EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 8 1 



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 em- 

 bryo the entodermal diverticulum 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 



Amniotic cavity 

 Coelom 

 Trophecto derm 

 Archenteron 



Entoderm 

 Mesoderm 



Ectoderm of amnion 

 Ectoderm of embryo 



/Immune aunty 



TrophecToderm 



YolK-sac 

 Entoderm 



Splanchnic 

 r meSoderm 



Ectoderm of embryo 

 Cavity of amnion 

 Mesoderm of amnion 



Ectoderm of chorion 



Cavity of yolK 

 sac 



Entoderm of. 

 yolK-sac 



Mesoderm of 



Allantois 



Extraembryonic yolk-Sac 

 coelom ' 



Extraembryonic 

 coelom 



Mesoderm of Chorion 



Chorionic villi 



Fig. 70. — Four diagrams showing hypothetical stages of early human embryos (based on figures of 



Robinson and Minot). 



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

 trophoderm (syncytial and nutrient layer). When the developing embryo comes 

 into contact with the uterine wall the trophoderm destroys the maternal tissues. 

 The destruction of the uterine mucosa serves two purposes: (i) 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 nutri- 

 6 



