MYLUM CHORDATA 



575 



of this ectodermal floor (B) begin to grow upwards, giving rise to a 

 layer which extends over the soof on the inner side of the chorionic 

 ectoderm and eventually (C) forms a complete layer the ectodermal 

 layer of the amnion. 



In the Mole (Talpa) spaces appear in the layer of ectoderm of 

 the embryonal area, and these subsequently coalesce to form a 



FIG. 1232. A C, diagram illustrating the formation of the amnion and trophoblast in the 

 Hedgehog. Only the ectoderm is represented. A, early stage in which the amniotie 

 cavity has appeared, roofed over by chorionic ectoderm ; B, later stage in which the 

 amniotie ectoderm is growing up below the chorionic from the edges of the ectodermal floor ; 

 C, stage in which the amniotie ectoderm completely roofs over the cavity. (After 

 Hubrecht.)- 



single cavity the primitive amniotie cavity, but this has only a 

 temporary existence, the amnion arising later by the formation of 

 a series of folds. In Mus, Arvicola, and others (Fig. 1233, A), the 

 amnion is developed from a series of folds of the ectoderm which 

 arise beneath the trophoblast. In other Mammals (B) the amnion 

 arises in the manner already described, and the portion of the tro- 

 phoblast imme- 

 diately overlying - *-- 



the embryonic 

 part of the ecto- 

 derm eventually 

 disappears. 



The allantois 

 has, in all essen- 

 tial respects, the 

 same mode of 

 development as 

 in the Bird, aris- 

 ing in most cases 

 as a hollow out- 

 growth from the 

 hinder part of 



the alimentary canal ; this, growing out into the space (extra- 

 embryonic ccelome) between the chorion and the amnion, becomes 

 in all the Eutheria applied to the former, and unites with 

 it to contribute towards the formation of the placenta. But in 



N N 2 



Q 



FIG. 1233. Diagram illustrating the mode of formation of the 

 amnion in various Mammals. A, commencing formation of 

 the amnion in Mus, Arvicola, etc. The asterisk marks what 

 corresponds to the portion of the trophoblast overlying the 

 embryo in Fig. 1232, C. B, mode of formation of the amnion 

 in many Mammals. The portion of the trophoblast indicated 

 by the asterisk in A disappears before the amniotie folds make 

 their appearance. (After Hubrecht.) 



