Talk:Book - A Laboratory Manual and Text-book of Embryology 4

From Embryology


The fetal membranes of mammals include the amnion, chorion, yolk sac, and allanlois, structures which we have seen are present in chick embryos. Most important in mammals is the manner in which the embryo becomes attached to the uterine wall of the mother, and in this regard mammalian embryos fall into two groups. Among the Ungidates or hoofed mammals (e. g., the pig) the fetal membranes are of a primitive type, resembling those of the chick. Among Unguiculates (clawed animals like the bat and rabbit) and Primates (e. g., Man) the fetal membranes of the embryo show marked changes in development and structure.


The amnion and chorion develop very much as in the chick embryo (Fig. 70 .4 . B). Folds of the somatopleure form very early and envelop the whole embryo.

SIrsodrrmal sf^mtiil ' — * "^"- ^-^

TW anmi<>n (Fig. 7Jt is oU\<(tI in cmhr\-\»s with hut a few pain: of segments, but for sonio tinio rrniains att«oh(Hl to tho chorion by a strand of tissue (Keibel), The \i>ik stK *K'\<>loj\< o;»rl>' as in all mamnials. In the pig it is small and the greater (wrt of it s*Hm dcjs-noratos. It is imjvrtiUit only in the early growth of the



embryo, its functions then being transterred to the allantois. Branches of the vitelline vessels ramify in its wall, as in that of chick embryos, but soon degenerate. The trunks of the vitelline vessels, however, persist within the body of the embryo. The aUarUois, developing as in the chick from the ventral wall of the hind-gut (Fig. 70 A-D), appears when the embryo is still flattened out on the germinal disc. In an embryo 3.5 mm. long it is crescent-shaped and as large as the embryo. It soon becomes larger and its convex outer surface (splanchnic mesoderm) is applied to the inner surface (somatic mesoderm) of the chorion.

Entoderm of

phinthnU mtsodtrm

-Chorionic mtsoderm

Clwrionk cdodrim

^L Urine rpilhclium

•~~ Tiinicii proprin oj ulrrut

Ftc. 73. — Diagnm of the (etal membranes and allantoic placenta of a pit; embo'n in median safcittal section (based on figures of Heisler and Minot).

These surface layers fuse more or less completely, A pair of allantoic veins and arteries branch in the splanchnic layer of the allantois. These branches are brought into contact with the mesodermal layer of the chorion and invade it. The outer ectodermal layer of the chorion in the meantime has closely applied itself to the uterine epithelium, the ends of the uterine cells fitting into depressions in the chorionic cells (Fig. 73). When the allantoic circulation is established, waste products given off from the blood of the embryo must pass through the epithelia of both chorion and uterus to be taken up by the blood of the mother.


In the same way nutritive substances and oxygen must pass from the maternal blood through these layers to enter the allantoic vessels. This exchange does take place, however, and thus in Ungulates the allantois has become important not only as an organ of respiration and excretion but as an organ of ntUrition, Through its vessels it has t^ken on a function belonging to the yolk sac in birds, and we now see why the yolk sac becomes a rudimentary structure in the higher mammals. Excreta from the embryonic kidneys are passed into the cavity of the allantois which is relatively large. The name is derived from a Greek word meaning sausage-like, from its form in some animals. The chorion is important only as it brings the allantois into close relation to the uterine wall, but in man we shall see that it plays a more important rdle.


Pig Embryos. — In their early development the relation of the amnion, allantois, and yolk sac to each other and to the embryo is much the same as in the chick of five days (Fig. 71). With the increase in size of the embryo, however, the somatopleure in the region of the attachment of the amnion grows ventrad (Fig. 70 D). As a result, it is carried downward about the yolk sac and allantois, forming the umbilical cord (cf. Fig. 241). Thus in a pig embryo 10 to 12 mm. long the amnion is attached at a circular line about these structures some distance from the body of the embryo (cf. Fig. 119). The ccelom at first extends ventrad into the cord, but later the mesodermal layers of amnion, yolk stalk, and allantois fuse and form a solid cord of tissue. This is the umbilical cord of fetal life and its point of attachment to the body is the umbilicus or navel. The cord is covered by a layer of ectoderm continuous with that of the amnion and of the embryo and contains, embedded in a mesenchymal (mucous) tissue, (1) the yolk stalk and (in early stages) its vitelline vessels; (2) the allantoic stalk; (3) the allantoic vessels. These latter, two arteries and a single large vein, are termed, from their p>osition, the umbilical vessels. At certain stages (Figs. 122 and 123) the gut normally extends into the coelom of the cord, forming an umbilical hernia. Later, it returns to the ccelom of the embryo and the cavity of the cord disappears. The umbilical cord of the pig is very short.

