Talk:Book - A Laboratory Manual and Text-book of Embryology 3

From Embryology


Chick embryos may be studied whole and most of the structures identified up to the end of t he second day. The eggs should be opened in normal saline solution at 40° C. With wIsHors <ut around the germinal disc, float the embryo oflf the yolk, and remove the vitelline membrane. Then float the embryo dorsal side up on a glass slide, remove enough of the saline solution to straighten wrinkles, and carefully place over the embryo a circle of tissue paper with opening large enough to leave the germinal disc exposed. Add a few drops of fixative (5 per cent, nitric acid gives good fixation) and float embryo into a covered dish. After fixitig and hardening, stain in Conklin's acid haematoxylin or in acid carmine. Extract surphiM Htain, clear, and mount on slide supporting cover-slip to prevent crushing the embryo. Ai'ld hiematoxylin gives the best results for embryos of the first two days. For a detailed account of embryological technique see Lee*s **Microtomist's Vade Mecum."

In the following descriptions we shall use the terms dorsad and ventrad to indicate "toward the back*' or "toward the belly**; cephalad and craniad to denote "headward"; caudiui to denote "tailward**; laterad to indicate "toward the side**; and mesady "toward the middle line."

Embryos of about Twenty Hours' Incubation

The events of cleavage and the formation of the primary germ layers in birds have been described in an earlier chapter. The appearance on the disclike blastoderm (Fig. 3) of the primitive streak and groove (Fig. 23), and of its cranial extension, the head process (Fig. 25), has likewise received brief treatment (1>. 31).

In a chick embryo of twenty hours' incubation (Fig. 30) the primitive streak is formed as a linear opacity near the posterior border of the germinal disc. Over a somewhat pear-shaped dear area the yolk has been dissolved away from the overlying entinlerm. This area, from its appearance, is termed the area prUuiiita. It is surrounded by the darker and more granular area opaca. Whether or nol the primitive streak represents the fused lips of the blastopore, it is certain that it n^pn^sonts the |>oint of origin for the middle germ layer, the extent of which is indicatcxl by the shade<l area of Fig, 30. It also indicates the future longitudinal axis of the embryo. The mesoderm extends at first more rapidly caudal to the primitive streak, at the cranial end of which appears a shaded thi\ kotung, the primitixr knot or rnkU (of HensenV From the primitive knot it grows cranially, forming aloi\g the midline a thicker layer of tissue, the notechordal plate or head process, which is temporarily united with the entoderm (Fig. 30).

Fig. 30. — Dorsal surface view o( a twenty-hour chick enibr)'o showing primitive streak and extent of mesoderm (after Duval). X 17. The lines A, B, and C indicate the levels of the corresponding sections shon-n in Fig. 31.

Mesodrrm Fio. 31. — Transvtrae sections through the embryonic area 01 a [wenty^nour cnicK. ;n. loa. / the head process; B, through the primitive knot; C, IhrouKh the primiti\-e streak.

A transverse section through the primitive streak at twenty hours (see guide line C, Fig. 30) shows the three germ layers distinct laterally (Fig. 31 C). In the midline. a depression in the ectcxierm is the primitive groove. In this region there is no line of demarcation between ectoderm and mesoderm. A transverse section through the primitive knot (Fig. 31 B; guide line B, Fig. 30) shows the three germ layers intimately fused (cf. Fig. 51). There is a marked proliferation of cells, which are growing cephalad to form the nolochordal plate (bead process) (cf. Fig.

A transverse section through the notochordal plate, just beginning to form at this stage (,Fig, 31 .4; guide lini- .4, Fig. 30>. shows the thickening near the midline which will separate from the lateral mesoderm and form the notochord. It is fused with the entoderm but not with the ectoderm.

.After the notochordal plate becomes prominent at twenty hours the differentiation of the germinal disc is rapid. .\ cu^^-et^ fold, at first in\-oh'ing the ecti.xlerm and entoderm alone, is formed cephalad oi the notochordal process. This is the head fold and is the anlage of the head of the embrvv ' Figs. 25 and 32). The ectixlerm has thickened on each side of the nud-dor<aI tine, forming the ntural \'ids. The s:rw\-e betw«n these is thf Kt-jt/---,' ^fiTc. The ciceure of this grvx>\e » ill t-.'rni the n^ttr^ litbr, the anU*:*- oi the nen-ous s>'stem. The tn.>li.vh>.>n.i is cv^w dmercntiated from the meso^ierm dr.d c^ay Sf seen in the mid4n.;«r-je™-^-i-v.e i^acliuvii:. MA dorsil line :~:\'t;jrfi the eci^.-derm. In the

rees<.x!ern'. i^tcrj.! to the noti.vhoni and " cepc-ilaii to the primitiN^e ccsie, tnins^^ersf ti:rro»> ~i\t vu5erer.:ij:ed tww pairs ji riixk-Like meiMn-na^ -efn^is. one ir.o.'n--t';c:e cra:'.LJ.".:> Xs ce\-eIopm«it ^c:<-i-f'-i :=ese [=crea^ b =w-Sfr, successive poW bei::-: cevcUt>;(.: ciiaiiUy, Tbt-v \C be describe-i ir: ieCoil '.i;^r.

it iinbriiU. ji v'lita iht


gether, they constitute the angioblasi from which arises the extra-embryonic blood \'ascular system. The area pellucida has the form of the sole of a shoe with broad toe directed forward. The head fold has become cylindrical and the head of the embryo is free for a short distance from the germinal disc. The mesoderm extends on each side beyond the head lea\'ing a median clear space, the proamnioiic area. The entoderm is carried forward in the head fold as \i\eJore-gul, from

AMtriot Hturopore Fore-brain

Frtr potlioH of head

Fig. 33,— Dors»l view of a twenty-five- hour chick cmbrjo willi seven primitixt segments. X 20.

