Book - A Text-book of Embryology 5

From Embryology


Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

   Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

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The formation of the fetal membranes occurs coincidcDtally with the production of the external form of the body of the embryo. These changes mark the division of the hollow sphere or vesicle of which the germ consists up to this stage into two essentially diflFerent parts — namely, the embryonic body and the fetal appendages, the latter of which are destined for the nutrition and protection of the growing embryo. Although the several processes by which are produced the diflFerent parts of the embryo and its various appendages go on simultaneously, it is necessary, for the sake of clearness, to consider successively the development of each structure from its inception to its completion.

In the stages of development thus far considered, the part of the ovuni that is to become the embryo — that is, the embryonic area — is represented by a localized thickening of the wall of the blastodermic vesicle, of the shape and relative size shown in Fig. 33, which presents a surface view of the germ. On each side of the embryonic axis, represented by the notochord, is the paraxial mass of mesoderm, which has undergone partial segmentation to form the somites ; on the distal side of the paraxial column, the mesoderm has split into the somatic or parietal, and the splanchnic or visceral lamellae, between which is the body-cavity or coelom. The cavity of the germ until the occurrence of the transformations about to be described is one undivided compartment which is bounded by splanchnopleure ; and a conspicuous feature of the changes under consideration is the division of this cavity into two by the folding-in of the splanchnopleure composing its walls. It will be well to consider first the formatior of the body-wall and the accessory structures of the chick, and then to take up the special mollifications that are presented by mammals generally and by the human ovum in particular. The first indication of the foldings that lead to the differentiation of the embryo from the fetal appendages is seen upon the surface of the germ at a very early stage. A surface view of the germ — in the case of the chick on the first day of incubation — shows, at what becomes the head-end of the embryonic area, a transverse crescentic groove, with its concavity looking backward (Fig. 41) ; a similar groove is seen at the opposite extremity of the area, and also one at each lateral margin. These marginal grooves are depressions in the somatopleure. The elevated outer edges of the grooves form folds of somatopleure, designated respectively the head-fold, the tail-fold, and the lateral folds of the amnion. As these marginal grooves increase in length they meet each other and now constitute one continuous furrow, which encircles the embryonic area ; its outer elevated edge is the aninionfsihL This furrow, which may be called an inverted fold composed of splanchnopleure and somatopleure, progressively deepens and at the same time its bottom is carried inward toward a point vertically under the central region of the cmbryimic area ; that is, a fold composed of somato})leure •■4 gfJanchnopleure grows from all parts of the periphery «f Ac embryonic area toward the point indicated above, a fMik which corresponds to the site of the future umbilicus. Bf At iDgiowth <rf the edges of the fold, the cavity of the

» moie and more constricted (Plate II., Figs.

Sy, vatil Cmdly, with the completion of the infolding, Avifcd iaio two parts of unequal size ; the smaller

<# t)liii»-fi|Ma^iftdiagliiiMnci, or intestinal canal of the em Vaffyil^ wWi» di^ luf j u m the yolk-sac or umbilical vesicle.

which the gut-tract commute vttelliiic duct (Plate II.,

kfcf of the ingrowing fold

niBgninu lUusLrntiiit: tha fbrmatlon of Itau fl-tal membniiiGs {miHlllIed frum Rnule). "The ipapes ■ lioclj-uaviiy ' In Flgun^s X and 4 nn; merely Itie etirs-embri'oiilo portions of llie body-cavlly. For more recent mncepUona »a in the romiatiuii «t tbOM'

thos outlines and forms the walls of the intestinal canal, the aomatopleuric layer, which acconiiianios it, constitutes the lateral and ventral body-walls of the enibrvo. During the progress of this infolding of the splanchnopleure and the somatopleure, the part of the latter nicn)l)rane that forms the outer wall of the groove becomes lifted up to constitute the anmion-fold (Plate II., Fig. 3) ; by the continued upward growth of this amnion-fold and the {simultaneous settling down of the embryo upon tlio yolk-sac, the margins of the fold come to lie above the cml)ryonic IkkIv, and, approacliing each other, they fuse over its ba(^k, in this manner enclosing it in a cavity. It is obvious that the fold just described is a double layer of somatopleure. After the union of its edges, the two layers become completely scjwinited, the inner one constituting the amnion, while the outer layer is the false amnion, or serosa, or chorion (Plate 11., Fi^s. 4-()).

