xiii PHYLUM CHORDATA 598 



effects of shocks. As in the case of the Bird, the folds giving 

 rise to the am n ion and chorion or serous membrane may consist 

 from the first (except the head-fold, which, being formed from the 

 proamnion, consists solely of ectoderm and endoderm) of somatic 

 mesoderm as well as ectoderm (trophoblast) : or mesoderm may 

 extend into them later, so that, either from the first, or as a result 

 of outgrowth which takes place subsequently, the serous membrane 

 contains mesoderm as well as ectoderm. The ectodermal cells 

 trophoblast cells of the chorion may enter into close relationship 

 with the mucous membrane of the wall of the uterus, and send 

 out processes or primary villi (Fig. 1219, EK) by means of which 

 the ovum becomes intimately attached, and by means of which 

 perhaps nourishment is absorbed. 



In certain Mammals the history of the amnion is very different 

 from that above described. In the Hedgehog (Fig. 1220), for 



FIG. 1220. A C, diagram illustrating the formation of tin amnion and trophoblast in the 

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

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

 ectoderm is growing up below the chorionic from the edges of the ectodermal floor ; C", stage 

 in which the amniotic ectodenn completely roofs over the cavity. (After Hubrecht.) 



example, a cavity appears in the ectoderm of the embryonic 

 area ; this is destined to give rise to the cavity of the amnion. 

 The ectoderm which forms its roof is entirely trophoblastic or 

 chorionic ; that which forms its floor is partly destined to become 

 amniotic ectoderm, partly embryonal ectoderm. After the meso- 

 derm has begun to become differentiated, the margins of the 

 amniotic part of this ectodermal floor (B} begin to grow upwards, 

 giving rise to a layer which extends over the roof 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 

 single cavity the primitive amniotic 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. 1221, A), 



