74 



HUMAN EMBRYOS AND FETAL MEMBRANES 



The Chorion. The human chorion is derived directly from the outer 

 trophectoderm layer of the blastodermic vesicle and from the extra- 

 embryonic somatic mesoderm. At first, its structure resembles that of 

 thej pig's chorion. The trophectodeim of the human embryo, however, 

 early gives rise to a thickened outer layer, the trophoderm (syncytial and 

 nutrient layer Figs. 74 C and 239). When the developing embryo comes 



Ectoderm of amnion 

 Ectoderm of embryo \ 



Allantoi 



D 



Body stalk 



Ectoderm of 

 mniotic cavity Cavity O f amnion 



Mesoderm of 



rophectoderm amnion 



Yolk sac Ectoderm of 

 chorion 

 Entoderm Cavity of yol 



sac 



Splanchnic Entoderm 

 mesoderm yo ik S ac 



'ra-embry- Mesoderm' 

 omc ccdom O f yo ik sac 

 Extra-embry- 

 onic ccelom 



Mesoderm 

 of chorion 



~Choriomc meso- 

 derm 

 -Trophoderm 



Chorionic vitii 



FIG. 74. Four diagrams of early human embryos (based on figures of Robinson and 

 Minot). A, Hypothetical stage; B, Bryce-Teacher embryo (modified); C, Peters' embryo; 

 D, Graf Spee's embryo. 



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 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 from the embryo as the um- 



