560 



ZOOLOGY 



SECT. 



no means the most primitive. In certain Mammals the history 

 of the amnion is very different, and an explanation of the facts 

 appears to be best sought by regarding certain of those modes of 

 formation as more primitive than what we find in the Rabbit or 

 the Fowl. In the Hedgehog (Fig. 1151), for example, there is no up- 



B 



Fie. 1151. A C, diagram illustrating the formation of the amnion and trophoblast in the 

 Hedgehog. Only the ectoderm is represented. A, early stage in which a layer of 

 ectoderm destined to form both amnion and the neighbouring portion of the trophoblast has 

 become separated off from the embryonic ectoderm (represented by the thick band) by a 

 distinct .space which will become the anmiotic cavity ; B, later stage in which the trophoblast 

 is becoming separated from the amnion at the sides ; C, stage of the complete separation of 

 the trophoblast as a vesicle enclosing the entire blastodermic vesicle as well as the amnion. 

 (After Hubrecht.) 



growth and coalescence of folds of the ectoderm or somatopleure ; 

 instead, the upper portion of the ectoderm over the embryonic 

 area becomes split off from the underlying ectoderm destined 

 to become the ectoderm of the embryo (A), remaining continuous 

 with it >round the edges, but separated by a distinct space 



FIG. 1152. 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. 1151, C ; B, mode of 

 formation of the arnnion in many Mammals. The portion of the trophoblast indicated by 

 the asterisk in-^A disappears before the anmiotic folds make their appearance. (After 

 Hubrecht.) 



(destined to give rise to the amniotic cavity) over the body of 

 the embryo itself. This layer of ectoderm then divides (B) into 

 two strata an outer, the troplioblast or ectoderm of the serous 

 membrane, and an inner, the ectoderm of the amnion. The 

 former becomes afterwards extended, by the extension of the split- 

 ting off of the surface layer of ectoderm, over the entire surface 



