Book - The development of the chick (1919) 9

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

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

Part II The Forth Day to Hatching, Organogeny, Development of the Organs

Chapter IX Organs Of Special Sense

I. The Eye

The development of the eye up to the stage of 36 somites has been already described. We shall now consider the subsequent changes in the following order: (1) optic cup, (2) vitreous body, (3) lens, (4) anterior chamber, cornea, iris, etc., (5) choroid and sclerotic, (6) the conjunctival sac and eyelids, (7) the choroid fissure and the optic nerve.

1. The optic cup at the stage of 36 somites is composed of two layers, an inner, thicker layer, known as the retinal layer, and an outer, thinner layer, known as the pigment layer; these are continuous with one another at the pupil and choroid fissure. The inner and outer layers come into contact first in the region of the fundus, and the cavity of the original optic vesicle is gradually obliterated. The choroid fissure is in the ventral face of the optic cup; it is very narrow at this time, and opens distally into the pupil; centrally it ends at the junction of optic stalk and cup, not being continued on the stalk as it is in mammals (Fig. 157).

The walls of the optic cup may be divided into a lenticular zone {pars lenticularis or pars cceca) and a retinal zone; the former includes the zone adjacent to the pupil, not sharply demarcated at first from the remainder or retinal zone, but later bounded distinctly by the ora serrata. The retinal zone alone becomes the sensitive portion of the eye; the lenticular zone develops into the epithelium of the iris and ciliary processes.

In the lenticular zone the inner and outer layers become actually fused, but in the retinal zone they may always be separated; indeed, in most preparations they are separated by an actual space produced by unequal shrinkage.

The differentiation of the lenticular from the retinal zone begins about the seventh day, when a marked difference in thick 271


272


THE DEVELOPMENT OF THE CHICK


ness appears. The transition from the thinner lenticular to the thicker retinal zone soon becomes rather sudden in the region of the future ora serrata. About the eighth or ninth day a further differentiation arises within the lenticular zone, marking off the regions of the iris and ciliary processes (Fig. 159). The region



ep ^es p r


157 ■ 158

Fig. 157. — Section through the eye of a chick embryo at the

beginning; of the fourth day of incubation. (After Froriep.)

ch. Fis. I., Lip of the choroid fissure. Di., Lateral wall of the diencephalon. V, \", Distal and proximal walls of the lens, st., Optic stalk.

Fig. 158. — Section of the distal portion of the eye of a chick,

second half of the fifth day of incubation. (After Froriep.)

c. ep. int., Internal epithelium of the cornea. Corn, pr., Cornea propria. Ect., Ectoderm, ep.. Epidermis. ir.,Iris. mes.. Mesoderm, p., Pigment layer of the optic cup. r., Retinal layer of the optic cup.

of the iris is a narrow zone bounding the pupil in which the two la3'ers of the optic cup become blended so that pigment from the outer layer invades the inner layer; the epithelia are decidedly


ORGANS OF SPECIAL SENSE


273


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C/2.

cj7.


irjs


ant. (•/?. Corri.


ITj^



^ — op.n .


■olf.


/i .m


Fig. 159. — Frontal section of the eye of an eight day chick. Shrinking in the process of preparation has caused a separation between the retinal and pigment layers, ant. ch., Anterior chamber of the eye. ch., Choroid coat, cil., Ciliary processes. Corn., Cornea. 1. e. 1., Lower eyelid, n. m., Nictitating membrane, olf., Olfactory sac. op. n., Optic nerve, o. s., Ora serrata. p., Pigment layer of the optic cup. post, ch., Posterior chamber of the eye. ret.. Retina, scl., Sclerotic coat. scl. C, Sclerotic cartilage, u. e. 1., Upper eyelid.

thinner than in the ciliary region. The mesenchyme overlying the iris early becomes condensed to form the stroma of the iris; the epithelia form the uvea of the developed iris (Fig. 159). The muscles of the iris (sphincter pupillse) are stated by


274


THE DEVELOPMENT OF THE CHICK


Nussbaum, Szily, and Lewis to arise from epithelial buds of the pupillary margin and the adjacent portion of the pigment layer of the iris. The marginal buds (Fig. 160) begin to form during the seventh day, the more peripheral ones somewhat later; the former are less numerous and larger than the latter. The observations are well supported, and appear to leave no doubt that the specificity of the ectoderm cells of the iris is not fixed. According to Lewis the wandering pigmented cells of the anterior portion, at least, of the choroid also arise from the pigment layer of the optic cup.

The ciliary processes begin to form from the ciliary region of the lenticular zone on the eighth day (Fig. 159) ; the epithelium

^M" Sph. Sfih. ^-"


ES.


3pM



B.


Fig. 160. — Two sections of the pupillary margin of the eye of a chick of 13 days' incubation. A., X 260. B., 130. (After Lewis.) c. P., Ciliary process. E. B., Epithelial bud. P., Margin of pupil, p. 1., Pigment layer of Iris. r. 1., Retinal layer of iris. Sph., Bud for the formation of the sphincter muscle of the iris, derived from the margin. Sph.', Sph.", Submarginal buds of the sphincter.

becomes thrown into folds projecting towards the posterior chamber, the cavity of the folds being filled by the mesenchyme of the developing choroid coat. The muscles of the ciliary body develop from the mesenchyme of the processes, which acquire a connection with the lens through a special differentiation of the vitreous body, the zonula ciliaris (zonula Zinnii).

In the retinal portion of the optic cup the inner la3^er forms the entire retina proper from the internal limiting membrane to the rods and cones inclusive. The outer layer forms the pig


ORGANS OF SPECIAL SENSE 275

ment layer of the retina. About the middle of the fourth day pigment begins to develop in the outer layer and extends throughout it, even to the distal portion of the optic-stalk at first (Ucke, '91). The histogenesis of the retina of the chick has been described by Weysse (1906).