I which later arise the pharynx, esophagus, stomach, and a portion of the small intestine. The opening into the fore-gut faces caudad and is xhe fovea cardiaca. The way in which the entoderm is folded up from the germinal disc and forward into the head is shown well in a longitudinal section of an older embryo (Fig. 42), The tubular heart lies ventral to the fore-gut and cranial to the fovea cardiaca. In later stages it is bent to the right. Converging forward to the heart, on each side

I of the fovea, are the vilellinc veins, just making their appearance at this stage.

The lips of the neural folds have met throughout the cranial two-thirds of the iMuhryo but have not fused. The neural iube, formed thus by the closing of the orUHlornial folds, is open at either end at the neuropares. Cephalad, the neural tube has begun to expand to form the brain vesicles. Of these only the fare-brain is i>roniinent» and from it the optic vesicles are budding out laterally. The paraxial mostHlerm is divided by transverse furrows into seven pairs of block-like f^rimih'xr scf^pptcptts. Caudally, between the segments and the primitive streak, ihcro is undilTercntiated mesoderm, but new pairs of segments will develop in this rojjion. LiH)king through the open neural tube (rhomboidal sinus), one may see in iho midline the notochord extending from the primitive node cephalad until it is losl U^nealh the neural tube in the region of the primitive segments. The frimitiir streak is still prominent at the posterior end of the area pellucida, forming about one fiUirth the length of the embryo. Transverse sections through the primitive streak and o|hmi neural groove show approximately the same conditions ;^> in the twenty hour embryo (Figs. iQ and 31).

k Transverse Section through the Fifth Primitive Segment (Fig. 34) is characterized l^\ iho tlilWixMUiation of the incsiHlorm. the appn)ximation of the neural folds and the presence ot t\x\> \\^s>ci?*» ihr i/iw. * »;./i»ijc .h'r/ir. on each side Inn ween the mesodermal segments and the c»\t\HUMtu. rho ncuirti l\^lds an* thick aiul the einiHlorm is thickened over the embryo. The

The neural tube at this level forms the third brain vesicle or hind-brain. The neural folds have not yet fused and at their dorsal angles are the neural crestSj the anlages of the spinal ganglia. Mesodermal segments do not develop in this region; instead a diffuse network of mesoderm partly fills the space between ectoderm, entoderm, and mesothelium. This is termed mesenchyme and will be described later.

Transverse Section through the Fovea Cardiaca (Fig. 36). — This section passes through a vertical fold of entoderm at the point where the latter is reflexed into the head as the fore-gut (cf. Fig. 42). The entoderm forms a continuous mass of tissue between the vitelline veins thereby closing the fore-gut ventrally. The splanchnic mesoderm is differentiated into a thick- walled pouch on each side, lateral to the endothelial layer of the veins.

Fig. 35. — Transverse section caudal to the fovea cardiaca of a twenty-five-hour chick embryo. X 90.

Transverse Section through the Heart (Fig. 37). — Passing cephalad in the series of sections the vitelline veins open into the heart just in front of the fovea cardiaca. The entoderm in the head fold now forms the crescentic pharynx ox fore-gut, separated by the heart and splanchnic mesothelium from the entoderm of the germinal disc. The descending aortae are larger, forming conspicuous spaces between the neural tube (hind-brain) and the pharynx. The heart, as will be seen, is formed by the union of two endothelial tubes , similar 10 those constituting the vitelline veins in the preceding sections. The median walls of these tubes disappear at a slightly later stage to form a single tube, the endocardium. Thickened layers of splanchnic mesoderm, which, in the preceding section, invested the vitelline veins laterally, now form the mesothelial wall of the heart. In the median ventral line, the layers of splanchnic mesoderm of each side have fused and separated from the splanchnic mesothelium of the germinal disc; thus the two pleuro-pericardia) cavities are put in communication. The mesothelial wall of the heart forms the myocardium and epicardittm of the adult. Dorsally, the splanchnic mesoderm, as the dorsal mesocardium, suspends the heart, while still more dorsally it is continuous with the somatic mesoderm.

Fig. 36. — Transverse section through the fovea cardiaca of a twenty-five-hour chick embryo. X 90.

Origin of Primitive Heart

From the two sections last described, it is seen that the heart arises as a pair of endothelial tubes lying in the pockets of the splanchnic mesoderm. Later, the endothelial tubes fuse to form a single tube. The heart then consists of an endothelial tube within a thick-walled tube of mesoderm. The origin of the endothelial cells of the heart — whether they arise from entoderm or mesoderm— is not surely known. The vascular system is prinutively a paired system, the heart arising as a double tube with two veins entering and two arteries leaving it.

Origin of the Blood Vessels and Blood

We have seen that in the area opaca a network of blood vessels and blood islands is differentiated as the angtoWast. This tissue gives rise to primitive blood vessels and blood cells and probably b derived from the splanchnic mesoderm. The vessels arise first as reticular masses of celb, the so-called Mood islands. These cellular thickenings undergo differentiation into two cell types, the innermost becoming blood ceils, the outermost forming a flattened endolhelial layer which encloses the blood cells. 11 the primitive blood vessels of the embryo are composed of an endolkdial layer only. The endothelial cells continue to divide, forming vascular sprouts and in this way new vessels are in part produced. The first vessels arising in the vascular area of a chick embryo unite into a close network, some of the branches of which enlarge to form vascular trunks. One pair of such trunks, the vitelline veins, is differentiated adjacent to the posterior end of the heart and later connects with it. Another pair, the vitelline arteries, are developed in continuation with the aorlsc of ihe embryo. The vessels of the vascular area thus appear before those of the embryo have developed; they probably arise from the splanchnic mesoderm, and, both arteries and veins, are composed of a simple endothelial wall. As the ccelom develops in the region of the vascular area of the embryo soon after the difTerentiation of the angioblast, (he anlages of the blood ves.<'els arc formed only in the splanchnic layer. (For the development of the heart and blood vessels see Chapter IX.)

