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 attain&d 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 



Em 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. 

 119). The embryonic 

 area, by means of the- 

 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 : fir&t the 

 metamorphosis of the flattened embryonic area sato 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, bvit 

 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 



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



BALFOUB. 

 Em, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; av, arteria 



vitellina ; vv, vena vitellina. 