2. The Vitreous Humor (Corpus Vitreum). Until comparatively recently embryologists have adhered to the view stated by Schoeler (1848) and Kolliker (1861) that the vitreous body arises from mesenchymal cells that enter the e3^eball through the choroid fissure. The fact that the embryonic vitreous humor of birds is almost entirely devoid of cells was a serious difficulty. The cells are in fact so scanty as to be absent in many entire sections. Moreover, in character they resemble embryonic blood-cells and not mesenchyme, and disappear entirely by the eighth day. It seems impossible that they should play any important part in the origin of the massive vitreous body. Researches of the last few years have demonstrated that the vitreous body is primarily of ectodermal origin, its fibers arising as processes of cells of the inner layer of the optic cup and the matrix as secretion. According to some the cells of the lens are responsible wholly (Lenhossek) or in part (S/ili) for the fibers; this view, however, has been strongly combatted (Kolliker and Rabl) and requires further evidence to substantiate it.

Both retinal and csecal parts of the cup take part in the formation of the fibers of the vitreous body; the retinal part is at first the most important, and the primary vitreous body is almost entirely retinal in its origin. But after the caecal part is differentiated the activity of the retinal part becomes less, and the greater part of the fillers of the vitreous body appears to be formed from cells of the csecal part, that send out branching and anastomosing processes into the posterior chamber. There is no sharp boundary between the fibers that form the vitreous body and those that form the zonula; and the fibers of the latter may be regarded as homologous to those of the former. The matrix of the embryonic vitreous body may be regarded as a secretion of the walls of the optic cup. Later, the secretion appears to be confined to the ciliary processes. It is possible that the mesenchyme plays some part in the formation of the vitreous body after the formation of the pecten begins; but there is no evidence that it does so at first.


276 THE DEVELOPMENT OF THE CHICK

3. The Lens. The account of the development of the lens is mainly after Rabl. The wall of the lens-sac is everywhere a single-layered epithelium, though the nuclei are at different levels in

the cells.

Shortly after the lens-sac has become separated from the ectoderm the proximal wall (that next the cavity of the optic cup) begins to thicken by elongation of the constituent epithelial cells (Figs. 157 and 158). During the fourth day the elongation of the cells increases greatly as the first step in the formation of the lens fibers, while those of the distal wall remain practically unchanged, being destined to form the epithelium of the lens. Between the cells of the proximal and distal walls are found ceUs of an intermediate character, bounding the equator of the lens (Fig. 158).

During the fifth day the elongation of the cells of the proximal waU proceeds apace; those in the center of the wall are most elongated and there is a gradual decrease towards the equator of the lens. In this way the face of the proximal wall gradually approaches the distal wall and meets it on the fifth day, thus obliterating the central part of the lens ca\ity, though the peripheral part remains open for a considerably longer time (Fig. 158). The nuclei of the lens fibers occupy approximately their center, and thus form a fairly broad curved band, concave towards the optic cup. At the same time the lens is increasing very rapidly

in size.

During the sixth, seventh, and eighth days the same processes continue and the elongation of the lens fibers makes itself felt on the inner face of the lens which becomes convex. The form and arrangement of the parts is shown in Figure 159. The fibers already present are destined to form only the core of the adult lens; and a new process begins at this time, leading to the formation of fibers that wrap themselves around this core in a meridional direction and form many concentric layers (666 according to Rabl). These new concentric fibers proceed from cells situated between the core fibers and the lens epithelium, that is, around the equator of the lens. There is a very rapid multiplication of cells here; those next the core transform into fibers arranged meridionally on the surface of the core; others develop over these and thus the original fibers come to be surrounded by more and more concentric layers. At first these are disposed rather irregularly, but soon the arrangement becomes extraordinarily regular.


ORGANS OF SPECIAL SENSE


277


This process is kept up not only during embryonic life, but during the entire growth of the fowl; thus the thickness of the superimposed lamellae is only 0.60 mm. at hatching, but is 2.345 mm. in the adult (Rabl).

In the fowl the lens includes three concentric layers of fibers: (1) the central mass or core formed by the proximal wall of the original lens-sac; this has the same diameter (0.80 mm.) as the entire fiber mass at eight days. Nuclei are entirely absent. (2) An intermediate layer of meridional rows of fibers rather irregularly arranged, which shade gradually into the fibers of the core and into those of (3) the radial lamellae, which form the greater part of the substance of the adult lens. The meridional rows and the radial lamellae proceed from the cells of the intermediate zone of the original lens-sac. Fig. 161 shows a sector of an equatorial section through the lens of a chick. The three zones are well marked; the extraordinary regularity of the superimposed layers of the radial lamellae is well shown.

The lens epithelium of birds and reptiles also produces a peculiar structure which may be called the equatorial ring (Ringwulst, Rabl).

It will be seen in the figures




^


f&^ #'


Fig. 161. — Equatorial section through the lens of a chick embryo of eight days. The main mass of the entire lens is represented by irregularly arranged central fibers. Towards the surface (above) the fibers are arranged in rows and are quite regularly six sided. (After Rabl.)


278 THE DEVELOPMENT OF THE CHICK

that the epithelium is originally thinnest distally and thickens towards the equator. This condition increases up to the eighth day, at which time the thickening increases more a short distance from the equator, so that there is a broad ring-shaped thickening of the anterior epithelium separated by a narrow thinner zone from the cells of the equatorial zone (cf. Fig. 159). This ring increases in thickness during the greater part of the period of incubation, and its cells become fibers arranged in a radial direction. The meaning of this curious structure is somewhat obscure, but from the fact that it shows on its surface the impression of the ciliary processes, Rabl w^as of the opinion that it served in accommodation of the eye as an intermediary between the ciliary processes and the true lens-fibers.

4. Anterior Chamber and Cornea, etc. When the optic vesicle is first formed it is in immediate contact with the ectoderm. After its invagination the lips of the optic cup withdraw a short distance from the surface. At the same time the lens invaginates and is cut off from the ectoderm, but rem.ains in contact with it during the third day. There is thus a ring-shaped space between the lens and optic cup on the one hand and the ectoderm on the other, which is the beginning of the anterior chamber of the eye (cf. Fig. 96 C). With the formation of the cornea the lens withdraws somewhat from the surface and the space spreads over the whole external surface of the lens; at first it is very narrow, but increases in size by the formation of the iris and the bulging of the cornea.

The cornea itself develops from two sources: (1) the external epithelium is derived from the ectoderm overlying the anterior chamber, (2) the cornea propria and the internal epithelium lining the anterior chamber develop from the surrounding mesenchyme but in somewhat different ways.