Fig. . — Transverse section through the heart of a twenty-five-hour chick embryo. X 90

Transverse Section through the Pharyngeal Membrane (Fig. 38).— This section passes through the head fold and shows the head free from the underlying germinal disc (cf. Fig. 42). The ectoderm surrounds the head and near the mid-ventral line it is bent dorsad, is somewhat thickened, and comes in contact with the thick entoderm of the pharynx. The area of contact between ectoderm and pharvngeal entoderm forms the pharyngeal plate oi membrane. Later, this membrane breaks through and thus the oral cavity arises. The expanded neural tube is closed in this region and fonns the middle brain vesicle or mid-brain. The descending aortx appear as small vessels dorsal to the lateral folds of the pharynx. The blastoderm in the region beneath the head is composed of ectoderm and entoderm only. This is the proamniolic area. Laterad may be seen the layers of the mesoderm.

Fic. 38. — Transverse section through the pharyngeal membrane of a twenty-five-hour chick embryo

Transverse Section through the Fore-brsin and Optic Vesicle (Fig. 39).— The neural tube is open here and constitutes the first brain vesicle or fore-brain. The opening is ir neuropore. The ectoderm Is composed o( two or three layers of nuclei and is cons with the much thicker wall of the fore-brain. The lateral expansions of the forebrain are the opik vesicles, which eventually give rise to the retina of the eye. The two ectodermal layers are in contact with each other except in the mid-ventral region, where the mescnchyma is beginning to penetrate between and separate them. The proamnion c< merely of a layer of ectoderm and of entoderm.

Fig. -W.^Transvi through the fore-brain and optic vesicles of a twenly-fi\e-hour chick. X 90.

Chick Embryo at Seventeen Primitive Segments (Thirty-Eight Hours)

The long axis of this embryo is nearly straight {Fig. 40), the area pellucida is dumb-bell shaped and the vascular network is well differentiated throughout the area opaca. The tubular heart is bent to the embryo's right, and opposite its posterior end the vascular network converges and becomes continuous with the trunks of the vitelline veins. Coimections have also been formed between the descending aorUe and the vascular area, but as yet the vitelline arteries have not appeared as distinct trunks. The proamniotic area is reduced to a small region in front of the head, which latter is now larger and more prominent. In the piistcrior third of the vascular area blood islands are still prominent.

Central Nervous System and Sense 0^;an8. — The neural tube is closed save at the caudal end where the open neural folds form the rhomboidal sinus. In the lu-ail the neural tube is dilTercntiated into the three brain vesicles, marked off frmn each nther by constrictions. The fore-hrain (prosencephalon) is characU'lizt-d by the outttrowing optic \esicies. The mid-brain (mesencephalon) is uinlilTrn-tiliattxl. The hind-briiiti (rhonil>eiicephalon') is elongated and gradually merp's caudally with the spinal ami, II shows a number of secondary constriclions, the nriiromrrcs. The ectmlerni is thickened laterally over the optic vesicles to form the lens placode of the eye (Fig. 43). The optic veacle is flattened at this point and will soon invaginate to produce the inner, nervous layer of the retina. In the hind-brain region, dorso-laterally the ectoderm is thickened and invaginated as the auditory placode (Fig. 45). This placode later forms the

Fig. 40.— View of the dorsal surface o( a thirty-eight-hour chick embrjo. X 20.

Fic. 41.— VenUal otocysl or otic vesicle from which is differentiated the epithelium of the internal ear (membranous labyrinth).

Digestive Tube

The entoderm is still flattened out over the surface of the yolk caudal to the fovea cardiacs. In Fig. 41 the greater part of the entoderm is cut away. The flattened fore-gut, folded inward at the fovea, shows indications of three lateral diverticula, the pharyngeal pouches. ' Cephalad the pharynx is closed ventrally by the pharyngeal membrane.

Heart and Blood Vessels

After receiving the vitelline veins cephalad to the fovea cardiaca the double-walled tube of the heart dilates and bends ventrad and to the embryo's right (Fig. 41). It then is flexed dorsad and to the median line, and narrows to form the ventral aorta. The aorta lies ventrad to the pharynx and divides at the boundary line between the mid- and hind-brain into two ventral aortce. These diverge and course dorsad around the pharynx. Before reaching the optic vesicles they bend sharply dorsad and caudad, and, as the paired descending aortce, may be traced to a point opposite the last primitive segments. In the region of the fovea cardiaca they lie close together and have fused to form a single vessel, the dorsal aorta. They soon separ;ate and opposite the last primitive segments they are connected by numerous capillaries with the vascular network. In this region at a later stage the trunks of the paired vitelline arteries will be differentiated. The heart beats at this stage; the blood flows from the vascular area by way of the vitelline veins to the heart, thence by the aortae and vitelline arteries back again. This constitutes the vitelline circulation and through it the embryo receives nutriment from the yolk for its future development.

Fig.. 42.— A median longitudinal section of the head of a thirty-eight-hour chick embryo. X about 50.

In studying transverse sections of the embryo it is not sufficient merely to identify the structures seen. The student should determine also the exact level of each section with respect to Figs. 40, 41 and 42, and trace the organs from section to section in the series. It is important to remember that the transverse sectii>ns figured and describeil in this manual (except those of the fifty-hour chick) arc all drawn viewed from the cephalic surface; hence the right side of the embryo is at the reader's left.

Transverse Section through the Fore-brain and Optic Vencles (Fig. 43), — The optic stalks coanect the epiic vesicles laterally with the ventral portion of the fore-brain. Dorsally the sectioa passes through the mid-brain due to the somewhat ventrally flexed head (d. Fig. 42). We have alluded to the thickening of the lens pituode. Note that there is now . a considerable amount of mesenchyme between the ectoderm and the neural tube. Layers of mesoderm are present in the underlying blastoderm.

Fig. 43. — Transverse through the fore-brain of a thirty-eight-hour chick embiyo. X 75.

—Splanckiupkure Fig. 44. — Transverse gectioD through the pharyngeal membrane of a thirty-eight-hour chick embryo.

