRD DAY: THE HIND—GUT 375

duct into which the two original ducts empty. This common duct is called the ductus choledochus, and is a temporary structure (Fig. 196).

The Gall Bladder. -—- While the above processes have been going on, the gall bladder has arisen as a posterior evagination from the posterior

the gall bladder is drawn out to form the cystic bile duct.

All of these hepatic structures it should be noted are covered by the splanchnic mesoderm of the ventral mesentery within which they have developed. This rnesentery, here termed the gastro-hepatic ligament, serves permanently to attach the whole mass to the gut and stomach.

The Panct'eas.——This organ first appears on the third day as a thickening on the dorsal wall of the intestine within the dorsal mesontery about opposite the posterior liver diverticulum. The rudiment thus indicated gives rise to only about a third of the entire organ whose further development will be described as it occurs (Fig. 196) .

THE MID—GUT

There is no great change in the mid-gut region during the third day except that it becomes more clearly marked oil as the lateral folds continue to press in toward one another.

THE HIND—GUT

The Postanal Gut.——-It will be recalled that at the close of the second day the ectoderm had taken so slight a part in the tail fold that the anal plate retained a dorsal position. On the third day, however, the fold becomes more marked, and soon takes on the character of a posterior outgrowth, which is at first anterior to the anal plate. This outgrowth is the tail bud. As its development progresses it becomes first postero-dorsal, and then by turning downward postero-ventral, to the anal plate, which itself becomes ventral instead of dorsal (Figs. 197., 198). Also as a result of this process there is drawn out into the bud an extension of the hind-gut, constituting a temporary structure known as the postanal gut (Fig. 197).

The Allantois. -——The most important structure to appear in connection with the hind-gut during early embryonic life is the allantois. The rudiment of this organ is usually indicated at about the beginning of the third day. The method of its development and its ﬁnal structure have been described above (Figs. 190, 193) . In connection with the diagrams presented in Figure 198, however, a further word about its early origin should be said. These diagrams represent the behavior of this re376 THE CHICK

gion as described in the text, and according to Gruenwald (°4«1). It must he added, however, that in spite of the fact that there is apparent agreement regarding the movements which are taking place, Gruenwald puts a somewhat different interpretation on them than do certain other an s.A. Am. Am.cav. Ect. N'ch. n.1.

Fig. 197.-Sagittal section through the tail of an embryo of about 35 somites. From Lillie (Development of the Clzic/:3.

All. Allantois. Am. Amnion. Am.cav. Amniotic cavity. An.pl. Anal plate. A0. Dorsal aorta. Bl.v. Blood-vessels in wall of allantois. c.C. Central canal of spinal cord. Cl. Cloaca. Ect. Ectoderm. Ectam. Ectoderm of amnion. E.E.B.C. Exocoelom. Mesrzm. Mesoderm of amnion. N’c}L. Notochord. n..T. Nerve cord. p’a.C. Post-anal gut. p.i.p. Posterior intestinal portal. s.A. Segmental arteries, between the somites. Spl’pl. Splanchnopleure and yolk-sac entoderm. T.B. Tail bud.

thors, e.g., Lillie and the present writer. Gruenwald, following an old interpretation presented by Duval in his atlas, chooses to regard the original “ hind-gut 7 as already “ allantois.” As can be seen from the ﬁgures, it is true that a considerable portion of the original hind-gut is eventually included in the allantoic outgrowth. It has also been shown that the elimination of this region results in more or less complete elimination of this organ (Zwilling, ’46) . Nevertheless, it seems to the writer confusing to identify this gut in its primary condition with the allantois, THIRD DAY: THE HEART 377

involving as it= certainly does at that time the anal plate. It seems preferable to say that the allantois grows out from the part of this hind-gut which, by the processes shown, eventually comes to lie anterior to the

anal plate.

an I plate

tall bud anal plate

A

region of allantoic origln B

beginning of post anal gut

/Post anal gut

posterior Intestinal, portal——-“ can bud

beglnnlng of allantols

Fig. 198.——Diagrams representing changes in the tail and hind-gut region of the Chick during the third day. up to the 30 somite stage. After Gruenwald with slight modiﬁcations. The successive stages are indicated in the order of the letters.

THE CIRCULATORY SYSTEM THE HEART

There are no very marked changes in the form of the heart during the third day, though the atrium becomes slightly more prominent, and the hendings and constrictions already described‘ are somewhat emphasized (Fig. 199). Internally toward the end of the (lay sections reveal the appearance of a slight ingrowth from the atrial wall just to the left of the sinus venosus. It is the beginning of the interatrial septum (Quiring, ’33) . In the ventricular region the myocardium is becoming thick378 THE CHICK

enecl and spongy, but in the bulbus arteriosus, on the other hand, endothelial thickening has occurred, while the myocardium remains thin (Fig. 201).

THE EMBRYONIC BLOOD VESSELS

The Arteries. .

The Dorsal Aortae.—During the third day these vessels continue their development by beginning to form posterior to the point at which the vitelline arteries leave the body. These latter arteries thus become lateral branches of the dorsal aortae, instead of their continuations, while the further posterior growth of these aortae brings them eventually to the extremity of the tail bud. Meanwhile anteriorly they have become fused, so that by the end of the third day a single aorta extends from just back of the aortic arches almost to the origin of the vitelline arteries. Finally during the ﬁfth and sixth days the fusion of these vessels progresses

Fig. 199.——Heart of 21 Chick embryo of 72 hours, dissected out and drawn from the dorsal surface. From Lillie (Development of the Chick). _ _ , _

Aur.l. Left atrium. Aur.r. Right Into the tall region 3150, resulting

Elirggin-afii“-Igglgtiirgrﬁrlgfgéf in the formation of asinglé caudal

Ductus venosus. s.V. Sinus venosus. artery. It will not be necessary, Tina. Truncus arteriosus. V.r. Right however, to trace these processes limb of ventricle. , , _ of growth and fusion in detail.

The Aortic Arches.———During the third day each original carotid loop plus the anterior part of each original ventral aorta disappears. At the same time the part of each original ventral aorta which occupied the ventral four-fifths of each mandibular arch becomes directly connected with its respective dorsal aorta through the upper ﬁfth of each of these arches (Fig. 200). In this way the actual first aortic arches are completed.‘ However, before the end of the day the dorso-ventral con» nections of these vessels in the mandibular arches have been broken,

1 This statement is based on ﬁgures from both Duval and Lillie. It should be pointed out, however, that Lillie does not actually say that such a direct dorsal connection occurs, and the writer has not been able. to verify the point at first

hand. If such a connection is established it is certainly for a very brief time, and conﬁrmation would require the study of closely graded embryos. THIRD DAY: EMBRYONIC BLOOD VESSELS 379

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§!"'!*"!’:.’.’.£%‘§‘,*.'£."‘e“‘\ \‘. Fig. 200.—Chick embryo with adjacent portion of area vasculosa, with 35 pairs of somites (about 72 hours). Dorsal view. From Lillie (Development of the Chick). ma. 1, 2, 3, 4. First to fourth aortic arches. Am. Amnion. Ar. Branches of vitelline arteries. Atr. Atrium (Auricle) . A.V. Vitelline artery. B./1. Bulbus arteriosns. cerv. Fl. Cervical ﬂexure. cr.F l. Cranial ﬂexure. D.C. Ductus Cuvicri. D.V. Ductus venosus. Ep. Epiphysis. Gn.V. Ganglion of V cranial nerve. Iszh. Isthmus. Jug. External jugular vein. Md. Mandibular arch. M.M. Maxillo-mandibular branch of V cranial nerve. Myel. Myelencephalon. olf.P. Olfactory pit. Ophth. Ophthalmic branch of V cranial nerve. 0t. Otocyst. 5.2, 5.10, 5.20, etc. Second, tenth, twentieth, etc., somites. V. Branches of the vitelline veins. V.c.p. Posterior cardinal vein. V.umb. Umbilical vein. VJ’. Vitelline vein. V.V.p. Posterior vein. W.B. Wing-bud. 380 THE CHICK

and thus the first aortic arches vanish after a very brief existence. The dorsal aortae in this region do not disappear, however, but extend anteriorly as the internal carotids. Ventrally the stump of each first aortic arch persists, and presently produces an anteriorly growing twig which becomes the primary external carotid. (See ﬁfth day for ﬁnal development.) Meanwhile a fourth aortic arch arises in each of the fourth visceral arches.

Chor. P_ c_ Lens p. Ch. pl. gr. Am.

1

Fig. 201.—~Transverse section, passing tlirough the eyes and heart, of an embryo with about 35 pairs of somites (about 72 hours). Compare with F ig. 200. From Lillie (Development of the C/lick’) . V

Am. Amnion. A0. Dorsal aorta. Atr. Atrium. B.A. Bulbus artcriosus. cI'1..Fis. Choroid ﬁssure. Chor. Chorion. D.C. Ductus Cuvieri. Dienc. Diencephalon. Lg. Rudiment of lung branches. P.C. Pericardial cavity. p.Ch. Posterior (vitreous) chamber. pl.gr. Pleural groove. V.c. Posterior cardinal vein. Y.S. Yolk-sac.

The Pulmonary Arteries. — During the third ‘day, these arteries appear as rudiments within the walls of the lungs.

The Veins.

The Cardinals and Jugulars.——During the third day, the anterior cardinals continue to branch considerably in the brain region and may now be known as the internal jugulars. At the same time a vessel from the floor of the pharynx joins each anterior cardinal (internal jugular) just at its point of union with the duct of Cuvier. These new veins are the external jugulars (Fig. 200). Late on the third day also a new pair of cardinals begins to develop. They arise from a series of anastomosing vessels on the ventral side of the mesonephros just lateral to the dorsal aorta, and are known as the subcardinals. They are scarcely apparent as definite vessels before the fourth day.

, a i . THIRD DAY: EMBRYONIC BLOOD VESSELS 381

The Vitelline Veins. —- Before leaving the body of the embryo, these veins become united by a short transverse vessel which passes over the intestine just posterior to the dorsal pancreatic rudiment. In this manner,

the intestine is surrounded by a venous rinv. The anterior ventral part of this ring is formed by the posterior end of the ductus venosus. The lateral parts consist of the portions of the vitelline veins lying between the ductus venosus and the transverse vessel, and the posterior dorsal part is constituted of the transverse vessel itself (Fig. 211, A, B; see Chapter 12). Meanwhile, as indicated in the account of the liver, the portion of -the ductus venosus which lies within that organ is beginning to give of? capillaries among the branches of the liver diverticula.

The Untbilical Veins. —Early on the third day, a vein develops in the body wall on each side of the embryo, and opens anteriorly into the respective duct of Cuvier. These are the beginnings of the umbilical veins, although at this

Fig. 202.———Part of a transverse section through the lateral mesocardia of a Chick with 35 pairs of somites (about 72 hours).

From Kellicott After Lillie.

a. Atrium. arm. Accessory mesentery. am. Amnion. ac. Dorsal aorta. be. Bulbus arteriosus. ch. Chorion. cv. Posterior cardinal vein. dC. Ductus Cuvieri. dm. Dorsal mesentery. 1. Liver. lm. Lateral mesocardium. pc. Pericardial cavity. pe. Pulmoenteric recess. pg. Pleural groove. 5. Stomach. sv. Sinus venosus. um. Ventral mesentery.

(Chordate Development).

time they have no connection with the allantois (Fig. 203). Until such a connection has been established the blood from this organ is conducted to the lateral vitelline veins as follows: A transitory vessel, the subintestinal vein, develops upon the dorsal surface of the allantois, from whence it proceeds up onto the ventral side of the gut, along which it passes to the posterior intestinal portal. Here it divides into two parts

'which pass auteriorly around either side of the yolk-stalk to open into

the vitellines as these vessels run from the yolk-sac ‘along the margins of the anterior intestinal portal to the ductus venosus. 382 THE CHICK

THE EXTRA—-EMBRYONIC BLOOD VESSELS

The Arteries.—The vitelline arteries reach further out into the area vasculosa than during the second day, terminating near its border in a network of capillaries which empty into the sinus terminalis.

Fig. 203.—Injected Chick embryo of the third day, showing the arrangement of the cardinal veins and the formation of the umbilical vein from capillary networks. From Evans.

A.C. V. Anterior cardinal vein. P.C.V. Posterior cardinal vein. U.V. Umbilical vein.

The Veins.——Posterior to the point where the anterior vitelline veins have fused, the right vein becomes greatly reduced. During this period, also, the lateral vitelline veins passing backward and outward along the margins of the anterior intestinal portal continue to form from the vascular network lying close to either side of the embryo. In this manner, they presently reach the region where the vitelline arteries turn rather directly outward into the area vasculosa, and at this point they also begin to pass outward just dorsal to the arteries. These veins

5 i 3 . e s I THIRD DAY: THE MESENCEPHALON 333

never extend all the we)’ t° the sinus terminalis, but branch widely in the more central part of the vascular area. They receive blood from the terminalis, however, through several intermediate veins (venous trunks), which cross the outer network of arterial capillaries to reach them. Before the end of the third day, one other new extra-embryonic vessel starts to appear, the posterior vitelline vein. At this time it is scarcely more than a mass of capillaries, but very shortly begins to become distinct. It runs forward from the posterior side of the sinus terminalis, and empties into the left lateral vitelline vein near its base (Fig. 182).

THE NERVOUS SYSTEM THE FLEXURES

These have already been discussed under the description oi external changes.

THE FORE—BRAIN OR PROSENCEPHALON

The Telencephalon. ——The indentation which marks the velum transversum becomes much more prominent, while the rudiments of the cerebral hemispheres grow in size and their walls increase in thickness. In about the center of the lamina terminalis, a thickening appears called the torus transversus. ltcorresponds to the similarly named structure in the Frog, and as in that case it represents the rudiment of the future anterior commissure.

The Di'encephalon.—The more anterior (ventral) portion of the diencephalon is now sometimes distinguished as the parencepkalon, and the posterior (dorsal) portion as the synencephalon (Fig. 204.-). Between them is a slight constriction, while the parencephalon is approximately boundcd below by the marked indentation of the velum transversum. Thus the roof of the parencephalic. region constitutes a relatively raised area from which the epiphysis begins to develop at the close of the day as a small out-pushing. Upon the ﬂoor of the diencephalon, the optic recess, the region of the optic chiasma, and the infundibulum all become more pronounced than they were at the end of the second day.

THE MESENCEPHALON

The roof of the mid-brain grows rapidly and becomes prominently arched, while its walls increase uniformly in thickness. This arching of 384 THE CHICK

the mid-brain causes the boundary between it and the roof of the diencephalon to appear gradually more constricted. Likewise posteriorly at the connection between mid- and hind-brain, a slight constriction in the roof and lateral walls, indicated during the second day, also becomes very pronounced. This latter constricted region is henceforth known as the isthmus.

Fig. 204. —— Optical longitudinal section of the head of an embryo of 30s. The heart is represented entire. From Lillie (Development of the Chick).

Atr. Atrium. B.a. Bulbus arteriosus. D.v. Ductus venosus. Isth..Isthmus. Lg. Laryngotracheal groove. Oes. Oesophagus. or.pI. Oral plate, which has begun to rupture. Parenc. Parcncephalon. Ph. Pharynx. Stain. Stomach. Synenc. Synenceph alon. Th. Thyroid. S.v. Sinus venosus. Ven.R. Right ventricle. Other abbreviations as before.

THE RHOMBENCEPHALON

The Metencephalon. ——After the isthmus has become established the thickening roof of the metencephalon consists largely of the wall forming the posterior side of the constriction. By the end of the day, the lateral walls of the metencephalon have also begun to thicken.

The Myelencephalon. —— The roof of the myelencephalon remains thin, while its ventro-lateral walls have started to thicken somewhat.

The Spinal Cord. —— At the end of the second day, the wallsof the spinal cord were seen to consist chieﬂy of ependymal supporting cells and germinal cells. During the third day, the latter continue to multiply, and theirdescendants migrate out somewhat from their position THIRD DAY: THE RHOMBENCEPHALON 385

near the central canal. In their new location, they presently become transformed either into neuroblasts, i.e., primitive nerve cells, or into primordial glia cells. The nerve cells even at this time have begun to send out the axones and dendrites typical of the adult neurones. The central parts of these neurones together 'with glia cells eventually come

to constitute the gray matter of the cord, while the axones form its white matter.

Fig. 205. —Transverse section through the spinal cord and ganglion of a Chick about the end of the third day; prepared by the method of Golgi. From Lillie (Development of the Chick). After Ramon y Cajal.

c. Cones of growth at the ends of growing nerve ﬁbers. Nbl. 1 and 2. Neuroblasts of the lateral wall. Nbl. 3. Neuroblasts of the spinal ganglion. Nbl. 4. Neuroblasts of the ventral horn (motor neurohlasts).

As regards the ﬁnal condition of the cord, the following may be said: Internally, the central canal is obliterated, save for a small ventral portion lined by the inner ciliated ends of the ependymai cells. Surrounding this and ﬁlling the central part of the cord is the gray matter with dorso-lateral and ventro-lateral extensions or horns reaching out into the white substance. Externally, there develops along both the dorsal and ventral sides a median longitudinal ﬁssure. These ﬁssures are

' formed mainly as a result of the enlargement of the lateral regions

through the accumulation of the nerve ﬁbers within them.

The Spinal Nerves. -—The spinal nerves are sometimes described as constituting parts of two systems, (1) the somatic, and (2) part of the parasympathetic and the sympathetic; both systems start to develop on the third day. We shall consider the somatic system. first.

I. The Somatic System. — From bipolar nerve cells within each spinal ganglion one bundle of ﬁbers (dorsal root) grows into the spinal 386 THE CHICK

cord, and another outward in a ventro-lateral direction. Together these constitute the aﬂerent or sensory nerve ﬁbers. At the same time from the ventro-lateral side of the nerve cord beneath each spinal ganglion, ﬁbers (ventral root) are growing out from nerve cells located within the cord. These are eﬂerent or motor ﬁbers which mingle with those of the respective outgrowing afferent bundle just at the point where the latter leave their ganglion. The mixed ﬁbers thus form the common trunk of a somatic spinal nerve. This trunk then divides again into a dorsal and ventral part, each part containing ﬁbers of both the above types. The condition thus indicated is approximately the stage reached in the development of the somatic nerves at the end of the third day or early on the fourth (Fig. 205; common trunk not shown).

Inasmuch as it will not be proﬁtable in a work of this scope to follow further the detailed development of the somatic spinal nerves from clay to day, their future arrangement will be summed up at this time, as follows: The fibers of the divided trunks increase in number and at the same time grow outward. Hence, they almost immediately come into contact with the muscular and dermal plates, which are the rudiments of the future voluntary musculature and dermis of the Chick. Thus nervous connections are early established with these elements, and as the latter develop, the nerves (-motor and sensory) develop with them.

It should be noted that some of this musculature just indicated is destined for the limbs, and hence certain groups of the spinal nerves will constitute the brachial and the sciatic plexuses. In this connection certain experimental results are of interest. Thus it has been shown that when limb buds are transplanted to abnormal locations as described above, spinal nerves nearby, which would normally have nothing to do with limbs, are apparently “ attracted to them,” even forming a characteristic plexus before entering them (Hamburger, ’39). (However, see conclusions of Detwiler and Piatt on this matter in the section on the Frog). Hamburger (’39, ’44, ’49), Bueker (’45) and others have also shown that the number of motor neurons in the cord may be respectively decreased or increased by the extirpation of an adjacent limb bud or the implantation of an extra one. Hamburger also showed that the variation in number of motor neurons was apparently not caused by a difference in the total number of cells, but rather by the differentiation of more or less of this particular type of cell as compared with other types. These results show the effect of developing limb buds on nerves. Lastly, however, Hunt (’32) and Eastlick (’4-3) have demonstrated that in transw_@aﬂ >.

THIRD DAY: THE RHOMBENCEPHALON 387

planted limbs which for any reason fail to be innervated few muscle ﬁbers develop, and those that do, degenerate after about ten days. In conclusion it thus appears that there are reciprocal inﬂuences between a growing limb bud and its musculature on the one hand, and the devel. opment of neurons and their ﬁbers on the other.

11. The Sympathetic and Sacral Parasympathetic Systems. — As in the Frog there has been much disagreement concerning certain details of the origin of parts of these systems. For some time all postganglionic neurons at least were alleged to arise from neuroblasts in the dorsal root ganglia, i.e., originally from the neural crests. Later Jones, ’37, ’39, ’4I asserted that cells within the neural tube were the exclusive source for these systems. Further experimental study by Hammond, ’49 and Yntema and Hammond, ’54 ’55 seem now to have resolved the problem as follows: It appears that all postganglionic ne_urons and their ﬁbers are derived from the neural crest. All preganglionic ﬁbers, both sacral parasympathetic and thoraco-lumbar sympathetic arise from special aggregations of motor neurons within the spinal cord. The sheath cells of all the fibers are from the crest and tube (Brizzee, ’49), and possibly some mesoderm.

At the end of the third day or early on the fourth the postganglionic cells derived from the crest collect just above and to either side of the dorsal aorta. Here they send out ﬁbers anteriorly and posteriorly, forming a pair of delicate longitudinal cords running from the cervical region to the tail, with thickenings (ganglia) opposite each somatic ganglion. These are the primary sympathetic and sacral parasympathetic cords and ganglia, and each of these ganglia is connected with a somatic ganglion by a strand of ﬁbers, the primary rami communicantes. Lastly there are a few cells in the dorsal mesentery, probably from the crest, and destined to form Remak’s ganglion (Chap. 12, Fig. 216).

The Cranial Ganglia and Nerves.—-The ganglia of the V, VII, VIII, IX and X nerves have already been described as appearing on the second day. During the third day, the V ganglion shifts its position of attachment to the brain somewhat, and its characteristic YM shape becomes more marked. The VII and VIII ganglionic mass also shifts to a more dorsal position. Otherwise the cranial ganglia show no marked alterations at this time (Fig. 200).

The Mixed Character of Certain Cranial Nerves.——In the Chick, as in the Frog, it is possible to distinguish the V, VII, IX and X nerves as mixed, i.e., as containing both sensory and motor elements. In this respect they are of course not different from the spinal nerves, except as 388 THE CHICK

regards the point at which the two types of ﬁbers become mingled. Thus in the region of the cord, the ventral or motor ﬁbers of any nerve join the dorsal or sensory ﬁbers of that nerve slightly peripheral to the dorsal ganglion. In the mixed cranial nerves, on the other hand, the two types of ﬁbers issue from the brain very close together and mingle before entering the ganglion of the respective nerve. It may be further noted that though the ganglion of the VIII nerve is very closely associated at this time with that of the VII, -its ﬁbers are wholly sensory.

The III or Oculo-Motor Nerve. ——-Besides the mixed or wholly sensory nerves in the Chick, there are also, as in the Frog, certain cranial nerves which are purely motor and without any connection with the cranial ganglia. They take their origin from neuroblasts within the brain itself, just as spinal motor ﬁbers arise from neuroblasts within the spinal cord. The III or oculo-motor nerve arises in this manner from the median line of the ventral side of the mid-brain, at about sixty hours. Its history will be traced a few steps further in connection with the IV and VI nerves which arise on subsequent days.