Since the infolding of the sphmchnopleure begins at the periphery of the much elongatcMl eml>ryonic area, the resulting gut-tract has the form of a straight tube extending from the head-end to the tail-end of the embryo (Plate IWX When the caudal and the cephalic portions of the s|)]am'lm(H pleuric fold have advanced but a comparatively short distance, in consequence of which the coinnuinicatinn between the gut-tnict and the umbilical vesi<*le is still widely o]>on, as shown in Plate II., Fig. 5, there is a eiil-de-^iie or ]>oeket formed of splanchnopleure at the head-end (»!' tlut embryo and a similar one at its tail-end ; these reee<ses are respectively the foregut and the hindgut, the orifiees of whieli are designated the intestinal portals. At this particular sta^rts therefore, the cavity of x\w crut-tnirt i< ineompletely closed off from that of the uinbilieal vesicle.

It is evident that the gut-tract, beiuL^ a tiiluilar cavitv encloseleurc, is lined with entodernial cells; this simple strai<j:ht tube develops subsequently into the adult intestinal canal and its associated ^^andular apparatus.

It has alreadv been pointed out that the lavcT of somatopleure which is iohh'd under the embryonie area in coni]>anv with the splanchnopleure constitutes the lateral and the ventral wall of the Ixxly of the embryo. The fold continues to 'jji\\"dnoM from each side and from each end, and its erJ^ri? t'jniiti together and fuse in the median line of the v*?fjtral hurface of the bo<Iy.* At one place, however, fusion of the mli^t^aof the fold dfies not occur; this r^ion correkf{x>tMlif U} the umbilicus and is often designated the dermal nsreL Here the {jart of the somatopleure that form^ the UmIv'^wiiII ih c;ontinuous with that part of this membrane whi^'h ^^>n»^tituUfH the amnion (Plate II., Fig. 6). By the iiifoldiiiir of the H^imat^>pleure the body-cavity or pleuro|X'rit/>fj<-sil K|KUM« \HH^mien divided into an intra-embryonic and an ^'Xtni'^frnbryonic [lortion, the two communicating for a liffie through the Hmail annular space that encircles the proximal t'tnl of the vitelline duct; this is represented in the a/'X'.'/miia living figures.

Uy iUii- Huiphf prrntesH of folding, associated with the iin<jiial growth of different parts, the leaf-like fundament i'diui^ilutU^l bv the embryoiiie arc«i is differentiated into the \p4nly of the I'lnbryo; the ventral jK)rtion of. this body now i'4nmti^tH of two tiibeh, one within the other, of which the hrrmlh'r, lK>iiiidi'd by the Hphiiiehnopleure, is the intestinal canal, and the Iarg<'r, I'lu-lowd by the somatopleure, is the bodjr-caritjr, th** wallK of which are tin? walls of the body of the embryo. In the doival region is a third tube, the medullary canal; iM'twecn it and the dorsal wall of the intestine is the notochord, on itwh side of wliieh an* the somites (Fig. 44). The further evolution of this biKJv and the differentiation of its various orgjuis and systems will be? described in subse^juent sections.

The Amnion

The amnion is a inembmnous fluid-filled sac, which surrounds the fetus of certain gmups of vertebrate animals during a part of their perio<l of (l<>velopment. In mati, it is

' Failuro of uni<m of tlio wimntoplciiric fohlM in the median line of the

thorax jjrrKluww the deformity known uh cleft Htennim ; while lack of fusion

of the hiteral halven of the alxloiniiial wall n^Mnlts in an extra-alxlominal

  • INmition of the inteHtincH, or, if in lesKer dej^ree, in exstrophy %£ the bladder.

found as early as the fourteenth day,^ before the medullary groove has closed to form the neural canal ; it attains its maximum size by the end of the sixth month and persists until the end of gestation. It constitutes a loose envelope for the fetus, being attached to the abdominal wall of the latter at the margins of the umbilicus, and loosely enveloping the umbilical cord (see Plate III., Fig. 2).

