572 



ZOOLOGY 



SECT 



and inwards, eventually meeting in the middle over the body 

 of the embryo, and uniting in such a way as to form two layers. 

 Of the two layers thus formed the outer, consisting of trophoblastic 

 ectoderm and somatic mesoderm, simply constitutes a part of 

 the extra-embryonic somatopleure which forms a complete 

 investment for the entire ovum, and is known as the chorion 

 or sub-zonal membrane (Fig. 1230, 2 and 3). In the account of the 

 development of the Bird it has been referred to as the false amnion 

 or serous membrane. The inner layer or true amnion, as in the Bird, 



do 



ajf 



FIG. 1229. Embryo Rabbit, of about nine clays, from the dorsal side. a. aorta ; aft, optic 

 vesicle; a/, fold of amnion; ao. area opaca; ap, area pelludda ; h, hz, heart; h', h", 

 medullary plate in the region of the future fore-brain ; h", medullary plate in the region 

 of the future mid-brain ; Maud hh'", hind-brain ; mh, mid-brain ; ph, pericardial section 

 of body cavity ; pr, primitive streak ; pz, lateral zone ; rf, medullary groove ; wio, proto- 

 vertebrae; stz, vertebral zone; vd, pharynx; vo, vitelline vein. (From Balfour, after 

 Kolliker.) 



forms the wall of a cavity the amniotic cavity (4 and 5, ah) which 

 becomes tensely filled with fluid (the liquor amnii) over the body of 

 the embryo ; this serves the purpose of protecting the delicate 

 embryo from the effects of shocks. As in the case of the Bird, the 

 folds giving rise to the amnion and serous membrane may consist 

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

 pro-amnion, consists solely of ectoderm and endoderm) of somatic 

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