THE ORGANS OF SPEClAL SENSE THE EYE

The Optic Cup. -—— There are two main changes connected with the optic cup during the third day. The first change is the rapid increase in its size. Thus at the end of the second day the lens rudiment practically ﬁlled the cavity of the cup, and came in contact with its inner wall. At the end of seventy-two hours, on the other hand, the lens is entirely separated from the wall of the cup, and simply rests within its rim. The second change is the thickening of the inner wall, from whose neuroblasts axones start to grow at the 30-somite stage (courtesy Rogers, K. T.) . The optic stalk is still ventral at the point of attachment to the cup, the region surrounding this point being called the fundus (Fig. 201).

The Lens. ——The lens becomes detached from the superﬁcial ectoderm during the third day, and forms a hollow ball, whose walls are at ﬂrst of almost uniform thickness. Presently, however, the cells of the inner wall (i.e., the one next to the optic cup) begin to lengthen, in a direction at right angles to this wall, so that the latter is thereby thickened. By the end of the day this thickening has progressed to a considerable extent, the elongated cells which cause it being destined to form the lens ﬁbers, which constitute the core of the lens. THIRD DAY: THE EAR 339

THE EAR

At the end of the second day, the auditory pit had been transformed into the auditory sac, whose mouth was still partly open to the exterior.

' By virtue of the method of the closure of the pit, described in the previ ous chapter, the major part of the sac lies below the level of its external

Fig. 206.———Two stages in the development of the auditory organ of the Chick. From Kellicott (Chmrclare Development. A. Hemisected model of left auditory sac posterior view, just before the separation from the head ectoderm, at about 72 hours. After Krause. B. Median view of a model of the left membranous labyrinth of an embryo of 7 days and 17 hours. After Riithig and Brugsch.

a. Anterior vertical semicircular canal. aa. Ampulla of anterior vertical semicircular canal. up. Ampulla of posterior vertical semicircular canal. d. Ductus endolymphaticus. e. Superﬁcial ectoderm of head. l. Lagena (cochlea). p. Rudiment of posterior vertical semicircular canal. s. Rudiment of saccule. u. Utricle. 9:. Connection between auditory. sac and superﬁcial ectoderm.

oriﬁce. The connection of this oriﬁce with the dorsal portion of the sac is then drawn out into a narrow tube, while the dorsal part of the sac itself is at the same time slightly constricted away from the major ventral part. The former, or dorsal portion, is the rudiment of the endalymphatic duct, which presently ‘grows upward somewhat so that its roof is slightly dorsal to the level at which the tube leading from it opens to the exterior (Fig. 206, A).

THE OLFACTORY ORGANS

Early on the third day a small circular spot of ectoderm on each ventrodateral side of the head somewhat in front of the eye becomes thickened, in consequence of a lengthening of its cells. These patches 390 THE CHICK

Fig. 207.—Tlze development of the mesonephros. A.B. Transverse sections through the mesonephric tubules of the Duck embryo with 4-5 pairs of somites. From Kellicott (Chordate Development). After Schreiner. C. Transverse section through the middle of the mesonephros of a Chick of 96 hours. From Lillie (Development of the Clzickt.

A0. Dorsal aorta. B. Rudiment of Bowman’s capsule. c. Conducting part of a primary tubule. coel. Coelom. Cal.T. Collecting tubule. cl. Dorsal outgrowth of the Wolﬁian duct to form a collecting tubule (see fourth day). Glam. Glomerulus. gcrm.Ep. Germinal epithelium. M’s't. Mesentery. n.t. Nephrogenous tissue. rc. Rudiment of conducting portion of primary tubule. T. 1, 2, 3. Primary,

secondary, and tertiary mesonephric tubules. V.c.p. Posterior cardinal vein. W.D. Wolﬂian duct. '

then begin to invaginate, and thus form the olfactory pits (Fig. 200). The thickened epithelium which lines them is the olfactory epithelium, and is said to consist of two types of cells, simple epithelial cells and germinal cells. The latter type later give rise to neuroblasts which eventually produce the sensory cells of the olfactory epithelium, while they

in turn give rise to axones which constitute the olfactory nerve. (See next chapter.) THIRD DAY: SUMMARY 391

THE URINOGENITAL SYSTEM

During the third day, the pronephros degenerates, while the mesonephros continues to develop, and soon becomes the primary excretory organ during embryonic life in a manner about to be indicated. Neither the metanephros nor the reproductive system appears during the third day.

As regards the changes in the mesonephric region, it will be recalled that at the end of the second day the Wolﬂian or mesonephric portion of the pronephric duct was just beginning to acquire a lumen. Its backward-growmg end, however, was still solid, and had not yet reached the cloaca. On the third day, this cellular rod connects with the cloaca, and by the end of the day a lumen has formed throughout its length. Concerning the mesonephros proper, at 4-8 hours the rudiments of the mesonephric tubules were forming in the neighborhood of the twentieth somite or segment, i.e., in the most anterior region of the future organ. At that time, these rudiments, of which there were two or more to the somite, consisted merely of spherical condensations of the nephrotome, which were beginning to become vesicular. Now at the end of seventytwo hours, however, the vesicles opposite the most anterior mesonephric somites are giving rise to small, hollow evaginafions in the direction of the Wolfhan duct (Fig. 207, A). There is one evagination to each vesicle, and it is the part of the vesicle which is destined to form the actual mesonephric tubule. Indeed, just anterior to the twentieth somite or mesonephric region proper, some of the out-pushings have already become tubules and are connected through conducting portions with the Wolflian duct (Fig. 207, B). In this region also Malpighian bodies have appeared in connection with some of the tubules. These most anterior tubules and glomeruli, however, never become functional.

SUMMARY OF THE CONDITION AT THE END OF THE THIRD DAY OF INCUBATION

I. GENERAL APPEARANCE

The cranial and cervical ﬂexures have increased, especially the latter. A small caudal ﬂexure has appeared, and the region in between has developed a slight ventral curvature. The lateral’ rotation has progressed so that the embryo is on its side as far back as the twenty-first somite. The four limb buds are clearly visible. 392 THE CHICK

II. THE SOMITES

The number of pairs of somites has increased to thirty-six and in the more anterior pairs dermatomes and Inyotomes are completely developed. Sclerotomal tissue is still collecting about the notochord and the sides of the nerve cord.

III. THE ALIMENTARY TRACT

The Fore-gut.-——The oral plate has broken through to complete the oral cavity, and Rathke’s pocket reaches nearly to the infundibulum. Subsequent development of these parts to form the pituitary is described in this chapter. The second pair of visceral pouches has acquired clefts, and the fourth pair has fused with the ectoderm. The thyroid depression has become a sac. The depression indicating the respiratory system has deepened in the laryngotracheal groove, and the rudiments of the lungs have appeared. The esophagus and stomach are beginning to be deﬁned. Finally, the liver diverticula have grown forward and anastomosed about the posterior part of the ductus venosus; the rudiment of the gall bladder is visible, and the dorsal portion of the pancreas has appeared.

The Mid-gut. —— It has become more clearly defined.

The Hind-gut.~—The anal plate has been carried around to the ventral side by the growth of the tail bud, and at the same time the

postanal gut has been formed. The rudiment of the allantois has appeared.

IV. THE CIRCULATORY SYSTEM

The Hea.rt.——There are no external changes aside from an emphasis of curvatures and constrictions already present. In the ventricular region myocardial thickening has occurred, and in the bulbus arteriosus the same is true of the endothelium. The interatrial septum has started to form.

Embryonic Arteries. —— Fusion of the aortae has progressed. The first pair of aortic arches has been completed and then disappeared. The dorsal aortae extend anteriorly as the internal carotids, while the stumps of the first arches produce the external carotials. The fourth pair of arches has developed, and the rudiments of the pulmonary arteries have arisen in the lungs. '

Embryonic Veins. —— The anterior cardinals have branched considerably in the brain region and are now known as the internal jugulars THIRD DAY: SUMMARY 393

which receive the external jugulars just at the union of the former with the ducts of Cuvier. The ductus venosus is beginning to develop capillaries among the branching liver diverticula. A new vessel passes over the intestine in the neighborhood of the pancreas and unites the vitelline veins to form a ring about the alimentary tract. A longitudinal vein has developed in each body wall; they are the umbilical veins, though at this time neither has acquired a connection with the allantois. The rudiments of the subcardinal veins may be visible on the "ventral side of the mesonephros. The transitory subintestinal vein is present.

Extra-embryonic Arteries.——The vitelline arteries have pushed out into the area vasculosa until their branches nearly reach the sinus terminalis.

Extra—embryonic Veins.~¥The right anterior oitelline vein has almost disappeared; the posterior and intermediate vitelline veins have started to arise, and the lateral vitelline veins have developed further.

V. THE NERVOUS SYSTEM

The Flexures and the Brain. —— As noted under external appear- '

ance the cranial and cervical flexures are both increased. The cerebral hemispheres have grown somewhat, and the epiphysis has started to develop. The optic chiasma, the optic recess. and the infundibulum have all become more clearly marked. The roof of the mid-brain. is more prominently arched and the isthmus has appeared. There has also been thickening and thinning of the brain walls at various points.

The Spinal Cord and Spinal Nerves. ——The germinal cells have changed their position and have begun to develop into neurones and glia cells. The sensory and motor nerve ﬁbers issue respectively from the spinal ganglia and the ventral portion of the cord, the two types uniting to form the common trunks of the somatic spinal nerves. The primary sympathetic trunks, ganglia and communicating rami have appeared. The completion of the somatic portion of the spinal nervous system is described in this chapter.

The Cranial Ganglia and Nerves.—-The ganglia have shifted their position slightly, and the third or oculo-motor nerves have appeared.

VI. ORGANS OF SPECIAL SENSE

The Eye. —— The optic cup has increased in size and its inner wall has thickened. The lens has become detached from the ectoderm, and its inner wall is also thickening. 394 THE CHICK

The Eat. -——The rudiment of the endolymphatic duct has appeared on the dorsal portion of the auditory sac.

The Olfactory 0rgans.~—The olfactory pits have been formed, with walls consisting of epithelial and germinal cells. ’

VII. THE URINOGENITAL SYSTEM

The proneplzros has begun to degenerate, while the mesonephros has started to develop tubules and glomeruli in its most anterior portion.

The Wolﬂian. duct has reached the cloaca and acquired a lumen throughout its length. °

VIII. THE AMNION AND ALLANTOIS

The folds of the amnion have approached one another above the posterior portion of the embryo and formed the amniotic umbilicus. The allcmtois, by about the middle of the day, has the appearance of a slight out-pushing from the hind-gut, and by the close of the day has extended’ well into the somatic umbilicus. 12

HE CHICK: DEVELOPMENT DURING THE FOURTH DAY OF INCUBATION

GENERAL APPEARANCE FLEXURES AND TORSION

T H E cranial ﬂexure remains about as on the previous day, but the cervical flexure has increased so in degree and extent as to bring the whole head further posterior. Also it brings the region of the dieti 1:. cephalon around so that it and the anterior part of the optic vesicles face almost directly caudad. At the same time the mid-region of the cervical ﬂexure is now the most anterior part of the embryo. From the anterior to the posterior limb buds the longitudinal axis has in most cases lost its ventral curvature, and has become virtually straight. Caudad to the posterior limb bud the caudal flexure is more marked so that the tip of the tail is curled around beneath the body. The lateral torsion now extends throughout the whole embryo so that it lies entirely on its side.

THE LIMB BUDS

All the limb buds have increased in prominence.

THE SOMITES

THE COMPLETION OF THEIR FORMATION

By the end of the fourth day the number of somites has reached 42, and subsequent to this time ten more are added posteriorly. These last ten, however, later disappear, together with the four most anterior ones (head somites), which become fused with the skull. Thus at 96 hours the Chick possesses all the somites which take any part in the development of the adult Bird. The development of the myotomal and derma l tomal elements progresses posteriorly in the manner already described. ~ I». V caud. Sci. int's. F.

int'v. F. caud. Sci.

X int's. F.

' ‘ Gn. Derm. ceph.ScL

snt{v§ 5.

My.

‘caud{ Sci. 4‘ im's. F. perm. V ceph. Sci. My. int'v. F, .

caud. Sci. int’s.- F. Derm. «ceph. Sci.

. My_. int'v. F.

caud. Sci.

int's. F.

ceph. sec. Ep. M

Fig. 208. ——Frontal section through the base of the tail of a Chick embryo of 96 hours. The anterior end of the section (above in the figure) is at a higher plane than the posterior end. From Lillie (Development

of the Chiclt).

caud.Scl. Caudal division of the sclerotome. ceph.Scl. Cephalic division of the sclerotome. Derm. Dermatome. Ep. Epidermis. Gn. Ganglion. int’s.F. lntersomitic ﬁssure. int’v.F. Intervertebral ﬁssure. My. Myotome.

N’ch. Notochord. N.T. Neural tube. per’ch.Sh. Petichordal sheath. s.A. Segmental artery.

THE CHICK FOURTH DAY: THE SCLEROTOMES 397

THE ULTIMATE FATE OF MYOTOMES AND DERMATOMES

Although the ultimate disposition of these elements of the somites is not accomplished until some time later, it is not desirable to follow their development longer by one-day periods. Regarding the dermatomes, or cutis plates, it has already been stated that their substance gradually moves out beneath the ectoderm, and ultimately forms the dermis in the dorsal regions, the dermis in the more ventral parts being derived from the underlying -somatopleure. Likewise Straus and Rawles, ’53 have now shown by carbon marking that the myotomes also are the source of only about the upper one third of the voluntary body muscles plus parts of three in the abdomen, the rest being somatopleural in origin. Head musculature and involuntary muscles develop from mesenchyme.

THE SCLEROTOMES

During the third and fourth days the mesenchyme of the sclerotomes comes to occupy all spaces about the notochord and between the latter and the myotomes. Indeed, immediately around the notochord itself it forms a thin continuous layer, the perichordal sheath. Further peripherally, however, a concentration of the mesenchyme in the cephalic and caudal portion of each sclerotome, as well as a slight division between these portions, has long made these parts distinguishable as such. Upon

the fourth day, moreover, it begins to appear that upon either side of I

the notochord the cephalic half of each sclerotome is beginning to become fused with the caudal half of the one anterior to it, thereby establishing a new segmental arrangement (Fig. 208). From the method of their formation, it follows that the segments thus arising do not coincide with the myotomes; instead, they alternate with them just as they did in the Frog. In this manner, blocks of mesenchyme are being marked out on either side of the notochord; these are the rudiments of the right and left halves of the future vertebrae. Lastly, from the cephalic and caudal portion of each sclerotome, mesenchymatous tissue has new extended well upward around the sides of the nerve cord. This

forms the rudiments of the neural arches, the cephalic arch of one

sclerotome later fusing with the caudal. of the next to form single arches corresponding to the vertebrae. The reason for the development of the alternative arrangement between vertebrae and myotomes, i.e., muscles, should be quite evident. In order to bend the back or neck it is apparent that each set of muscles must be attached at each of its ends to a different vertebra. 398 THE CHICK

THE ALIMENTARY TRACT THE REGION OF THE FORE—GUT

The Tongue. — The tongue appears on the fourth day as two papilliform outgrowths from the ﬂoor of the pharynx, one in front of and one behind the thyroid. These two rudiments then grow forward and fuse with one another. Eventually the structure thus constituted unites with a pair of lateral folds to form the tongue of the adult.

The Visceral Pouches and Arches.

The Pouches.——During the fourth day, the third pair of pouches acquire dorsal and ventral clefts like those of the second, while the clefts of the latter pouches and of the ﬁrst (hyomandibulars) become closed. The second pouches then gradually disappear, whereas the dorsal portions of the first pair extend dorso-posteriorly toward the respective otocysts; here each eventually forms a part of the tubo-tympanic cavity (see ﬁfth day).

The Arches.—The ﬁve pairs of arches reach their maximum development as such during the fourth day, and certain changes in their blood vessels take place; these changes will be described below.

The Thyroid.——The thyroid sac at this time completely separates from the ﬂoor of the pharynx. Subsequently it becomes divided into two massive lobes which move backward and take up'a position at the junction of the subclavian and the common carotid arteries. The effect of the pituitary upon the later development of this gland has been determined experimentally as follows:

Transplants have been made of thyroid glands from twelve-day old Chicks to the chorio-allantoic membranes of Chicks with and without pituitaries. It was found that only in Chicks possessing the pituitary does either a transplanted thyroid‘ or that of the host develop beyond the twelve-day stage (Martindale, ’4l).

The Respiratory Tract.——It will be recalled that at the end of the third day, the posterior part of the pharynx had deepened and narrowed to form the laryngotracheal groove, with the lung primordia at its posterior extremity. During the fourth day, the posterior portion of this groove, including the lung diverticula, separates from the ventral part of the alimentary tract. The anterior portion of the new tube thus

formed is the larynx which continues to open into the pharynx through a slit-like aperture, the glottis. The remainder of the tube is the trachea, FOURTH DAY: THE REGION OF THE FORE—_GUT 399

which divides into the lung primordia, really only the primary bronchi, at its posterior end. This is the condition of the respiratory apparatus at the end of 96 hours.

The Esophagus, the Stomach, and the Duodenum. —— At the end of the third day, the fore-gut region posterior to the pharynx consisted of an elongated tube——the esophagus, a slight dilation——the stomach, and finally another elongated region to which were attached the rudiments of the liver and pancreas. This last section of the foregut may from now on be termed the duodenum. During the fourth day the elongation of these parts continues, and also a certain curvature becomes evident. This latter process extends from the posterior region of the esophagus to the end of the duodenum, and the direction of the bending is such that the convex side of the curve is toward the left.

The Liver.——It will be recalled that at the end of the third day the main body of this organ had formed an anastornosing network about the ductus venosus, and that it extended somewhat further forward on the left side than on the right. During the fourth day, this network increases, together with its interstitial blood vessels (Fig. 196, B). As this enlargement proceeds, it will be found that the larger part of the organ comes to lie more and more upon the right side of the body, in the hollow made by the bend of the stomach.

The Pancreas. -——At the close of the third day, a thickening in the dorsal wall of the intestine opposite the posterior liver diverticulum was noted as the first rudiment of the pancreas. Upon the fourth day this thickening becomes a solid outgrowth, somewhat hollowed at its base. By the end of the day, two similar ventral rudiments may also be visible as antero-lateral outgrowths from the common bile duct (the ductus choledochus) . The subsequent union of these three elements will be described in the following chapter.

The Spleen.--Although this organ is not really a part of the digestive tract at all, it is convenient to describe its development at this point. During the fourth day a proliferation of cells occurs in the peritoneum at the base of the dorsal rnesentery just above" the dorsal pancreatic element. These cells become mingled with the surrounding mesenchymal tissue, thus forming the main substance of the spleen. Subsequent development results in the formation of a considerable mass, ﬁlled with sinuses which communicate directly with the splenic veins. Cells from the spleen are buddedyoff into these spaces and pass into the circulation, where they apparently become transformed into blood corpuscles. 4.09 V THE CHICK

THE REGION OF THE MID—GUT

For purposes of deﬁnition, the fore-gut region may be said to terminate at the end of the duodenum, and this point is marked approximately by the opening of the bile duct. The mid~gut, therefore, is the portion of the alimentary tract extending from the opening of this duct to the point at which the gut contained in the tail fold begins. It is difficult to deﬁne the latter point exactly at this time, except to say that since the tail fold never becomes very deep, it is relatively near the posterior end of the embryo, a short distance in front of the origin of the allantois. This boundary between the mid- and hind-gut is marked later by the intestinal caeca (see Chapter 13).

During the third and fourth days the folding-in process has been going on rapidly in the region of the mid-gut, and due to this, and to the growth of the entire body, the somatic umbilicus is so relatively constricted as to be called the umbilical stalk. Within it, as already noted, are the allantoic stalk and the yolk-stalk. The former has always been small, and the latter has necessarily shared in the constriction of the urnbilical walls. The result of these processes is obviously a mid-gut closed in at every point save the relatively narrow opening into the yolk-stalk; it is also a gut which still remains virtually straight. The section of alimentary tract which has thus been deﬁned is destined to become the small intestine of the adult bird.

In concluding the discussion of this topic it is well for the student to realize that there are two aspects to the umbilical constrictions just indicated. There is, on the one hand, the absolute narrowing of the umbilical opening. There is also in addition to this the immense growth of the remainder of the embryo. The girth of the umbilicus is thus a relative as well as an absolute matter, and the apparent reduction in its

size is due as much or more to the increase in size of the embryo as to its own constriction.

THE REGION OF THE HIND—GUT The remainder of the digestive tract posterior to the small intestine is, by the above definition, the hind-gut, and constitutes the large intestine or rectum. This opens into a terminal chamber, the cloaca. There is little to be said about the development of the rectum at this time, since it remains short, uncoiled, and without appendages.

The cloaca at 96 hours consists of a chamber into whose anterodorsal wall there opens, as indicated, the rectum. Just back of the rectal FOURTH DAY: THE HEART 401

oriﬁce, the cloacal cavity also receives the Wolﬁian ducts. Antero-ventrally below the rectal opening is the aperture of the allantois, while just behind this on the ventral side of the chamber is the original anal plate, or cloacal membrane (Fig. 193). It consists, as will be recalled, of a fused plate of endoderm and ectoderm, and during embryonic life separates the cavity of the cloaca from the exterior. Posterior to these apertures and the cloacal membrane, the cloacal chamber shows a marked lateral compression.

THE CIRCULATORY SYSTEM

THE HEART

In order to understand the development of the heart during the fourth and subsequent days, it will be necessary for the reader to refer to the description of that organ at the end of the second day. Assuming that this description is clearly in mind, we may then continue the account of the development on the fourth day, as follows:

Changes in the Proportion of the Parts. --The entire loop has gradually been expanding so that its parts have tended to approach one another. This has also resulted in a relative shortening of the two ascending limbs, i.e., the posterior limb comprising the atrium and part of the ventricle, and the anterior limb comprising another part of the ventricle and the bulbus arteriosus. At the same time so great has been the expansion of the transverse portion of the loop connecting these two limbs that the limbs as such have almost disappeared. What remains of the posterior one is marked by what amounts to a constriction just below the developing atrium. This apparent constriction, however, is brought about not so much by an actual contracting of this region

as by the relative expansion of the parts above and below it. Since the‘

part above forms the atrium, and the part below is a portion of the ventricle, the constriction between constitutes the atria-ventricular canal.