Transverae Section through the Pharyngeal Membrane and Hid-brain (Fig. 44). — In the mid-ventral line the thickened ectoderm bends up into contact with the entoderm of the rounded pharynx of the fore-gut. At this point the oral opening wiU break through.

On either side of the pharynx a pair of large vessek are seen; the ventral pair are the ventral aorta. Two sections cephalad their cavities open into those of the dorsal pair, the descending aorlee. The section is thus just caudad of the point where the ventral aorts bend dorsad and caudad to fonn the descending aorte. The section passes through the caudal end of the

Fig. 45. — Trausveise section through the hind-braia and auditor)' placodes of a thirty-right-boui chick embrjo. X 75,

ih an oval cavity. Note the large amount of The structure of the blastoderm is complicated

mesencephalon which is here thick walled undifferentiated mescnchynie in the sectioi by the presence of collapsed blood vcsseb.

Transverse Section through the Hind-brain and Auditory Placodes (Fig. 45).— Besides the auditory placodes already described as the anlages of the internal ear, this sec

Fic. 46. — Tnnsi-erse SpAuhAnif memiirm Ilcdrl Myociirdiiim a through the caudal rnd of the heart of a thirty-eight -hour chick embryo.

tion is characterized by (1) the large hind-braitt. somewhat flattened dorsad: (31 the broad dorso- vent rally flattened pharynx, above which on each side lie the dfscrnJhig .i.vta: (3) the presence of the bulbar andtentricuiar portions of the hean. The bulbuf^ is suspended dorsally by the mesodenn, which here forms the dorsal mesocardium. The ventricle lies on the right side of the embryo; a few sections caudad in the series it is continuous with the ventral aorta (cf. Fig. 41). Between the somatic and splanchnic mesoderm is the large pericardial cavity. It surrounds the heart in this section. Dorsal to the aorta; are the anterior cardinal veins, which return blood from the head region.

Transverse Section through the Caudal End of the Heart (Fig. 46).— The section passes through the hind-brain. The descending aorta are separated only by a thin septum which is ruptured in this section. The anterior cardinal veins are cut at the levei where they bend ventrad to enter the heart. The mesothclial wall of the heart is continuous with the splanchnic mesoderm. On the right side of the section there is apparent fusion between the myocardium of the heart and the somatic mesoderm. A pair of primitive mesodermal segments may be seen in this section lateral to (he hind. brain. It may be noted here that the primitive segments were not present in the sections of the head previously studied.

Transverse Section through the Fovea Cardiaca (Fig. 47).— The descending aorta now form a single vessel, the dorsal aorta, the medium septum having disappeared. The section passes through the entoderm at the point where it is folded dorsad and cephalad into the head as Xhc fore-gut (cf. Fig. 42). Two sections caudad is found the opening {fovea cardiaca) where the fore-gut communicates with the flattened open gut between the entoderm and the yolk. On each side of the fore-gut are the large vitelline veins, sectioned obliquely. As the splanchnic mesoderm overlies these veins dorsad, it is pressed by them on each side against the somatic mesoderm and the cavity of the ccelom is thus interrupted.

^ l-'.ilra-cmbryottic tirlom ^ linSadrrm ^Lrjt vildline rein iMi-aodr-rm F.iilndcrm t Fic. 47.— Tran.s\-etse section through the fovea cardiaca of a thirty-eight-hour chick embr)o. X 90.

Transverse Section Caudal to the Fovea Cardiaca (Fig. 48).— This section resembles the preceding save that the primitive gut is without a ventral wall. The right viteUine vein is stiU large.

Section through the Fourteenth Pair of Primitive Segments (Fig. 49)— The body of the embryo b now flattened on the surface of the yolk. Here the descending aorte are still separate and occupy the depressions lateral to the primitive segments. The section is characterized by the notochord and the differentiated mesoderm which forms the primitive segments, nephrotomes, somatic and splanchnic mesoderm, structures soon to be described. Arising from the nepbiotomes are sprout-like pronephric tubules. The tips of these hollow out and unite to form the primary excretory or mesonepkric duct.

Transverse Section through the Rhomboidal Sinus (Fig. 50).— The neural groove b open, the notockord is ova) in form. The ectoderm is characterized by the columnar form of its cells. At the point where the ectoderm joins the neural fold a ridge of cells projects ventrally on either side. These projecting cells form the neural cresls. and from them the spinal ganglia are formed. The mesodermal plates have split laterally into layers, but the ccelomic cavities are mere slits. Between the splanchnic mesoderm and the entoderm blood vessels may be seen.

Fig. 48. — Transverse section caudal to the fovea cardiaca of a thirty-eight-hour chick embryo. X 90.

Fig. 49, — Ttaasverse section through tlte fourteenth pair of mesodermal segments of a thirty-tight-hour chick embryo. X 90.

Fig. 50. — Transverse section through the rhomboidal sinus of a thirty-eight -hour chick embryo. X 90.

Transverse Section through the Primitive (Hensen's) Knot or Node (Fig. 51). The section shows the three germ layers fused inseparably at the '^knot into a mass of undifferentiated tissue. The mesoderm is split laterally into the somatic and splanchnic layers.

Fig. 51. — Transverse section through the primitive (Hensen's) knot of a thirty-eight-hour chick embryo. X90.

Transverse Section through the Primitive Streak (Fig. 52). — In the mid-dorsal line is the primitive groove. The germ layers may be seen taking their origin from the undifferentiated tissue of the primitive streak beneath the primitive groove. Between the splanchnic mesoderm and entoderm blood vessels are present laterad as in the preceding sections.

Entoderm Fig. 52. — Transverse section through the primitive streak of a thirty-eight-hour chick embryo. X 90.