An amnion is found in birds, reptiles, and mammals, these groups being classed together as Anmiota, while fishes and amphibians, which are without an amnion, constitute the class Anamnia.

The first indication of the growth of the amnion is apparent at a comparatively early stage of development. A surfaceview of the blastodermic vesicle of the first- day of incubation in the case of the chick shows a curved line or marking at the anterior edge of the embryonic area (Fig. 41) ; this is the anterior marginal groove, in front of which is another marking, the head-fold of the amnion. Very soon the lateral and posterior marginal grooves appear at the sides and posterior edge respectively of the embryonic area ; the outer elevated edges of these marginal grooves constitute the lateral folds and the tail-fold of the amnion. The grooves and folds increase in length in each direction until they meet, when they form one continuous furrow, which circumscribes the embryonic area, and the outer elevated edge of which is the amnion fold. The groove involves both the somatopleure and the splanchnopleure, constituting the inverted fold of these two structures that grows in to form the body- wall and the wall of the gut-tract, while the amnion fold is composed of somatopleure alone (Plate IT.). This separation of the somatopleure and the splanchnopleure enlarges the extraembryonic portion of the bofly-cavity. The amnion fold continues to grow upward, and finally its edges meet and fuse over the back of the -embryo, the line of union being the amniotic suture ; the suture closes first at the head-end of the embryo and last at the tail-end. After the union of the edges

' Ret*enlly it lias been found complete in an ovum estimated to be four (lavs old.

of the fold, its inner layer, consisting of ectoderm and parietal mesoderm, separates from the outer layer to constitute the true amnion, whose enclosed s|>ace is the amniotic cavity ; the outer layer, which is merely a part of the general somatopleure, is the false amnion or serosa. It is apparent from this description that the amniotic cavity is lined with ectodermal epithelium and that its walls consist of somatopleure — that is, of ectoderm and parietal mesoilerm.

While the amnion fold is growing upward, the embryonic area — now undergoing ditferentiation into the embryonic body — is sinking down ujK)n the yolk-sac. The amnion fold does not grow uniformly in all parts of its periphery. The head-fold is produced first and constitutes a cap or hood covering the head of the embryo, which is forming simultaneously by the vcntnid growth of the somatopleure at the bottom of the marginal groove. It is only after the development of the head-fold is well a<lvanced that the lateral, and, later, the caudal, portions of the amnion-fold grow up to meet it. The head-fold is, for a time, destitute of mesodermic tissue, since it corresponds to that region of the wall of the blastodermic vesicle described on ])age 64 as the proamnion.

It htis lK»en shown (p. 54) that the amniotic cavity of mammals is produced not by the upgrowth of folds of somatt>pleure, but by a vacuolation of a portion of the cells of the inner ii»ll-mass (Fig. 28, p. 5")). Since the enveloping layer, which forms the roof or vault of the amniotic cavitv, constitutes the extra-embryonic cct<Mlerin (p. 05 1, this cavitv in mammals as in binls is lin<»d with ccttMhTmal cells, the floor t»t the cavitv being also ectodermal since it is i'ormed by the <Mni^r^'«mic disk, the amniotic* surface of which constitutes the •'mnrviioic eou>ilerm (Fig. 21>). Covering the ectodermal roof > A .uv^-r '^t nu*SiHlenn continuous with tlu* mesiHJerni of the Li'n- -iiic dLsk. The embryonic bud or <lisk, at first con"■' •«! rs> ^mwiitfii* <urf?HH\ the future dorsjil surface, becomes

