PHYLUM CHORDATA 



591 



FiQ. 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 amnion about to close ; 4. embryo 

 with villous chorion, larger allantois, and mouth and anus ; 5, cmbr^'o in which 

 the mesoderm of ;tbe allantois has extended round the itnicr 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 

 Teaicle ; ah. amniotic cavity; al. allantois; am. amnion; ch. fcetal part of placenta; cliz, 

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

 endoderm ; df. area vasculesa ; dg, stalk of umbilical vesicle ; ds, cavity of umbilical 

 vesicle ; e. embryo ; ?ih, pericardial cavity ; i, non-embryonic endoderm ; Kk, cavity of 

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

 meacdenn ; r, space between investing membrane and amnion ; sIl, chorion ; sa, tail-fold of 

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



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