Changes in the Relative Position of the Parts.——At the same time that these changes in shape and proportion have been occurring, changes in the relative positions of the parts are also progressing. Of these there are three principal ones which may be indicated thus: (1) The bulbus arteriosus is swinging toward the median line beneath the atrium (Fig. 209, D). (2) The ventricular region is moving backward behind the atrium and also somewhat toward the median line, the region of the future apex pointing posteriorly. (3) To some degree as 402

THE CHICK

Fi . 209. —- The development of the heart of the Chick. From Kellicott (C ordaze Development). A, F, after Hochstetter. B—E, after Greil. A—E, ventral views of the heart. A. of a 40-hour embryo; B. of an embryo of 2.1 mm. head-length; C. of an embryo of 3.0 mm. head-length; D. of an embryo of 5.0 mm. head-length; E. of an embryo of 6.5 mm. head-length. F. Frontal section through the heart of an embryo of 9 mm. head-length.

a. Atrium. b. Bulbus. d. Roots of dorsal aorta. e. Median endothelial cushion (i.e., the cushion septum}. i. Interventricular groove. la. Left atrium. le. Lateral endothelial cushion. to. Left ventricle. om. Vitelline veins. p. Left pulmonary artery. ra. Right atrium. rv. Right ventricle. 5. Intfrrventricular septum. sa. Interatrial septum. t. Roots of aortic arches.

entnc e. E 1 »

FOURTH DAY: EMBRYONIC BLOOD VESSELS 403

a part of the latter movement, the posterior portion of the atrium into which the sinus venosus opens is rotating forward. In this manner, it is brought just over and then anterior to the atrio-ventricular canal, the latter remaining at a comparatively ﬁxed point between the ventricular and atrial regions. Though not completed during the fourth day, these movements are well under way at this time. Their progress, moreover, is suflicient to show that their tendency is to place the parts of the heart more nearly in their adult positions; i.e., the atrium anterior and dorsal, and the ventricle posterior and ventral.

Interior Changes Involving the Growth of Septa.——Whi1e the above external alterations in the form of the heart have been going on, further internal changes are occurring as follows: (1) the interatrial septum which started to form on the third day becomes more clearly evident as, a sickle shaped membrane extending postero-ventrally from the curved antero-dorsal wall, the back of the sickle being attached to the wall. Eventually of course this septum, augmented by certain other elements, completely divides the atrium into right and left chambers (the atria). (2) At the apex of the ventricle, the interventricular septum arises, and grows forward. Now since the ventricular apex has be</ome posterior to both the atrio-ventricular canal and the bulbus arteriosus, it is possible for the forward extension of this septum to meet them both. This, it eventually does ( see Chapter 13). (3) At the same time these septa are developing, a third one is beginning to arise within the atrio-ventricular canal; it starts as two endothelial thickenings, one in the ﬂoor, and the other in the roof of this canal. These are destined to grow towards one another until they unite in the center of the atrioventricular aperture, thus dividing it into right and left parts. When completed, this partition isknown as the cushion sept‘-tun (Fig. 209, F ) .

EMBRYONIC BLOOD VESSELS

The Arteries.

The Aortic Arches.-—— It will be recalled that during the third day, the first pair of aortic arches disappeared, leaving the anterior extensions of the dorsal aortae as the internal carotids. In a similar manner, extensions from the bases of the ﬁrst arches continue anteriorly as the external carotids. Upon the fourth day, the second aortic arches are likewise obliterated, and the two pairs of oarotids continue posteriorly to the dorsal and ventral ends of the third pair of arches. At the same time two new pairs of aortic blood vessels develop in the vestigial ﬁfth visceral arches behind the fourth and last pair of visceral pouches.

£3 5

g, 9'

2 l 404 THE CHICK

These are the fifth and sixth aortic arches (Fig. 210, A). The ﬁfth pair is small and quite transitory, being actually attached both dorsally and ventrally to the anterior sides of the sixth pair. Shortly after the sixth arches have thus arisen a small branch develops from about the middle of each and connects with the rudiments of the pulmonary arteries growing out from the lungs. In this manner the pulmonary arterial sysstem is completed, though throughout embryonic life the branches just indicated remain small.

LEFT SIDE RIGHT SIDE 4} day 8 day ductus Batallt subclavian artery

internal carotid

dorsal aorta artery ' ‘ __: carotid artery h ‘ ' ';. 3 3rd aortic arch 5th BOFUC arches I ' j: 4th (systemic) vitelline artery mm‘ arch I ’d pulmonary artery th aortic arch externa caroti

ITIGTY runcus arterlosus

A B

cruncus urtertosus '

Fig. 210.—Aortic arches of the Chick. Left side from a 45-day injected embryo.

Modiﬁed from Lillie, after Locy. Right side reconstructed from saggital sections of an 8-day embryo. Modiﬁed from Lillie.

From this description, it is clear that only the ventral portions of the sixth arches take part in the formation of the pulmonary arteries. The dorsal portion of each arch, on the other hand, is known as the duct of Botallo or ductus arteriosus, which, as will be noted below, atrophies at the time of hatchinv.

The Subclavian Arteries. ——- As noted under the description of external features the primordia of the anterior and posterior limb buds appear by the end of the day as broad swellings on the sides of the body. Correlated with this we ﬁnd that on the fourth day the eighteenth segmental artery on each side gives rise to a branch which extends out toward the respective bud. It is the primary subclavian artery. From it, at the point where it enters the limb, a branch also extends anteriorly toward the third aortic arch. This is destined to form the permanent subclavian (see ﬁfth day).

The Sciatic Arteries.-—-Posteriorly, a pair of segmental arteries also enlarge and grow out toward the hind limb buds. These vessels become Fig. 211.——Diagrarns illustrating the formation of the omphalomesenteric and umbilical veins, in the Chick, ventral view. From Kellicott (Chordate Development). Alter Hochstetter. A. At about 58 hours. B. At about 65 hours the veins are joined dorsal to the gut by a short transverse vessel. C. At about 75 hours the anterior intestinal portal has moved posteriorly somewhat so that the transverse vessel appears to be more anterior. At the same time, the left side of the loop, which its development created. has disappeared. D. At 80 hours a second loop has been formed by the fusion of the vitellineveins beneath the gut. E. At about one hundred hours the right side of this new loop has also disappeared. F. At about 130 hours, just before the disappearance of the main portion of the ductus venosus within the liver. This ﬁgure is obviously on a much smaller scale than E. A

c. Vena cava posterior (inferior). dC. Ductus Cuvieri. dv. Ductus venosus. g. Gut. hl. Left hepatic vein. hr. Right hepatic vein. 2. Liver. a. Omphalomesenteric or vitelline vein (the posterior continuation of the ductus venmsus). p. Anterior intestinal portal. pa. Rudiment of pancreas. ul. Left umbilical vein. ur. Right umbilical vein. 1;. Vilelline vein. I, II. Primary and secondary venous rings around the gut. 406 THE CHICK

the sciatic arteries, and as the legs develop they grow with and supply them.

The Umbilical Arteries. — During the fourth day, each sciatic artery gives off at its base a branch which extends into the allantois. These are the umbilical or allantoic arteries. Later {eighth day), the right member of this pair starts to disappear, while the left becomes a very important embryonic vessel, furnishing blood to the allantois. Indeed, so large does it become that the left sciatic seems for a time to be merely a branch from it.

The Renal Arteries and Those of the Conads. -—-Numerous branches from the dorsal aorta supply the mesonephros at this time, and later on a few of these persist as the renal arteries. Branches from the aorta also supply the reproductive organs as these develop.

The Veins. _

The Vitelline Veirzs. —- It will be recalled that at the close of the third day, the vitelline veins within the embryo had been united by a transverse vessel dorsal to the intestine, so that the latter was surrounded by a venous ring. Between this time and the close of the fourth day, ‘inither changes have taken place in this region, as follows: Very shortly after the transverse vessel has been formed the left side of the above ring disappears (Fig: 211, C ). Later, as the anterior intestinal portal moves backward, the vitelline veins between the poltal and the transverse vessel fuse with one another beneath the intestine. In this manner, a venous ring is again formed around the posterior extremity of the fore-gut, and in this case the right side presently begins to grow smaller. Anterior to the vitelline veins the ductus venosus continues to receive capillaries from the surrounding liver (Flo: 211, D).

The Cardinal Veins. -— The anterior cardinals, as indicated -in the previous chapter, have, by this time, reached a stage when they may be known as jugulars, while the posterior cardinals continue as previously described. The subcardinals which started to form on the third day become distinct vessels and presently acquire several direct connections with the posterior cardinals lying on the dorso-lateral sides of the mesonephros (Fig. 212).

The Inferior or Posterior Vena Cam. -~— This important vessel of the adult Bird begins to develop at this time out of some of the capillaries in the dorsal part of the liver on the right side. Slightly further back it is also augmented by venous islands in a fold (the caval fold) of one of the liver mesenteries. These capillaries and venous islands soon fuse Fig. 212. -— Reconstruction of the venous system of a Chick, 90 hours, ventral view. From Lillie (Development of the Chick). After Miller.

A.o.m. Omphalomesenteric. (vitelline) artery. a.sc.s. Left sciatic artery. A.u.s. Left umbilical artery. 6. Vessels enclosed within ventral side of mesonephros c. One of the direct connections of subcardinal with posterior cardinal. V.c.p.d.,s. Right and left posterior cardinal veins. V.c.i. Venn cava inferior. V ..sc.d.,s. Right and left subcardinal veins.

407 408 THE CHICK

together so as to form a deﬁnite vein which empties anteriorly into the ductus venosus (Fig. 211, E), and posteriorly establishes a connection with the right subcardinal (Fig. 212). Its subsequent development will be described in the following chapter.

The Umbilical Veins. ——— Upon the fourth day, the veins of the lateral body wall acquire connections with efferent vessels which have developed in the allantois, and at the same time, the right vein begins to disappear, along with the transitory subintestinal vein. The left vein on the other hand persists, but presently loses its anterior outlet into the ductus Cuvieri. At the same time, however, it develops new connections with the anterior half of the ductus venosus (Fig. 211, D, E). Through these, therefore, blood from the allantois ﬂows quite directly into the latter vessel, without taking any extensive part in the hepatic portal circulation. Later, these connections with the ductus venosus/fuse into one, which thus constitutes _the anterior extremity of the single umbilical vein (Fig. 211, F). Eventually this vein acquires a median position in the embryo instead of its original lateral one. Subsequent to hatching, its proximal portion persists as a vein of the ventral body wall.

The Pulmonary V eins.—-These vessels also develop at about this period in connection with the rudiments of the lungs, and presently become connected with the heart in the region of its left atrium.

EXTRA-—EMBRYONIC BLOOD VESSELS

The Arteries.——-During the fourth day the proximal portions of the vitelline arteries become fused with one another so as to leave the dorsal aorta as a single vessel. This fusion, however, occurs for only a relatively short distance, and never passes beyond the end of the umbilical stalk. From that point, the two main vessels continue to run out laterally, branching as they go, and terminatingin a network of capillaries just inside the sinus terminalis. Subsequent development does not fundamentally alter the arterial plan except that as the septa of the splanchnopleure develop in the yolk-sac, the arterial capillaries come to occupy the deeper portions of these septa.

The Veins.-—By the end of this "day the right anterior vitelline vein has disappeared, while the left anterior vitelline vein and the posterior vein, are well developed. The lateral vitelline veins have also become larger and more deﬁnite at the point where they extend outward in company with the arteries. Furtherout in the area vasculosa, they continue to branch extensively, the branches connecting with the intermediate veins as already noted. By this time, however, these conFOURTH DAY: THE PROSENCEPHALON 409

motions are so pronounced that the intermediate vessels appear merely as the finer endings of the lateral vitellines, uniting these veins with the sinus terminalis (Fig. 182). Subsequent to the tenth day, the anterior and posterior vitelline veins are gradually eliminated, the lateral veins persisting as the main efferent vessels of the yolk-sac. After the tenth day, the sinus terminalis is no longer distinct, becoming obliterated by a mass of capillaries. These capillaries and the vessels with which they are connected, forming the area vasculosa, then continue to spread over the yolk in company with the yolk»sac mesoderm. Thus, like the latter, they come at last virtually to surround it.

THE NERVOUS SYSTEM

This system, like the others, continues to develop through embryonic life. The differences observed in it between the fourth and ﬁfth days, however, are not, in most respects, very great. Therefore, since it is not proposed to carry a detailed chronological description of any of the organs beyond the ﬁfth day, we shall conclude the account of the nervous system in the present chapter.

THE FLEXURES

The cranial and cervical ﬂexures of the brain and nervous system have already been noted in the account of external changes through the fourth day. As has been indicated in the general discussion of this matter, only one of the ﬂexures just named, i.e., the cranial, is permanent, the cervical gradually straightening out until it is entirely gone. Also, though to a smaller extent than in the Frog, even the cranial ﬂexure is partly obscured in the adult brain by the development of the cerebral hemispheres and other parts. There is now to be noted a third ﬂexure, which though barely visible on the fourth day, later becomes quite marked. It, like the cranial, is permanent and also like the cranial is never entirely obscured. This is the pontine ﬂexure which consists of a ventral bulge in the thickened ﬂoor of the myelencephalon (Fig. 214) .

THE PROSENCEPHALON

The Te1encepha1on.——-The cerebral hemispheres continue to increase in size during the fourth day, and their lateral walls in particular, are thickening to form the corpora striata, The other features already noted as characteristic of this portion of the brain have also increased in prominence. As regards subsequent development the cere410 THE CHICK

bral hemispheres ultimately become one of the most noticeable portions of the brain, their backward growth causing them to overlap, and to conceal partially the large optic lobes. Their surface, however, never attains the complicated convolutions so characteristic of the Mammal. Anteriorly, beginning about the eighth day, small portions of these hem

Fig. 213.—-Optical longitudinal section of the head of an embryo of 395. From Lillie (Development of the Chick).

Atr. Atrium. 8.1:. Bulbus arteriosus. D.v. Ductus venosus. Lg. Laryngotracheal groove. Es. Oesophagus. or.pl. Oral plate, which has now ruptured; Parenc. Parencephalon. Plz. Pharynx. Slam. Stomach. Synenc. Synencephalon. Th. Thyroid. 5.12. Sinus vcnosus. V en. Ventricle. Other abbreviations as before.

ispheres become partially constricted away from the main posterior parts to form the olfactory lobes.

Concerning other parts of the telencephalon, as already indicated, the anterior commissure appears in the midst of the torus transversus. On the ﬁfth day, also, an evagination develops at the antero-dorsal boundary of the lamina terminalis just between it and the velum transversum; it is the paraphysis. This structure virtually marks the boundary between the telencephalon and diencephalon, Lillie placing it in the former. and some anatomists in the latter. Above this body occurs the inward bend of the wall which constitutes the velum transversum, whose

more dorsal half at least, according to most authorities, lies deﬁnitely in the diencephalon. FOURTH DAY: THE PROSENCEPHALON 411

i e - Hyp. pant. Ft.

Com. ant. Rec. op.‘

<

' Fig. 214.—~Dissection of the brain of an 8-day Chick. From Lillie { Development

of the Chick). The arrows shown in the ﬁgure lie near the dorsal and ventral boundaries of the foramen of Monro.

ch.Pl. Choroid plexus (anterior). Com.ant. Anterior commissurc Com.I’ost. Posterior commissure. C.str. Corpus striatum. Ep. Epiphysis. H. Hemisphere. Hyp. Hypophysis (anterior stomodaeal, part). L.t. Lamina terminalis. Myel. Myelencephalon. olf. Olfactory nerve. ap.N. Optic chiasma. op.L. Optic lobe. Par. Paraphysis. Ptzren. Parencephalon. pl.enc.v. Plica encephali ventralis. pon.t.Fl. Pontine flexure. Recap. Recessus opticus. S.Inf. Saccus infundihuli. Tel.med. Telencephalon medium. Th. Thalamus. T.tr. Torus transversus. Tr. Commissura trochlearis.

hThe lines a-a, b-b, c-c, d-d, e-e, f-f, represent the planes of sections not ﬁgured in t is text.

The wall of this portion of the fore-brain, therefore, gives rise to the anterior comrnissure and the cerebral hemispheres. Its cavity forms the anterior part of the third ventricle into which the lateral ventricles of the hemispheres open through the foramina of Monro.

The Diencepha1on.~——The anterior part of the roof in this region of the brain, as noted, apparently consists of the dorsal half of the velum transversum which later becomes folded to form the anterior choroid plexus. Eventually this plexus develops anterior branches extending forward into the lateral ventricles of the cerebral hemispheres. Posterior to the plexus the epiphysis shows no great change on the fourth day. Later, however, it grows out into a long narrow tube, whose 412 THE CHICK

end is dilated and possessed of numerous buds, the epiphysial or pineal gland. Just posterior to this organ at the boundary between the fore- and mid-brains, the posterior commissure eventually develops within the broad constriction which has marked this point from the first.

During the fourth day no striking development occurs in the lateral

Fig. 215.——Median sagittal section through the brain of the Chick of 12_to 13 days. From Kupiier (He-rtwig's Handbuch; etc.).

c. Cerebellum. ca. Anterior commissure. cd. Notochord. ch. Habenular commissure. ci. Infundibular commissure. ck. Central canal of spinal cord. cp. Posterior commissure. cpa. Anterior pallial commissure. cs. Spinal commissure. cu. Cavum cerebelli. cw. Optic chiasma. dr. Epiphysial (Pineal) gland. dt. Decussation of the trochlear (IV) nerve. e. Epiphysis. ex. Paraphysis. hm. Cerebral hemisphere. hy. Hypophysis (anterior part). 1'. Infundibulum. le. Ependymal lamina of the roof of the fourth ventricle. lo. Olfactory lobe. 1p. Posterior lobe of cerebral hemisphere. M. Mesencephalon. opt. Optic chiasma. pch. Choroid plexus third ventricle. pl. Choroid plexus of fourth ventricle. re. Epiphysial recess. ro. Optic recess. 5. Saccus infundibuli. si. Posterior intracephalic furrow. tp. _Tuberculum posterius. lpi. Tuberculum mammillare. tr. Torus transversus. wz. Velum medullare anterius. vi. Median ventricle of telencephalon. up. Velum medullare posterius.

or ventral region of the diencephalon. Subsequently, however, the former region becomes greatly thickened to form the thalami. On the ventral side, the fate of the infundibulum has already been described (see discussion of fore-gut, third day) while the optic chiasma comes to comprise a thick bundle of fibers from the optic nerves.

The floor of this posterior division of the fore-brain thus gives rise to the optic stalks, the optic chiasma and the infundibulum, while the optic thalami develop within the lateral walls. The roof forms the anterior

choroid plexus and the epiphysis; the cavity constitutes the posterior part of the third ventricle. C FOURTH DAY: SPINAL CORD AND NERVES 413

THE MESENCEPHALON

There is nothing in particular to be said concerning the development of this region on the fourth day. Later we find that the growth and thickening of the dorso—lateral parts of the mid-brain greatly exceed that of a narrow dorso-median strip, thus producing the two large optic lobes, which the median strip separates_ from one another by a ﬁssure. Ventro. laterally, the sides and floor of the mid-brain also become thickened, constituting the crura cerebri. This thickening ﬁnally results in narrowing the central canal to form the aqueduct of Sylvius or iter, which con. nects the cavities of the third and fourth ventricles.

THE RHOMBENCEPHALON

The Metencephalon. ~ The thickening which was noted in the roof of this region on the third day continues to increase, resulting ﬁnally in the production of a large median lobe, and two small lateral lobes united with it. The body thus formed extends backward somewhat so that it partially overhangs the myelencephalon. It is the cerebellum. About the ninth day, transverse ﬁssures appear on the surface of this organ, which deepen as development proceeds. The ventro-lateral walls of the metencephal on, which have also been thickening, come eventually to form the pans Varolii.

The Myelencephalon. —— It has already been stated that the roof of this region of the brain remains thin; it eventually forms the choroid plexus of the fourth ventricle. The ventral and ventro—latcral walls, however, showed signs of thickening on the third day. This tendency increases, and these walls ﬁnally constitute the medulla oblongata.

THE SPINAL CORD AND SPINAL NERVES

The description of the development. of the cord and of the somatic spinal nerves was completed in Chapter 11. The completion of the sympathetic. and parasympathetic systems, i.e., the autonomic, will now be noted.

The Sympathetic and Sacral Parasympathetic Systems.———It will be recalled that at the end of the third day the primary sympathetic and sacral parasympathetic systems had just been established. They consisted of two slender cords and their ganglia lying just dorso-lateral to the dorsal aorta, and extending from the region of the vagus ganglion to the tail. On the fourth and ﬁfth days neuroblasts migrate from each ganglion of the primary systems to positions above the primary cords just median to where each somatic trunk divides (Fig. 216) . Each such 414 THE CHICK

aggregation of neuroblasts, or ganglion, forms neurons which again send axones anteriorly and posteriorly to form the paravertebral or permanent sympathetic and sacral parasympathetic cords. For a time both primary and secondary cords exist to some degree, but eventually the primary cord is mostly eliminated. It is generally thought that neuroblasts from the ganglia ‘of the permanent cords also migrate to the mesentery and viscera to form the visceral plexuses, but, save for the sacral ganglia, Yntema, ’55 denies this, and claims that in the Chick at least, all these visceral plexus neuroblasts are from the vagus crest (see below). Though unorthodox this view is supported by extensive investigations.

It should be emphasized at this point that all the neurons so far de

Fig. Z16.-—Diagram of the chief

elements of the sympathetic nervous system of the Chick, in trans verse section: From Kellicott (Chordate Development). After His, Jr.

a. Dorsal aorta. op. Aortic plexus. J. Dorsal (aﬁercnt) root of spinal nerve. g. Spinal ganglion. i. Intestine. m. Me-sentery. n. Notochord. R. Remak’s ganglion. s. Splanchnic plexus. sg Sympathetic elements in intestinal wall. 1!. Mesonephric tubules. v. Ventral (efferent) root of spinal nerve. I. Primary sympatheticcord. 11. Secondary sympathetic cord. The rami communicantes are only partially

scribed as originating from the neural crests, constitute only the postganglionic elements of the systems under discussion. The preganglionic neurons on the other hand are all derived from

neuroblasts in the neural tube. These

cells at first occupy the ventro-lateral parts of the tube along with the somatic motor neurons. From here the sympathetic and sacral parasympathetic neuroblasts separate from the somatic neuroblasts, and migrate dor 5l‘°‘””° sally taking up positions on either

nuclei of Term’. From these, cell ﬁbers grow out through the ventral somatic nerve roots to the points‘ where

these roots join their respective dorsal roots. The preganglionic sympathetic and sacral parasympathetic ﬁbers then leave the somatic roots and through short connections, the secondary or permanent rami commurzicantes, enter the ganglia of the permanent sympathetic and sacral parasympathetic cords. Either in these ganglia (sympathetic) or in the

ganglia of the visceral plexuses (parasympathetic) they synapse with the postganglionic ﬁbers of these plexuses. '

side of the neural canal in the FOURTH DAY: SPINAL com) AND NERVES 415

It should now be noted that all the nerves and ﬁbers of the autonomic system, i.e., the sympathetic and sacral parasympathetic already dis. cussed, and the cranial parasympathetic described below, are strictly motor. Nevertheless there are sensory ﬁbers which convey sensations from the viscera. These arise from neurons in the cranial and spinal ganglia where all sensory neurons outside the nose, eye, and ear are located. They leave the dorsal roots through the rami communicantes and accompany the motor ﬁbers of the autonomic system to the viscera, though not part of that system.

The Cranial Ganglia, Mixed Nerves, and Cranial Parasyrnpathetics. Trigeminal Ganglion and Nerve.—-—It has been stated that this ganglion has the form of an inverted Y. On the fourth day ﬁbers from the anterodorsal branch, i.e., the ophthalmic, pass anteriorly along the dorsomedian wall of the optic vesicle. Eventually these ophthalmic ﬁbers, mostly sensory, reach the face and beak. The other branch of the Y extends toward the angle of the mouth, where it also divides, one part, the mandibular, is a mixed nerve, and supplies the lower jaw. The other all sensory branch, the maxillary, supplies the upper jaw. As usual all sensory ﬁbers arise from neurons in the ganglion, while the motor ﬁbers are from neurons in the brain.

