FCETAL MEMBRANES. 579 



pulled away from the maternal placentae; while in the discoidal placenta of 

 man, maternal and foetal parts are so closely related that both come away to- 

 gether as the after-birth or decidua. 



THE FCETAL MEMBRANES IN MAN. 



The fcetal membranes in man are characterized by the early development of 

 the amnion, the development of an extremely complicated discoidal placenta and 

 the rudimentary condition of the yolk sac and allantois. The high develop- 

 ment of the placenta the organ of interchange between fcetal and maternal 

 circulation is undoubtedly dependent upon the very long period of gestation 

 during which the human foetus leads an entirely parasitic existence, being 

 dependent wholly upon the mother for nutrition and respiration. The exten- 

 sive development of the placenta in turn explains the rudimentary condition of 

 the yolk sac and stalk and of the allantois, the nutritional and respiratory func- 

 tions of these large and important organs in some of the lower animals, being in 

 man taken up by the placenta. 



The Amnion. 



In describing the development of the germ layers in the human embryo, 

 comparisons were made between one of the youngest known human embryos 

 that of Peters and the embryos of the bat and mole (p. 99). Reference to this 

 description and to the figures shows that in the bat and mole the amnion is 

 formed, not as in the chick and rabbit by dorsal foldings of the somatopleure 

 and fusion of these folds, but in situ by a breaking down of some of the cells of 

 the inner cell mass and consequent cavity formation. In Peters' embryo the 

 amnion is already present as a closed cavity. The earlier stages in its forma- 

 tion are not known. As in the case of the germ layers, however, the appear- 

 ances in sections are so closely similar as to suggest at least, that the human 

 amnion is formed in the same manner as that of the bat and mole. 



In Peters 7 ovum (Fig. 74), also in Bryce-Teacher's (Fig. 493), the 

 amniotic cavity is seen already formed. It is roofed by a single layer 

 of flat cells apparently analogous to the trophoderm of the bat (Fig. 59). 

 As in the bat and chick this layer is continuous with the higher ecto- 

 derm of the embryo proper as represented here by the embryonic disk. The 

 extraembryonic mesoderm is already present at this stage between the ecto- 

 derm of the amnion and the trophoderm, the epithelial cells of the latter 

 being seen on the surface. Ventrally lies the yolk sac lined with entoderm, 

 while laterally between the entoderm and ectoderm is seen the embryonic 

 mesoderm. This formation of the amnion in situ considerably shortens the 

 process of amnion formation as compared with that in most of the lower 

 animals, where it is formed by dorsal foldings. This results in the very early 