The cornea propria appears on the fourth day as a delicate structureless membrane beneath the corneal epithelium. During the fifth day it increases to about the thickness of the overlying ectoderm (Fig. 158). About this time mesenchyme cells from the margin of the optic cup begin to migrate between the cornea propria and lens, and soon form a single complete layer of cells on the inner face of the cornea propria; this layer becomes the inner epithelium of the cornea (Fig. 158). The cornea propria is still devoid of cells, but on the sixth and


ORGANS OF SPECIAL SENSE 279

seventh days the mesenchyme surroimding the eyeball begins to penetrate it from all sides in the form of a compact wedge, which, advancing in the substance of the cornea propria, soon meets in the center. These cells form the so-called corpuscles of the cornea. They appear arranged in strata from a very early period.

The anterior chamber is bounded by the cornea externally; its margins, which are at first coincident with the lips of the optic cup, soon extend peripherally over the iris (Fig. 159). The inner epithelium ceases at the margin of the cavity or is continuous with the cells of the sclerotic; it does not appear, in an eight-day chick at any rate, to be reflected over the iris, but the epithelium of this structure next the anterior chamber appears to be simply a special differentiation of its own superficial cells. The anterior chamber is closed centrally by the lens, but communicates more or less for a considerable period around its margin with the posterior chamber. This is at least the appearance in good sections; it seems probable, though, that in life there is contact between the optic cup and lens.

The stroma of the iris proceeds from that portion of the mesenchyme left in association with the pars iridis retinae after the peripheral extension of the anterior chamber. It becomes very vascular at an early stage. The canal of Schlemm arises as a series of vacuoles just peripheral to the margin of the anterior chamber about the eighth day. These soon run together to form a ring, which is separated from the anterior chamber by the ligamentum pectinatum iridis.

5. The choroid and sclerotic coats are differentiations of the mesenchyme surrounding the optic cup. But little is known concerning the details of their development in the chick. A figure of Kessler's shows chromatophores developed in the choroid coat at twelve days; I find a very few already formed at eight days. Cartilage begins to appear in the sclerotic at eight days, the forerunner of the sclerotic ossicles (Fig. 159).

6. The Eyelids and Conjunctival Sac. The integument over the embrvonic eveball remains unmodified until about the seventh day. At this time a circular fold of the integument forms around the eyeball with the pupil as its center. At the same time a semi-lunar fold develops within the first on the side of the eyeball next the beak. (See Figs. 122-124.) From the


280 THE DEVELOPMENT OF THE CHICK

first fold the upper and lower eyelids are developed, and from the second the third eyelid or nictitating membrane. The area bounded by the outer ring-shaped fold becomes the conjunctival sac.

From their place of origin the free edges of these folds then grow towards the center, and thus a cavity, the conjunctival sac, is formed between the folds and the integument over the eyeball (conjunctiva sclerse). The outer fold grows more rapidly above and below than at the sides and the opening narrows, becoming, therefore, gradually elhptical and finally somewhat spindle-shaped. Thus the upper and lower eyelids are established. The semi-lunar fold of the embryonic nictitating membrane also grows towards the pupil, most rapidly in its center. The conjunctival sac also expands peripherally, especially at the inner angle of the eye, and thus accommodates itself to the increasing size of the eyeball (Fig. 159).

The Harderian gland is visible on the eighth day as a solid ingrowth of ectodermal cells of the conjunctival sac at the innermost angle of the nictitating membrane.

Feather germs develop on the outer surface of both upper and lower lids especially at their edges. The ectoderm covering the inner faces of the upper and lower lids, both faces of the nictitating membrane and the remainder of the conjunctival sac becomes modified into a moist mucous membrane. Over the cornea the ectoderm is especially modified as already noted.

Papillce Conjunctiva Sclerce. On the seventh day of incubation papillae begin to appear on the surface of the conjunctiva sclerse and soon form a ring surrounding the iris at some distance peripheral to its margin (Figs. 122, 123 and 124). The number of these papillae appears to be quite constantly fourteen. They are at first fully exposed owing to the undeveloped condition of the eyelids, but the latter overgrow them about the eleventh or twelfth days. Degeneration of the papillae begins about this time, and on the thirteenth day they have entirely disappeared. In section they are found to be thickenings of the ectoderm, produced by multiplication of the cells. They may rise above the surface; but more frequently project inwards towards the connective tissue. There is apparently no accompanying hypertrophy of the latter. Thus they differ quite essentially from feather germs with which it seems natural to compare them; and their significance is entirely problematical (see Xussbaum).


ORGANS OF SPECIAL SENSE 281

7. Choroid Fissure, Pecten, and Optic Nerve. The pecten of the hen's eye is a pigmented vascular plate inserted in the depression occupying the center of the elongated blind spot, or entrance of the optic nerve, which extends meridionally from the fundus nearly to the ora serrata. The pecten projects a considerable distance into the posterior chamber and its free edge is much longer than its base, being consequently folded like a fan; hence the name. The optic nerve runs along the base of the pecten, its fibers passing off on either side into the retina; thus it continually diminishes in size until it disappears. The pecten is consequently separated from the choroid coat by the optic nerve. It is supposed to function as a nutrient organ for the layers of the retina, by means of lymph channels that pass off from its base into the retina. There is no arteria centralis retinae in the

bird's eye.

These structures develop in connection with the choroid fissure as follows: On the fourth day the choroid fissure has become a very narrow slit, and by the middle of the day its edges are in apposition in the pars cceca of the bulbus. Proximally, however, the meeting of the lips of the fissure is prevented by the mesoblast, in which the basal blood-vessel runs along the entire length of the open portion of the fissure. During the fourth day this blood-vessel enters the posterior chamber Avith its enveloping mesenchyme along the entire length of the open portion of the choroid fissure, and forms a low mesenchymal ridge connected by a narrow neck of mesenchyme in the fissure with the mesenchyme outside. During the fifth day the ridge becomes higher and keel-shaped, and a thickening appears along part of its free edge above the blood-vessel. During this day also fusion of the lips of the choroid fissure has taken place in the pars caeca. At the same time an important change begins in the proximal portion of the choroid fissure that leads to the formation of the pecten proper. This is an involution of the lips of the optic cup bounding the choroid fissure on each side of the mesodermal keel, and their continuous ingrowth until they meet over the keel and fuse above it in a mass in which the outer and inner layers of the retina are indistinguishably fused. Thus the proximal portion of the mesodermal keel is enclosed in a kind of tunnel composed of the involuted edges of the optic cup. The formation of this tunnel progresses gradually from the fundus towards


282


THE DEVELOPMENT OF THE CHICK


the ora serrata by the same process of involution, until on the eighth day the mesodermal keel is completely covered up.