T he Acustico-facialis Ganglion and Nerves.-—As indicated above, the ganglion which gives rise to the VII and VIII nerves is at first in a single mass. During the fourth day, however, the antero-ventral portion separates from the remainder, and gives rise to a nerve which extends chieﬂy along the hyoid arch, though possessing also a small branch to the mandibular. This is the rudiment of the future VII or facial nerve with :1 motor component from the medulla. The remainder of the ganglion gives rise to the VIII or auditory nerve which is purely sensory, and which communicates with the inner ear as described below.

The Glossopharyngeal Ganglion and Nerve. —-The origin of the IX cranial ganglion was noted in the account of the second day, where it was indicated as lying above the third arch. The IX nerve appears on the fourth day and extends into this arch. Later another branch enters the second arch, and together they eventually supply the tongue and pharyngeal region. ,

The V agus and Cranial Parasympathetic S'ystem..——Neuroblasts in the crest and an adjacent placode above the third branchial pouch, together with neuroblasts within the brain, produce the vagus complex as follows: Upon‘ the fourth day the crest part of the X ganglion separates from the placodal portion, and eventually produces the ganglion jagu~ 416 THE CHICK

lare, the placodal part producing the ganglion nodosum. The exact origin of all the neural elements of the X nerve complex in the Chick is still uncertain, but the situation seems to be thus: Neuroblasts of the ganglion jugulare produce the somatic sensory neurons, the somatic motor neurons arising from within the medulla. The crest produces all postganglionic neurons of the cranial parasympathetic system (Yntema and Hammond, ’55) except possibly those of the ciliary ganglion, said by Levi-Montalcini and Amprino, ’47, to be derived from mesenchyme; the preganglionic neurons of this system arise within the medulla. From the ganglion nodosum nerves pass into the fourth and ﬁfth neural arches and posteriorly to the heart, lungs, stomach, and intestine, while the ganglion moves back into the thorax. Eventually a part of the nodosum is detached as the ganglion cervical primum.

THE CRANIAL MOTOR NERVES

The Mo’cor—ocu1ar or III Nerve. — The early development of this cranial motor nerve has already been described. During the fourth day, it passes down beneath the optic stalk, and there enters a ganglion. This receives a connection from the ophthalmic branch of the V nerve, and is known as the ciliary ganglion. The III nerve ends by innervating the superior, inferior, and internal rectus, and the inferior oblique muscles of the eye when these develop.

The IV or Trochlearis Nerve.———This motor nerve does not appear until the ﬁfth or sixth day, but will be described at this point. It is peculiar as a motor nerve, in that it arises from the dorsal side of the brain, at the bottom of the isthmus. It has no connection with any ganglion, and ultimately innervates the superior oblique eye muscles.

The VI or Abducent Nerve. —— This is a perfectly typical motor nerve, appearing toward the end of the fourth day. It has no ganglion, and arises from the ventral side of the medulla median to the point of origin of the ﬁfth nerve. It innervates the external rectus muscle of the eye.

The XI or Spinal Accessory Nerve. ——There is no data on the development of this nerve in the Chick (Lillie).

The XII or Hypoglossus Nerve. — This nerve develops during the fourth day from two pairs of ventral roots on the medulla at the level of the third and fourth somites. There are no ganglia, and the roots are evidently serially homologous with the ventral roots of the spinal nerves.

The nerve to which they give rise eventually innervates the floor of the pharynx. ' FOURTH DAY: THE EYE 417

THE ORGANS OF SPECIAL SENSE

THE EYE

At the end of the third day the inner wall of the optic cup had thickened, and the whole cup was in the process of enlarging. The lens, meanwhile, had separated from the external ectoderm, and the side of the lens toward the cup had also begun to thicken. The further development of the eye may be described as follows:

Parts Connected with the Optic Cup.—-During the fourth day, pigment begins to appear in the wall of the optic cup nearest the brain, i.e., its outer wall. At the same time, there is developing upon the innermost surface of the inner wall, the internal limiting membrane. Beneath this membrane, but still toward the inner side of the inner wall, as noted on the second day, neuroblasts near the fundus have sent out axones. These have passed over the retinal elements just beneath the limiting membrane, and have reached the optic stalk through the proximal part of the choroid ﬁssure. Here they proceed among cells of the ventral wall of the stalk, and late on the fourth or early on the fifth day, reach the brain and form the optic chiasma. Later many more-ﬁbers grow through the ventral part of the optic stalk, causing it to swell so that the original internal cavity is obliterated. It may then be termed the II or optic nerve. In this connection it may further be noted that during the ﬁfth and sixth days the processes of growth occur in such a manner as to alter the relative position of the point of attachment of the optic stalk to the cup. The result is that at the completion of these processes the point in question is no longer at the ventral edge of the cup, but approximately at its center, opposite to the lens.

Subsequent to the fourth day, other changes are also occurring in the walls of the optic cup. As the various cell layers of the retina are formed in the inner wall, this wall shows diﬁerentiation into two zones. The central and larger of these, which includes the fundus, is called the retinal zone, i.e., the retina proper, and it is only within this zone that the above retinal elements are developed. The remainder of the inner wall consists merely of a band around the rim of the cup, and is known as the lenticular zone. The line of separation between the two is known as the cm serrata (Fig. 217). Within the retinal zone, the outer wall forms the pigmented layer of the retina,'but never completely fuses with it. In the lenticular zone, on the contrary, fusion between inner and

outer walls is complete, pigment penetrates them both, and both remain 418 THE CHICK

ant. ch. ‘ ' corn.

iris

Fig. 217. ——Frontal section of the eye of an eight-day Chick. From Lillie (Development of the Chick).

ant.ch. Anterior chamber of the eye. ch. Choroid coat. cil. Ciliary processes. Corn. Cornea. l.e.l. Lower eyelid. n.m. Nictitating membrane. olf. Olfactory sac.

op.n.- Optic nerve. as. Ora serrate. p. Pigment layer of the optic cup. post.ch.. Posterior (vitreous) chamher oi the eye. ret. Retina. scl. Sclerotlc coat. scl.C. Sclerotic

cartilage. u.e.l. Upper eyelid‘

relatively thin. From this zone, in connection with certain mesenchymal elements, are differentiated the iris and the ciliary processes. While these parts are forming, the cavity of the optic cup is being ﬁlled with a gelatinous matrix containing ﬁbers. Both elements are probably derived from certain cells of the retinal and lenticular zones, and together are known as the vitreous humor. Certain of the ﬁbers of the humor are con

» l 8 i FOURTH DAY: THE EYE 419

nected with the ciliary processes, and help to support the lens. Finally, the outside of the cup is gradually covered by two layers of mesenchymal origin. The inner is the choroid coat, and the outer the sclerotic coat, the latter being partly cartilaginous.

The Pecten. -—This body is_also developed in connection with the optic cup and choroid ﬁssure, but is entirely peculiar to the Birds. It

Fig. 218. — Diagrammatic reconstruction of the pecten of the eye of (Chick embryo of 71: days’ incubation. From Lillie (Development of the Chick). After Bernd.

Ch.ﬁs.l. Lip of the choroid ﬁssure. Ch./iss. Choroid ﬁssure. Mes. Mesenchyme. Mes.b. Upper edge of the rpesenchymal ridge covered by the lips of the choroid ﬁssure. Mes.K. Thickening of the

edge of the mesenchyrnal ridge. op.C. Optic cup. 0.St. Optic stalk. P. Pecten. P.B. Base of the pecten.

The arrow indicates the direction of growth of the lips of the choroid ﬁssure over the mesenchymal ridge. The line d shows the plane of the section reproduced in Fig. 219.

has seemed well, therefore, to emphasize it by a separate description. It arises during the fourth day in the form of a blood vessel embedded in mesenchyme. This mesenchymal mass is in the shape of a ridge which enters the cavity of the cup through the choroid ﬁssure near its proximal end. The distal end of the ﬁssure between this mesenchyme and the rim of the cup has, meanwhile, been closed. On subsequent days, the mesenchymal ridge pushes up into the cavity, while at the same time it is being gradually covered over by the in-turning and up-growth of the edges of the choroid ﬁssure on either side of it. This covering soon becomes more prominent than the relatively thin ridge of mcsenchyme 420 THE CHICK

which it has overgrown, and presently (eighth day) the two parts he. come indistinguishable. Though remaining constricted at its base, the ridge of fused tissues inside the cavity of the cup continues to grow somewhat, and later becomes folded, assuming the appearance of a fan, though in most Birds it is more comb-like, and hence is named the pecten. It is very vascular and probably helps to nourish the retina. The E opening in the choroid ﬁssure between pecten and optic stalk provides ‘

Fig. 219.-S£-ction in the plane of (1. of Fig. 218. to show the histological structure. From Lillie (Development of the Chick). After Bernd. Bl.v. Blood vessel in mesenchymal ridge. il. Retinal layer of op~ tic cup. Other abbreviations as in Fig. 218.

the exit for the optic nerve ﬁbers from the retina. A few of these ﬁbers runrdirectly to this point, but the majority come to the base of the pecten, and run along its sides to the place of exit (Figs. 218 and 219).

The Lens. — At the end of the third day, the inner wall of the lens vesicle had thickened considerably by virtue of the lengthening of its cells. This process continues for several days until the cavity of the vesicle is entirely obliterated. Moreover, inasmuch as the lengthening of the central cells is greater than that of those at the periphery, the inner surface of the lens becomes distinctly convex (Fio. 217). These lengthened cells of the inner wall form the core of the future lens, while the cells of the outer layer toward the ectoderm form a simple ﬂat epithelium. The lens now grows, largely by the production of cells at its equa- ‘V tor where the inner and outer walls meet. These cells become ﬁber-like

. x FOURTH DAY: THE EAR 4.21

and wrap themselves around the original elements which form the core, thus increasing the size of the lens by the addition of concentric layers of cells.

The Cornea, the Anterior Chamber, and the Lids. ——The cornea at first consists merely of the external ectoderm opposite the lens. On the fourth day, however, this layer is augmented internally by a thin non-cellular layer of mesenchymal origin. On the ﬁfth day, this thickens slightly, and begins to be covered on the side toward the lens by a third layer formed of mesenchymal cells. Later, the middle layer becomes cellular by the migration into it of cells from the mesenchyme, while the third and innermost layer forms a typical epithelium. The latter ﬁnally becomes continuous at its edges with the cells of the sclerotic coat. The cornea thus constituted arches outward slightly, and thus a chamber is formed between its inner layer and the front of the lens. This is the anterior chamber, and it becomes ﬁlled with the aqueous humor.

The lids begin to develop about the seventh day as folds of the integument surrounding the cornea (Fig. 217).

THE EAR

The Internal Ear.—-At the end of the third day, the otocyst, or future internal ear, was in the form of a sac. The uppermost portion of the sac had been slightly constricted away from the lower major portion, and had started to grow upward somewhat as the rudiment of the endolymphatic duct. This upper portion, furthermore, still retained its narrow tubular connection with the exterior (Fig. 206). There is, in these parts, no marked change characteristic of the fourth day. Upon the fifth day, however, the connection of the endolymphatic duct with the exterior is entirely lost. Moreover, the opening of the duct’ into the sac is being gradually shifted ventrally along the median side of the latter. At the same time, the dorsal part of the duct is continuing to grow upward, and expanding to form the means endolymphaticus. Eventually, this becomes embedded in mesenchyme above the hind-brain.

While these events are taking place in connection with the formation of the endolymphatic duct the remaining major portion of the otocyst is developing further, as follows: Upon the early part of the ﬁfth day, there arises from its dorsal half a vertically elongated, hollow out-pushing in the direction of the ectoderm. Then a horizontal out-pushing appears just beneath the ﬁrst, and therefore at about the equator of the otocyst. Presently a vertical split develops in the ventral part of the vertical out-pushing and soon extends dorsally, thus dividing it into an

..q,,,,,,.,/,,.,,a...,..~.,«._.. was 1—,.,~.«— <,..,..« 422 THE CHICK

anterior and a posterior ridge. The anterior, posterior, and horizontal ridges which have thus arisen are the rudiments of the respective semicircular canals. These canals eventually develop by a gradual constricting away of the hollow ridges, so that they become separated from the

Fig. 220.——Model of the auditory labyrinth of the the right side of a Chick embryo of 8 days and 17 hours; external view. From Lillie (Development of

the Chick). After Riithig and Brugsch.

A.zz. Ampulla of the anterior vertical semicircular canal. A.l. Ampulla of the lateral horizontal semicircular canal. A.p. Ampulla of the posterior vertical semicircular canal. C.a. Anterior vertical semicircular canal. C.l. Lateral horizontal semicircular canal. C.p. Posterior vertical semicircular canal. D.c. Ductus cochlearis. D.e. Endolymphatic duct. La. Lagena. Sa.c. Endolymphatic sac. U. Utriculus (utricle).

otocyst everywhere except at their ends. During this process a dilation occurs" on each canal to form its ampulla. The remainder of the dorsal portion of the otocyst into which the canals open is the utricle. Meanwhile, most of the ventral part of the otocyst has grown downward and also turned backward and toward the median line of the head. _,Itsi end forms the lagena, and the portion connecting this with the utricle ‘is the ductus cochlearis or cochlear duct. The sacculus arises about the seventh day as a pouch on the median side of the uppermost portion FOURTH DAY: ORGANS or SPECIAL SENSE 423

of the ventral part of the otocyst, i.e., just above the point where the latter receives the ductus cochlearis (Fig. 206, B).

The parts of the inner ear thus "far described constitute the membranous labyrinth (Fig. 220). The walls of this labyrinth are composed of epithelium, and its cavity is soon ﬁlled with the endolymphatic ﬂuid. Except for small areas within the ampullae and at certain other points, the above epithelium becomes ﬂat. At these points, however, elongated sensory cells end in hairs which project into the fluid, and among these cells grow the endings of nerve ﬁbers (axones) coming from the VIII cranial ganglion.

On ‘the sixth day, the mesenchyme which immediately surrounds the developing labyrinth begins to form a membrane (membrana propria) in close contact with it. At the same time the more peripheral mesenchyme is forming a cartilaginous case, separated slightly from the labyrinth and its membrane, but following all its contours. The space between the two is called the perilymphatic space. It is bridged by tissue which carries the nerves and blood vessels, and is ﬁlled by the perilymphatic ﬂuid derived from loose mesenchyme tissue left within the space. The cartilaginous case later becomes ossiﬁed, and is known as the bony labyrinth. In it, on the side toward the middle ear, are two small openings, the fenestra ovalis, and the fenestra rotunda.

The Middle Ear, or Tubo-tympanic Cavity. — As was stated in connection with the alimentary tract, the first visceral clefts are closed during the fourth day, and the ventral portion of the pouch of each disappears. The dorsal portion, however, grows up toward the respective otocyst, and during the ﬁfth and sixth days comes between it and the external epithelium. Each pouch then starts to enlarge, and the space within it is the dorso-lateral portion of one of the two tuba-tympanic cavities. Meanwhile," beginning on the fourth day, the ventro-median portion of each cavity is developed, as follows. In the antero-dorsal region of the pharynx, a horizontal shelf has grown backward, so as to produce a dorsal chamber virtually separate from the space beneath. Laterally, the part of each tubo-tympanic cavity already developed opens into this newly constituted dorso-median chamber. Then, as growth proceeds, an increasing portion of this chamber becomes drawn out into the respective cavities. Thus eventually the larger part of each middle ear space is really developed in this manner, rather than directly from the original “ gill” pouch. When these processes axrgeorri-1 .’~. plete the median part of the dorso-median chamber still $hains‘“as‘* *' such. while its lateral parts constitute the Eustachian tubes e({ have a \‘f‘_

V S Q Alhlnbcd ) O ‘ O

4- K “'r\""'/I lg‘ 424 THE CHICK

common opening into the mouth by a single median slit-like aperture in the horizontal shelf. With regard to the cavities themselves two other points remain to be noted. First as in the case of the Frog, each tubotympanic cavity contains a bone, the columella. Its development can best be described, however, in connection with the tympanum. Secondly there is the peculiar relation which exists between the tubo-tympanic cavities and certain of the other bones of the Bird’s skull. These bones like bones in other parts of the Bird skeleton to be described later contain spaces which give lightness to the body. The case of the head bones is noteworthy at this point, however, because in some of them the spaces are formed and ﬁlled by outgrowths from the tubo-tympanic cavities (Bremer, ’40). v

The External Auditory Meatus and the Tympanum. —-It will be recalled that the temporary external opening of the first visceral pouch occurs only at its dorsal end. Ventrally, however, there is a fusion with the ectoderm which causes the latter to form a vertically elongated pit. When the dorsal perforation closes, that point also is marked by a pit. These pits presently disappear, and on the sixth day the point between them becomes marked by a new depression, the beginning of the external auditory meatus. It gradually deepens until, except for a thin layer of mesenchyme, the external ectoderm is in contact with the endoderm of the tympanic cavity. These thin layers of ectoderm, mesenchyme, and endoderm which thus separate the middle ear from the outside, constitute the tympanum or ear drum.

To the inside of the tympanum of an adult Bird is attached one end of the columella. The other end is in contact with a membrane covering the fenestra ovalis of the bony labyrinth, i.e., the bony case which finally surrounds the membranous labyrinth. The columella is, therefore, like a bridge stretching across the tympanic cavity from the tympanum to the inner ear. It is chieﬂy developed from mesenchyme which lies in the dorsal wall of the enlarged tubo-tympanic portion of the gill pouch. This mesenchymal rudiment, it may be noted, is thought to be derived from the dorsal end of the second or hyoid arch. However that may be, as the cavity increases in size, it extends upward on each side of the above mesenchyrne until it has surrounded it except at its inner and outer ends. Then as this mesenchyme becomes cartilaginous and finally ossiﬁes, it forms a bone (the columella), occupying the position already described‘. Lastly, it should be added that the inner end of this bone in contact with the membrane of the fenestra ovalis seems to arise, at least in some_Birds, from an element (the stapes) which, though at ﬁrst disFOURTH DAY: ORGANS or SPECIAL SENSE 425

Fig. 221.—Sagittal section through the head of a Chick embryo of 5 days, showing the floor of fore~brain, olfactory pit, and developing olfactory nerve between. From Lillie (Development of the Chick). After Disse.

a. Unipolar neuroblasts near the olfactory epithelium. b. Bipolar cell in the olfactory nerve. c. Unipolar cell near the brain. F.B. Floor of fore-brain. N'bl. Neuro blast in the olfactory epithelium. olf.Ep. Olfactory epithelium. alf.N. Olfactory nerve. olf.P. Cavity of olfactory pit.

tinct, eventually fuses with the columella. This stapedial element in the Bird would thus apparently correspond to the opercular element in the ear of the Frog.‘

The Olfactory Organs.———lt will be recalled that, at the close of the third day, the olfactory epithelium consisted of two types of cells:

1 Some writers recognize a third element, the stylohyal, which enters into the formation of the columella of Birds. It must be stated. however. that the exact origins, as well as the homologies of the bones of the middle ear in the various groups of Vertebrates are not yet completely known. 426 THE CHICK

simple epithelial cells and germinal cells. It had also become depressed to form the olfactory pits. During the fourth day this process of depres. sion continues to a considerable extent, and thus the specialized olfactory epithelium lying at the bottom of the pits is carried in some dis. tance from the surface. The epithelium forming the sides of the pits, on the other hand, is unmodiﬁed and similar to that outside. The position of the pits has also shifted somewhat with the growing of the head, so that their months now lie just on the antero-lateral border of the oral cavity.

At the same time that these processes are taking place, the germinal cells referred to are transformed into neuroblasts, and the latter in turn into typical neurones. On the external side, these neurones send short processes to the surface of the olfactory epithelium. On the other side, they produce axones which extend in toward the brain, the region of whose future olfactory lobes they do not enter, however, until about the sixth day. Along the course of these axones are a few bipolar neurones and also numerous epithelial cells, the latter serving as supporting and sheath cells for the ﬁbers. Both types are said to migrate from the olfactory epithelium, to their ﬁnal position during the growth of the axones. The axones, together with the other cells just indicated, constitute the I cranial nerve (Fig. 221). _

On the ﬁfth and succeeding days, the nasal cavities continue to deepen somewhat, and become greatly modiﬁed in shape. This is partly the result of the appearance of certain folds in the nasal wall; these folds are the rudiments of the three nasal turbinals, only two of which are ﬁnally covered by epithelium of the olfactory type.

While the internal development of the olfactory organ is thus progressing, certain external changes are also going on in connection with the apertures. However, since these changes have more to do with the development of the face than with that of the olfactory organs proper,

they will be discussed under the heading of general external changes in Chapter 13.

THE URINOGENITAL SYSTEM

THE EXCRETORY SYSTEM

The Mesonephros.——At the end of the third dayithe pronephros had virtually disappeared, while the typical mesonephros was beginning to develop, posterior to the twentieth somite. During the fourth day, the FOURTH DAY: THE REPRODUCTIVE SYSTEM 427

primary me5°“ePh1"iC tubules are developed from the most ventral vesicles thro11gh°ut_the greater part of the mesonephric region. The remaining vesicles which occur in every mesonephric segment are, moreover, each giVi1'1g rise '50 a tubule. Thus besides the primary tubules, there are formed eventually secondary and tertiary tubules and sometimes even more, all of a similar nature, developing from the nephrotomal mass opposite each somite. As suggested in the previous chapter, the primary tubules thus formed soon connect directly, through a non-secretory or conducting portion, with the Wolﬂian duct. The others as they develop empty into outgrowths from that duct, which receive the name of collecting tubules (Fig. 207).

At the time that these tubules are developing, the remaining portion of each vesicle is forming a Malpighian body or corpuscle consisting of a glomerulus and its capsule. These Malpighian corpuscles are similar in essential respects to those found in the Frog, and need not be described further. Though its development is still incomplete, the mesonephros apparently starts to function as a kidney at this time (Boyden, ’24). In this connection it is of interest to note that in the Bird a few of the more cephalic rnesonephric tubules also establish rudimentary nephrostomal relations with the coelom in the manner characteristic of all these tubules in the Frog.

The Metanephros.—The rudiment of the ureter and collecting tubules of the metanephros, or permanent kidney of the Chick appears at the end of the fourth day as a diverticulum from the mesonephric duct. It arises from the dorsal side of this duct just at the point where the latter bends to enter the cloaca. During the fourth day, also, the nephrotomal tissue, just posterior to the thirtieth somite or end of the mesonephros, begins to degenerate for a short distance (see Chapter 13, Fig. 240). Thus anterior to this point, the mesonephros, and any undifferentiated nephrogenous tissue overlying it, become entirely cut off from the nephrotomal tissue posterior to this region. The latter tissue thus cut oi? accompanies the forward growth of the ureter and its collecting tubules, and is destined to form the secreting portion of the entire metanephros (see Chapter 13, Fig. 240).