Human Umbilical Cord. — This develops like that of the pig and may attain a length of more than 50 cm. It becomes spirally twisted, just how is not known. In embryos from 10 to 40 mm. long the gut extends into the ccelom of the cord (Figs. 179 and 180). At the 42 mm. stage, according to Lewis and Mall, the gut returns to the coelom of the body. The mucous tissue peculiar to the cord


arises from mesenchyme. It contains no capillaries and no nerves, but embedded in it are the large mnbilical vein, the two arteries, the allantois, and the yolk stalk. The umbilical cord may become wound about the neck of the fetus, causing its death and abortion, or by coiling about the extremities it may lead to their atrophy or amputation.


Referring to the blastodermic vesicle of the mammal (Figs. 17 and 18), it is found to consist of an outer layer, which we have called the trophectoderm, and the inner cell mass (p. 36). The trophectoderm forms the primitive ectodermal layer of the chorion in the higher mammals and probably in man. From the inner cell mass are derived the primary ectoderm, entoderm, and mesoderm. In the earliest known human embryos described by Teacher, Bryce, and Peters, the germ layers and amnion are present, indicating that they are formed very early. We can only infer their early origin from what is known of other mammals. The diagrams (Fig. 74 A and B) show two hypothetical stages seen in median longitudinal section. In the first stage (A) the blastodermic vesicle is surrounded by the trophectoderm layer. The inner cell mass is differentiated into a dorsal mass of ectoderm and a ventral mass of entoderm. Mesoderm more or less completely fills the space between entoderm and trophectoderm. It is assumed that as the embryo grows (B) a split occurs in the mass of ectoderm cells, giving rise to the amniotic cavity and dividing these cells into the ectodermal layer of the embryo and into the extra-embryonic ectoderm of the amnion. At the same time a cavity may be assumed to form in the entoderm, giving rise to the primitive gut. About this stage the embryo embeds itself in the uterine mucosa. In the third stage, based on Peter's embryo (C), the extra-embryonic mesoderm has extended between the trophectoderm and the ectoderm of the amnion and the extraembryonic ccelom appears. At first strands of mesoderm, known as the magma reticulare, bridge across the ccelom between the somatic and splanchnic layers of mesoderm (Fig. 76). The anmiotic cavity has increased in size and the embryo is attached to the trophectoderm by the unsplit layer of mesoderm between the ectoderm of the amnion and the trophectoderm of the chorion. The latter shows thickenings which are the anlages of the chorionic villi surrounded by trophoderm cells. In the fourth stage, based on Graf Spee's embryo (Z)), the chorionic villi are longer and branched. The mesoderm now remains unsplit only at the posterior end of the embryo, where it forms the body stalk peculiar to Unguiculates and Primates. It connects the mesoderm of the embryo with the mesoderm of the



chorion. Into it there has grown from the gut of the embryo the entodermal diverticulum of the atlantois.

The Chorion. — The human chorion is derived directly from the outer trophectcderm layer of the blastodermic vesicle and from the extra-embryonic somatic mesoderm. Its early structure resembles that of the pig's chorion. The trophectoderm of the human embryo early gives rise to a thickened outer layer, the Irophoderm (syncytial and nutrient layer — Figs, 74 C and 239). When the develop



Chorionic vilti

Fig. 74.— Four diagrams of early human cmbo'os (based on fibres of Robinson and Minot). A, Hypothetical stage; B, Bryce-Teacher embryo (modified); C, Peter's embryo; D, Graf Spec's embiyo.