Mesodermal Segments

We have seen that these are developed by the appearance of transverse furrows in the mesoderm (Fig. 53). Later a longitudinal furrow partially separates the paired segments from the lateral unsegmented mesoderm. The segments are block-like with rounded angles when viewed dorsally, triangular in transverse sections (Figs. 49 and 53). They are formed cranio- caudally, the most cephalad being the first to appear. The first four lie in the head region. The segments contain no definite cavity but a potential cavity representing a portion of the coelom is filled with cells, and the other cells of the segments form a thick mesothelial layer about them (Fig. 49). The ventral wall and a portion of the median wall of each primitive segment become transformed into mesenchyma which surrounds the neural tube and notochord (Fig. 290). The remaining portion of the segments persist as the dermo-muscular plates. The cells of the mesial portions of the plates, the myotomes, elongate and give rise to the voluntary muscle of the body. The voluntary or skeletal muscles are thus at first all segmented but later many of the segments fuse. In the trunk muscles of the adult fish the primitive segmented condition is retained.

The Intermediate Cell HasBes or Nephrotomes

The bridge of cells connecting the primitive segments with the lateral mesodermal layers constitutes the nephrolome (Figs. 49 and 55). In the chick the nephrotomes of the fifth to sixteenth segments give rise dorsad to pairs of small cellular sprouts, the rudimentary kidney tubules of the pronepkroi, segmentally arranged in the furrow lateral to the primitive segments. By the union of these cell masses distally solid cords are formed which run lengthwise in the furrow. These cords hollow out, grow caudad, and become the primary excretory (mesonephric) ducis (Fig. 53). More caudally the intermediate cell masses form the embryonic kidney or mesonephros, the tubules of which open into the primary excretory duct. Further details concerning these provisional kidneys are given on pages 195-199. Since the genital glands develop in connection with the mesonephros, and the kidney of the adult (metanephros) is partly developed as an outgrowth of the primary excretory duct, the intermediate cell mass may be regarded as the anlage of Ihe urogenilal glands and their ducts. These structures are thus of mesodermal origin.

Fig. 53. — Semi-diagrammalic reconstruction of five mesodermal segments of a forty-eight-bour chick embryo. The ectoderm is removed from the dorsal surface of the embryo.

Somatopleure and Splanchnopleure

In the embryo of seven primitive segments the mesoderm was seen to split laterally into two layers, the somatic (dorsal) and the splanchnic (ventral) mesoderm (Fig. 34). These layers persist in the adult, the somatic mesoderm giving rise to the pericardium of the heart, to the parietal pleura of the thorax and to the peritoneum of the abdomen, while the splanchnic layer forms the epicardium and myocardium of the heart, the visceral pleura of the lungs, and the mesenteries and mesodermal layer of the gut. The somatic mesoderm and the ectoderm, with the tissue developed between them, constitute the body wall, which is termed the somatopleure. In the same way the splanchnic mesoderm and the entoderm, with the mesenchjinal tissue between them, constitute the wall of the gut, or the splanchnopleure.


The cavity between the somatopleure and splanchnopleure is the ctElom (body cavity). With the splitting of the mesoderm, isolated cavities are produced. These unite on each side and eventually form one cavity — the coelom. With the extension of the mesoderm, the ccelom surrounds the heart and gut ventrally (Fig. 54). Later, it is subdivided into the pericardial cavity about the heart, the pleural cavity of the thorax, and the peritoneal cavity of the abdominal region. In the stages already studied, the embryo was flattened on the surface of the yolk and the somatopleure and splanchnopleure did not meet ventrally. If this union occurred they would conform to the structural relations shown in Fig. 54, which is essentially the ground plan of the vertebrate body.


In the sections through the head of this embryo, and through that of the preceding stage, but four primitive segments were found. The greater part of the mesoderm in the head appears in the form of an undifferentiated network of cells which fill in the spaces between the definite layers (epithelial. This tissue is mesetKhyme (Fig. 55). The mesoderm may be largely converted into mesenchyme, as in the head, or any of the mesodermal layers may contribute to its formation. Thus it may be derived from the primitive segments and from the somatic and splanchnic mesoderm. The ceUs of the mesenchyme form a syncytium or network, and are at first packed closely together. Later, they may form a more open network with cytoplasmic processes extending from cell to cell (Fig. 55). The mesenchyme is an important tissue of the embryo; from it are differentiated the blood and lymphatic systems, together with most of the smooth muscle, connective tissue, and skeletal tissue of the body.

Fig. 54 — Diagrammatic transverse section of a vertebrate embryo (adapted (rom Minot).

The body of the embryo is now composed (1) of cells arranged in layers — epitkelia, and (2) of diffuse mesenchyme. The term "epithelium" may be used in a general sense, or restricted to layers covering the surface of the body or lining the digestive canal and its derivatives. Layers lining the body cavities are termed mesolkelia, while those lining the blood vessels and heart are called endothelia.

Fig. 55. — Mesenchyme from the head of a thirty-cight-hour chick embryo. X 495.


Derivatives of the Germ Layers

The tissues of the adult are derived from the cpithelia and mesenchyme of the three germ layers as follows:


1. Epidermis and lis derivatii'cs (hair, nails, Klands). 1. Conjunctiva and lens of eye. J. Sensory cpithelia of organs of

4. Kpilhclium of moulh, enamel of teeth, oral glands. Hypo++++5. ICpithclium of anus.

6. Mole urethra (dislad).

7. LCpilhelium of amnion and chorion. H. Nervous, neuioitlia. and chn>malVm cells of nervous sysleni. Retina and optic ner\-e. <J, Not.«-li.>r,l {.}).

iiimi li muscle of sweat glands nd of iris.


A. Mesothelium. \. Pericardium.

2. Pleura.

3. Peritoneum.

4. Serouslayerofintestine.

5. Epithelium of most of urogeni++++tal organs.

6. Striated muscle.

1. Skeletal.

2. Canliac.

B. Mesenchyme. 1. Blood cells.

3. Kndothelium of blood vessels.

4. Endothelium of lymphatics. ."i. Spleen and lymphoid organs.

6. Su|)piirtin([ tissues. (Connect++++in): lissue, cartilage, and bone.)

7. Smooth muscle.


1. Epithelium of digestive tract.

3. Pancreas.

4. Epithelium of pharynx.

Eustachian tube. Tonsils. Thymus, Thyreoids. PaiB thyreoids.

