Vertehrata. 



353 



Segmentation of the ovum ia total in some of the 

 Vertebrata — AmphioxuSj Cyclostomi, Ganoidei, Amphibia (with the 

 exception of Coeoilia and several others), and most Mammalia ; in 

 others, in which the egg is large, segmentation is partial (Selachii, 

 Teleostei, Eeptilia, Aves, Monotrema). As in the lower animals 

 a gastrula is formed, Amphioxus offering the simplest instance 

 (see p. 43), the formation in others being more complicated (pp. 43-45) ; 

 the mode of gastrula formation in Mammalia is not yet fully 

 elucidated. Most Vertebrate embryos are for a long time provided 

 with a yolk-sac {see p. 49), which attains a huge size in some {e.g., the 

 Selachians), but has usually vanished or is no longer visible when the 

 animal is born {i.e., leaves the egg-shell or the body of the parent.) 

 In Reptilia, Aves, and Mammalia (the amniote Vertebrata), certain 

 peculiar conditions may be observed : the embryo is surrounded by 

 several embryonic membranes, which develop as special 

 outgrowths of the young animal. These are embryonic organs and 

 are thrown off at birth. 



In the Hen's egg, at a very early stage of development, a fold, consisting of 

 epiblast and the outer layer of mesoblast, is formed round that portion which 

 will develop into the embiyo itself, as distinct from the yolk-sac portion. This 

 fold gi-adually gi-ows round the whole embiyo, its walls meet and fuse, and thus 

 a cavity is formed above, limited by the inner layer of the coalesced fold.. 

 This inner layer is now called the amnion whilst the oiiter layer, which is 



Fig. 292. Illustrating the development of the embryonic membranes in a 

 bird embryo ; diagrammatic longitudinal sections of various stages. In A the development 

 of the membranes has not begun, ek epiblast, en hypoblast, m mesoblast (broader line), 

 am amnion, aw/ folds from which the amnion and serous membrane originate, s serous 

 membrane, al allantois, II food yolk, t gut. — Orig. (partly after older figures). 



A A 