ing embryo comes into contact with the uterine wall the trophoderm destroys the maternal tissues. The destruction of the uterine mucosa serves two purproes: (1) the embedding and attachment of the embryo, it being grafted, so to speak, to the uterine wall; and (2) it supplies the embryo with a new source of nutrition. To obtain nutriment to better advantage, there grow out from the chorion into the uterine mucosa branched processes or nUi. The villi are bathed in maternal blood, and in them blood vessels are developed, the trunks of which pass to and



from the embryo as the umbilical vessels. The embryo receives its nutriment and oxygen, and gets rid of waste products through the walls of the villi. The repon where the attachment of the chorionic villi to the uterine wall i>ersists during fetal life is known as the placenta. It will be described later with the decidual membranes of the uterus. We saw how the allantois of Ungulates had assumed the



Bmbryenk tctoAerm

L bat embryos (after Van Beneden).

nutritive functions performed by the yolk sac in birds, with a consequent degeneration of the ungulate yolk sac. In man and Unguiculatcs the functions of the allantois are transferred to the chorion, and the allantois. in turn, becomes a rudimentary structure.

The Amnion.— This is formed precociously in Unguiculates and in a manner quite different from its mode of origin in Ungulates and birds. It is assumed that



its cavity arises as a split in the primitive ectoderm of human embryos, as in bat embryos (Fig, 75). Later, a somatic layer of mesoderm envelops its ectodermal layer, its component parts then being the same as in birds and Ungulates — an inner layer of ectoderm and an outer layer of mesoderm (Fig, 74 D). It becomes a thin, pellucid, non-vascular membrane and about a month before birth is in contact with the chorion. It then contains about a liter of amniotic fluid, the origin of which is unknown. During the early months of pregnancy the embryo, suspended by the umbilical cord, floats in the amniotic fluid. The embryo is protected from maceration by a white fatty secretion, the vernix

At birth the amnion is ruptured either normally or artificially. It not ruptured, the child may be born enveloped in the amnion, popularly known as a veU or "caul." The

Fig. 76. — Section of Peter's embryo of 0.2 mm. (about fifteen days), ed., Ectoderm of chorion; ma., mesoderm; am., amniotic cavity; em. pf., embryonic plate; y.s., yolk sac; en/,, entoderm; ex. e«., portion of extra-embiyonic axiom limited by a strand of the magma reticulare.

c fluid may be present in excessive amount, the condition being known as hydramnios. If less than the normal amount of fluid is present, the amnion may adhere to the embryo and produce malformations. It has been found, too, that fibrous bands or cords of tissue may extend across the amniotic cavity, and, pressing upon parts of the embryo during its growth, may cause scars and splitting of eyelids or lips. Such amniotic threads may oven amputate a limb or cause the bifurcation of a digit.

The Allantois. — The allantois appears very early in the human embryo before the development of the fore-gut or hind-gut. In Peter's embryo the amnion, chorion, and yolk sac are present, but not the allantois (Fig. 76). In an embryo 1.54 mm. long, described by Von Spee (Fig. 77), there is no hind-gut, but the


allantoic diverticulixm of the entoderm has invaded the mesoderm of the body stalk. This embryo, seen from the dorsal side with the amnion cut away, shows

Fic. 77. — Vkmof a human embiyo 1.54 mm. long (Graf Spee). X 23. A, Dorsal surface; B, mediao sagittal section.

a marked neural groove and primitive streak. In front of the primitive knot a pore is figured leading from the neural groove into the primitive intestinal


cavity, and hence called the neurenUric canal (p. iS). The fore-gut and head

fold have formed at this stage and there are branched chorionic villi. Somewhat

more advanced conditions are found in

an embryo of 1.8 mm. with five to six

pairs of segments (Fig, 78),

A reconstruction by Dandy of Mall's embryo, about 2 mm. long with seven pairs of segments, shows well the embryonic appendages (Fig. 79). The fore- and hind-gut are well developed, the amniotic cavity is large, and the yolk sac still communicates with the gut through a wide opening. The allantois is present as a long curved tube somewhat dilated near its blind end and embedded in the mesoderm of the body stalk. As the hind-gut

Fit;. V). — A human embryo of 2 mm. in median sagittal section (adapted from reconstructions of Mall's embryo by F. T. Lewis and Dandy). X 23.

develops, the allantois comes to open into its ventral wall. A large umbilical artery and vein are present m the body stalk.