5. Epithelium of rt^iiratoiy

6. Epithelium of most of bladder, of female urethra, male prostatic urethra and prostate.

7. Notochoid (?).

For the histological dovolopment thistogeneas) of the various tissues from tilt' primary norm layt-rs sco Chapter X.

Chick Embryo of Twenty-Seven Segments (Fifty Hours)

This embryo, of nearly fifty hours' incubation, lies in the center of the vascular area and is peculiar in that the head is twisted 90' to the right. In a dorsal view, therefore, one sees the right side of the head but the dorsal side of the body. In the region of the tnid-brain is a very marked bend, the cephalic flexure. Below the head, and ventral in position, hes the tubular heart, now bent in the form of a letter S - Dorsal to the heart, in the region of the pharynx, three transverse grooves or slits may be seen. These are the branchial clefts or gill slits. The head of the embryo is now covered by a double fold of the somatoplcure, the head fold of the amnion. It envelops the head like a veil. Caudally, a fold and opacity mark the position of the lail bud from which develops the caudal end of the body. The curved fold embracing this is the tail fold of the amnion, which will eventually meet the head fold and completely envelop the embryo.

Fig. 56.— Dond view o( a fifty-hour chick embryo, stained and mounted in balsam.

Central Nervous System and Sense Organs {Fig. 57). — Cephalad, the neural tube is divided by constrictions into four vesicles. The fore-brain of the previous stage is now subdivided into two regions, the telencephalon and diencephahn. The cephalic flexure has been established in the region of the mesencephalon. The hind-brain, as yet undivided, equals the combined length of the other three vesicles. The lens of the eye has invaginatcd, pushing in the wall of the optic vesicle and thus forming a double-walled structure, the optic cup. The auditory placcHlc has Iwcomc a sac, the olorysl, which overlies the hind-brain opposite the second branchial groove and is still connected with the outer ectoderm, cut away in Fig. 57. The rhomboi<lal sinus is still open at the caudal end of the neural tube.

Fig, 57. — Semi -diagrammatic reconstruction of a fifty-hour chick embryo, in ventral view. X 18. The cnlmlcnn has been removed save in the regioa of the fovea cardiacs and of the hind-gut. Owing to the torsion of the embryo, the cranial third of the embryo >s seen from the left side, the caud&l twothirds in ventral view.

Digestive Canal (Fig. 57).— In a reconstruction from the ventral side, the digestive canal shows differentiation into three regions. Of these, the fore-gut has been seen in earlier stages. A greater part of the mid-gut has been cut away to show the underlying structures; it is without a ventral wall and overlies the yolk. Caudad, a small fovea leads into the Innd-gut which is just beginning to evaginate into the tail fold. The pharyngeal membrane now lies in a considerable cavity, the stomodcBuniy formed by the invaginated ectoderm. The median ectodermal pouch next the brain wall is known as Rathke's pocket and is the anlage of the anterior lobe of the hypophysis. The pharynx shows laterally three outpocketings, of which the first is wing-like and is the largest. These pharyngeal pouches occur opposite the three branchial grooves and here entoderm and ectoderm are in contact, forming the closing plates. At about this stage the first closing plate ruptures, thereby forming a free opening, or branchial cleft, into the pharynx. Between the pouches are developed the branchial arches, in which course the paired cuyrtic arches. Towards the fovea cardiaca the fore-gut is flattened laterally and before it opens out into the mid-gut there is budded off ventrally a bilobed structure, the anlage of the liver (Figs. 57 and 63). It lies between the vitelline veins and in its later development the veins are broken up into the sinusoids or blood spaces of the liver.

Just as the entoderm participates in the head fold to form the fore-gut so in the tail fold it forms the hind-gut. This at once gives rise to a tubular outgrowth which becomes the aUantois, one of the fetal membranes to be described later (Fig. 70).

Blood Vascular System

The tubular heart is flexed in the form of a letter S when seen from the ventral side. Four regions may be distinguished: (1) the sinus venosus, into which open the veins; (2) a dilated dorsal chamber, the atrium; (3) a tubular ventral portion flexed in the form of a U, of which the left limb is the ventricle, the right limb (4) the bulbus cordis. From the bulbus is given off the ventral aorta. There arc now developed three pairs of aortic arches which open into the paired descending aortae. The first aortic arch passes cranial to the first pharyngeal pouch and is the primitive arch seen in the thirtysix-hour embryo. The second and third arches course on either side of the second pharyngeal pouch. They are developed by the enlargement of channels in primitive capillary networks between ventral and descending aortae. Opposite the sinus venosus the paired aortic tnmks fuse to form the single dorsal aorta which extends as far back as the fifteenth pair of primitive segments. At this point the aortae again separate, and, opposite the twentieth segments, each connects with the trunk of a vitelline artery which was developed in the vascular area and conveys the blood to it (Fig. 57). Caudal to the vitelline arteries the dorsal aorUe rapidly decrease in size and soon end.

As in the previous stage, the blood is conveyed from the vascular area to the heart by the vitelline veins, now two large trunks. In the body of the embryo there have developed two pairs of veins. In the head have appeared the anterior cardinal veins, already of large size and lying lateral to the ventral region of the the more important structures. The approximate plane and level of each section may be ascertained by referring to Figs. 56 and 57.

Transverse Sections Section through the Fore-bnin and Eyes (Fig. 58).— Tbe section passes cranial to the optic stalks, consequently the optic vesicles appear unconnected with the fore-brain. The thickened ectoderm is invaginated to form the aniages of the lens vesicles. The thicker waH oi the optic vesicles Dtxt Ihe Unsanlage will give rise to the nervous layer of the retina, the thinner outer wall becomes the pigment layer of tbe retina. Ventrad in the section are the waU and cavity of the /ore-brain, dorsad the hind-brain with its thin, dorsal ependymal layer.

