XU1 



PHYLUM CHORDATA 



573 



FIG. 1230. Five diagrammatic sections illustrating the formation of the foetal membranes 

 of a Mammal. In 1, 2, 3, 4 the embryo is represented in longitudinal section. 1, Embryo 

 with zona pellucida, blastodermic vesicle, and embryonic area ; 2, embryo with com- 

 mencing formation of yolk-sac and amnion ; 3, embryo with amnion about to close ; 4, 

 embryo with villous chorion, larger allantois, and mouth and anus ; 5, embryo in which 

 the mesoderm of the allantois has extended round the inner surface of the chorion and 

 united with it to form the foetal part of the placenta ; the cavity of the allantois is aborted. 

 a, ectoderm of embryo ; a', ectoderm of non-embryonic part of the blastodermic vesicle ; 

 ah. amniotic cavity ; al. allantois ; am. amnion ; ch. foetal part of placenta ; chz, placental 

 villi ; d, in 1 zona radiata, in 2 and 3 chorion ; d', processes of zona radiata and chorion ; 

 dd, embryonic endoderm ; df. area vasculosa ; dg, stalk of umbilical vesicle ; ds, cavity 

 of umbilical vesicle ; e. embryo ; hh, pericardial cavity ; i, non-embryonic endoderm ; 

 Kh, cavity of blastodermic vesicle ; Ks. head-fold of amnion ; m. embryonic mesoderm ; 

 m', non-embryonic mesoderm ; r, space between chorion and amnion ; sh, subzonal 

 membrane (chorion); ss, tail-fold of amnion; si, sinus terminalis ; sz, villi of chorion; 

 r/. ventral body wall. (From Foster and Balfour, after Kolliker.) 

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