In an embryo of 23 somites 2.5 mm. long, described by Thompson, the allan



t(HS has elongated and shows three irregular dilatations (Fig. 80). A large cavity never appears distally in the human allantois as in Ungulates. When it becomes included in the imibilical cord its distal pwrtion is tubular and it eventually atrophies. That part of the allantois extending from the umbilicus to the cloaca of the hind-gut possibly takes part in forming the bladder and the urachus, a rudiment extending as a solid cord from the fundus of the bladder to the umbilicus.

The human allantois is thus small and rudimentary as compared with that of birds and Ungulates. As we have seen, the cavity is very large in the pig, and Haller found an allantoic sac two feet li)ng connected with a goat embryo of two inches. In human embryos it ap

Clowal memhtant Clooca

Tic. 80. — Median sagittal section of a mm. human embryo showing digestive I (after Thompson). X 40.

pears very early and is not free, but embedded in the body stalk. Its functions, so important in birds and Ungulates, are in man performed by the chorion. Tolk Sac and Tolk Stalk. — In the youngest human embryos described, the entoderm forms a somewhat elongated vesicle (Fig. 76). With the development of the fore-gut and hind-gut in embryos of 1.54 and 2 mm. (Figs. 77 and 78), the entodermal vesicle is di\'ided into the dorsal intestine and ventral yolk sac, the two being connected by a somewhat narrower region. This condition persists in an embryo 2.5 mm. long (Fig. 80). In the figure most of the yolk sac has been


cut away. Embryos with 9 and 14 pairs of segments, with three brain vesicles and with the amnion cut away are seen in Figs. 81 and 324, The relation of the

Fig. 82,— Human embryo ol about 2.5 mm. (His, after Coste). X IS.

fetal appendages to the embryo shows well in the embryo of Coste (Fig. 82). The dorsal concavity is probably abnonnal. A robust body stalk attaches the

Flc. B3. — Human embryo 2.6

m. lonR shomng amnion, yolk stalk am! body stalk (Hfe), "< 25.

embryo to the inner wall of the chorion. With the growth of the head- and tail folds of the embryo, there is an apparent constriction of the yolk sac where it



joins the embryo. This will become more marked in later stages and form the yolk stalk. His* embryo, 2.6 mm. long, shows the relative size of yolk sac and embryo and the yolk stalk (Fig. 83). The relations of the fetal membranes to the embryo are much the same as in the chick embryo of five days, save that the allantois of the human embryo is embedded in the body stalk. The embryo shows a regular convex dorsal curvature, there is a marked cephalic ben^ in the region of the mid-brain and there are three gill clefts. The head is twisted to the left, the tail to the right. At the side of the oral sinus are two large processes; the dorsal of these is the maxillary, the ventral the mandibular process. The heart is large and flexed in much the same way as the heart of the fifty-hour chick embryo.

In later stages, with the development of the umbilical cord, the yolk stalk becomes a slender thread extending from the dividing line between the fore- and hind-guts to the yolk sac or umbilical vesicle (Figs. 84 and 119). It loses its attachment to the gut in 7 mm. embryos. A blind pocket may persist at its point of

Fig. 84. — Yolk sac and stalk of a 20 mm, human embryo. X 1 1.

union with the intestine and is known as MeckeVs diverticulum^ a structure of clinical importance because it may telescope and cause the occlusion of the intestinal lumen. The yolk stalk may remain embedded in the umbilical cord and extend some distance to the yolk sac which is found between the amnion and chorion. The yolk sac may be persistent at birth.


This embryo, studied and described by His, is regarded by Keibel as not quite normal. Viewed from the left side (Fig. 85), with the amnion cut away close to its line of attachment, there may be seen the yolk stalk, and a portion of the yolk sac and of the body stalk. There is an indication of the primitive segments along the dorso-lateral line of the trunk. The head is bent ventrad almost at right angles in the mid-brain region {cephalic flexure). There are also marked cervical and caudal flexures, the trunk ending in a short blunt tail. The heart is large and



Slomodaum St ondibtiltiT

\ililery vesielt

flexed as in the earlier stage. Three gill clefts separate the four branchial arches. The first has developed two ventral processes. Of these, the maxillary process is small and may be seen dorsal to the slomodaum. The mandibular process is large and has met its fellow of the right side to form the mandible or lower jaw. Dorsal to the second gill cleft may be seen the position of the oval otocyst, now a closed sac. Opposite the atrial portion of the heart, and in the region of the caudal flexure, bud-like outgrowths indicate the anlages of the upper and lower extremities.