Fig.— Transveise section through the optic statics and hypophysis of a fifty-hour chick embryo. X50.

Between the brain vesicles on either side are sections of the first aortic arches and bteral to the hind-brain are the smaller paired anterior cardinal veins, which convey the blood from the bead to the heart.

Section through the Optic Stalks and Hypophysis (Fig. 59).— The section passes just caudal to the lens which does not show. The optic vesicles are connected with the wall of tbt fore-brain by the optic stalks which later form the path by which the fibers of the oplic nerve pass from the retina to the brain. Both the ventral and the descending aorta are seen in section about the cephalad end of the pharynx. Between the ventral wall of the fore-brain mnd the pharynx is an invagination of the ectoderm, Rathke's pocket (anterior lobe of hypophysis).

Section tlirough the Otocysts and Second Aortic Arch (Fig. 60). — The otic vesktes arc sectioned caudal to their apertures and appear as closed sacs lateral to the wall of the hind-brain. The cavity of the pharynx is somewhat triangular and its dorsal wall is thin. The anterior cardinal veins pass between the otocysts and the waU of the hind-brain. Ventral to the pharynx the buibus cordis is sectioned obliquely where it leaves the heart, and at this level gives off laterad the second pair of aortic arches which connect dorsad with the descending aortx. Surrounding the buibus cordis is the large pericardial cavity. The student should note that in the sections of this stage so far studied, the mesenchyme of the head is undifferentiated, the tissues peculiar to the adult not yet having been formed.

Fig. 60. — Transverse n through the otic vesicles and second a embiyo. X 50.

Mfsodtrm of buibus Endolhiiium of bidbus arches of a fifty-hour chick

Section through the Second Pharyngeal Pouches and Thyreoid Anlage (Fig. 61). fi this section is taken at a level between the second and third aortic arches, the descending aorta; and heart are unconnected. Tangential shavings have been cut from the walls of the olocysls. Extending laterally from the pharynx arc the second pair of pharyngeal pouches which have already come in contact with the ectoderm to form closing plates. A pocket-like depression in the mid-ventral floor of the pharynx represents the thyreoid anlage; later it becomes saccular and loses its connection with the pharyngeal entoderm. The splanchnic mesodermal wall of the heart is destined to give rise later to the cpi- and myocardium.

Section through the Sinus Venosus and Common Cardinal Veins (Fig. 62).— At this level, the common trunk formed by the anterior and posterior cardinal veins opens into the thin-walled sinus venosus. The sinus receives all of the blood passing to the heart and is separan-d only by a slight constriction from the larger atrium. The muscle plates of the first mesodermal segments are seen, and the descending aortae have united to form a single dorsal vessel. On either side of the pharynx are subdivisions of the cceIodi which will form the pleural cavities. These cavities are separated from the pericardial cavity by the

Epi -myocardium of biilbut F.ndolheliuin of bulbus

Fig. 61. — Tranivetse section through the second pharyngeal pouches and thyreoid anlage of a fifty*hour chick embryo. X 50.

Spinal cord itttodermol setmenl septum transversHut (anUgc of diaphragm) in which the common cardinal veins cross to the sinus venosus.

The somatopleuric folds of the amnion envelop the right side of the embryo and the ectoderm of these folds now forms the outer layer of the chorion and the inner layer of the amnion. The mesodermal components of these folds have not united.

Section through the Anlage of the Liver (Fig. 63). — In this section the cavity of the fore-gut is narrow, the gut being flattened from side to side. Ventrad there are evaginated from the entoderm two elongate diverticula which form the anlages of the liver. On either side of the anlages of the liver are sections of the viUUine veins on their way to the sinus venosus at a higher level in the scries. Note the intimate relation between the entodermal epithelium of the liver and the endothelium of the vitelline veins. In later stages, as the liver anlages branch, there is, as Minot aptly expresses it, "an intercrescence of the entodermal celb constituting the hver and of the vascular endothelium" of the vitelline veins. Thus are formed the hepatic sinusoids of the portal system, which surround the cords of hepatic cells.

Fig. 63. — Transverse section through the anlage o( the liver of a fifty-hour chick embryo.

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

Section through the Cranial Portion of Qie Open Intestine (Fig. 64).— The intestine is now open ventrad, its splanchnopleure passing directly over to that of the vascular area. The folds of the amnion do not join, leaving the amniotic cavity open. The dorsal aorta is divided by a septum into its primitive compronents, the right and left descending aorta. Lateral to the aorta; are the small posterior cardinal veins. The coelom is in communication with the extra-embryonic body cavity.

Section through the Seventeenth Pair of Mesodermal Segments (Fig. 65). — The body of the embryo is now no longer flexed to the right. On the left side of the figure the mesodermal segment shows a dorsolateral muscle plate. The median and ventral portion of the segment is being converted jnlo mesenchyme. On the right side appears a section of the primary excretory or mesonephric duct. The embryonic somatopleure is arched and will form the future ventro-laterai body wall of the embryo. The fold lateral to the arch of the somatopleuTc gives indicatioD of the later approximation of the ventral body walb, by which the embryo is separated from the underlying layeii of the blastoderm.

Fig. 64. — Transverse section through the cranial portioo of the open intestine of a fifty-hour chick embryo. X 5O.

Section through the Origin of the Vitelline Arteries (Fig. 66).— At this level the embryo is more flattened and simpler in structure, the section resembling one through the mid-gut region of a thirty -eight -hour chick (Fig. 49). The amniotic folds have not appeared.

Fig.. 65. — Transverse section through the seventeenth pair of mesodermal segments of a lifly-hour chick embrj'o. X 50.

