XIII 



PHYLUM CHORDATA 



59] 



Fki. 1218.— Five diagrammatic sections illustrating the formation of tho 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 commencing 

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

 witli 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 ftetal part of the placenta ; the cavity of the allantois is 

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

 vesicle ; ah. amniotic cavity ; al. allantois ; am. amnion ; cli. foetal part of placenta ; chz, 

 placental villi; rf, investing membrane ; d'. processes of in vesting membrane ; tW, embryonic 

 endoderm ; df. area vasculosa ; «'r/, stalk of umbilical vesicle ; ds, cavity of umbilical 

 vesicle ; c. embryo ; /</*, pericardial cavity ; i, non-embryonic endoderm ; Kh, cavity of 

 blastodermic vesicle ; A'.s. head-fold of amnion ; m. embryonic mesoderm ; m'. non-embryonic 

 mescderm ; /•, space between investing membrane and amnion; nh, chorion; w, tail-fold of 

 amnion ; st, sinus terminalis ; vt. ventral body-wall. (From Balfour, after Kolliker.) 



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