Central Nervous System and Sense Organs. — The neural tube is closed throughout its extent and is differentiated into brain and spinal cord. The brain tube, or encepkalon, is divided ~ — by constrictions into four regions, or

vesicles, as in the fifty-hour chick (Fig. 57). Of these, the most cephalad is the Itlencepkalon. It is a paired outgrowth from the fore-brain, the persisting portion of which is the diencepkalon. The mid-brain or mesencephalon, located at the cephalic flexure, is not subdivided. The hindbrain, or rhombencephalon, which is long and continuous with the spinal cord, later is subdi\'ided into the melencephalon (region of the cerebellum and pons) and myelencephalon (medulla oblongata). The spinal cord forms a closed tube extending from the brain to the tail and containing the neural cavity, flattened from side to side.

The eye is represented by the optic vesicles and the thickened ectodermal anlagc of the lens. Its stage of development is between that of the thirty-six and fifty-hour chick embryos.

The otocyst is a closed sac, no longer connected with the outer ectoderm as in the fifty-hour chick.

Digestive Canal. — In a reconstruction of the \'iscera viewed from the right side (Fig. 86), the entire cvtcnt of the digestive canal may be seen. The pharyngeal membrane, which wc s;iw developed in the chick between the stomodfeum

Body Ualk —Left side of a human embiyo of 4.2 n

(His). X 15.


and the pharynx, has broken through so that these cavities are now in communication. The fore-gut, which extends from the oral cavity to the yolk stalk, is differentiated into pharynx, thyreoid, trachea and lungs, esophagus and stomach, small intestine and digestive glands (pancreas and liver). The gut is suspended from the dorsal body wall by the dorsal mesentery.

Pliaryiigeal pouchii 1-4

Fic. 86. — Diagrammatii

4.2 mm. human cmbrjo, vi fromamixlel by His). X 2.^.

ed from the right side (adaplrd

The ectodermal limits of the oral cavity are indicated dorsad by the diverticulum of the hypophysis iRathke's pocket). The fore-gut proper begins with a shallow out-pocketing known as Seessel's pocket. As the pharyngeal membrane disappears between these pockets, it would seem that Seessel's pocket represents the persistence of the blind anterior end of the fore-gut. No other significance has been as.signed to it.

The pharynx is widened laterally and at this stage shows four pharjngeal



pouches (Fig. 87). Later a fifth pair of pouches is developed {Fig. 168). The four pairs of pharyngeal pouches are important as they fonn respectively the following adult structures: (1) the auditory tubes; (2) the palatine tonsils; (3) the thymus anlages; (4) the paralkyreoids or epUkelial bodies. Between the pharyngeal pouches are the five branchial arches in which are developed five pairs of aortic arches. Between the bases of the first and second branchial arches, on

Pharyngeal peiuhts I-',

Htpatic diterticulii Ven'ral pamrr

Mtsoncphrk liibule vil/i gl

Fig. S7. — Diagiammatic ventral view of phaiynx, digestive tube, and mesonephroi of a 4-5 mm. embryo (based on reconstructions by Grosser and His). X about 30. The liver aod yolk sac are cut away. The tubules of the right mesonephros are shown diagrammatically.

the floor of the pharynx, is developed the tuberculum impar which perhaps forms a portion of the anterior part of the tongue. Posterior to this unpaired anlage of the tongue there grows out ventrally the anlage of the thyreoid gland. From the caudal end of the trachea have appeared ventrally the lung buds. The trachea is still largely a groove in the ventral wall of the pharynx and esophagus (Fig. 86). Caudal to the lungs a slight dilation of the digestive tube indicates the position of the stomach. The liver diverticulum has grown out from the fore-gut

ildlo-umhilical w

Fic. 88.— Venlral reconstruction of a 3.2 mm, cmbrjo, showing vessels (His).

