XIII 



PHYLUM CHORDATA 



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FIG. 1218. 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 commencing 

 formation of yolk-sac and amnion ; 3, embryo with atniiion 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 ; , ectoderm of embryo ; a', ectoderm of non-embryonic part of the blastodermic 

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

 placental villi ; d, investing membrane ; d'. processes of investing membrane ; dd, embryonic 

 endodertn ; df. area vasculesa ; dy, stalk of umbilical vesicle ; ds, cavity of umbilical 

 vesicle ; e. embryo ; hit, pericardia! cavity ; i, non-embryonic endoderm ; Kk, cavity of 

 blastodermic vesicle ; Kn. head-fold of amnion ; j/t. embryonic mesoderm ; m'. non-embryonic 

 rnescderm ; /, space between investing membrane and amnion ; ah, chorion ; an, tail-fold of 

 amnion ; at, sinus terminalis ; vl. ventral body- wall. (From Balfour, after Kb'lliker.) 



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