THE REPRODUCTIVE SYSTEM

The Gonads.-—-The rudiments of the two gonads appear on the fourth day as thickenings of the peritoneal epithelium on each side of the dorsal mesentery, between it and the respective mesonephros. These thickenings occur just posterior to the origin of the vitelline arteries 428 THE CHICK

and extend for seven or eight somites, i.e., through the posterior half or third of the mesonephric region. Presently primordial germ cells appear in this epithelial tissue, near to which they have been transported from the anterior part of the germ wall, where they are said to be distinguishable as early as the primitive streak stage. According to the remarkable observations of Swift (714) and Goldsmith (’28) they are conveyed to their new location by the blood stream. No sex differentiation is apparent at this time.

The Gonoducts.——The future male gonoducts or vasa cleferentia are the mesonephric ducts whose development has already been described.

‘The oviducts or Mzillerian ducts begin their development at this time in both sexes in the form of two ridges, the tubal ridges. Each ridge is a strip of thickened peritoneum which appears on the fourth day. It lies on the dorso-lateral face of each mesonephros next to the body wall and near to the Woliiian duct. It is first found at about the level of the twen tieth somite; from this point it differentiates posteriorly (see Chapter 13, Fig. 246)

THE ADRENALS

These bodies, though not really a part of the renal system, are closely connected with it, and their development may, therefore, best be described at this point.

As in the Frog, the adrenal organs are composed partly of cells de-, rived from the peritoneum, and partly of cells from the sympathetic nervous system. The former element, known as the cortical substance, arises from the coelomic epithelium slightly anterior to the germinal region, and proliferations of this substance presently penetrate the mesenchyme between the Wolfiian body and the dorsal aorta. The element derived from the sympathetic nervous system (mainly the primary sympathetic system) is known as the medullary substance, which comes

into contact with the cortical material by the end of the fourth day

(Willier, ’30). SUMMARY OF THE CONDITION AT THE END OF THE FOURTH DAY OF INCUBATION

I. GENERAL APPEARANCE

The cervical ﬂexure has increased so that its mid-region is anterior and the diencephalon faces posteriorly. The caudal ﬂexure has also in a..;..ys.u.e_.»....m.=»..-,..a» FOURTH DAY: SUMMARY 429

creased, and the embryo between it and the end of the cervical ﬂexure is virtually straight. The entire embryo is on its side, and the limb buds have increased in prominence.

II. THE SOMITES

The number of pairs of somites has increased to forty-two, including all those which take part in the formation of adult structures, while the myotomal, dermatomal, and sclerotonzal elements have been developed in each pair. The last named element forms a nearly complete sheath about the nerve cord and notochord, and shows slight indications of the vertebral segments. The account of the further developnﬁnt of the my otomal and dermatornal elements is completed in this chapter.

III. THE ALIMENTARY TRACT

The Fore-gut. ——The rudiments of the tongue have appeared. The first and second visceral clefts have closed, and the third opened; the visceral arches reach their maximum development as such. The thyroid has completely separated from the ﬂoor of the pharynx. Subsequent development of the tongue and thyroid are indicated in this chapter.

The posterior end of the laryngotracheal groove and the lung rudiments have separated from the alimentary tract.

The esophagus, the stomach, and the duodenum have increased in length, and the two latter parts of the tract have developed a curve to the left. The liver has increased in size and come to lie somewhat in the

curve of the stomach. The dorsal pancreatic rudiment has become a solid outgrowth and a pair of ventral pancreatic rudiments have arisen from the ductus choledochus. The spleen (not really a part of the alimentary tract) has started to develop.

The Mid-gut. —-—The mid-gut or region of the small intestine is now a virtually straight tube open to the yolk only by the relatively con’ stricted aperture of the yolk-stalls.

The Hind—gut.—The anterior portion of the hind-gut constitutes the rectum, while its terminal portion becomes the cloaca. The latter is still separated from the exterior by the cloacal membrane, and its posterior part is laterally compressed.

IV. THE CIRCULATORY SYSTEM

The Heart. — The ventricular region, especially the transverse portion, has expanded and moved posteriorly. The bulbus arteriosus has swung toward the median line, and the atrium has rotated forward. The interventricular, the interatrial, and the cushion septa are developing. 430 THE CHICK

The Embryonic Arteries.———The second aortic arches have dis. appeared, and the fifth and sixth pairs have developed. From the latter have arisen the roots of the pulmonary arteries which grow out and connect with the rudiments coming from the lungs. The primary subclavian, the rudiment of the permanent subclavian and the sciatic arteries have appeared, while the last named have given rise to the umbilical or allantoic arteries. The history of the sciatic and allantoic vessels is concluded in this chapter.

The Embryonic Veins.——The ring about the alimentary tract, which is formed in connection with the vitelline veins, has been broken by the disappearance of its left half. A fusion of the above vessels has occurred beneath the fore-gut, forming a second ring. The capillaries of the ductus venosus among the branches of the liver diverticula are becoming more numerous. Posteriorly, on the ventral side of the mesonephros, the rudiments of the subcardinals have become distinct vessels and have acquired direct connections with the posterior cardinals. The inferior vena cava has begun to form in the liver and caval fold, and posteriorly has connected with the right subcardinal. The longitudinal vein in the right body wall is disappearing, along with the transitory subirztestina.-l vein, and the left, having acquired a connection with the allantoic vessels, has become the functional umbilical vein. The account of its development is completed. The pulmonary veins appear in connection with the developing lungs.

The Extra-Embryonic Arteries. —The vitelline arteries have

‘ fused with one another for a short distance as they leave the aorta. Their

branches in the area vasculosa continue to develop in company with the

growth of that region, but are without features requiring further note. The Extra-Embryonic Veins. -—The right anterior vitelline vein

has disappeared, but the left anterior, posterior, and lateral veins are

well developed. Subsequent development of the extra-embryonic veins is included in this chapter.

V. THE NERVOUS SYSTEM

The Brain.——The cranial and cervical ﬂexures have increased slightly; the porztine ﬁexure may be in evidence. The cerebral hemispheres have increased in size, and their lateral walls are thicker. The optic lobes are also becoming steadily more prominent. There are no other marked changes evident at this time.

The Spinal Cord and Spinal Nerves.—-There is no special development on the fourth day. FOURTH DAY: SUMMARY 431

The Cranial Ganglia and Mixed Nerves.—From the V nerve ganglion a branch (ophthalmic) has extended toward the future beak and another (mandibular) toward the angle of the mouth. The VII nerve ganglion has become separated from the VIII, and has given rise to the hyoial and mandibular branches. The IX ganglion has sent a nerve into the third arch. The X ganglion has divided into the ganglion jugulare and ganglion nodosum, and the latter is giving rise to the vagus nerve.

The Cranial Motor Nerves. —~ The III nerve has entered the ciliary

ganglion, and the VI nerve has just appeared. The XII nerve has also begun to develop.

VI. THE ORGANS OF SPECIAL SENSE

The Eye. -— Pigment is presented in the outer wall of the optic cup. On the inner wall the internal limiting membrane is developing and beneath this in the region of the fundus, axones of the retinal neuroblasts are growing into the optic stalk. The choroid ﬁssure has partly closed, and its proximal end is ﬁlled with the ingrowing pecten. The inner wall of the lens is continuing to thicken. The middle layer of the cornea has begun to develop. '

The Ear. -—There is no characteristic change directly connected with the ear at this time. Within the pharynx, however, the formation of the tu-bo—tympanic cavities has begun.

The Olfactory Organs. —The depression of the pits has greatly increased, and their openings now lie on the border of the oral cavity. The olfactory epithelium is giving rise to the elements of the I nerve.

Besides describing the events of the fourth day, this chapter also in cludes an account of the subsequent development of the nervous system and the organs of special sense.

VII. THE URINOGENITAL SYSTEM

The Excretory System. -— Primary tubules have developed throughout most of the mesonephros, while secondary and tertiary tubules are arising. Collecting tubules are springing from the Wolilian duct to connect with the two latter types. The Malpighian bodies are beginning to appear in the functional portion of the organ which starts to act as a kidney at this time. Rudiments of the metanephros are evident as a divcrticulum from the posterior end of each mesonephric duct.

The nephrotomal tissue just behind the mesonephros is beginning to degenerate. 432 THE CHICK

The Genital System. —— The Gonads are represented by thickenings of the peritoneal epithelium on either side of the dorsal mesentery, and contain primordial germ cells. The oviducts are present in both sexes in the form of the tubal ridges.

VIII. THE ADRENALS

The cortical substance of the adrenal bodies appears on the peritoneal wall near the mesonephros, and material from the primary sympathetic nervous system which is to form the medullary substance comes in contact with it.

IX. THE AMNION AND ALLANTOIS

The amnion is completed upon the fourth day, while the allantois has pushed out somewhat further into the extra~embryonic coelom. 13

HE CHICK: DEVELOPMENT DURING THE FIFTH AND SUBSEQUENT DAYS

THE EXTERNAL APPEARANCE GENERAL

DURING the ﬁfth day, the cervical flexure reaches its maximum curvature and from then on becomes less and less marked, while the protuberance caused hy the mid-brain also attains its greatest relative prominence at this time. The third and last visceral cleft closes during the fifth day, and the future neck is slightly indicated; the first three visceral arches, however, are still somewhat in evidence in this region. The limb buds which were merely rounded swellings on the fourth day are beginning to give evidence of joints.

By the seventh day the second and third arches are no longer visible externally, the heart has moved backward so that the neck is clearly defined, and the external auditory meatus has appeared, as indicated in the previous chapter. The limbs are distinctly jointed, and by the eighth day, the fore limbs begin to appear winglike. Upon the eighth day feather germs are also visible, the tail is relatively much shorter, and the position of the abdominal viscera is quite clearly marked by an external protrusion. From this time on, the embryo gradually assumes a typical bircllike form, one of the most striking changes being the relative increase in the size of the body as compared with that of the head due to mitosis and rearrangeinent of cells (Gaertner, ’49} (Fig. 222).

THE FACE

In connection with the development of the nose and mouth, the face undergoes so great a change between the fourth and eighth days, that it seems best to treat the subject separately.

At four days the openings of the olfactory pits are separated by a median projection overhanging the mouth. It is the naso-frontal process. Dorso-laterally each pit is further bounded by the lateral nasal process lying between the pit and the antero-dorsal part of the eye. Just below each lateral process there is also another slight out-pushing adjacent to 434 THE CHICK

the antero-ventral side of the eye, termed the maxillary process (Fi«_ 223). During the ﬁfth day the lateral nasal process of either side hecomes more closely united with the maxillary process heneath it. the two being separated only by the shallow lac/Lrymal groove. At the same

time an extension of these united processes crosses each nasal pit and fuses with the frontal process, thus dividing the pits into antacdorsal and postero-verma? halves. Thereafter as detet. opment proceeds the f0I:‘:‘;t:’.‘ are carried forward as: ti-to external hares while the Hiter are drawn back ~-.{t:~.'I;; the mouth as the z';.::.r»~,;.-1,." nares (Fig. ‘724«). It is _ evident that the midriic: ;.zs;~:‘— tion of the upper jaw to be derived from the nascfrontal process, and the lateral parts chieﬂy from the maxillary process. The lower jaw is molded upon the ventral and main part of the mandibular arches (see he Fig. 222.-—Embryo of 7 days’ and 7 hours‘ low)‘ By Virtue of tiles"

incubation 3:5. From Lillie (Development of changes the eighth day ﬁnds the Chick). After Keihel and Abraham.

the nares and rudimentary beak quite clearly defined, the latter being developed by the co1‘niiication of epidermal cells about the margins of the jaws. Further growth of these parts, accompanied by a relative diminution in the size of the

eye and the development of the eyelids, brings the face to the condition found’ at the time of hatchinrr.

FEATHERS

In a preceding paragraph feather germs were mentioned, and because of the peculiarly characteristic nature of these structures in the

whole class of Birds, it seems desirable to indicate very brieﬂy the essentials of their development.

«.... ..»..: FIFTH DAY: FEATHERS

Feathers, like hair, which we shall consider briefly in connection with the Mammal, are epidermal structures. That is to say, the feather consists of hardened tightly pacl-zed epidermal (er.-toclermal) cells, not of secretion by cells. Initially

1 point on the skin where the feather is to appear develops a slight depression, in the midst of which rises. 51 small tipgroxrtli or papilla. The apex ml the papillzi at iirst is at about the level of the rim of the surrounding depression, or sli_s;l1t'i§.* shove it. It consists of 21 C3iT.' of dermis (inesoilerm) covered by the

Fig. 22-t.—~Head of an embryo of about 5 days from the oral surface. (N.L. 8 mm.) From Lillie (Development of the Chielc). ch.F. Choroid ﬁssure. E.L. Eye-lid (nictitating; membrane’). cx.mzr. External nares. l.Gr. Lachrymal groove. Other abbreviations as in Fig. 223.

435

Fig. 223. -—-Head of an emhryo of 4 days’ incubation. from the oral surface (‘N.L. 6 mm.). From Lillie iﬂarclopnzent of the Chick).

E12. Epipliysis. H0111. Cerebral hemisphere. Hy. llyoid arch. I.nas.pr. Lateral nasal process. Id. i\l:tmlihulur arch. flfx. Maxillary procx.-ss. nas.fr. Nusu-f-rontal process. Olf. Olfactory pit. Or. Oral cavity. Ph. Pharynx. 1).A.3. Third visceral arch.

usual Malpighian layer of the epidermis, and a thin layer of stratified and corniﬁed epithelium cells, the corrzeum. In other words it possesses the same cell layers which constitute the other regions. Very shortly this papilla grows outward so that it protrudes deﬁnitely above the Ievel of the rim of the depression, at which stage it is known as a feather germ. Within this germ the vascular dermal core is now known as the feather pulp. At

skin in 436 THE CHICK

the same time the Malpighian layer of the epidermis at the distal end ‘of the germ forms folds whose cells are modiﬁed to make the barbs. More proximally the folds arise from a nonfolded part of the Malpighian layer whose cells produce a single axis, the quill. The latter structure pushes upward and soon throws off its sheath of coreum, emerging as a down feather, i.e., a short quill with many short, soft barbs. At the base of the down feather the dermis produces the pulp of the permanent feather, while the Malpighian layer here forms two main folds opposite each other, the rachis, other lesser folds again producing the barbs. It is interesting to note that transplantation experiments by Cairns, ’54, have shown that the underlying dermis determines the special type of epidermal structure which will be formed, i.e., wing feather, leg feather, claw, or scale. '

THE SKELETON

As in the case of the Frog, only a brief description of the development of the skeletal system will be given. For a more extended study, the reader is referred to LilIie’s Development of the Chick, and the books of reference cited therein.

THE VERTEBRAE, THE RIBS, AND THE STERNUM

At the end of the fourth day the cephalic portion of each sclerotome was beginning to fuse with the caudal portion of the one anterior to it to form the rudiment of the right or left half of a vertebra. The occurrence of these vertebral rudiments thus necessarily alternated with the myotomes. An extension of mesenchyme had also grown up on either ‘side of the nerve cord above both the cephalic and the caudal divisions of every sclerotome, forming in each case the respective posterior and anterior rudiment of a future neural arch. This reversed cephalic and caudal relationship between the original sclerotome on the one hand, and the future vertebrae and their arches on the other, is of course a corollary to the alternative arrangement between the vertebrae and myotomes just indicated.

Upon the fifth day, the fusion of the cephalic portion of each sclerotome with the caudal portion of the next anterior to it is completed. The sclerotomes upon one side of the notochord also have become fused above and beneath it with the corresponding sclerotomes upon the other. Furthermore, as a result of concentration, all of the sclerotomal tissue is beginning to become membranous, and ire; the region of each future vertebra certain portions of this membrane appear especially condensed. FIFTH DAY: VERTEBRAE, RIBS, HSTERNUM 437

One such condensation surrounds the notochord as a ring, constituting the rudiment of a vertebral cenzrzmz. Another occurs in each of the upgrowing primordia of the neural arches, and still another arises in the membranous mesenchyme extending outward between the myotomes on either side of the notochord. Each of the latter extensions represents a transverse or costal process.

During the sixth to the eighth days these eostal processes develop Iurther, and in the thoracic region give rise to the membranous primordia of the dorsal two thirds of the upper parts of the true ribs, i.e.,

Fig. 225. —The right side of four bisected vertebrae of the trunk of an 8-day Chick. From Lillie (Development of the Chick). After Schauinsland.

caud.v.A. Caudal division of vertebral arch. ceph.v.A. Cephalic division of vertebral arch. N’ch. Notochord.

those movably articulated to the vertebrae. The cervical costal processes which are not movably articulated are often called cervical ribs.‘ The ﬁrst true rib primordlia are those of the fifteenth vertebra, which are followed by six other pairs. The third to the seventh pair of these ribs possess ventral parts which develop from separate centers, and like the ventral one third of the dorsal parts come from lateral plate mesoderm, not sclerotome (Straus and Rawls, ’53). The third to the sixth of these parts later fuse to the sternum. Further ventrally, the sternum itself develops from bilateral membranous plates also arising within the lateral plate mesoderm. Presently the membrane of the neural arch primordia unites above the nerve cord, and their normal development seems to be

. conditioned by both nerve cord and notochord (Waterson, ’54-) . Carti lagepformation now starts in all of the regions indicated, and in the last five pairs of ribs the dorsal and ventral part of each has its own center of chondriﬁcation. The sternum or breast bone of the chick, including

1 Since there is no clear cut distinction between cervical and thoracic vertebrae

in the Bird, the writer is arbitrarily deﬁning as thoracic all vetebrae with freely articulating or true ribs. 438 THE CHICK

heel likewise has two cartilage forming centers, one in each of the lateral membranous plates; these, however, soon fuse. Following chondriﬁcation the cartilage is in turn replaced by actual bone; during this procedure the remains of the notochord are completely eliminated. Such ossiﬁcation is well advanced by the sixteenth day.

Subsequent to this time several of the thoracic and lumbar vertebrae become rather ﬁrmly united with one another, and these in turn are fused to the coalesced vertebrae of the sacral region. To this mass there is also added posteriorly a number of the caudal vertebrae, so that a considerable portion of the spinal column is virtually inﬂexible, a condition peculiar to the Birds. Lastly, the extreme terminal vertebrae are likewise fused into a single piece termed the pygostyle.

C THE APPENDICULAR SKELETON

The Fore-limb. ——- On the fourth day a concentrated mesenchymal mass—probably of sclerotomal origin appears in the base of each forelimb bud, and on the ﬁfth day there grow out from this membranous mass four processes. One, the primordium of the limb bones, grows out into the lengthening wing bud; a second, the scapula, grows backward and dorsally above the ribs; a third, the coracoid, grows down posteriorly toward the region of the sternum; and a fourth, the clavicle, grows in front of the coracoid toward the median line. The last three elements represent the rudiments of the pectoral girdle. Centers of chondriﬁcation occur’ in the membranous primordia of the scapula and coracoid on the sixth day, followed later by ossiﬁcation. The clavicle, on the other hand, ossiﬁes directly from membrane, about the eighth day. Like the coracoid and scapula, all the bones of the fore-limb pass through both a membranous and cartilaginous stage previous to ossiﬁcation. It is interesting to note that in the wrist there are 13 membranous elements which as a result of fusions produce only two deﬁnitive carpals. Likewise in the hand ﬁve digits are represented in the membrane, but the first and ﬁfth soon disappear.

The Hind—1irnb. —-Like the fore-limb, the parts of the pelvic girdle and hind-limb bones arise about the ﬁfth day as four processes from a common mass of mesenchyme in the region of each hind-limb bud. The membranous process representing the limb bones grows out into the bud; another process, the ilium, which is elongated in an anterior posterior direction, grows dorsally; a third, the pubis, grows anteroventrally, and a fourth, the ischium, grows postero-ventrally. By the FIFTH DAY: APPENDICULAR 439

eighth day, the distal ends of the pubis and ischium have both rotated posteriorly so that they are parallel with one another, and with the ilium. Chondriﬁcation and ossiﬁcation follow the membranous stage, and the limb develops in a manner fundamentally similar to that of the fore-limb. There are three tarsal elements and ﬁve digits present in cartilage, but the rudiment of the ﬁfth digit soon disappears. Later the two proximal tarsals fuse with the tibia, and the distal one with the three long metatarsals; subsequent to ossiﬁcation the latter become united, thus forming with the distal tarsal element the single tarso-nzetatan

sus. As regards the details of ossiﬁcation in the long

bones of the Chick, we I , _ ., endochondng ﬁnd that the situation dif- , _ - ‘‘ b°"°

remains of :‘ - — ‘ ‘ _ diaphysial that 111 the Frog, and ca.-mag: from what we shall. later see in the Mammal. As noted the membranous

stage is as usual followed

by cartilage, and as in the Fig 226__The head of a long bane (femur, in Frog in the region of the the Chiizk. From Lillie, aftc; Br:11chet.1Thed.siﬁlua _ - _ -- lion wit I respect to the epip ysia rarti age i ers Shaft or d1al’h)“”i"'” ll“: from that in the Ft‘H,[_‘. but the -nizuation in the cartilage is overlaid by «Iiapi; Z: i» :-in:El'ir to the extent that, save at the uncjs, horn is little or no bone except that produced by the p:,'rin$t+"um.

fers somewhat, both from

marrow

perlosteum cavity

periosteal bone. In this case, however, the cartilage is presently destroyed, and partly replaced by true endochondral bone, though of a cancellcus character. Throughout the shaft this cancellous endochondral bone is then likewise removed to be replaced to a considerable extent by marrow. Thus in respect to having most of each long bone ultimately of periosteal and membranous origin the Bird approaches, but does not quite equal the condition in the Frog. There is in the Chick some endo«::honrlral ossiﬁcation of a permanent nature in these bones which comes about because of their method of longitudinal growth which takes place as follows:

The epiphyses or ends in the Chick bones, unlike those in the Frog, only remain cartilaginous during the increase in length of the diaphysis. 440 THE CHICK

This increase occurs through ossiﬁcation of the cartilaginous ends on their diaphyseal sides, with simultaneous addition of more cartilage distally (Fig. 226). Finally as growth is completed the cartilage of the epiphyses is entirely replaced by cancellous bone. In this manner it happens that a little spongy bone at the ends of the diaphysis, and all of that in the completely ossiﬁed epiphyses is of endochondral origin. In concluding this topic it should be noted that among the long bones of the Bird the humerus is peculiar in one respect. In this bone there is relatively little marrow, the extensive cavity therein being largely occupied, as will presently be noted, by one of the lung outgrowths called air sacs. (See below.)

THE SKULL

The Primordial Craniu~m.——The primordial or cartilaginous cranium of the Chick is first indicated by concentrations of mesenchyme during the fourth and ﬁfth days. Then, during the sixth, seventh, and eighth days, these mesenchymal concentrations develop into the following fused elements of cartilage. Along either side of and encasing the anterior end of the notochord, appear the parachordal plates. In the Chick these elements develop from the first as a single piece, and are often known, therefore, as the basilar plate. Anterior to it are developed simultaneously upon either side another pair of plates — the trabeculae. Posteriorly, these are continuous with the parachordals, with which they form an angle corresponding to the cranial ﬂexure, while anteriorly, their ends meet and fuse with one another. This fusion then extends somewhat, so that eventually the central space is closed, except for a small opening containing the pituitary body. Thus the trabeculae and parachordals together form the entire cartilaginous floor of the skull.