  • ia\vx^ Its eilgi*s curving towanl th<' o])jM»site

r -^HTTs. -=iriL'^- It should not be forgotten that the -5P-tt-:=. — -^^^ "iis ventral surface and at the ]MTipherv

Jeti'p surface of the enveloping layer, and also that the embryonic ectoderm is likewise continuous at the periphery of the embryonic bud with that part of the enveloping layer which forms the vault of the amniotic cavity ; hence, after the ventral curvature of the embryonic bud, the periphery of which carries with it toward the ventral surface the amniotic ectoderm and mesoderm, we have practically the same conditions as obtain in the avian embryo as shown in Plate II., Figs. 4 and 5. While in the latter case the amnion is produced by the formation of folds, in the mammalian germ the same result is attained by the vacuolation of the inner cell-mass.

As the curving ventrally of the embryonic bud continues, the originally flat mass of cells composing it is converted into an imperfect tube, the lateral and ventral surfaces of which correspond with the former dorsal surface. This ventral folding of the embryonic bud produces the body of the embryo. As the folding includes the entodermal layer on the ventral surface of the embryonic bud, the blastodermic cavity is divided, as in the bird's germ, into the primitive intestinal canal and the yolk-sac (Fig. 48).

The amnion of man presents an important variation from that of all other Amniota, since the inner layer of the amnion-fold does not entirely sever its connection with the outer layer, but remains attached to it over the caudal pole of the embryo. In consequence of this attachment the true amnion is connected with the false amnion, and since the true amnion is continuous with the body- wall of the embryo, the caudal end of the embryonic body is attached to the false amnion or chorion bv a mass of mesodermic tissue called the allantoic stalk or belly-stalk, as seen in Fig. 47. The relation of the belly-stalk to the development of the allantois will be pointed out hereafter.

The space within the amnion — the amniotic cavity — is filled with the amniotic fluid or liquor amnii.

The amnion at first envelops only the sides and dorsum of the embryonic body, occupying the upper part of the cavity enclosed by the chorion, as shown in Plate II., Figs. 5 and 6 ; the groove, or farrow, however, of which the amnion fold is the peripheral or outer elevated edge, becomes deeper, and the bottom of the groove is carried toward the middle of the future ventral surface of the embryo, its ventrad growth continuing until it reaches the position of the future umbilicus (Plate II. : Fig. 4, transverse section ; Figs. 5 and 6, longitudinal section). The layer of somatopleure constituting the inner wall of the groove — that is, on the side toward the embryonic area — becomes the lateral and ventral walls of the body of the embryo, as described above ; in this manner is effected the transition from the flattened or layer-like embryonic area to the definite form of the embryonic body. The ventral body-wall is continuous at the margins of the umbilicus witli the amnion, since the somatopleure, forming the outer boundary of the original groove, is a part of that membrane. After its completion, therefore, the amnion envelops tlic body of the embryo on every side, lying closely applied to it, since the amniotic cavity is at first very small. With the progress of development and the increase of the fluid the amnion requires more room, until, in the third month, it fills out the entire space within the chorion, with the inner surface of which it acquires a loose connection. The umbilical vesicle and the alhintois have meanwhile imdergone regression. The walls of the amniotic sjic contain contractile fibers; it is to these that the rhvthmical contractions observed in the amnion are due. Its lining is at first a single layer of flattened epithelial cells; at the fourth month the cells are cubical for the most part, but to some extent columnar.

The liquor amnii is a watery fluid having a s]>ecific gravity of 1.007, and containing about 1 per cent, of solids (albumin, urea, and grape sugar). The origin of the fluid is believed to be in the blood of the mother, the liquid portion of which transudes into the amniotic cjivity. The amniotic fluid increases in quantity until the sixth month of pregnancy ; from this time until the close of gestation it generally diminishes about one half. A pathological excess of the fluid constitutes the condition of hydramnios.