Plattnlal iiltii<hmriU

Fic:. 8*).— lateral vicwof human cmliryoiif 4.2 mm.. shouin);.-u)rtirHrchrs ami venous tnmks (His). w.r, Muxillnry imM-rs-s; >.. nnlrrior cnnlinal vi'in: <~.:'.. |M>sleriiir cunliniil vein; nl, i<1ih'\>i.


into the ventral mesentery cranial to the wall of the yolk stalk. It is much larger than in the fifty-hour chick, where its paired anlage was seen cranial to the fovea cardiaca, and is separated from the heart by the septum transversum. The small intestine between the liver and yolk stalk is short and broad. In later stages it becomes enormously elongated as compared with the rest of the digestive tube. The yolk stalk is still expansive. The region of its attachment to the gut corresponds to the open mid-gut of the chick embryo. The hind-gut and tail fold of this embryo are greatly elongated as compared with the chick embryo of fifty hours. The hind-gut termiqates blindly in the tail. Near its caudal end it is dilated to form the cloaca. Into the ventral side of the cloaca opens the stalk of the allantois, Dorso-laterally the primary excretory (Wolffian) ducts which we saw developed in the fifty-hour chick have connected with the cloaca and open into it. Caudal to the cloaca, on the ventral side, is the cloacal membrane, which later divides and breaks through to form the genital aperture and anus. That part of the hind-gut between the cloaca and the yolk stalk forms the rectum, colon, caecum, and appendix, with a portion of the small intestine (ileum).

Urogenital Organs. — The opening of the primary excretory (Wolffian) ducts into the cloaca has been noted. These arc the ducts of the mid-kidney or mesonephros. At this stage the nephrotomes, which in the chick embryos formed the anlages of these ducts, are also forming the kidney tubules of the mesonephros which oi>en into the ducts (Fig. 87). The mid-kidneys project into the peritoneal cavity

as ridges on each side. A thickening of the mesothelium along the median halves of the mesonephroi forms the anlage of the genital glands or gonads (Fig. 220). 

Circulatory System. — The heart is an S-shaped double tube as in the fiftyhour chick. The outer myocardium is confined to the heart while the inner endothelial layer is continuous, at one end with the veins, at the other end with the arteries. The disposition of the heart tube is well seen in a ventral view of a younger embryo (Fig. 88). The veins enter the sinus vcnosus just cranial to the yolk sac. Next in front is the atrium with the convexity of its flexure directed cephalad. The ventricular portion of the heart is U-shaped and is flexed to the right of the embryo. The left limb is the ventricle, the right the bulbus. The arteries begin with the ventral aorta which bends back to the midline and dix-ides into five branches on each side of the pharynx (Figs. 88 and 89) . These are the aortic arches and they unite dorsally to form two trunks, the descending aorta. The aortic arches pass around the pharynx between the gill clefts in the


Fig. 90,— Erabrjos of Ui.

11 mm.}. From His' Normcnlatcl (Keibel and Elzc)


ftltfanchial arches. The arrangement is like that of the adult fish which has gill fits, branchial arches, and aortic arches to supply the gills. The descending

J »!.— Embrj-os at six to eight weeks (12.5

X 2.5. Stagu wi22) marks th« 

■Kortx run caudad and opposite the lung buds unite to form a single median dorsal prta. Tiiis, in the region of the posterior limb buds, divides into the two um

.}. From His' Normentafel (Kt:ibcl and Elzc). from embryo to fetus.


bilical arteries, which, curving cephalad and ventrad, enter the body stalk on each side of the allantois and eventually ramify in the villi of the chorion. The vitelline arteries^ large and paired in the chick, are represented by a single small trunk which branches on the surface of the yolk sac (Fig. 271). Compared with the arterial circulation of the chick of fifty hoiurs the important differences are (1) the development of the fourth and the fifth pairs of aortic arches, and (2) the presence of the chorionic circulation by way of the umbilical arteries in addition to the vitelline circulation found in the fifty-hour chick.