Fig. 162 gives a diagrammatic view of the condition of the pecten in the middle of the seventh day of incubation. Figs. 163 and 164 show sections through this at the points a, h, c, d, e, indicated in the figure. The formation of the tunnel will be readily understood by study of the figures. It will be seen that the major portion of the embryonic pecten is of ectodermal origin, and that the mesoderm forms a relatively inconspicuous part of it. Later, on the same day, it becomes increasingly difficult


CAfi^^


P.




i PB.


H^



Fig. 162. — Diagrammatic reconstruction of the pecten of the eye of a chick embryo of 1\ days' incubation. (After Bernd.)

Ch. fis. 1., Lip of the choroid fissure. Ch. fiss., Choroid fissure. Mes., Mesoblast. Mes. b., Boundary of the mesoblast within the choroid fissure. Mes. K., Thickening of the mesoblastic keel. op. C, Optic cup. O. St., Optic stalk. P., Pecten. P. B., Base of the pecten.

The arrow indicates the direction of growth of the ectodermal tunnel.

The lines a, b, c, d, e show the planes of the sections reproduced in Fig. 163 (a, b, c, e) and in Fig. 164 (d).

to distinguish ectodermal and mesodermal portions of the pecten, and thereafter it is quite impossible to say which parts of it are of ectodermal and which are of mesodermal origin. During the eighth and ninth days the pecten increases greatly in height, and becomes relatively very much narrower.

The folds of the pecten now begin to develop and, b}^ the seventeenth day their number is 17-18, the same as in the adult. The pigment does not begin to appear until about the twelfth day. The details of the development of the blood-vessels are not known.


ORGANS OF SPECIAL SENSE


283


The Optic Nerve. Owing to the relations established by the choroid fissure, the floor of the optic stalk is continuous from the first with the inner layer of the retina (Fig. 96 B), and it furnishes the path along which the optic nerve grows. The axones of the optic nerve originate, for the most part, from the retinal neuroblasts, composing the layer next to the cavity of the optic cup, and their growth is thus centripetal. They are first formed in the fundus part of the retina, and grow in the direction of the



Mes ft




Fig. 163. — Outlines of sections in the planes a, b, c, e, of

Fig. 163. (After Bernd.)

bl. v., Blood vessel, i. 1., Inner or retinal layer of the optic cup. o. 1., Outer or pigment layer of the optic cup. P. inv., Angle of invagination of the pecten. Other abbreviations as before. (Fig. 162.)


optic stalk between the internal limiting membrane and the neuroblast layer (ganglion cell layer), thus forming a superficial layer of axones; their formation begins on the fourth day, and there is a period about the end of this day when axones are found in the distal part of the optic stalk, next to the bulbus oculi, but not in the proximal part, next to the brain. This observation affords conclusive proof of the retinal origin of the fibers of the optic


284


THE DEVELOPMENT OF THE CHICK


nerve; moreover, at an early stage of their differentiation it is possible to trace their connection with retinal neuroblasts.

The first fibers of the optic nerve are formed, as already stated, from the fundus part of the retina; the fibers, therefore, pass directly to the floor of the optic stalk; but on the fifth day the formation of fibers begins from more distal portions of the retina and these do not grow towards the insertion of the optic stalk, l3ut towards the choroid fissure; arrived there, they bend centrally and run in a bundle on each side along the floor of the bulbus oculi to the optic stalk, where they join with the fibers first formed. The later formed fibers pass to still more distal portions



■ Mes/I


P.,






t-«*f;


«^:Mfe^2^




Fig. 164. — Section in the plane of d of Fig. 162, to show the histological structure. (After Bernd.) Abbreviations as before.


of the choroid fissure, and, as the pecten forms in the manner already described, the fibers of the optic nerve all unite beneath it on their way to the original optic stalk. Thus, the optic nerve obtains an insertion coincident in length with the base of the pecten, and its fibers, radiating off into the retina on each side of the pecten, separate the latter completely from the choroid coat of the eyeball.

The optic stalk is at first a tubular communication between the optic vesicle and the fore-brain, and its walls are an epithelial layer of the same thickness throughout. The fibers of the optic


ORGANS OF SPECIAL SENSE 285

nerve grow into its ventral wall exclusively, between its epithelial cells, which gradually become disarranged and irregular. Thus the ventral wall becomes increasingly thick and the lumen excentric. By the sixth day the lumen appears in cross-section as a narrow lenticular space with an epithelial roof, above the large optic nerve. Soon after, the lumen disappears entirely; no trace of its former existence is to be found on the eighth day.

II. The Development of the Olfactory Organ

The origin of the olfactory pit, external and internal nares, and olfactory nerve, has already been considered (pp. 169, 215, and 263). Before the formation of the internal and external nares, not only has the entire olfactory epithelium become invaginated, but, owing to the elevation of internal and external nasal processes, the pit has become so deepened that the margin of the olfactory epithelium proper now lies a considerable distance within the cavity. That part of the nasal cavity thus lined with indifferent epithelium is known as the olfactory vestibule. After the fusion of the internal nasal process with the external nasal and maxillary processes, the cavity deepens still more.

The choanse lie at first just within the oral cavity, but the palatine processes of the maxillary process, growing inwards across the primitive oral cavity (pp. 298, 299), unite on the sixth or seventh day at their anterior ends with the internal nasal processes, and thus cut off an upper division of the primitive oral cavity at its anterior end from the remainder; in this way the internal openings of the nasal cavities into the oral cavity are carried back of the primitive choanae; they are henceforward known as the secondary choanse. Further growth of the palatine processes brings them nearly together in the middle line along the remainder of their length, about the eleventh day; but fusion does not take place, the birds possessing a split palate. Thus the superior division of the primitive oral cavity is added to the respiratory part of the nasal passages.