The Fuction of the amniotic fluid is two-fold ; it serves as a buffer for the fetus, protecting it from mechanical violence, and it supplies the fetal tissues with water, since portions of it are from time to time swallowed. Evidence that the fetus swallows the fluid is aflTorded by direct observation of chicken embryos, and by the presence of epidermal cells, hairs, and fatty matter in the fetal alimentary canal. After the development of the bladder, the urine of the fetus is from time to time evacuated into the amniotic cavity.

The epidermis of the child in utero is protected against maceration in the amniotic fluid by the presence of a fatty coating, the vernix caseosa, which is a modified sebaceous secretion.

At the end of pregnancy, the amnion is loosely united with the chorion and the deciduae ; during birth it ruptures, and its fluid escapes.


The yolk-sac, or umbilical vesicle, as seen in the higher vertebrates, is a capacious sac attached by a narrow i>edicle, the vitelline duct, to the ventral surface of the embryonic intestinal canal, the duct passing through the umbilical aperture (Plate II., Fig. 6).

In order to appreciate more fully the function and the morphological relations of this structure, it is necessar}' to glance at the conditions that obtain in the several classes of vertebrate animals. In ova that develop outside of the body of the parent organism, a special dower of pabulum is provided for the nutrition of the embryo ; this dower is represented by the deutoplasm so abundant in telolecithal ova. In the case of amphibians, whose cleavage, it will be remembered, is holoblastic or total, the cells richest in deutoplasm are accumulated, after segmentation, in the floor of the archenteron ; this accumulation produces on the future ventral surface of the embryo a marked bulging, which constitutes the amphibian yolk-sac. As the embryo grows, it draws upon this store for its nutrition, in consequence of which the sac gradually shrinks, its cells being, for the most part.

liquefied nod absort)ed, while Bome of them cx>ntribute to the lining of the iiiteittiiial canal.

Ill a UghaT type, as exemplifled in sharks and dog-fiahGS, the yolk-sac is produced by a folding-in of the spIaDchnopleure and the soraatopleure, the walls of the sac being therefore constituted by both of these layers ; this folding-ia divides the arehenteroa into a smaller part, the intestinal canal, lying within the body of the embryo, and a larger cavity, the yolk-sac, situated outside of that body. The eplanchnopleuric layer of the yolk-sac is continuous with the wall of the intestuial canal, while its somatopleuric layer is continuous with the body-wall. A system of blood-vessels develops upon the yolk-sac, their function being to convey the nutritive material into the Ixidy of the embryo. These blood-vessels constitute the so-called Tascnlar area, which appears, in surface views, as a zone encircling the embryonic area, and, later, the embryo, since the latter reposes upon the proportionately much larger yolk-sac. As the contents of the sac becj)me absorbed, the latter shrinks, the splanchnopleurie layer slipping into the abdomen iif the embryo through the umbilical oi>ening, the somatopleuric layer contracting to close that aperture.

In the Asmiota (p. 83) the development and structure of the yolk-sac are modified by the presence of the amnion. In these groups the yolk-sac and the gut result from the division of the blastodermic cavity by the folding-in of the eplancli■oplcure alone, since the somatopleure grows away from the Bplanchnopleure to form the amnion-fold, and thus only partially invests the yolk-sac (Plate 11., Fig. 4),

Since the yolk-sac contains the store of food destined for the nutrition of embryos that develop outside of the maternal body, and since the mammalian embryo, which le.idsan intranterine existence, is endowed with a relatively small quantity of such store, the yolk-sac of mammals would seem to indicate the dcwent f)f the latter from oviparous ancestors. Further and strimger evidence of such descent is found in the fact that the eggs of the lowest order of mammals, the Monotrcniata, comprising the echidna and the ornithorhynchus, are " laid" and undergo triro-uterine development.

In the human embryo the umbilical vesicle is found partially constricted off from the intestinal canal by the end of the second week ; by the end of the third week the separation of the two cavities has advanced to such an extent that the vitelline duct is present, the sac attaining its maximum size by about the fourth week.