The veins are all paired and symmetrically arranged (Figs. 88 and 279). There are three sets of them: (1) The blood from the body of the embryo is drained, from the head end by the anterior cardinal veins; from the tail end of the body by the posterior cardinal veins. These veins on each side unite dorsal to the heart and form a single common cardinal vein which receives the vitelline and umbilical veins of the same side before joining the heart. (2) Paired vitelline veins in the early stages of the embryo drain from the yolk sac the blood carried to it by the vitelKne arteries. The trunks of these veins pass back into the body on each side of the yolk stalk and liver, and with the paired umbilical veins form a trunk which empties into the sinus venosus of the heart* As the liver develops it may be seen that the vitelline veins break up into blood spstces called by Minot sinusoids (Fig. 279). When the liver becomes large and the yolk sac rudimentary the vdtelline veins receive blood chiefly from the liver and intestine. (3) A pair of large umbiliccU veins which drain the blood from the villi of the chorion and are the first veins to appear. These unite in the body stalk, and, again separating, enter the somatopleure on each side. They run cephalad to the septum transversum where they unite with the vitelline veins to form a common vitello-umbilical trunk which joins the common cardinal and empties into the sinus venosus.

The veins of this embryo are thus like those of the fifty-hour chick save that the umbilical vessels are now present and take the place of the allantoic veins of later chick embryos. The veins, like the heart and arteries, are primitively paired and symmetrically arranged. As development proceeds, their sjonmetry is largely lost and the asymmetrical venous system of the adult results.

The later stages of the human embryo cannot be described in detail here. The student is referred to the texts of Minot, and Keibel and Mall. Two embryos will be compared with the pig embryos described in Chapter V. Figs. 90 and 91 show the human embryos described by His, the ages of which were estimated by him at from two weeks to two months. The figures show as well as could any description the changes which lead to the adult form when the embryo may be


called SL fetus (stage w). The external metamorphosis is due principally: (1) to changes in the flexures of the embryo ; (2) to the development of the face ; (3) to the development of the external structure of the sense organs (nose, eye, and ear) ; (4) to the development of the extremities and disappearance of the tail. The more important of these changes will be dealt with in later chapters.


The ages of the human embryos which have been obtained and described cannot be determined with certainty, because fertilization does not necessarily follow directly after coitus. It has been shown also that ovulation does not always coincide with menstruation so that the menstrual period cannot be taken as the starting point of pregnancy. In 1868, Reichert, from studying the corpus luteum in ovaries obtained during menstruation, concluded that ovulation takes place as a rule just before menstruation and that if the ovum is fertilized the approaching menstruation does not occur. Reichert then decided that a human embryo of 5. 5 mm., which he had obtained from a woman two weeks after menstruation failed to occur, must be two weeks, not six weeks, old. His accepted Reichert's views and since then the ages of embryos have largely been estimated on this basis. According to this view, Peter's ovum, obtained thirty days after the last period, is only three or four days old. This does not agree at all ^^-ith what is known of the age of other mammalian embryos.

From the observations of Mall and obstetricians of the present day, we must conclude that ovulation does not immediately precede menstruation, but that most pregnancies take place during the first or second week after the menstrual period. // is therefore more correct to compute the age of the embryo from the end of the last menstruation, or, according to Grosser, from the tcKth to the twelfth day before the first missed menstrual period. Peter's embryo then would be about fifteen days old. To compare an embryo with one of known age, the crown-rump length (i. e., from vertex to breech) is usually taken. Embryos of the same age vary greatly in size so that their structure must be taken into account. At the present time the exact relation of ovulation to menstruation is not known, nor the exact time required for the fertilized ovum to reach the uterus. The computed age of the embryo thus can be only approximate.

The period of gestation of the human fetus is usually computed from the beginning of the last menstrual period. Forty weeks or two hundred and eighty days is the time usually allowed. As some women menstruate once or more after becoming pregnant this is not a certain basis for computation.



The following are the estimated ages and lengths of human embryos, according to Mall, and their weights, according to Fehling:

Crown-heel Length Age. (CH), or Standing

Height (mm.).

Twenty-one days 0.5

Twenty-eight days 2.5

Thirty-five days 5.5

Forty-two days 11.0

Forty-nine days 19.0

Second lunar month 30.0

Third lunar month 98.0

Fourth lunar month 180.0

Fifth lunar month 250.0

Sixth lunar month 315.0

Seventh lunar month 370.0

Eighth lunar month 425.0

Ninth lunar month 470.0

Tenth lunar month 500.0

Crown-rump Length


(CR), OR Sitting


Height (mm.).



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