The nasal cavity is further elaborated between the fourth and eighth days by ingrowths from the lateral wall (turljinals) and by the formation of the supraorbital sinus as an evagination that grows outwards above the orbit. Three turbinals are formed in the nasal cavities, viz., the superior, middle, and inferior turbinals. These arise as folds of the lateral wall projecting into


286 THE DEVELOPMENT OF THE CHICK

the lumen, the superior and middle from the olfactory division proper, and the inferior from the vestibulum; on the middle turbinal, however, the sensory epithelium gradually flattens out to the indifferent type. The middle turbinal appears first in the ventral part of the olfactory division, about the beginning of the fifth day, and the superior somewhat later, immediately above the former, the two being separated by a deep groove (Fig. 165). The vestibular turbinal arises still later, and is well formed on the eighth day.

Fig. 166 shows a reconstruction of the nasal cavity, seen from the lateral side, of an embryo of about seven days. It is a reconstruction of the epithelium, and thus practically a mold of the cavity; therefore projections into the cavity appear as depressions in the model, and the grooves and outgrowths of the external wall as projections. The superior turbinal has an oval shape with the long axis in an apical direction; it is bounded by a fairly deep depression, the elevated margin of the model, from the lower end of which the supra-orbital sinus (S. s'o.) passes off ventrally and externally. The deep depression immediately below the superior turbinal lodges the median turbinal. A fairly long passage leads off from its neighborhood to the choanse and a shorter one, the vestibulum, to the external nares. The depression in the wall of the vestibulum is caused by the vestibular or inferior turbinal. The palatine and maxillary sinuses are not yet formed.

The external nares are closed during the greater part of the period of incubation by apposition of their walls. The form and dimensions of the nasal cavities change greatly during incubation, owing to shifting in the original positions of the turbinals, outgrowth of the facial region, and development of sinuses. The details are not very well investigated, and an examination of them would lead too far.

There has been a good deal of discussion as to the existence of an organ of Jacobson in the nose of birds; it has usually been assumed that it is entirely absent even in the embryo. Others have identified the ducts of nasal glands as a modification of this organ. Recently, however, Cohn has described a slight evagination in the median wall of the primary olfactory pit, that agrees precisely in its form and relationship with the first rudiment of the organ of Jacobson in reptiles. Although it persists only from the stage of about 5.3 mm. to about the stage of


ORGANS OF SPECIAL SENSE


287


5.9 mm. head-length, he identifies it positively as a rudimentary organ of Jacobson.

The septal gland arises on the eighth day from the inner wall of the vestibulum, opposite the base of the vestibular tur


FiG. 165. — Transverse section of the olfactory organ of a chick embryo, of 7.5 mm. head length. (After Cohn.) f., Line of fusion, e. n., External nasal process, i. n., Internal nasal process. T. 1, T. 2, Intermediate and superior turbinals.

binal, as a solid cord of cells. This grows backwards in the nasal septum and passes to the outer side and branches, subsequentlyj acquiring a lumen.


288


THE DEVELOPMENT OF THE CHICK


III. The Developmext of the Ear

The ear develops from two entirely different primary sources, viz., the otocyst,and the first visceral or hyomandibular cleft : The former furnishes the epithelium of the membranous lab^-rinth; the entodermal pouch of the latter becomes the tympano-eustachian cavity; and part of the external furrow forms the external auditory meatus; the tissue between the internal pouch and the external furrow develops into the tympanum. The mesenchyme in the neighborhood of each of these primordia becomes modified,




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Fig. 166. — Reconstruction of the nasal cavity of a chick

embryo of about 7 days; lateral view. (After Cohn.)

Ch., Choanal, e. N., External nares. S. s'o., Supraorbital sinus. T. 1, T. 2, T. 3, Intermediate, superior and inferior (vestibular) turbinals.

(1) to form the bony labyrinth, perilymph, and other mesenchymal parts of the internal ear, and (2) to form the auditory ossicles of the middle ear. Thus the ear furnishes a striking example of the combination of originally diverse components in the formation of a single organ. The course of evolution of this complex senseorgan is thus illustrated in the embryonic development; in the Selachia the hyomandibular cleft is a communication between pharynx and exterior, like the branchial clefts, and still preserves to a certain extent the respiratory function. The embryonic history furnishes a summary of the way in which it was gradually


ORGANS OF SPECIAL SENSE


289



drawn into the service of the otocyst in the course of evohition.

Development of the Otocyst and Associated Parts. In Chapter VI we took up the formation of the otocyst and the origin of the endolymphatic duct. The Letter is at first an apical outgrowth from the otocyst, but its attachment soon becomes shifted to the median side of the otocyst, owing to the expansion of the dorsal external wall of the latter (Fig. 167). Three divisions of the otocyst may now be distinguished: (a) ductus endolymphaticus or recessus labyrinthi; (6) pars superior labyrinthi; (c) pars inferior labvrinthi. The boundarv between the two latter is rather indistinctly indicated at this stage by a shallow groove on the median face of the otocyst. The development of these parts may now be followed separately.

(a) The Development of the Ductus Endolymphaticus. It was noted in Chapter VI that the ductus endolymphaticus is united to the epidermis by a strand of cells that preserves a lumen up to the stage of 104 hours at least (Fig. 98). Shortly after, this connection is entirely lost.

The opening of the endolymphatic duct into the otocyst appears to be shifted more and more ventrally along the median surface, with the progress of differentiation of the other parts of the otocyst, until it lies in the region of communication of the utriculus, sacculus and lagena (Figs. 168 and 171). This is brought about by the various foldings and expansions of the wall of the otocyst described in b and c. In the meantime the endolymphatic duct has increased in length with the growth of the surrounding parts, and on the sixth day the distal half begins to expand to form the saccus endolymphaticus, lying between the utriculus and the hind-brain. The elongation of the entire endolymphatic duct and the enlargement of the saccus continue during the seventh day, and on the eighth day the saccus overtops


Fig. 167. — Model of the otocyst of a chick embryo shortly before its separation from the ectoderm. (After Krause.)

D. e., Endolymphatic duct. Ect., Ectoderm, p. v., Pocket for formation of vertical semicircular canals. X indicates the strand of cells uniting the endolymphatic duct to the ectoderm.