At the same time that these plates are forming, cartilage also develops around the auditory sacs and the olfactory organs, forming respectively the auditory and olfactory capsules. These are in direct continuity eventually with the plates. From the postero-dorsal part of each auditory capsule, processes now grow toward one another and fuse above the hind-brain. Thus is constituted the only portion of the roof of the cranium which is preformed in cartilage. Posterior to each auditory capsule, a dorso-lateral plate of cartilage develops, while anterior to and in contact with the capsule, a transverse partition arises between it and the orbit. This partition extends medially somewhat, so as partially to bound the brain cavity in front. Anterior to the cranial cavity, midFIFTH DAY: THE SKULL 441

way between the two orbits, and between the nasal capsules, a continuous longitudinal partition appears and fuses ventrally with the trabeculae. It is the interorbital and internasal septum.

The remaining part of the skull which is preformed in cartilage is known as the visceral skeleton or cartilaginous splanc/mocralziunz, and arises from the first three pairs of visceral arches. During the fifth day, these arches are chieﬂy membranous. and the antero-ventral or distal ends of the first mandibular pair have fused with one another in the middle line. Subsequent to the fifth day, the ventral or main parts of each mandibular arch become chondriﬁed, and are known as Mec-kel’s cartilages; they form the core of each side of the lower jaw. From the proximal (i.e., hinder and upper) end of each of these arches, there develops a tri-radiate piece of cartilage, the palate-quadrate, which eventually ossiﬁes as a separate bone. It is termed simply the quadrate, and constitutes the articulation between the lower and upper jaws. The second (hyoid) and third visceral pairs of arches later‘ form the hyoid apparatus, consisting respectively of the paired lesser and greater cornuae and the two median copulae. Moreover, the upper ends of the second arches are thought to give rise to parts of the colurnellae, as noted in the account of the ear (Chapter 12) .

Altogether, the ﬁnal bones of the Bird’s skull which have been preformed in cartilage are the following: the basi-occipital, exoccipztals, and supra-occipitals about the foramen magnum; the proiitic, epiotic, and opisthotic about each auditory capsule; the basisp/zenoid, orbitasphenoids. alisphenoids, and interorbital and internasal septum about the eyes and nasal capsules; the quadrate, and Meckel’s cartilages in connection with the lower jaw; and the hyozd apparatus in the region of the throat.

The Membrane Bones. ——These are bones which are not preformed in cartilage, but ossify directly from the condensed mesenchymc or membrane. They constitute a good share of the Bird’s skull, and begin to develop about the ninth day. The bones thus formed are as follows: the parietals, jrontals, and squamosals, forming together the main part of the cranium proper; the lachrymals, nasals, and premaxillae, form ing the face and part of the upper jaw; the maxillae, jugals, quadratojugals, pterygoids, palatines, parasphenoids, and vomer, forming the rest of the upper jaw and the base of the cranium; and the angulars,

supra-angulars, operculars, and dentals, forming the coveringbones for the lower jaw. 442 THE CHICK

THE ALIMENTARY TRACT THE FORE—GUT REGION

The development of the mouth proper has already been suliiciently described in connection with the discussions of the alimentary tract and the middle ear in Chapter 12, and of the skull in the preceding paragraph. We shall proceed, therefore, to an account of the further development of the remainder of this tract and its appendages.

Fig. 227.-—— Derivatives of the visceral pouches and associated organs, in the Chick. From Lillie (Development of the Chick). After Verdun (Maurer). Combined from frontal sections. A. In embryo of 7 days. B. In embryo of 8 days.

Ep. 3,Ep.4-. Epithelial vestiges derived from ventral portions of the third ancl=fourth visceral pouches. J. Jugular vein. p’br.,p’br.(V). Postbranchial bodies derived from ﬁfth visceral pouch. Ph. Pharynx. T h.3.,TH.4. Thymus bodies derived from dorsal portions of the third and fourth visceral pouches. T’r. Thyroid body. 111, IV. Remains of third visceral cleft and position of fourth which never becomes a real cleft.

The Visceral Pouches and Arches.

The Pouc-hes.——At the end of the fourth day, the first and second visceral clefts had closed, and the third had opened; during the ﬁfth day, this latter cleft also closes, whereas the fourth pouch, it will be recalled, has never developed an outer opening. About the seventh or eighth day, the third and fourth pouches sever their connections with the pharynx, and thus remain as patches of epithelium in the mesenchyme of the neck, adjacent to the jugular vein. The dorsal portion of the epi thelium from the third pouch then fuses with the dorsal portion from

the fourth to form a thymus body on each side of the throat of the FIFTH DAY: THE FORE—GUT REGION 443

young Chick. Though thus apparently endoderrnal, I-lamrnond, ‘S4, states that the clefts rather than the pouches may be the source of the thymus and hence that it is ectodermal. Epithelial vestiges of the third and fourth pouches are gener:.il.ly thought to produce the para:/iyroirls, while each fourth pouch also produces a posterior outpushing sometimes regarded as a vestigial fifth pouch. These separate from the pouches, and the left one becomes the pose‘-bram:/zial body, somewhat like a small parathyroid. while the right one clegenerates (Fiﬁ. 227). Dudley, 7112, thinks these outpushings may be rudimentary sixth pouches, the ﬁlth having {used with the fourth.

The Arches. —-The fate of the first three pairs of Visceral arches has already been suiiiciently described above in ('0I1IlB('li()]] with the visceral chondrocranium. The fourth pair of arches never develop beyond a inesenchymal state and eventually disappear. The lifth pair are vestigial and even more transitory.

The Respiratory Tract. —— At the end of the fourth day, the respiratory tract consisted of the glottis, the larynx, the trachea, and a pair of posterior outgrowths from the latter. the rudiments of the bronchi and lungs. All these parts, having arisen from the fore-gut, are necessarily lined by encloderm. Upon the fifth day, however, the mesenchyme about them begins to condense to form true mesoderm. through which the lung rudiments continue to grow posteriorl_v as a pair of tubes. Upon the sixth day, these tubes begin to branch, and thus it appears that the original rudiments really represented the lining of only the two main or primary bronchi. Their branches then constitute the linings of the secondary bronchi’, and the intercommunicating terticlry or parnbronclzi, together with the finer ramiﬁcations from the latter known as air capillaries. This network of air capillaries, it is to he noted. takes the place of the blind terminal sacs or alveoli found in the Marnmals. Thus there are no pockets of residual air in the lungs of the Bird, but continuous passages which make possible a. complete circulation. The mesoderm indicated above eventually gives rise in the region of the larynx and trachea to the cartilages and muscles of these organs. Further hack it surrounds the endodermal lining of the various bronchi and air capillaries, and ultimately forms the connective tissue substance of the lung. Through this tissue the blood vessels later rarnify among the tubes and capillaries.

In the case of the Bird, besides these tubes and respiratory capillaries,.there are also connectgd with the lungs the various air sacs. These arise, with one exception, as outgrowths from the secondary bronchi, the exceptional case being the abdominal sacs which originate directly

l i r 3 444 THE CHICK

from the posterior ends of the primary bronchi. The rudiments of the abdominal and cervical sacs are said by some to be distinguishable as early as the ﬁfth day, while the others appear somewhat later (Fio. 228). In the course of development these peculiar sacs which have thus originated, gradually push their way to their respective positions among

M°5»'"9‘-‘"' -1 ' ‘ -— -Lat.moi.

Rec.Br.-:=.' ‘ . . ___Me5.mOi_

Rec. Br. Abd. Sc.--""“'

Fig. 228. —A. Lateral view of the left lung of a 9-day embryo, showing branches of the bronchi within it. B. Ventral View of the lungs and air-sacs of a 12day embryo, with internal branches of the bronchi not shown. After Locy and

Larsell.

Abd. Sc. Abdominal air-sac. A. Int. Sc. Anterior Intermediate air-sac. Br. Extrapulmonary bronchus. Cerv. Sc. Cervical air-sac. Ect. 1. An ectobronchus. Ent. 1. An entobronchus. Lat. I, 2, 3. Laterobronchi. The ecto. ento. and laterobronchi are all classed as secondary bronchi in the text description. Lat. moi., Mes. mai. Lateral and mesial moieties of interclavicular air-sac. 0e. Oesophagus. Par. Parabronchi. P. Int. Sc. Posterior Intermediate air-sac. Rec. Br. Recurrent bronchi.

the viscera. Here they come to occupy considerable space, while a branch of the interclavicular sac extends eventually even into the upper bone (humerus) of each wing.“ Besides being connected with the respiratory passages by the bronchi from which they arose, each sac, with the exception of the cervicals, also develops secondary connections with the parabronchi. In the adult these connections always convey air from the

3 In the latter case the bone is said to undergo a kind of dissolution to make way for the ingrowing sac, and the dissolution is thought to depend on parathy-K

mid activity which in turn is due to oestrogens derived from the yolk-sac

(Bremer, ’40) . FIFTH DAY: THE FORE—GUT REGION 44.-5

sacs to the lungs, and are, therefore, termed recurrent bronchi. The cervical sacs, though possessing no recurrent bronchi, are indirectly connected with branches of the most anterior pair of secondary bronchi, and these branches probably act as recurrents. The functions of the sacs are apparently to lighten the Bird’s body, to help maintain air currents and, in the case of the abdominal sacs, to cool the testes.

Fig. 229.—Partially dissected viscera of the Chick, from the right side. From Kellicott (Chordate Development). After Duval. A. Of a 6-day Chick, enlarged slightly less than six times. B. of a 13-day Chick, enlarged two and one half times, showing the elongated intestine and its extension into the umbilical stalk. _

zz. Right atrium. al. Allantois. as. Abdominal air-sac. b. Bulbus arteriosus. c. Caecal processes. zl. Loop of duodenum. dj. Duodenaljejunal ﬂexure (a relatively ﬁxed point during the elongation of the intestine). f. Fore-limb bud (cut through}. g. Gizzard. go. Gonacl. h. Hind-limb bud (cut through _). i. Loops of small intestine. l. Liver. lg. Lung: ll. Left lobe of liver. lv. Left ventricle. M. Rudiment of Mullerian duct (tubal ridge). p. Pancreas. r. Rectum. rl. Right lobe of liver. To. Right ventricle. s. Yoll-:-stalk. U. Umbilical stalk. W. Wolfﬁan body or mesonephros.

Finally, in connection with the development of the respiratory system, it is to be noted that about the ﬁfth day, the glottis begins to close. Both larynx and glottis later become entirely shut, but subsequent to the eleventh day, the opening is gradually re-established.

The Esophagus, the Stomach, and the Duodenum.-—At the end of the fourth day, the esophagus was a straight tube, while the region of the stomach and duodenum was indicated by a slight curvature to the left. The esophagus does not alter much on the ﬁfth day, except to continue to elongate. The stomach, however, is becoming distinguished from the duodenum by its greater dilation. Also, at the extreme left of the gastric duodenal curve, a slight pouch is forming. This 446 THE CHICK

marks the end of the gastric region. Later this pouch enlarges to form the muscular gizzard. while the part between it and the esophagus develops the gastric glands and comprises the proventriculus. The crop is evident by the eighth day as a dilation of the esophagus at the base of the neck. Anterior to the crop at that time, the lumen of the esophagus is temporarily closed. ‘

The duodenum is not very clearly deﬁned on the ﬁfth day, but shortly afterward it begins to develop as a loop in the tract just beyond the gizzard. From the gizzard, the proximal limb of the loop descends a short distance, and then bends upward to form the ascending branch. Ultimately the pancreas comes to lie in between the limbs of this loop. The end of the ascending branch marks the termination of the original fore-gut region and the beginning of" the small intestine (Fig. 229).

The Liver.~——On the fifth and subsequent days, as on the fourth day, development of the liver consists chiefly in further growth in size. This is accomplished as already indicated by continuous branching and anastomosing of the original diverticula together with the accompanying blood capillaries. These diverticular branches are at first solid, but on the fifth day many of them have acquired a lumen, and this process continues as growth proceeds. '

As regards the bile ducts, it is to be noted that on the sixth day the common duct disappears, and the two bile ducts which emptied into it again empty directly into the duodenum.

The Pancreas.——The pancreas at four days, it will be recalled, consisted of three separate outgrowths: a dorsal one from the wall of the duodenum opposite the common bile duct, and the beginnings of two ventral ones from the duct itself. During the ﬁfth day all three diverticula continue to grow and branch (Fig. 230). On the sixth day, the right ventral pancreatic mass becomes united with the dorsal, whose duct shifts ventrally on to the left side of the duodenum. As noted above, the common bile duct disappears at this time, and thus the two ventral pancreatic ducts come to open directly into the intestine. Later, the left pancreas becomes fused with the other two, and there remains a single glandular mass lying in the mesentery within the loop of the duodenum. lts three ducts continue to remain separate, however, and they open into the distal limb of the duodenal loop near the bile ducts.

THE MID-GUT REGION

It has been indicated that the mid-gut or rudimentary small intestine’ begins at the end of the duodenum. At the close of the fourth day, it

n F IFTH. DAY: THE MID—GUT REGION 44?

was noted that it extended from this point as a virtually straight tube across the region of the umbilicus to the beginning of the tail fold and hind-gut. In zibout the middle, it gave off the yolk-stalk. During the fifth day a very slight downward bend (the duodeno-jejunal flexure)

Fig. 230.—-Reconstruction of gizzard; duodenum, and liepato-pancreatic ducts of a Chiclc embryo of 124 hours. From Lillie (_Development of {he Clzz'c'/.15. Alter Broulia.

D.clI. Duclus (:l10le(lOCllllS. D,r‘y. Ductus cystivus. DJ1.cy. Duutus hepato-cysticus. 11.11.11. Do1'.~:ul or hepato-enteriv duct. Du. Duodenum. G.bl. Gall bladder. Ciz. Gizzard. }’u.rI. Dorsal pancreas. Pa.2'.u’. Right ventral pancreas. Pa.-v.5. Left \‘entr:il pancreas.

appears just at the point where the duodenum ends and the mid-gut begins. From this bend, the latter extends postero-ventrally for about half its length; at this point, as noted, it connects with the yolk-stalk. lt then ascends again to its termination, which is now marked by a small bilateral swelling, the rudiment of the intestinal caeca. The entire midgut region thus indicated is still quite short, and its dip down into the umbilical stalk very slight. t

On the sixth day, however, the ventral dip of the small intestine reaches well down into the above stalk, thus forming in the intestine 448 THE CHICK

as a whole a second distinct loop (Fig. 229, A). The latter soon becomes much more pronounced than the duodenal loop, and during later development acquires numerous convolutions (Fig. 229, B). These convolutions lie within the umbilical stalk until about the eighteenth clay and are then drawn into the body. They are soon followed. by the

EVIL ._. W. D.

An. PL

Fig. 231.—Chick embryo of 11 days, sagittal section through the region of the cloaca. Reconstructed from several sections. (After Minot.) From Lillie (Development of the Chick). Anterior end toward the reader’s left.

All’. Ascending limb of the allantois. Al ". Descending limb of the allantois. An. Anal invagination, or proctodaeum. An.pl. Anal plate or cloacal membrane. Art. Umbilical artery. B.F. Bursa Fabricii. b.f. Duct of the bursa. Clo. Cloaca, i.e., the urodaeal portion. Eb. Ectoderm. Ent. Entoderm of the rectum. Ly. Nodules of crowded cells, probably primordia of lymphoid structures in the wall of the large intestine. W.D. Wolﬁian duct.

remains of the yolk-sac. The intestinal caeca which were barely indicated on the ﬁfth day ultimately grow out into two ﬁngerlike processes.

THE HIND—GUT REGION

On the ﬁfth day, as on the fourth, there is no particular change in the rectum. On the seventh and eighth days, however, its cavity becomes occluded. Later, the lumen is restored except for a small plug separating it from the cloaca, and just anterior to this plug a slight dilation develops. This dilation is the coprodaeum. The plug persists until about the time of hatching.

The chief change in the cloaca during the ﬁfth day is the fusion of FIFTH DAY: THE HIND—-GUT REGION 449

the laterally compressed walls of the posterior part. During subsequent development, a cavity is re-established in the postero-dorsal part of this closed portion; it constitutes the bursa F abricii of the adult. This is a sac which remains separate from the original cloaca, but which opens into another cavity, communicating directly with the exterior. This

mesonephric duct

mecanephrlc duct

coprodaeu m

allantoic stalk urodaeum

proctodaeum

Fig. 232.—A diagram of a sagittal section of the posterior end of an approximately eleven-day embryo to indicate better the relations of the parts partially shown in Fig. 231. The metanephric duct opening separately into the urodaeum (a condition attained on the sixth day) is shown, though for some reason it does not appear in Fig. 231. The anal plate separating urodaeum from proctodaeum is shown in the diagram, but is unlabelled.

latter cavity is the proctodaeum, and has arisen by an outpushing of the ectodermal _walls around the edges of the anal plate or cloacal membrane (Figs. 231, 232; compare Fig. 193, Chapter 10). At hatching the latter disappears and thus the proctodaeum is ﬁnally placed in communication with the original embryonic cloaca minus the posterior portion of the latter which went to form the bursa Fabricii. At. about the same time the plug which closes the rectum disappears. Thus, theadult cloaca consists of three parts, the coprodaeum, a part of the original cloacal chamber now called the urodaeum, and the proctodaeum. The latter opens to the outside through the anus. 450 ‘THE CHICK

THE CIIRCULATORY svsrsm THE HEART

During the fourth day a series of changes in the position of the various parts of the heart in relation to each other were indicated. During the ﬁfth day these changes progress rapidly, and upon the sixth day are virtually completed.

Besides these movements, there were also noticed on the fourth day the beginnings of certain partitions within the heart. These were the interatrial, the interventricular, and the cushion septa. During the ﬁfth and part of the sixth days, all these are practically completed. This process involves, ﬁrst, the meeting of the two parts of the cushion septum so as entirely to divide the atrio-ventricular canal into right and left channels. The interatrial septuin then unites with the cushion septum on the antero-dorsal side of the latter, while the ventricular septum joins it postero-ventrally. These fusions, though described separately, occur more or less simultaneously (Fig. 209, F

In connection with these processes there remain to be added certain details as follows: As the division of the originally single atrium into two atria occurs communication between them is preserved by the concomitant development of perforations in the newly formed septum. These perforations correspond functionally to the foramen ovale in the heart of the Mammal, and their physiological signiﬁcance is described below. It must also be noted that the interatrial septum as thus far described is augmented in the adult Bird by the addition of another part as follows: Upon the seventh day the proximal portions of the left precava and the pulmonary vein start to be incorporated into the atria, and as this occurs the tissue between them is added to the septum. This new part is called the pars cauo-pulmonalis (Quirring, ’33). Lastly, there is also a small ventricular foramen whose ﬁnal closure will be described presently in connection with the development of . the aortic division of the bulbus.

This completes the description of the septa within the heart proper. Upon the ﬁfth day, however, another septum develops within the truncus arteriosus. It appears first at the anterior end of this vessel in such a position as to separate the oriﬁce leading to the sixth aortic arches and hence to the pulmonary arteries, from that which leads to the third and fourth "aortic arches. This partition then grows backward through the FIFTH DAY: THE HEART 451

distal portion of the bulbus, and on the sixth and seventh days it connects with a septum which has formed within the proximal portion of that vessel. Thus a continuous somewhat spirally twisted partition has been produced extending through the truncns .-md hullaus clear In to the interventricular septum of the heart. It is to he noterl that the entire bul bus, though now ventral,

still lies somewhat to the ca;,¢om_ l\

right of this latter septum. ;'

Nevertheless, the fusion of p 6 xi 3

the hulbus septum and inter- Au 4 ventricular septum is eiiectetl ' " ,4»

in such a way that in cmmec- ‘pp .1?‘ S_ GL5.

tion with subsequent changes in the cushion septum the aortic division (i.e., the division from the third and fourth arches) of the bulbus comes to open through the foramen in the ventricular septum directly into the left ventricle. The pulmonary division, on the other hand, continues to open into the right ventricle (Fig. 233). Subsequent to the ﬁfth day also, certain other changes

are Completed as f°u°w5'The Fig. 233.——The heart and aortic arches of a

semilunar valves develop in Chick embryo the latter part of the sixth day.

both the aortic and pulmo- Efogeacgézﬁﬁtiggiefrgﬁiyllie (Development

nary divisions of the bulbus, Au. Atria. Car.com. Common carotid ar and the parts of that vessel Zi§’$ia§°’;'§§'fé?‘ §'i'2'?’ls.“"i’h?f§°"€ii.-‘fulﬁl:

proximal to these valves are fourth (systemic) and sixth (pulmonary) incorporated into the ventri- acme arches’

cles. The two divisions of the bulbus and truncus arteriosus distal to this point are gradually separated so as to form distinct vessels, i.e., the proximal portions of the aortic and pulmonary arteries. As noted in a previous chapter, the sinus venosus becomes a part of the right atrium into which empty all the systemic veins, and ﬁnally both atria acquire small auricular appendages or auricles. L2

452 THE CHICK

THE EMBRYONIC BLOOD VESSELS

The Arteries.

The Aortic Arches.——At the end of the fourth day, the pairs of aortic arches which remained fully developed were the third, fourth, and sixth. The third pair, it will be recalled, ran upward from the ventral aorta. and continued anteriorly as the internal carotids, while pos teriorly the dorsal end of each of these arches was still connected with the dorsal end of each fourth arch. From the base of each of the third

LEFT SIDE RIGHT SIDE 6th day 6th day

Internal carotid

i 3 r

aortlcv3rches{-‘i-th : ‘ systemic arch ’ 4 h} “me h gh < . ' r are es ‘ ' ' 6th

external carotid — ' ‘ . external carotid

common carotid

. \tfUn€US 3l’t¢l‘lOSl.|S

Fig, 234.——Reconstruction of the aortic arches of a 6-day Chick embryo from a. series of sagittal sections. Modiﬁed from Lillie. '

arches, on the other hand, another vessel ran forward as an external carotid.

Upon the ﬁfth day three further changes are initiated as follows. First, on each side, the portion of each dorsal aorta between the third and fourth arches begins to disappear. Secondly, the fourth arch on the left side diminishes in size (Fig. 234-). Thirdly, there occurs anteriorly an anastomosis between the internal and external carotids, while the portion of the latter between this point and the base of the third arch (primary external carotid in Fig. 235) begins to atrophy.

