198 



EMBRYOLOGY. 



voluminous the accumulated yolk-material, the more time it requires : 

 thus, for example, in the case of Birds it is completed at a very late 

 stage of development, when the embryo has already attained a high 

 state of perfection (Plate I., fig. 5). 



In the case of meroblastic eggs, the part of the germ-layers 

 on which the first fundaments of the organs (neural tube, chorda, 

 primitive segments, etc.) appear has been distinguished as the 

 embryonic area from the remaining part, or the extra-embryonic area. 

 The distinction is both fitting and necessary ; but the names might 

 have been more appropriate than " embryonic and extra-embryonic," 

 since obviously everything that arises from the egg-cell, and con- 

 sequently even that 



which originates in 



the extra-embryonic 

 area, must be rec- 

 koned as belonging 

 to the embryo. The 

 differentiation into 

 two areas persists in 

 the course of further 

 development, and be- 

 comes expressed still 

 more sharply (fig. 



Fig. 119. Advanced embryo of a Shark (Pristiurus), after 119). The embryonic 

 BALFOUR. , , 



Em, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; av, aiteria area > DV means Or the 



vitellina ; w, vena vitellina. folding of its flattened 



layers into tubes, 



alone forms the elongated, fish-like body which all Vertebrates a' 

 first exhibit ; the extra-embryonic area, on the contrary, becomes 

 a sac filled with yolk (ds), which, like an enormous hernia, is united 

 to the embryo (Em) by means of a stalk (st) attached to its belly, 

 sometimes even while the embryo is still remarkably small. 



We must now explain more minutely the details of the processes 

 of development which take place in this connection: first the 

 metamorphosis of the flattened embryonic area iato the fish-like 

 embryonal body, and secondly the formation of the yolk-sac. 



In the presentation we shall adhere chiefly to the Hen's egg, but 

 for the time being we shall leave out of consideration the formation 

 of the embryonic membranes. 



The body of the Chick is developed by a folding of the flattened 

 layers, and by the constricting off of the tubular structures thus formed 



