72 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



chorion. Into it there has grown from the gut of the embryo the entodermal diver- 

 ticulum of the allantois. 



The Chorion. — The human chorion is derived directly from the outer troph- 

 ectoderm layer of the blastodermic vesicle and from the extra-embryonic somatic 

 mesoderm. Its early structure resembles that of the pig's chorion. The troph- 

 ectoderm of the human embryo early gives rise to a thickened outer layer, the 

 trophoderm (syncytial and nutrient layer — Figs. 74 C and 239) . When the develop- 



Ertoderm 



Amniotic cavity 



Ccelom 

 Trophectoderm 

 Archenterori' 

 Entoderm 



Mesoderm 



C 



Ectoderm of embryo 



Ectoderm of amnion 



Allantois 



D 



Body stalk 



Ectoderm of embryo 



Cavity of amnion 



Mesoderm of 



amnion 



Ectoderm of 



chorion 



Cavity of yolk 



sac' 



Entoderm of 



yolk sac 



Mesoderm 



of yolk sac 



Exlra-embry- 



„, . . onic calom 



Lnortomcmeso- 



Amniolic cavity 



Trophectoderm 

 Yolk sac 

 Entoderm 



Splanchnic 

 mesoderm 

 Extra-embry- 

 onic ca:lom 



derm 

 Trophoderm 



Mesoderm, 

 of chorion 



Chorionic villi 



^^w^ 



%"■ 



Fig. 74. — Four diagrams of early human embryos (based on figures of Robinson and Minot). A, Hj-- 

 pothetical stage; B, Bryce-Teacher embryo (modified); C, Peter's embryo; D, Graf Spee's embryo. 



ing embryo comes into contact with the uterine wall the trophoderpi destroys the 

 maternal tissues. The destruction of the uterine mucosa serves two purposes: 

 (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 villi. The villi are bathed in maternal 

 blood, and in them blood vessels are developed, the trunks of which pass to and 



