54 



HUMAN EMBEYOLOGY. 



growth of foetal vessels into the foetal mesodermal cores. The secondary villi, 

 therefore, consist of a mesodermal core covered by a layer of cellular trophoblast 

 and a layer of plasmodium, the latter lying outside the former. Still later the 



secondary villi send out numer- 



_-4 ; Mesoderm of amnion 



- Ectoderm of amnion 

 Allantoic diverticulum 

 of entoderm vesicle 



Body stalk mesoderm 

 Extra-embryonic ccelom 

 Entoderm 



Mesoderm covering of 

 entoderm vesicle 



Neurenteric canal 



Cavity of entodermal vesicle 

 FIG. 70. SCHEMA OF SAGITTAL SECTION OF ZYGOTE ALONG LINE A. 



( Plasmodial trophoblast Neural groove 



Chorion { Cellular trophoblast 



\ Mesoderm lining of trophoblast^ 



Amnion cavity 

 Extra-embryonic coelom 



Mesoderm of amnion - 

 Ectoderm of amnio 



Mesoderm covering 

 entoderm 



Entoderm 



Cavity of entodermal 

 vesicle 



ous branches into the blood 

 [blast spaces,and thus increase greatly 



Mesoderm lining of tropho- in Complexity (FigS. 75, 76, 77). 



development progresses 

 still further a part of the chorion 

 is converted into the fcetal 

 portion of an organ called the 

 placenta, and thus the chorion 

 is divided into placental and 

 non - placental regions. Upon 

 the placental part the villi con- 

 tinue to increase, but they dis- 

 appear entirely from the non- 

 placental part, which is then 

 called the chorion Iseve (Fig. 



The Amnion, the Body- 

 Stalk (Allantoic Stalk), and 

 the Umbilical Cord. The 



amnion is formed from that 

 portion of the wall of the larger 

 of the two inner vesicles of the 

 zygote, the ecto - mesodermal 

 vesicle (p. 22), which does 

 not take part in the formation 

 of the embryo. It consists of 

 ectoderm cells covered exter- 

 nally by a layer of extra-em- 

 bryonic mesoderm, and it is 

 continuous with the margin 

 of the embryonic area (Figs. 

 70, 71). 



The cavity of the ecto- 

 mesodermal vesicle, enclosed 



between the amnion and the embryonic area, is the cavity of the amnion ; it 



is filled with fluid, which raises the amnion in the form of a cupola over the 



embryonic region (Fig. 70). 



The Body-Stalk (Allantoic Stalk). It has been noted already that the mesoderm 



of the median part of the posterior or caudal portion of the amnion becomes 



Notochord 



FIG. 71. SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG 

 LINE B (Fig. 31). 



Plasmodi 

 trophoblast 



Plasmodial 

 tropho 

 Cellular 

 trophobl 

 Mesoder: 

 Ectode 

 of amnion 



Plasmodial 

 trophoblast 



Cellular 



trophoblast 



Efferent vessel 



of villus 



Fused mesod 



of c 



and amnio: 

 Ectoderm/ 

 of amnion 



'Afferent vessel of villus 



Fused mesoderm of 

 r - "amnion and chorion 

 Ectoderm of amnion 



FIG. 72. SCHEMA OF THREE STAGES IN THE FORMATION OP A CHORIONIC VILLUS. 



thickened. In the thickened strand lies the allantoic diverticulum of the 

 entodermal vesicle (Fig. 70), whilst through it, on either side of the allantoic 

 diverticulum, pass the umbilical arteries and veins, by means of which blood is 

 conveyed between the embryo and the chorion. 



This segment of the wall of the amnion vesicle was termed by His the 

 body-stalk. It takes no direct part in the formation of the embryo, and as it 