By the eighth day the changes thus begun have been completed. so that the condition then obtaining is as follows: First as regards the systemic and pulmonary arches, it is to be noted that on the left side, the entire fourth arch together with the dorsal aorta between the third and the sixth arch has vanished. On the right side the dorsal connection between the third and fourth arches is gone, but the fourth arch itself is well developed It persists as the main systemic arch of the FIFTH DAY: EMBRYONIC BLOOD VESSELS 4153

adult (Fig. 210, B). It is to be noted that the Bird differs from the Mammal in that in the latter, it is the left arch which remains. The immediate cause of this interesting difference between Bird and Mammal according to Bremer (’28) is as follows: In the first place in the Bird the torsion of the heart tube is somewhat greater than in the Mammal. Secondly this is cor related Witll 3 greater . Eiigggim

backward movement of ‘°'°“d the heart in the in intemolcorotid ' connection with the great- _ 7 er length of the neck. This comm“ €°'°*‘d' . ‘°"""‘°" ‘°'°"d last ‘feature results in lengthening the aortic ves- NGHT

" internal carotid

LEFT sels and in involving them in the increased torsion of §f,;;‘,‘;'v‘f§,', _ §’:§,:{2C;°."..' the cardiac tube. Thus the left fourth arch is drawn dums \ [ mm; into a disadvantageous Borulh ‘ W Soto"! position on the ventral §?'r'§'.‘,‘,’"°'Y ‘ . °"“ °"°'Y side of the truncus, while p,i,m,y V primary the right assumes a dorsal ‘“""°"‘°" ‘”b°'°"'°" position with a much systemic artery \ 1 p

more direct connection ‘3'*\5¢9'“°"‘°‘°'*°'Y '3'“ “"9'“""°‘”'"'V

with the dorsal aorta Fig. 235.—-Diagram ca; tgi ionic archvés fand - connecting vesse s in t e ic as viewe rozn (F1g' 236)‘ In the Mam‘ the ventral side. The vessels in outline indicate mal on the other hand, the situation existing at one time or_ another in not only is this not true the embryo. Those shown in black indicate the 1

permanent arrangement. “ but according to Congdon and Wang (’26) the blood as it comes from the truncus on the right is necessarily directed toward the left. Hence the left arch receives the larger stream and so becomes the dominant vessel.

All parts of the sixth arches continue to be well developed on both sides throughout embryonic life. At the time of hatching, however, the upper portion of each vessel between the origin of the pulmonary arteries and the dorsal aorta (i.e., the duct of Botallo or ductus arteriosus, indicated above) becomes atrophied and remains only as an occasional vestige in the adult.‘ In the second place with respect to the carotids it appears that since the atrophy of each external carotid between

i the base of the respective third arch and the point of its anastomosie

3 In the Mammal a remnant of the left duct of Botallo always persists. 454 THE CHICK

with the internal carotid has been completed, each external and internal vessel now takes its origin and continues anteriorly from this point of fusion. Posterior to this point certain remaining parts constitute on either side a newly named vessel, the common carotid. Each common carotid consists of what was previously the postero-dorsal portion of the respective internal carotid, the respective third arch, and the part

dorsal aorta

aortlc arch

Fig. 236.——Diagrammatic ventral view of the truncus and the third and fourth aortic arches in A, the Mammal, and B, the Chick. After Bremer. Note that in the Chick the fourth arches are involved in the twist of the truncus, thus bringing the right fourth arch dorsal, and hence nearer to the dorsal aorta. The left fourth arch on the other hand is brought ventrad, and hence

further from the dorsal aorta, thus leading to its elimination in this form.

of each ventral aorta proximal to the base of this arch and the point of union with the systemic vessel (Fig. 235). It is to be noted in this connection that the point of anastomosis between each external and internal carotid is not shown in Fig. 234. Hence each vessel there indicated as an internal carotid eventually becomes part of a_ common carotid. Finally, it must be remembered that while these changes are occurring, the head of the Bird is being separated from the body by the development of the neck. This process results in the backward movement of the heart and all its arches, so that by the time they have reached the stage indicated on the eighth day, they lie entirely within the thorax. The carotids, on the other hand, are elongated into vessels which pass forward into the head.

The Physiological Signiﬁcance of the Embryological Structure of the Heart and Aortic /1rches.—Before considering the remainder of the FIFTH DAY: EMBRYONIC BLOOD VESSELS 455

blood vessels, it seems well to digress at this time in order to point out the physiological signiﬁcance of the heart and its arches as they have just been described.

The heart, as has been seen, becomes virtually four chambered. It fails to become entirely so during embryonic life, however, because of the persistence of the foramina in the interatrial septum. This fact, as well as the existence of the dorsal portions of the sixth arches, i.e., the ducts of Botallo, is correlated with the embryonic method of aerating the blood. This becomes clear upon a consideration of what this method involves, as follows:

It is obvious that previous to the hatching of a Bird or birth of a Mammal the lungs cannot act. Instead the allantois of the Bird, or as will later be explained, the partially homologous placenta of the Mammal, performs the function of blood aeration. There now remains to be described the relationship which the interatrial foramina and the ducts of Botallo bear to the distribution of the different classes of blood. The fully aerated blood from the allantois, the nutrient laden blood from the yolk-sac, and a relatively small amount of strictly venous blood from the posterior part of the body become mixed in the ductus venosus, and from thence are poured together into the right auricle. At the same time that this occurs the right auricle is also receiving blood through the ducts of Cuvier or anterior venae cavae (see below). This blood is returning from the head, and hence, save perhaps in the very early stages, is relatively depleted of oxygen and nutriment. Up to this point there is no question about the facts. From here on, however, there have been two distinct theories as to the fate of the two classes of blood just indicated. Both have been developed as a result of observations and experiments upon Mammals, but probably apply equally well in their essential points to Birds. 1

The first theory was somewhat obscurely outlined by Harvey in connection with his original discussion of the circulation of the blood in 1628. It can be very brieﬂy stated as follows: It holds simply that the two types of blood are completely mixed as they enter the right atrium, and hence that there. is no separation of aerated and unaerated blood in the embryo. This has been accounted for on the ground that the organism is suﬂiciently small and inactive and the circulation suﬂiciently swift so that such separation is unnecessary. The second theory was developed in 1798 by Sabatier, and may be described thus:

It is supposed that the structure of the right atrium is such that the blood entering it from the posterior part of the body through the ductus 456 _ THE CHICK

venosus (aerated blood) is turned away from the right ventricle and guided through the aperture or apertures in the interatrial septum into the left atrium. From here it passes into the left ventricle, and thence through the aortic division of the bulbus and truncus arteriosus into the third and fourth aortic arches. The third arches, as has been seen, convey this blood newly oxygenated and full of nutriment straight to the head; the rest passes through the fourth arches (later only one, the right or left) and backward along the dorsal aorta. On its way, however, it becomes mixed with the depleted blood which has returned from the head; this occurs as follows: It was noted above that this blood from the head also passes into the right atrium. According to the present theory, however, its direction of entrance, together with the structure of the cavity, is such that it is diverted from the openings into the left atrium, and emptied directly into the right ventricle. From here it passes out through the pulmonary division of the bulbus and truncus arteriosus, and thence a slight part of it ﬂows through the small pulmonary arteries into the rudimentary lungs. The larger part, however, continues through the dorsal portions of the sixth arches, i.e., the ducts (later only one duct) of Botallo, into the dorsal aortae; here, as indicated above, it inevitably mixes with the aerated blood from the fourth arches (later arch). Some of this mixture then supplies the body posterior to the head. The larger share of it, however, eventually reaches again the walls of either the allantois or the yolk-sac, where it receives respectively oxygen or food material, and is returned‘ to the heart in the manner already noted. Thus the posterior part of the body should get blood poorer in oxygen and nutriment, at least during later stages when the above arrangement would be in operation (Fig. 236X). Hence some think there may be a relation between this and the faster growth of the anterior end, if indeed that end is still growing faster at this time.

However, despite the theoretical considerations in favor of this second theory, all evidence until recently has supported the earlier view. Thus to begin with, in the human embryonic heart near term at least, it was shown anatomically that the interatrial aperture is not large enough to pass all of the blood delivered by the postcaval vein. Hence it would appear that some mixture of blood from the anterior and posterior veins must occur in the right atrium. Then Pohlman, in 1909, apparently settled the matter experimentally by injecting cornstarch into the vessels leading from -the "placenta of the Pig embryo into the right atrium. He then withdrew equal amounts of blood from each ventricle and found them to contain equal numbers of grains. This type of experiment with certain FIFTH DAY: EMBRYONIC BLOOD VESSELS 457

5 ‘nternal carotid artery

’ xternal carotid artery

internal carotid artery external carotid artery

common carotid artery common carotid artery

~ subclavion artery (3rd arch)

subclavian artery (3rd arch)

main aortic (4th arch)

duct of Botollo (arteriosus

pulmonary artery (6th arch‘ anterior caval vein

ulrnonory artery (6th arch) uct of Botallo (arteriosusl - ulmonory veins

trunc us arteriasus

RIGHT LEFT

posterior vena cav entrances of _

fight at,-.;um anterior cavol veins

entrances of pulmonary veins

eft atrium

atria-ventricular (mirrol) valves

hepatic vein liver

entrance of posterior vena cov

epotic portal vein

atrio-ventricular vaive ‘ oeliac artery ‘ ‘ esenteric vein

right ventricle

ductus venosu

posterior vena covo "

eft ventricle

As. . . .-,3! posterior mesenteric artery

.«

mbilical (ollantoic) vein

‘ caudal artery A

umbilical (allantoic) arteries

Fig. 236X.—— Diagrams to illustrate the circulation in the Chick embryo according to Lillie, and indicating at least a partial separation of aerated from unacrated blood. Solid arrows represent aerated blood and broken arrows unaerated blood, the relative amounts of each type being suggested by the size and heaviness of the respective arrows. A, the complete circulation. B; the heart alone. Note the numerous small foramina in the interatrial septum as compared with the one larger foramen ovale in the Mammal. The right atrio-ventricular valve is also different from either of the mammalian valves (Fig. 336). With the substitution of the placenta for the allantois, essentially the same type of circulation with the separation of the two classes of blood has been alternately denied and claimed in the case of the Mammal ever since Harvey. For a complete discussion of this controversy see the text. It seems now to be settled as shown.

Because this is an embryonic stage the sixth arches are connected with the aorta. Being ‘a Bird the right sixth unites with the aortic extension of the right fourth arch through the right ductus Botalli. In the Mammal it would be the left.

At hatching both ducts of Bot-allo will close and later atrophy (Coughlin and Walker, ’53) . 458 THE CHICK

reﬁnements was repeated by Kellogg on both the Pig and the Dog in 1923, and later by others with similar results. Therefore, it was reasonably concluded by both investigators that there had been a thorough mixture of the two classes of blood in the right atrium. And so the question seemed to be ﬁnally answered.

Regardless of all this seemingly overwhelming evidence in favor of the theory of mixture, however, many embryologists were still intrigued by the hypothetical desirability of a separation if it could only be proven. Consequently they have once more returned to the attack with both similar and improved techniques, and with most interesting results. In the first place Windle and Becker (’40) using the Cat and Guinea Pig, injected india ink instead of cornstarch. This probably did not reduce the velocity of flow as did cornstarch, thus providing more normal conditions, and their results supported the separation theory of Sabatier. Next, in 194-1, Barclay, Barcroft, Barron, Franklin, and Prichard performed the most ingenious experiment yet devised. They injected inert material, opaque to X-rays, into the blood stream of living Sheep fetuses. Then by means of X-ray moving pictures they showed that there is a fairly complete separation of the anterior and posterior streams in the right atrium. This brilliant experiment, especially if conﬁrmed, would seem to be conclusive. Finally, Whitehead (’42) has made a model of an embryo Cat heart in neoprene by the reconstruction method. With it he has demonstrated that the key to the separation of the streams entering its right atrium is the pressure at which each stream enters. He, moreover, believes that the pressures with which the blood streams do enter the actual Cat heart are such as to separate them. Thus the matter rests at the date this book is written, and we are back once more to the purely hypothetical conclusions of 1798.

However this may be in the embryo, it is of course certain that in the adult Bird or Mammal the completely aerated blood from the lungs (arterial blood) is normally entirely separated in the heart and arterial circulation from the venous blood. To achieve this at, or shortly after, the hatching of the Bird or the birth of the Mammal, all that is necessary' is the closure of the interatrial openings, or opening, in the septum and the occlusion of the ducts of Botallo (one duct in the Mammal).

Considering the matter of the septum ﬁrst, it will be recalled that by the end of the sixth day in the Chick this structure was closed except for the existence of numerous foramina. During the embryonic life of the Bird these foramina are kept open according to current theory in the FIFTH DAY: EMBRYONIC BLOOD VESSELS 459

following manner: The pressure on the septum from the side of the right atrium greatly exceeds that from the left side because of the relatively small amount of blood being returned to the left atrium from the non-functioning lungs. Hence the septum tends to belly out to the left, and to remain in a stretched condition with the foramina wide open. In the Bird, as indicated below, the lungs start functioning to some degree two or three days before hatching takes place. Hence the vessels of these developing organs receive more and more blood, and the pressure on the two sides of the septum is gradually equalized. This causes it to straighten out, the stretch is taken out of it, and as a consequence its wall thickens and the foramina are functionally closed. Later the tissue about the former openings presumably becomes entirely fused. The mechanism in the Mammal is somewhat different, but is supposed also to depend on an equalization of pressure in the two atria, and a functional closure of the single interatrial opening. The details of the process in this class will be discussed further in connection with the Pig.‘

The closure of the duct of Botallo (arteriosus), at least in the Mammal where it has been most studied, is apparently brought about by the contraction of muscle ﬁbers within its walls. This has been rather cleverly demonstrated in the Guinea Pig by Kennedy and Clark (111) . Under anesthesia living, almost full term, fetuses were removed from the uterus while leaving the umbilical cords attached. The fetuses themselves were then opened. so that the heart could be observed. When such a fetus was in the air it would breathe. and the duct of Botallo could be seen to close. When it was immersed in normal saline the embryonic respiratory situation was restored, and the duct of Botallo would promptly reopen. This could be repeated several times. Thus the closure would appear to be a result of the stimulus of breathing. Within a month or so after normal birth. however, the walls of the duct have grown together, and the structure is reduced to a cord.

In conclusion of this topic it may be noted that in man either a defect in the interatrial septum or a persistently patent duct of Botallo are among the causes of infantile cyanosis, “blue babies.” Where a patent

4 The sudden functioning of the lungs as a factor in increasing the blood ﬂow from them to the heart in the case of the Mammal has been questioned for the Cat and Guinea Pig by Abel and Windle (’39l. These authors claim that there is already a good deal of circulation here at term, and that subsequent increase is gradual. A similar situation is also claimed for other Mammals, including Man (Patten, ’46). As noted the condition in the Bird is such that in that case gradual

initiation of lung function, and hence of change in the course of the blood, must always occur. 460 THE CHICK

duct is the primary defect, it may be remedied by tying oii this vessel. A failure in septal closure, however, is more diﬂicult to cope with. Yet now even this may be greatly helped by a clever operation which involves rerouting part of the aortic blood to the lungs.

The Subclavian Arteries. -—-The primary subclavian arteries arise as outgrowths from the. eighteenth segmental arteries. On the ﬁfth day, however, an anteriorly growing branch of each primary artery connects with the respective third aortic arch, which as indicated eventually becomes a part of the common carotid (Bakst and Chaise, ’28; Figs. 233 and Q35) . These new branches then develop, while the original connections with the dorsal aorta through the segmental arteries become atrophied. Thus the permanent subclavians eventuallyarise from the carotids in the Bird. These arteries, of course, supply the wings, and in so doing, develop various branches. It will not be advisable, however, to follow them further in detail.

The Remaining Arteries. —— The only other major arteries whose development has not already been indicated in the account of the fourth day, are the coeliac, the anterior mesenteric and the posterior mesenteric. The coeliac arises from the anterior part of the dorsal aorta, and supplies the stomach, gizzard and part of the intestine. The anterior mesenteric originates as an outgrowth from the single vitelline artery close to the place where the latter leaves the aorta, and supplies the intestine. Lastly the posterior mesenteric develops from the aorta slightly caudal to the kidneys, and supplies the rectum and cloaca. These three arteries appear during the ﬁfth and subsequent days (Fig. 237).

The Veins.

The Vitelline Veins. --- At the end of the fourth day, a second venous ring had been formed about the intestine by a fusion of the vitelline veins for a short distance beneath it. This second ring was beginning to be destroyed by the disappearance of its right side, and during the ﬁfth day, this side is completely obliterated. From a review of the previous development of this region, it will be evident that the condition of the vitelline veins at this point has now become as follows. The two veins unite just in front of the anterior intestinal portal, and ventral to the intestine, to form a single trunk, which is really a posterior continuation of the ductus venosus. This trunk runs forward beneath the intestine for a short distance, and then curves upward and to the ‘left. It next turns sharply to the right and crosses over the intestine dorsally; ﬁnally it bends immediately downward and again runs anteriorly to pass into the FIFTH DAY: EMBRYONIC BLOOD VESSELS 461

liver (Fig 211, E). During subsequent stages as the anterior intestinal portal continues to move backward, it is closely followed by the fusion of the vitelline vessels. Indeed before very long this fusion passes beyond the region of the intestinal portal, and thus the single ductus venosus, or vitelline trunk, comes to extend a considerable distance into the umbilicus before dividing into its two branches.

Fig. 237. —Diagrammatic lateral view of the chief embryonic blood vessels of the Chick, during the sixth day. From Kellicott (Chordate Development). After Lillie.

a. Atrium. al. Allantoic stalk. ao. Dorsal aorta. c. Coeliac artery. ca. Caudal artery. cl. Cloaca. cv. Caudal vein. da. Ductus arteriosus. dv. Ducurs venosus. ec. External carotid artery. e]. External jugular vein. i. Intestine. ic. Internal carotid artery. ij. Internal jugular vein. 1. Liver. m. Mesone‘phros. ma. Mesenteric artery. mv. Mesenreric vein. p. Pulmonary artery. ,pc. Posterior cardinal vein. pv. Pulmonary vein. 5. Sciatic artery. 31:. Sulfclavian artery. scv. Subclavian vein. st. Yolk-stalk. sv. Subcardinal vein. ul. Left umbilical artery. ur. Right umbilical artery. 1112. Left umbilical vein. 1;. Ventricle. va. Vitelline artery. vca. Anterior vena cava (anterior cardinal vein). vp. Posterior vena cava. vv. Vitelline vein. y. Yolksac. 3, 4, 6. Third, fourth, and sixth aortic arches.

The Hepatic Portal System. -—— It will be recalled that within the liver the ductus venosus receives numerous capillaries. These capillaries increase during the ﬁfth day, while at the same time the main channel of the vein within the liver begins to disappear. This is brought about through the gradual occlusion of this channel by means of strands of the hepatic substance which grow into and across it. On the fifth day also, a vessel starts to develop in the dorsal mesentery of the gut; it is the mesenteric vein, and~presently acquires a connection with the vitelline trunk at about the region of the pancreas. By the seventh day the 462 THE CHICK

occlusion of the main part of the cluctus venosus within the hepatic sub stance has been completed. From now on, therefore, the blood enters the liver by the remaining posterior half of this vein, is distributed through the hepatic capillaries, and is ﬁnally collected again to enter the now separate anterior half of the same vessel through two main branches. When development has reached this stage the posterior half of the ductus venosus may be termed the hepatic portal vein, which receives the mesenteric vessel as its chief tributary. The two branches entering the anterior half of the ductus vencsus, upon the other hand, constitute the Izepagic veins (Fig. 211, F).

Upon the ﬁfth and immediately subsequent days the blood which enters the liver circulation is largely from the yolk-sac. Before long, however, the mesenteric vein has begun to send out branches which develop simultaneously with the various digestive organs and spleen. Thus these organs send an ever-increasing supply of blood through the hepatic portal rein to the liver. When the yolk-sac ﬁnally disappears they become the sole source of the blood which passes through the hepatic capillaries. The complete system of circulation which is developed in this manner is then called the hepatic portal system.

The Fate of the Cardinals and Development of the Caval and Renal Veins. -— On the fourth day, the subcardinals lying ventral to the mesonephros have direct connections with the posterior cardinals lying dorsolateral to it. Upon the ﬁfth day, however, these connections are severed and new ones established through capillaries within the mesonephric

-substance. At the same time, the subcardinals fuse with one another near their anterior ends, and the connection of the right one with the posterior end of the vena cava inferior (established on the fourth day) becomes larger (Fig. 238). Thus a part of the blood in the posterior cardinals now passes through the mesonephros and by way of the subcardinals and vena cava inferior to the heart. In other words, there is in the embryo of the Bird a typical renal portal circulation. On the fifth day also, or late upon the fourth, the subclavian veins begin to develop in connection with the fore-limb buds. They arise as branches of the posterior cardinal veins, a short distance behind the junction of the latter with the Cuvierian ducts.

Upon the sixth day, the section of each posterior cardinal between the entrance of the respective subclavian vein and the anterior end of the mesonephros disappears, thus forcing all the blood from the posterior part of the body to traverse the renal portal channels. In this manner also that portion of each posterior cardinal anterior to the entrance of wag-ea

2;:;.5.~,a....~,..»s,~.._,.~ .. ,.

FIFTH DAY: EMBRYONIC BLOOD VESSELS 463

c. /V. sc. d. V. sc. s.

Fig. 238.--Reconstruction of the venous system of a Chick of 5 days. Ventral view. From Lillie (Development of the Chick). After Miller.

a. i\-Iesonephric veins. A0. Aorta. A.o.m. Omphalomesem teric artery. A.u.s. Left umbilical artery. A.sc.s. Left sciatic artery. V.c.p.d.s. Right and left posterior cardinal veins. v.c.i. Vena cava inferior. V..sc.d.,s. Right and left subcardinal veins.

the subciavian becomes simply the proximal part of the latter vessel. From this time on, the ducts of Cuvier, which now receive the jugulars (anterior cardinals) and subclavians, may be termed the anterior or superior caval veins. At about this stage also, the anterior portion of the ductus venosus, which receives the two hepatic veins and the posterior vena cava (vena cava inferior), may be said to have become merely the anterior end of the latter vessel. Thus the posterior caval vein, like the 464 THE CHICK

two anterior cavals, now opens directly into the right atrium (Fig. 237). While the above changes are occurring subsequent to the ﬁfth day, there are a pair of new veins arising in connection with the metanephros

Fig. 239. -o- Reconstruction of the venous system of‘ a sparrow embryo, corresponding to a chick of about 14- days. From Lillie (Development of the Chick). After Miller.

V .c.i.H. Intra-hepatic part of the vena cava inferior. V.c.i.SC. Part of the venecava inferior derived from the suhcardinal vein. V.v.g. Genital veins. V.i.e.d.,s. Right and left vena iliaca externa. V.i.i. Vena iliaca interna, (or V.c.p.s. Posterior part of the left cardinal). V.i.l.d.,s. Right and left vena intervertehralis lumhalis. V.r.m.d.,s. Right and left great renal veins.

or permanent kidney. These are the renal veins which presently take blood from the permanent kidney to the anterior fused portion of the subcardinals (now really the posterior part of the posterior vena cava) . Just anterior to the kidney these renal veins also later establish direct connections with the ‘posterior cardinals. Thus a new channel is formed for the blood from the posterior part of the body via the cardinals and the anterior portion of the new renal veins to the posterior vena cava (Fig. 239). At the same time that this is occurring, the mesonephros toFIFTH DAY: SEPARATIO1'_V' OF BODY CAVITIES 465

gather with the renal portal system is disappearing. While the latter exists, however, it is essentially similar to the permanent system of the same name in the Frog and other more primitive Vertebrates, thus affording an excellent example of recapitulation. It remains to note that the hinder portions of the posterior cardinal veins persist in the adult Bird as the iliac veins, receiving branches from the hind-limbs. Also in subsequent stages, branches from the cardinals fuse with one another medially at the posterior end of the body and give rise to the caudal vein.