290


THE DEVELOPMENT OF THE CHICK


the hind-brain and bends in above it towards the middle line (Fig. 168). The right and left sacci are, however, still separated by a considerable space. The walls of the saccus already form a large number of low folds, presumably glandular, the first begin


FiG. 168. — Transverse section through the head of a chick embryo of eight

days in the region of the ear (photograph).

C. a., Anterior semicircular canal. C. h., Horizontal semicircular canal. Caps, and., Auditory capsule. Cav. Tymp., Tympanic cavity. Col., Columella. Duct end., Endolymphatic duct. ex. au. M., External auditory meatus. Fis. Tub., Tubal fissure. Lag., Lagena. M. C, Meckel's cartilage. Myel., Myelencephalon. N'ch., Notochord. p'l.. Perilymph. Sac, Sacculus. Sac. end.. Endolymphatic sac. Tub. Eust., Eustachian tube. Tymp., Tympanum. L^tr., Utriculus. X., Sac derived from the inner extremity of the tympanic cavity.


nings of which were visible on the sixth day. The form of the saccus and ductus endolymphaticus at a somewhat later stage is shown in the reconstruction (Fig. 173).


ORGANS OF SPECIAL SENSE


291



It is interesting to note that the epidermic attachment to the endolymphatic duct is about at the junction of the saccus endolymphaticus and ductus endolymphaticus s.s. If this may bear a phylogenetic interpretation, it would seem that the saccus should be regarded as an addition to the primitive ductus of Selachii, which opens on the surface.

(b) Development of the Pars Superior Lahyrintki; Origin of the Se7nicircular Canals. We have already seen that the shifting of the ductus endolymphaticus to the median surface of the otocyst is brought about by a vertical extension of the superior lateral wall of the otocyst, w'hich forms a shallow pocket opening widely into the otocyst (Fig. 167). Slightly later a second pocket is formed by a horizontally extended evagination of the lateral w^all of the pars superior directed towards the epidermis. These two pockets, known as the vertical and horizontal pockets, are the forerunners of the semicircular canals : the vertical of both anterior and posterior, and the horizontal of the horizontal semicircular canal. The horizontal pocket forms at about the middle of the external surface on the fifth day; immediately above it is a roughly triangular, pear-shaped depression in the wall of the otocyst, bounded by the vertical pocket on the other tw^o sides. Thus the vertical pocket consists of two divisions, anterior and posterior, meeting at the apex of the otocyst (Fig. 169) « 

The pockets gradually deepen; and the semicircular canals arise from them by the fusion of the walls of the central part of each pocket, thus occluding the lumen except at the periphery (Fig. 170). The fused areas subsequently break through. The peripheries thus form semicircular tubes communicating at each end with the remainder of the superior portion of the otocyst, or ntriculus, as it may now be called. Three semicircular canals are thus formed, one from each division of the original vertical pocket and one from the horizontal pocket. The upper ends of the anterior and posterior semicircular canals, formed from the anterior and posterior divisions of the vertical pocket, open together into


Fig. 169. — Model of the auditory labyrinth (otocyst) of a chick embryo of undetermined age ; view from behind. (After Rothig and Brugsch.)

C. 1., Pocket for the formation of the lateral (horizontal) semicircular canal.

C. v., pocket for formation of vertical semicircular canals.

D. C, PrimonHum of ductus cochlearis and lagena. D. e., endolymphatic duct.


292


THE DEVELOPMENT OF THE CHICK



the apex of the utricukis; and the horizontal canal formed from the external pocket extends between the separated lower ends of the other two.

We must now proceed to a more detailed examination. In

point of time the anterior (sagittal) semicircular canal is the first to be formed (Fig. 171) ; the external (horizontal or lateral) canal comes next, and considerably later the posterior (frontal). Thus the anterior canal is at first the largest, the external next, and the posterior the smallest. These differences are, however, largely compensated in the course of the embryonic development. The ampullae appear as dilations in the pockets even before the canals are Pig. 170. — Model of the auditory formed, and are conspicuous dilalabyrinth of a chick embryo of 6 tions by the time that the central days and 17 hours; external view, parts of the pockets have broken (After Rothig and Brugsch.) throuo'h (Fig. 172).

C. a., Pocket for formation of „. ^^^ ^^„ , ,, _^^u^+^

anterior semicircular canal. C.I., FlgS. 1/0-1/3 show the pocketS

Pocket for formation of lateral ^Tid canals at six days seventeen semicircular canal. C. p., Pocket , , _+^^„ u^tt,.^

for formation of posteriir semicir- hours, seven days seventeen houis,

cular canal. D. c, Ductus coch- eight days seventeen hours, and

learis. D. e., Endolymphatic duct. . ^ , ■> ,^ j La., LagenL eleven days seventeen hours. It

will be seen that, whereas the anterior and lateral canals are formed from the start in planes at right angles to one another, viz., the sagittal and horizontal, the posterior canal is not at first in the third or transverse plane, but gradually assumes it.

The form of the utriculus is gradually assumed during the formation of the semicircular canals; it becomes drawn out into a roughly triradiate form, so that it consists of a central cavity and three sinuses, viz., the median sinus which receives the end of the anterior and posterior semicircular canals, the posterior sinus situated above the ampulla of the external semicircular canal, and the anterior sinus in the region of the ampullae of the horizontal and sagittal semicircular canals (cf. Fig. 173). A short distance in front of the posterior sinus on the median face of


ORGANS OF SPECIAL SENSE


293


the utriculus occur the openings of the ductus endolymphaticus, sacculus, and ductus cochlearis; the two latter derived from the pars inferior of the otocyst, to the development of which we now turn.

(c) Development of the Pars Inferior Lahyrinthi; Lagena, Ductus Cochlearis, and Sacculus. During the changes described in the pars superior labyrinthi, the pars inferior has developed into the ductus cochlearis and lagena on the one hand, and the sacculus on the other. Throughout the series of the vertebrates



Fig. 171. — Model of the auditory labyrinth of the left side of a chick of 7 days and 17 hours. A. Median view. B. External view. (After Rothig and Brugsch.) A. a., Ampulla of the anterior semicircular canal. A. p., Ampulla

of the posterior semicircular canal. C. a., Anterior semicircular

canal. C. 1., Pocket for formation of the lateral semicircular canal.