On the left side of the figure the vitelline arlrry leaves the aorta. On the right side the connection of the vitelline artery with the aorta does not show, as the section is cut somewhat obliquely. The posterior cardinal rein is present just latcrad of the right mesonephric duct. The other structures were described in connection with Fig. 49. Section Caudal to the Mesodermal Segments (Fig. 67).— The mesodermal segments are replaced by the segmetilal zone, a. somewhat triangular mass of undifferentiated mesoderm from which later are formed the segments and nephroloma. The notochord is larger, the aortae smaller, and a few sections caudad they disappear. Laterally the somatopleure and splanchnopUure are straight and separated by the slit-like coelom.

Fig. 66. — Transverse section of a fifty-hour chick embryo at the level of the origin of the vitelline arteries. XSO.

Section through the PrimitiTe Knot Cranial to the Hind-gut (Fig. 68).— \^'ith the exception of the ectoderm, the structures near the median line are merged into an undiffentiated mass of tissue. The cavity of the neural lube and its dorsal outline may still be seen, but its ventral portion, the notochord. mesoderm, and entoderm, blend in a dense mass of tissue which is characteristic of the primitive knot. Laterally the segmental zone and the various layers arc differentiated.

Fig. 67. — Transverse section of a fifty-hour chick embryo through the segmental zone caudal to the mesodermal segments. X 50.

Fig. 68. — Transverse section o( a fifty-hour chick embrj'o through the primitive node cranial to the hind-gut. X 50.

Section Passing through the Hind-gut (Fig. 69).— In this embryo the caudal evagination to form the hind-gut has just begun. The section shows the small cavity of the hind-gut in the midline. Its wall is composed of columnar entodermal cells and it is an outgrowth of the entodermal layer. Dorsal to the hind-gut may be seen undifferentiated cells of the primitive streak continuous dorsad with the ectoderm, ventrad with the entoderm of the hind-gut, and laterally with the mesoderm.

Fig. 69. — Transverse section passing through the hind-gut of a fifty-hour chick embryo. X 50.

In the chick embryos which we have studied, there are large areas developed which are extra-embryonic, that is, lie outside the embryo. The splanchnopleure of the area vasculosa, for instance, forms the wall of the yolk sac, incomplete in the early stages. The amnion, chorion, and allantois are extra-embryonic membranes which make their appearance at the fifty-hour stage. These structures are important in mammalian and human embryos and a description of their further development in the chick, where their structure and mode of development is primitive, will lead up to the study of mammalian embryos in which the amnion and chorion are precociously developed.

Amnion and Chorion

These two membranes are developed in all amniote vertebrates (Reptiles, Birds, and Mammals). They are derived from the extraembryonic somatopleure. The amnion is purely a protective structure, but the chorion of mammals has a trophic function, as through it the embrj^o derives its nourishment from the uterine wall. Fig. 70 A shows the amnion and chorion developing. The head fold of the somatopleure forms first and envelops the head, the tail fold makes its appearance later. The two folds extend laterad, meet and fuse (Fig. 70 B, C). The inner leaf of the folds forms the amnion, the remainder of the extra-embryonic somatopleure becomes the chorion. The actual appearance of these structures and their relation to the embryo have been seen in Figs. 63 and 64. The amnion, with its ectodermal layer inside, completely surrounds the embryo at the end of the third day, enclosing a cavity filled with amniotic fluid (Fig. 71). In this the embryo floats and is thus protected from injury. The chorion is of little importance to the chick. It is at first incomplete, but eventually entirely surrounds the embryo and its other appendages.

Yolk Sac and Yolk Stalk

While the amnion and chorion are developing during the second and third day, the embryo grows rapidly. The head- and tailfolds elongate and the trunk expands laterally until only a relatively narrow stalk of the splanchnopleure connects the embryo with the yolk. This portion of the splanchnopleure has grown more slowly than the body of the embryo and is termed the yolk stalk. It is continuous with the splanchnopleure which envelops the yolk and forms the yolk sac. The process of unequal growth by which the embryo becomes separated from the yolk has been described as a process of constriction. This, as Minot points out, is an error. The splanchnopleure at first forms only an oval plate on the surface of the yolk, but eventually encloses it. In Fig. 70, C and Z?, the relation of the embryo to the yolk sac is seen kt the end of the first week of incubation. The vitelline vessels ramify on the surface of the yolk sac and through them all the food material of the yolk is conveyed to tlie chick during the incubation period (about twenty-one days).

Fig. 70. — Diagrams showing the development of the anmion, chorion and allantois in longitudinal section (Gegenbaur in McMurrich). Ectoderm, mesoderm, and entoderm represented by heavy, light, and dotted lines respectively. i4/., amnion folds; Al., allantois; Am.y amniotic cavity; CA., chorion; Fj., yolk sac.


We have seen that in the fifty-hour chick a ventral evagination^ the hind-gut, develops near its caudal end (Fig. 69). From it develops the anlage of the allantois, which, as an outgrowth of the splanchnopleure, is lined with entoderm and covered with splanchnic mesoderm (Fig. 70). It develops rapidly into a vesicle connected to the hind-gut by a narrow stalk, the allantoic stalk. At the fifth day the allantois is nearly as large as the embryo (Fig. 71). Its wall flattens out beneath the chorion and finallv it lies close to the shell but is attached only to the embryo. The functions of respiration and excretion are ascribed to it

In its wall ramify the ailanioic vessels, which have been compared to the umbilical arteries and veins of mammalian embryos.

The chick embryo is thus protected by the amnion which develops from the inner leaf of the folded somatopleure and is composed of an inner ectodermal and an outer mesodermal layer. Nutriment for the growth of the embr>-o is supplied by the yolk sac and carried to the embryo by the vitelline veins. The allantois, which takes its origin from the splanchnopleure of the hind-gut and is composed of an iimer layer of entoderm and an outer layer of splanchnic mesoderm, functions as an organ of respiration and serves as a reservoir for the excreta of the embryonic kidneys. As we shall see, the allantois becomes more important, the yolk sac less important in some mammals, while in human embryos both yolk sac and allantois are unimportant when compared to the chorion.

Fig. 71. — Diagram of a chick embryo at the end of the fifth day showing amnion, chorion and allantois (Marshall). X IS.