THE BODY CAVITIES

From previous discussion, it will be recalled that the space surrounding the heart has been designated as the pericardial cavity. Up to this time, however, there has been no mention made of any separation of this cavity from the peritoneal or general body cavity behind it. It now remains to describe how this separation is effected, together with the simultaneous closing 03 of 51 third space, the pleural cavity (see below). It will then be possible in conclusion to show also how the walls of the pericardial cavity come to form the independent pericardial sac of the

adult bird.

THE SEPARATION OF THE PERICARDIAL, PERITONEAL AND PLEURAL CAVITIES

The separation of the peritoneal and pericardial cavities is chieﬂy brought about by the development of a partition known as the septum transversum. This so-called septum in turn is composed of three parts, two of which have already been mentioned. The entire septum then is made up as follows: First, there is a median mass consisting of the liver and the sinus and ductus venosus, together with the dorsal and ventral ligaments which unite the liver to the gut and for a time to the ventral body wall. Second, there are the lateral mesocardia extending obliquely in an anterior and lateral direction from the median mass to the body walls. Above and below the lateral mesocardia, the pericardial cavity still communicates posteriorly with the peritoneal or general body cavity. About the ﬁfth day, however,'the ventral communication begins to be closed. This is accomplished by the development of the third part of the septum transversum, i.e., the lateral closing fold, extending from the mesocardia to the ventro-lateral body wall. By the eighth day, this closure is complete. In the meantime, the lungs have been developing in 466 THE CHICK

the portion of the peritoneal space which extends forward above the pericardial cavity. This space may be termed pleural cavity, and at this time (fifth day) the oblique lateral mesocardia have not yet entirely separated it anteriorly from the pericardial cavity beneath it; posteriorly also it still communicates with the general body cavity. Presently, however, with the further development of the lateral mesocardia and other parts, the opening between the pleural and pericardial cavities is closed, and a closure of that between the pleural and body cavities soon follows (tenth day). This latter is effected by the pleuro-peritoneal septum, which arises as an outgrowth from the sides of the esophagus. The median pericardial cavity is thus bounded dorsally largely by the mesocardia, laterally and ventrally by the peritoneum of the body wall, and posteriorly chieﬂy by the median mass of the septum transversum.

THE ESTABLISHMENT OF THE DEFINITIVE PERlCAR— DIUM

Eventually, however, the tissue upon the front of the median mass beu comes thickened and splits into two sheets. The anterior sheet then becomes the posterior wall of the pericardium, the posterior sheet covers the face of the liver, and the general body cavity extends between them. At the same time, the latter cavity is also pushing forward beneath and at the sides of the present pericardium, and as it does so, it apparently splits the peritoneum of the body wall into two layers. The outer layer forms the peritoneum of the general body cavity in this region, and the inner layer constitutes the ventral and lateral wall of the pericardium proper. In this manner, the ﬁnal pericardial wall or deﬁnitive pericardium of the adult bird comes to surround the heart as a relatively independent sac with a portion of the liver extending beneath it.

THE URINOGENITAL SYSTEM

THE EXCRETORY SYSTEM

The Mesonephrcs. — During the ﬁfth day, the increase in the numher of the mesonephric tubules ceases, while the organ becomes more active as a kidney. For a couple of days subsequent to this, however, the tubules continue to grow in length, thus greatly increasing the bulk of the organ. Degeneration begins about the eleventh day, and from then

on, the metanephros aids in performing the excretory functions which it later entirely takes over. FIFTH DAY: THE EXCRETORY SYSTEM 467

The Metanephros. —— At the end of the fourth day, the diverticulum (ureterl from the posterior end of the W/olﬁian duct had just appeared, and the nephrogenous tissue immediately behind the mesonephros had degenerated. During the ﬁfth day, the above diverticulum, accompanied by the nephrogenous tissue posterior to the region of degeneration, grows forward somewhat, and begins to branch dichotomously ( Fig. 240, representing a slightly later stage). Its position in this region is adjacent to the posterior cardinal vein, upon the median side of the latter and above the Wolllian duct. The accompanying nephrogenous tissue lies. in turn, adjacent to ‘the median side of the diverticulum, so that the latter, i.e., the diverticulum, lies between the vein and the tissue. The nephrogenous tissue, which is in immediate contact with the diverticulum and its branches, is called the inner zone. Lastly this inner zone is covered on its median sidelby a layer of dense mesenchyme which

Fig. 240.——Proﬁle reconstruction of the Wolfﬁan duct and primordium of the metanephrns of a Chick embryo of 6 days and 8 hours. From Lillie (Development of the Chick). After Schrei~ ner.

XXV to XXXUI, thetwemy-ﬁfth to thirty-third somites. ALN. The neck of the allantois. CI. The cloaca. Int. The intestine. M’s’n. The mesonephros. 71.7‘. The nephrogenous tissue of the metanephros

included within the dotted lines. W.D. The Wolfﬁan duct. Ur. The ureter. 468 THE CHIGK

differentiates in advance of the growing nephrogenous element and diverticulum. It is called the outer zone (Fig. 241).

During subsequent days, the posterior end of the mesonephric duct bearing the rnetanephric diverticulum (ureter) is drawn into the cloaca, and thus the ureter acquires an opening separate from that of the mesonephros (Fig. 24-0}; The other end of the rnetanephric duct, with its

’ inner and outer zones, meanwhile, grows still further forward till it reaches the region of the mesonephros, and then continues on dorsal to that organ, nearly to its anterior extremity. The inner zone of this tissue everywhere gives rise to the secreting tubules and glorneruli of the permanent kidney in a manner very similar to that dethe scribed for the mesoneplr ros. These tubules then

Fig. 241.—Transverse section through ‘ ureter and metanephrogenous tissue of a live tziygdghick. From Lillie (Dez1elopmen.t of the Connect with the diCh0tO_

A.umb. Umbilical artery. Coal. Coclom. M’s’t. Mesentery. n.(..i.z. Inner zone of the nephrogenous tissue. n.!.o.z. Outer zone of the nephrogenous tissue. Ur. UI‘€'l€!‘. V .c.p. Posterior cardinal vein. W13. Wolﬂian duct.

mous branches of the metanephric duct, which thus function as collecting tubules, while the duct itself becomes the ureter of the adult. Eventually the outer zone helps to form a connective tissue covering for the entire organ.

THE REPRODUCTIVE SYSTEM

The Gonads in the Male. ——~ During the fourth day, it is impossible to distinguish sex. Occasionally on the ﬁfth day, but more generally and deﬁnitely on the sixth, the distinction becomes possible by the fact that in the female the left gonad is slightly larger than the right. This is apparently due to the fact that the right gonad usually possesses relatively little cortex, and fewer germ cells. These latter facts according to Witschi (’35) are correlated. The left gonad in the female possesses more cortex because of the female chromosomal complex and the excess cortex this worker thinks acts as an inductor to attract more germ

LY’). FIFTH DAY: THE REPRODUCTIVE SYSTEM 469

cells. Be this as it may, in the male, which is to be considered first, there is virtually no difference between the gonads, and therefore the description of one will suffice for both.

It has been indicated in the introductory discussion of germ cells in general that the primordial germ cells of the Chick are said to be first

Fig. 242.—Section through the gonad of a Chick, the middle of the ﬁfth day, showing the sexual cords growing inward from the germinal epithelium. The connections of many of the cords with the epithelium have been cut across. From Kellicott (Chordate Derelopnzent). After Semen.

g. Germinal epithelium. m. Epithelium of the mesentery (peritoneum). o. Primordial germ cells. 5. Sexual cords. t. Connective-tissue stroma.

discernible well outside the embryo. Indeed, according to Swift (’l4) and Goldsmith (’28, ’35), these cells are ﬁrst found at the primitive streak stage in the zone of junction lateral to the proanmion. From here they are carried by the blood stream to the vicinity of the germinal epithelium, whence by amoeboid movements they enter this epithelium during the fourth and ﬁfth days.

More recently, so far as the representatives of these cells which actually reach the germinal epithelium are concerned, their initial transfer by means of the blood stream has been denied (Stanley and Witschi, ’40). These authors admit that primordial germ cells are indeed found 470 THE CHICK

I l l l l

Fig. 243. —-Cross-section through the genital primordium of Limosa aegocephalzz.

From Lillie (Development of the Chick). After I-ioffxnann, from Felix and Buhler.

The stage is about similar to that of a Chick embryo of 4; days, and shows the rote

cords extending from the Malpighian tubules to the germinal epithelium. The lat ter appears in the figure as a dark mass on the right ventral side of the nn:soneph ros next to the mesentery. Three primordial germ cells (light colored) are visible

in it. ; Germ. Germinal epithelium. Ms.t. Mesentery. S.C. Rete cord. V. Posterior cardi-

nal vein. W.D. Wolﬂian duct. j

in the blood in early stages, but claim that they are only cast offs, never destined to enter the gonads. According to them all movement of such cells really on their way to the germinal epithelium is by passive shifting accompanying growth and rearrangement of parts, and later by active migration as indicated? Be this as it may, by the ﬁfth day the germinal epithelium with the primordial germ cells in it is being drawn

~" It must be further noted that according to Firket (’20) and others all, or most, ‘ of these so-called primordial germ cells in the Chick, as in the Albino Rat, ulti-

mately degenerate and are replaced by deﬁnitive germ cells derived from the germinal epithelium itself. ‘ FIFTH DAY: THE REPRODUCTIVE SYSTEM 471

Fig. 244. —Cross-section through the periphery of the testis of a just hatched Chick. From Lillie (Development of the Chick). After Semen. The sexual cords have acquired a lumen, and the walls of the canals thus formed are lined within by the spermatogonia. Next to the latter come a layer of supporting or Sertoli cells. The connective tissue (stroma) lying between the sexual cords (now seminiferous tubules‘! connects at the periphery of the testis with the special layer of connective tissue (albuginea) which covers the entire organ beneath the thin outermost layer of coelomic epithelium.

Alb. Albuginea. c.T. Connective tissue of the stroma, or septulae testis. Ep. Remains of the germinal epithelium now forming the outermost or serous covering of the testis. L Lumen of the sexual cords. pr.o. Spermatogonia. s.C. Sexual cord, lined by supporting cells and spermatogoma.

somewhat on to the ventro-median surface of the mesonephros. Meanwhile from the capsules of the Malpighian bodies of that organ, strands of cells begin to grow out through the loose mesenchyme to the germinal epithelium. These strands are the rete cords, and are destined to form the vasa eﬂerentia which help to connect the future tubules of the testis with the vas deferens (see below). At about this period also the germinal epithelium begins to send processes inward among the mesenchyme cells and the rete cords. These new strands of tissue of epithelial origin are the sexual cards, which contain primordial germ cells (Figs. 24-2, 243) . Up to this point the condition of the male gonad is virtually iden472 THE CHICK

tical with that of the female. From now on, however, the former begins to be differentiated to form the adult testis in the following manner:

The sexual cords become separated from the epithelium, and increase in number so as to constitute the bulk of the organ (seventh day) , while the rete cords are pressed to the side nearest the mesonephros. Presently also (eleventh day) the mesenchyme, which has been scanty, begins to increase among the sexual cords, forming the connective tissue or stroma. Eventually it gives rise further to a layer, the albuginea, lying between these cords and the reduced sheet of epithelium which remains as the outer covering of the gonad. Meanwhile the sexual cords themselves (twentieth day) begin to acquire a lumen, and are thus transformed into the seminiferous tubules. The walls of the latter are composed of supporting cells which are lined internally by the multiplying primordial germ cells. The latter may now be termed spermatogonia, from which arise in turn the sperrnatocytes and sperm (Fig. 244) . It is to be noted in this connection that the spermatogonia, unlike the oiigonia in the Bird, continue to divide throughout the sexual life of the individual. The ends of the seminiferous tubules eventually become connected with the rete cords which, as indicated above, become tlt: vasa efferentia. These in turn connect with the modiﬁed mesonephric tuhules in the anterior or sexual half of that organ, which thus becomes the epididymis. The posterior and non-sexual portion of the mesonephros which remains becomes a vestige known as the paradidymis.

The Gonads in the Female. —Although differences in sex may be indicated by the disparity in the size of the gonads as early as the ﬁfth day, there is little else to distinguish male from female at this time. The description of the testes up to this point will, therefore, suffice also for the ovaries. The right and left ovary, however, are different in the Bird, and this difference appears at an early stage.

ln.the left ovary, following; the sixth day, a secondary set of sexual cords, the ovigeraus cords, grow inward from the germinal epithelium, and again carry primordial germ cells. The new cords press the original or primary cords into the medullary region, and the germinal cells in the latter cords degenerate. In the right’ ovary no such secondary growth occurs, and under normal conditions the primary cords develop only slightly, the whole structure remaining rudimentary unless artiﬁcially stimulated by injected male hormone to form a testis. In the left ovary, however, the secondary or ovigerous cords soon break up into nests, each containing at least one germ, surrounded by remaining epithelial cells which form its follicle. From this point on, the young egg cell begins to grow, FIFTH DAY: THE REPRODUCTIVE SYSTEM 473

and it may, therefore, be termed an oiicyte (Fiﬁ. 245) . This growth period is reached earlier by some ova than by others, but the oogonial or multiplication stage ceases for all about the time of hatching. The anterior portion of the mesonephros, which in the male forms the epicli(ly Fig. 245. -Cross-section of the ovary of a ﬂedgling of Numenius arouatus 3-4 days old. The germinal epithelium is below. From Lillie (1)0velopment of the Chick). After Hoflmann. Note numerous oiicytes surrounded by a single layer of follicle cells.

s.c. Sexual cords degenerating. Germ. Ep. Gerrninal epithelium pruducing ovigerous cords.

mis, remains as a minute rudiment, the epoophoron. The paradidymis of the male is sometimes evident in the hen as a still smaller vestige, the pa/'o6p/Loron.

The Gonoducts in the Male. — It has already been stated that in the male, the Wolfﬁan ducts become the vasa deferentia or sperm ducts of the adult. They connect with the testes through the vasa eflerentia and epididymis. Late in ‘development, they become muscular and somewhat convoluted, with a dilation at their posterior extremities. 474 I THE CHICK

The Gonoducts in.the Female. —— As has been stated, the oviduct: begin development on the fourth day as the tubal ridges, one on the lat eral side of each mesonephros adjacent to the respective Wolﬁian duct During the ﬁfth day, a groove-like invagination develops along the an

mttz

Fig. 246.——Trans\ erse section through the metanephros, rnesonephros, gonads and neighboring parts of an 8-day Chick. From Lillie (Development of the Chick).

A0. Aorta. bl.v. Blood vessels. BJ7. Body-wall. Coel. Coelom. COLT. Collecting tubule of the mesonephros. col.T.M’t’n. Collecting tubules of the metanephros. Glam. Glomerulus. Gon.l. Left gonad. Gon..r. Right gonad. M.D. Miillerian duct. M’s’t. Mesentery. n.t.i.z. Inner zone of nephrogenous tissue (metanephric). n.t.o.z. Outer zone of the nephrogenous tissue. Symp.Gn. Sympathetic ganglion of the

twenty-first spinal ganglion. V.C. Centrum of vertebra. V.s’c.l. Left subcardinal vein. W.D. Wolﬁian duct.

terior portion of each ridge, and the lips of the groove fuse with one another to make a tube open at its anterior end. This tube which is quite short, then grows backward independently between the remaining tissue of the ridge and the Wolﬁian duct (Fig. 24.6) .

Subsequent development is as follows: By the eighth day each duct has reached the cloaca, but does not open into it. At this time, there begins the atrophy of both ducts in the male and of the right duct in the FIFTH DAY: THE ADRENALS - 475

female, accompanied in both sexes by the disappearance of the remains of the tubal ridges. The left duct in the female, however, gradually enlarges and dillerentiates the infundibulum and glandular portions charaeteristi(- of the adult. It does not, however, effect. an entrance into the cloaca until the hen is about six months old (Lillie alter Casserl. It always remains attachetl to the body wall and the rudiments of the meso 'nephros by a ligament or mesentery-like fold.

THE ADRENALS

During the fifth day, the cortical substarree, noted as arising on the fourth day, increases in amount, and cornea into relation with the Malpighian capsules. On the sixth day it begins to be zirrzmged in deﬁnite cords. \'\'l'tlC‘.ll during subsequent days increase in size and number. while at the same time innervation of the organ begins. On the eighth day this mass of cords is becoming penetrated by blood sinuses and by the medullary material previously l!tLll(‘al€d. Within the latter, “ chromafﬁne ” cells are being differentiated, and eventually this medullary material also acquires a cord-like arrangement.

HATCHING

lt will be recalled that originally the embryo was orientated with its long axis transverse to that of the shell. and with the head away from the observer when the large end of the shell is to the obser\‘er’s left. Between the fifth and ninth days the position of the embryo varies considerably, and changes from time to time due to active contractions of the amnion. By the tenth day, however, a normal embryo agrain assumes the original position relative to the shell. But at this stage it is nearer to the large end of the latter, and lies with its back against the yolk-sac in ‘ stead of either its ventral parts or its side. In this position of course its

legs are pressed against the shell. Next, aided by contractions of the amnion, the ‘yolk-sac is moved first toward the small end of the shell, and then up over the ventral side of the embryo. This movement is usually completed by the thirteenth or fourteenth day. During the next three or four days the yolk-sac moves on over the ventral side of the embryo until the now partially emptied and ﬂabby sac occupies the large end of the shell. As this is occurring the embryo by means of vigorous wriggling turns itself so that when the process is completed its tail is at the small end of the shell, i.e., the long axis of the embryo and shell have 476 THE CHICK

now become parallel. According to the schedule indicated this condition is finally achieved on the seventeenth or eighteenth day.‘ The next step involves the piercing of the egg membrane by the beak so that breathing of air from the air chamber can begin. Some respiratory movements may occur, however, even before this, there being by this time small amounts of air in other parts of the egg. As respiration starts the amnion and allantois dry up and become detached, while movements of the abdomen draw the remains of the yolk-sac within the body. At the same time the necessary circulatory changes are occurring within the embryo as already described. About the last hour before hatching on the twenty-first day the Chick starts a vigorous counter clock-wise rotation within the shell aided by strong thrusting movements of the legs. Presently as a result of the thrusting of the legs and the stretching of the neck the shell is broken into two parts and the Chick is hatched.

The foregoing description of later positional changes and hatching is taken from the detailed account by Kuo (’32). One interesting feature which is not mentioned by this author, however, is the so-called egg tooth. This is a sharp cone shaped point of horny material developed on the dorsal side of the beak, and is said by other writers to function in

chipping the shell. At all events it is a transitory structure lost soon after hatching.

Summary Or The Condlt!On At The End Or The Fifth Day Or Incubation

I. THE EXTERNAL APPEARANCE

The cervical ﬂexure has reached its rnaicimum development, the third visceral cleft has closed, and the future neck is slightly indicated. The limb buds are beginning to appear jointed. The nasal apertures are sep arated into internal and external nares and the beak and mandible are just startingto form.

II. THE FEATHERS

A depression develops in the skin. At its bottom a slight outgrowth arises consisting of_a core of mesoderm, the pulp, with a covering of the Malpighian layer and a thin outer layer of corniﬁed epithelium. This

outgrowth is the papilla. The papilla emerges above the depression, and '

is known as the feather germ. With further growth and the throwing off

“Waters (’35) says usually not until the nineteenth or twentieth day. FIFTH DAY: SUMMARY 4??

of the cornified cells the Malpighian layer becomes folded and modified to form the quill and barbs of a feather. Feather germs appear in the Chick on about the eighth day.

III. THE SKELETON

The deﬁnitive or vertebral segmentation of the mesencliymal slzeatlz, about the notochord and nerve cord has become more marked, while all the sclerotomal tissue is becoming membranous. These membranous condensations are especially evident in certain regions, representing parts of the future vertebrae neural arches and costal processes. Mesenchymal concentrations representing the limb bones and the parts of the pectoral and pelvic girdles are also visible. The various parts of the primordial cartilaginous cranium and visceral skeleton. are discernible at this time as concentrations of mesenchyme about the head

IV. THE ALIMENTARY TRACT

The Fore-gut Region.———The third visceral cleft closes, the lung rudiments have grown posteriorly somewhat through a mass of developing mesoderm, and faint indications of the abclomirzal and cervical air sacs may be present. The glottis is partly closed.

The esophagus has continued to elongate, the stomach is slightly dilated, and a pouch representing the rudiment of the gizzard has appeared in connection with it. The duodenal loop is barely defined. The liver has continued to branch, and some of the branches have acquired lumens. The three pancreatic diverticula have also branched somewhat.

The Mid-Gut Region. The end of the duodenum is marked by a ventral bend, the duodeno-jejurzal ﬂexnre. From here the midgut or small intestine descends to connect with the yolk-sac, and passes dorsally again to its posterior end, marked by rudiments of the intestinal caecae.

The Hind—gut Region. —-The hind-gut or rectuniis not materially altered, but the laterally compressed walls of the posterior part of the cloaca have become fused.

V. THE CIRCULATORY SYSTEM

The Heart.———The alterations in the relative positions of the parts are nearly completed, as are also the septa within the heart. The septum of the truncus arteriosus has formed and that of the ‘bulbus has started to develop.

The Arteries. —-— The portions of the dorsal aortae between the third and fourth arches have begun to disappear, and the left fourth arch has 478 » THE CHICK

also diminished in size. The subclavian, arteries have become connected with the carotids and the anterior mesenteric and coeliac arteries are developed.

The Veins. —-—The right side of the second venous ring about the intestine has disappeared, so that in this region there is only a single vitelline trunk. Within the liver, the capillaries of the ductus venosus are continuing to develop, while the main channel is atrophying. The mesenteric vein has started to form.

The subcardinals have lost their original direct connections with the posterior cardinals, and have developed new ones through capillaries within each mesonephros. At the same time the subcardinals have ‘fused. with one another anteriorly, and by means of the previous connection with the vena cava inferior, have thus established a renal portal system. The subclavian veins have started to develop from the posterior cardinals.

VI. THE BODY CAVITIES

The ventral communication between the pericardial and peritoneal cavities has begun to he closed by the development of the lateral closing folds beneath the lateral mesocarzlia.

VII. THE NERVOUS SYSTEM

In connection with the description of this system in the preceding chapter, it was noted that there are few important developments occurring in it on the fifth day. The following events, however, may be mentioned as having taken place during this period.,The fourth cranial nerves have originated, and in connection with the ear the rudiments of the semicircular canals have appeared. In the eye the mesenchymal part of the pecten. is increasing, while the lips of the choroid ﬁssure are beginning to overgrow it.

VIII. The Urinogenital System

The Excfetory System. —— The mesonephric tubules have ceased to increase in number, but are continuing to grow in length as the organ becomes more active. The metanephric diverticulum, accompanied by its nephrogenous tissues or inner zone, has grown forward and begun to branch, while about the latter the outer zone is developing from mesonchyme.

The Genital System.——The primordial germ cells have begun-to pass into the germinal epithelium and the rete and sexual cords have 9 REFERENCES TO LITERATURE » 479

started to develop. The male and female gonads are similar except for occasional differences in size between the right and left organs in the female. In both sexes, the oviducts are present as small tubes growing

toward the cloaca.

IX. The Adrenals

The cortical substance of the adrenals increases in amount, and comes into relation with the Malpighian capsules.

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CHAPTERS VIII, IX, X, XI, XII, AND XIII

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