The function of the umbilical vesicle, as above intimated, is to serve as the organ of nutrition for the embryo during a certain period. The manner in which its blood-vessels develop will be considered in Chapter X. Their growth precedes that of the intra-embryonic portions of the vascular apparatus, the vascular area of the yolk-sac being the seat of the earliest blood-vessel formation. The vessels find their way into the body of the embryo along the vitelline duct, and consist of two vitelline arteries and two vitelline veins.

With the development of the allantois the yolk-sac retrogresses, the allantois succeeding it as the organ of nutrition and respiration. By the end of the sixth week the sac has shrunk to a narrow stalk, which is surrounded by the enlarged amnion, and which terminates in a knob ; at birth, the knob lies near the placenta (Plate V., Fig. 2), and the atrophic remnant of the stalk is one of the constituents of the umbilical cord.

The Allantois

The allantois is an embryonic structure which is found in those vertebrates possessing an amnion. Its growth is correlated with the retrogression of the umbilical vesicle, which structure it supplants as the organ of nutrition and respiration for the embryo.

Appearing at first as a little evagination or out-pocketing of the ventral wall of the gut-tract, the allantois finally becomes a pedunculated sac lying in the extra-embryonic part of the coelom (Plates II. and III.)> its stalk leaving the body-cavity proper through the umbilical opening. Being an outgrowth from the intestinal canal, the walls of the allantois are made up of splanchnopleure — that is, of entoderm and visceral mesoderm. Blood-vessels develop in the mesodermic stratum^ the principal trunks^ the two allantoic arteries and veins, being connected at their proximal ends with the primitive heart ; this system of vessels constitutes the allantoic circulation and is the avenue through which the growing eml)rj'o is supplied with nutritive material and oxygen. As the fundus of the allantois increases in size, it spreads itself out ujwn the inner surface of the false amnion or chorion (Plate III., Fig. 1), into whose villi its vascular tissue penetrates, and with which it becomes intimately blended. The union of the allantois and the false amnion produces the true chorion of some authors.

The human allantois presents a striking i)eculiarity as compared with that of birds and reptiles ; in man, the allantois

Fig. 47.— DIaprammatic sections roproRcntinp: growth nnd nrrnnpomcnt of the amnion in the earlioHt HtagcH of the human embryo (His).

develops not as a free sac projecting into the extra-embryonic bo<ly-cjivity, but as a mass of splanchnopleuric tissue which contains only a rudimentary cavity and which grows into the alxlominal stalk (Fig. 47 and Fig. 5S, hfd\^ Iwing guided bv that structure to the chorion. Moreover, while the human allantois is in effect an evagination of the ventral wall of the primitive gut-tract, its evagination begins before the gut-tract is const ricted-otf from the yolk-sac (Fig. 48).

The ftmctioii of the allantois in ogg-laying animals, and possibly in some others, is to serve as a nutritive and respiratory organ and as a receptacle for the fetal nrine : in man its cavity is exceedingly minute, and its chief function is to furnish a means of conveying blood-vessels from the embryo to the chorion.

Fig. 48.— Mesial section through an early human ovum (Oraf Spec) : a, AMominal Btalk ; b, amnion : r, yolk-.sac : d. hypoblaut ; c, me»obla!»t; /, vessels on wall of yolk-sac : g, primitive streak : h. allantois : i. medullary [»late ; j, early heart : k, mesoblast of chorion ; /, early villi ; m, chorionic mesoblast extending outward into villi.

The part of the allantois contained within the body of the embryo produces three structures of the adult organism : 1, the nrachus, an atrophic cord extending from the summit of the bladder to the umbilicus;^ 2, the urinary bladder; and 3, the first part of the urethra of the male, or the entire female urethra. The extra-embryonic portion shrinks after the appearance of the placenta and forms one of the constituents of the umbilical cord, its blood-vessels becoming the umbilical arteries and veins.