C. p., Pocket for formation of the posterior semicircular canal. Sa.,

Sacculus. Other abbreviations as before.

the structure of the pars superior is very uniform; the pars inferior, on the other hand, has a characteristic structure in each class and exhibits in general a progressive evolution. The condition in the chick is characteristic on the whole for the class of birds. At six days the lower division of the otocyst has grown out ventralward into a deep pouch which is curved posteriorly and towards the middle line (Fig. 170); the terminal portion is the nicUment of the lagena, and the intermediate portion of the ductus cochlearis; the tip of the lagena in its growth ventralward has reached the horizontal level of the notochord. The sacculus is barely indicated yet, but is clearly seen on the seventh day as a slight protuberance on the median surface of the uppermost part of the pars inferior; it lies in front of the lower end of the endolymphatic duct at a slightly lower level and is separated by two depressions above and below, from the anterior ampulla and the ductus cochlearis respectively. The furrows above the sacculus and below the ampulla of the frontal semicircular canal mark the boundary between the pars superior and inferior.


Fig. 172. — Model of the auditory labyrinth of the

right side of a chick embryo of 8 days and 17

hours ; external view. (After Rothig and Brugsch.)

A. a., Ampulla of the anterior semicircular canal. A. 1., Ampulla of the lateral semicircular canal. A. p., Ampulla of the posterior semicircular canal. C. a., Anterior semicircular canah C. 1., Lateral semicircular canal. C. p., Posterior semicircular canal. Sa. e., Endolymphatic sac. U., Utriculus. Other abbreviations as before.


A day later (Fig. 172), these furrows have cut in deeper and have become continuous on the median surface; the lagena has enlarged distally, and the sacculus is a hemispherical protuberance. The tip of the lagena lies beneath the hind-brain (Fig.


168). The condition shown in Fig. 173, at eleven days seventeen hours is substantially the same as in the adult.

(d) Development of the Auditory Nerve and Sensory Areas of the Labyrinth. During the changes in the form of the labyrinth described in the preceding section, the lining epithelium has become thin and flattened except in eight restricted areas: viz., the three cristce acusticce, one in each of the ampullae of the semicircular canals, the macula utriculi, the macula sacculi, the 'papilla



Fig. 173. — Model of the auditory labyrinth of the right side of a chick embryo of 11 days and 17 hours; external view. (After Rothig and Brugsch.) Abbreviations as before.

lagenoe, the papilla hasilaris and the macula neglecta. Each of these contains sensory cells ending in fine sensory hairs projecting into the endolymph, or fluid of the labyrinth, and receives a branch of the auditory nerve proceeding from the acustic ganglia. Returning to an early stage to follow the development of sensory areas and nerves, we note first that the acustic ganglion from w^hich the auditory nerve arises takes its origin from the acustico facialis ganglion which lies in front of and below the center of the auditory pit. During the closure of the latter, the acustic ganglion becomes fused with part of the wall of the otocyst in such a way that it becomes impossible to tell in ordinary sections where the epithelial cells leave off and the ganglionic cells begin. This fused area may be called the auditory neuro-epithelium. At the 36 somite stage the neuro-epithelium is confined to the lower (ventral) fourth of the otocyst, covering the entire tip, the anterior face, and a small portion of the median face (cf. Fig 98). The neuro-epithelium is the source of all the sensory areas, which arise from it by growth and subdivision. The branching of the auditory nerve follows the subdivision of the neuro-epithelium.

The exact manner in which the changes take place has not been made a subject of special investigation in the chick, so far as the author knows. However, it can be said in general that there is first a partial division of the neuro-epithelium into a pars superior and a pars inferior, and that the former divides into the cristse acusticse (sensory areas of the three ampullae) and the macula utriculi, while the latter furnishes the macula sacculi, papilla basilaris and papilla lagense.

The sensory cells differentiate from the epithelium of the labyrinth, and the nerve fibers from the bipolar neuroblasts of the acustic ganglion, the peripheral process growing into the epithelium and branching between the sensory cells, while the central process grows into the brain.

(e) Bony Labyrinth, Perihjmph, etc. The loose mesenchyme that entirely surrounds the otocyst, differentiates in the course of development into the membrana propria and perilymphatic tissue of the membranous labyrinth, the perilymph and the bony labyrinth in the following manner; on the sixth day a single layer of mesenchyme cells in contact with the cells of the otocyst are arranged with their long axes parallel to the wall, and show already in places a slight fibrous differentiation. These gradually form the membrana propria, which appears on the eighth day as an extremely thin adherent layer with protruding nuclei at intervals. The mesenchyme external to this delicate layer is already differentiated on the sixth day into a perilymphatic and a procartilaginous zone; in the former the mesenchyme is of loose consistency, and in the latter zone it has become dense as a precursor to chondrification. The distinction between the perilymphatic and cartilaginous zones is most distinct (on the sixth day) on the median surface of the ductus cochlearis and lagena. The differentiation proceeds rapidly, however, and on the eighth day the entire membranous labyrinth is surrounded by a mass of embryonic cartilage, the foundation of the bony labyrinth, excepting around the endolymphatic duct (Fig. 168). Between the bony and membranous labyrinths is a thick layer of perih'mphatic tissue composed of very loose-meshed mesenchyme, which in the course of the subsequent development breaks down to form the perilymphatic space. Portions of the perilymphatic tissue, however, remain attached to the membranous labyrinth and form a support for its blood-vessels and nerves.

The Development of the Tubo-tympanic Cavity, External Auditory Meatus and Tympanum

These structures develop directly or indirectly from the first or hyomandibular visceral cleft and the adjacent wall of the pharynx. In a preceding chapter the early development of this cleft was described; we saw that the pharyngeal pouch forms two connections with the ectoderm, a dorsal one corresponding to a pit-like depression of the ectoderm, and a ventral one corresponding to an ectodermal furrow. The latter connection is soon lost, the ectodermal furrow slowly disappears, and the ventral portion of the pouch flattens out. In the dorsal connection, however, an opening is formed which closes on the fourth day, and the dorsal division of the pouch then frees itself from the ectoderm and expands dorsally and posteriorly until it lies between the otocyst and the ectoderm, still preserving its connection with the pharynx (Fig. 102).