^ If the urachas remains patulous instead of becoming impervious, urine may escape at the umbilicus, and the condition is a variety o( urinary JUtukL

The Chorion

At the time when the false amnion is forming, the attenuated zona pellucida still surrounds the embryonic vesicle as the so-called prochorion, which unites with the false amnion, producing the primitive chorion. After the allantois has grown forth from the gut-tract and has spread itself over the inner surface of the primitive chorion, it becomes blended with the latter to constitute the tme chorion of some authors. The chorion, according to the above nomenclature, may be defined as the membrane which encloses the germ at the stage following the appearance of the amnion and the false amnion, and which has resulted from the fusion of the allantt)i8 with the primitive chorion ; or, ignoring the zona pellucida, the chorion results from the fusion of the allantois and the false amnion. Minot, however, defines the chorion as all that part of the extra-embryonic somatopleure which is not used in forming the true amnion, and hereafter in this work the wonl will be used in this sense. This definition limits the term to the outermost covering of the germ after the formation of the amnion (Plate III., Fig. 1).

The chorion consists of an outer ectodermic layer and an inner lamella of mesodermic tissue. The mesoblastic layer it thini being composed of from two to four layers of round,

OVtl|Or fusiform cells, and is at first devoid of blood-vessels.

The latter, in the form of capillaries, make their appearance

il KNne time during the second week, probably as extensions

^ the bUKxl-vessels of the allantois.

  • t\m outer ectodermic or epiblastic cells of the chorion at

ft ttty tftrly period, certainly as early as the third day, pffoliferatioD to form a layer of tissue called the wbioh IB from one to several layers of cells in The tfophoblast layer is thickest at the place of eC dM ovum to the uterine mucosa. The inner ef il» iMfhebbH aie cttbical, and have large, finely

•vtl niicleL In the youngest human

of Peters, estimated to be three

was found to present many

qC etiiiids and buds, these being the foundations of tlip future villi nf tlio cliorion. The tropliohlaet layer was not solid, but was honeycombed with little spacra or vacuoles filled with maternal blood, which spaces were partly lined with a nnclealed protoplasm, the early syncrtitun (Plate lA'., n). Even at this early stage, therefore, when the trophoblast strands or early villi are as yet devoid of a mesnblastic element, they are bathed with the maternal blood. Very soon the niesoblastic tissue of the chorion grows into the trophohlast strands, thus forming the permanent villus stems ; and during the seeond week capillaries extend into the stems, completing the foruiatioD of the fully developed villi of the chorion.

The early development of villi in characleristio of the human chorion {Fig. 40 and Plate II, Fig. 6). At first the

Fig 19.— Human ovum o



—Front view of

Keioherl), U vie*.


la tKitb ncurui the illi are limlled 1 rll

lea vine th

villi, either covering the entire surface of the chorion or leaving the two opposite poles free, are of uniform size ; in the latter half of the first month, however, there begins to be a differentiation into a r^on containing smaller, and one hiiving larger and more nnmerous, pmjections. The difference between the two area* becoming more marked, the relatively smooth part of the membrane, possessed of rudimentary villi, is designated the chorion love, while the region with well-develoiHsd villous projections is distinguished as the chorion frondoBom (Plate III., Figs. 1 and 2 the latter a jqim-es of the uterus and close relation with the mucous membrane i the fetal part of the placenta.

The villi in their earlier condition are somewhat club-shaped elevations, which later become branched to form secondary villi. Each fully developed villus consists of a core of mesoblast, covered with ectodermic epithelium and containing blood-vessels (Fig. 54 and Plate IV., a). Their microscopic appearance is so characteristic that they afford a means of positively determining whether a mass discharged from the uterus is or is not a product of conception. The further alterations in the villi as well as in the trophoblast in general, including the syncytium, will be considered in Chapter VI.

A chorion is present, as a rule, in those animals whose embryos develop witliin the uterus ; this would include the entire class Mammalia, with the exception of the monotremes, wliose eggs undergo extra-uterine development, and the marsupials, whose embryos, though nourislied in the womb, never acquire villi on the serosa, nutriment being absorbed by simple contact of the latter with the uterine mucous membrane. the Mammalia are therefore divided into the Achoria, comprising the monotremes and marsupials, and the Ohoriata, including all other mammals.