(a) The Tuho-tympanic Space. The dorsal portion of the first visceral pouch forms the lateral part of the tubo-tympanic space, but the greater portion of the latter is derived from the lateral wall of the pharynx itself, immediately adjacent to the entrance into the first visceral pouch; the region concerned extends from near the anterior edge of the second visceral pouch forwards, and ends a short distance in front of the first pouch. The original transverse diameter of the pharynx in this region increases in the course of development, and a frontal partition grows across the pharynx forming a dorsal median chamber into which the two tubo-tympanic cavities open. The median chamber communicates by a longitudinal slit (tubal fissure) in the roof of the pharynx with the oral cavity (Figs. 168 and 175).

The frontal partition in question is a posterior prolongation of the palatine processes of the maxillary arch, and forms as follows: If the head of a four-day chick be halved by a sagittal plane, and the interior of the pharynx and mouth cavity be then viewed by reflected light, an elongated lobe will be seen on the median surface of the mandibular arch and maxillary process (Fig. 174 A). This lobe begins far forward on the median surface of the maxillary process and may be followed posteriorly over the median surface of the mandibular arch to the first visceral pouch, where it ends with a free rounded extremity. The lobe itself is called by Moldenhauer the colliculus palato-pharyngeus; it is bounded above and below by depressions, viz., the sulcus tubo-tympanicus dorsally and the sulcus lingualis ventrally, both of which end behind in the first visceral pouch; anteriorly the ventral furrow disappears at the margin of the mouth, and the dorsal furrow near SeessePs pocket. The maxillary portion of the colliculus palato-pharyngeus corresponds to the palatine processes of mammals; the mandibular portion is peculiar to Saiiropsida.


Fig. 174. — A. Head of a chick embryo of 4 days, halved by median section and viewed from the cut surface. (After Moldenhauer.)

B. Internal view of the pharynx of a pigeon embryo, corresponding in development to a chick of 10 days. (After Moldenhauer.)

Col. 1., Colliculus lingualis. Col. p. p., Colliculus palato-pharyngeus. Cr. i., Crus inferior. Cr. s., Crus superius. Hyp., Hypophysis. Mx., Maxilla. N'ch., Notochord. O. Ph. T., Ostium tubse pharyngae. S. P., Seessell's pocket. 2, 3, 4, Second, third, and fourth visceral arches.


If the interior of the pharynx and oral cavity of a ten-day chick be examined (Fig. 174 B), it will be found that the colliculus has undergone important changes. Its maxillary or anterior division divides in two limbs, crura superior and inferior^ diverging anteriorly and separated by a depression which continues the nasal cavity backward; its free posterior end extends farther backwards than before, and is more elevated. The bounding sulci are both deeper than before. The sulcus tubotympanicus, with which we are specially concerned, now extends on to the median surface of the hyoid arch. Subsequently, the crura superiores of the opposite side meet in the middle line and fuse together; in a similar fashion the posterior ends of the colliculi fuse; thus the sulci tubo-tympanici open into a dorsal chamber common to both, which communicates with the ventral division of the pharynx by a slit remaining between the two fused areas. The crura inferiores also approach one another in the middle line but do not fuse, thus leaving the typical split palate of birds in front of the fused lower ends of the crura superiores. In this way the typical adult condition of the bird's palate is established.

From this description it will be seen that only the most lateral portion of the tubo-tympanic cavity is directly derived from the first visceral pouch. In later stages it is quite impossible to say exactly what part, but it is quite certain that it lies within the tympanic part of the cavity. About the end of the fifth or the beginning of the sixth day the tubo-tympanic canal begins to enlarge distally to form the tympanic cavity proper (cf. Fig. 168); the auditory ossicles (see chapter on skull) are beginning to form just above its dorsal extremity, and as the tympanic cavity enlarges it expands around them, displacing the mesenchyme, and finally meets above the auditory ossicles, so that these appear to lie within it, though as a matter of fact the relation is analogous to that of the entodermal alimentary tube to the body-cavity. The process of inclusion of the auditory ossicles is not, however, concluded until about the twelfth day. The blind end of the tympanic cavity attains a level dorsal to the external auditory meatus. (See below.)


During the seventh and eighth days the enlarging cartilaginous labyrinth presses down on the Eustachian tube and hinders its further enlargement. On the eighth day the tube is a wide but narrow slit which appears crescentic in a sagittal section of the head (Fig. 150).

Some rather obscure details about the formation of the tubo-tympanic canal are mentioned here as suggestions for further work on the subject. On the sixth day almost the entire roof is composed of flattened cells similar to the roof of the pharynx; the floor, however, is lined with a columnar epithelium which extends out to and surrounds the distal extremity; it seems probable that this terminal chamber lined on all sides by columnar epithelium represents the first visceral pouch proper. On the eighth day the cavity of this distal chamber is completely constricted off from the main tympanic cavity, though it is still connected with the latter by a solid rod of cells, which gives unequivocal evidence of its origin. I do not know what becomes of this separated cavity later. (See Fig. 168 X.)

(5) The External Auditory Meatus and the Tympanum. We have already seen that on the ectodermal side there are originally two depressions corresponding to the first visceral pouch, viz., a dorsal round one in which a temporary perforation is formed, and an elongated ventral furrow. Between these is a bridge of tissue within which the external auditory meatus arises as a new depression, first clearly visible on the sixth day, when it is surrounded by four slight elevations, tw^o on the mandibular and t'wo on the hyoid arch. The meatus gradually becomes deeper and tubular, mainly owdng, I think, to the elevation of the surrounding tissue, the bottom of the meatus, or tympanic plate, being held in position by the forming stapes. The meatus is directed in a general median direction Avith a slight slant dorsally and posteriorly, and the tympanic plate is placed obliquely, not opposite the lateral extremity of the tympanic cavity, but ventrally to this (cf. Fig. 168).

Even on the sixth day the position of the head of the stapes may be recognized by the density of the mesenchyme internal to the bottom of the meatus. During the seventh and eighth days the stapes becomes sharply differentiated, and the internal face of the tympanum is established in proportion as the tympanic cavity expands around the cartilage (cf. Fig. 168). Thus the tympanum is faced by ectoderm externally, by entoderm internally, and includes an intermediate mass of mesenchyme, which differentiates by degrees into the proper tympanic substances.


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

Cite this page: Hill, M.A. (2019, October 16) Embryology Book - The development of the chick (1919) 9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_development_of_the_chick_(1919)_9

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