204 



EMBRYOLOGY. 



of the embryo furnishes in the case of the Reptiles and Birds the 

 yolk-sac and certain embryonic membranes. I shall speak of the 

 development of these in the next chapter. 



The fate of the extra-embryonic area of the blastoderm in Fishes 

 is more simple, since there is formed from it only a sac for the 

 reception of the yolk. 



Fig. 123 exhibits the embryo (Em) of a Selachian, which has 

 arisen by the infolding of a small area of the germ-layers in the 



manner described for 



Era> the Chick. All the 



remaining part of 

 the egg has become 

 a great yolk-sac (ds), 

 which is united with 

 the middle of the 

 belly by means of a 

 long stalk. 



The Teleosts (Plate 

 I., fig. 6) show us 

 transitions from this 

 (Pristiurus), after condition to one in 

 which the yolk-sac, 

 as in Amphibians, 

 is not separated by 



a stalk from the mesenteron, but represents only a capacious 

 enlargement of the latter and of the belly-wall. 



Let us now examine more carefully the structure of the yolk-sac. 

 As has been remarked already, all four of the germ-layers spread 

 themselves out one after another around the unsegmented yolk-mass 

 of meroblastic eggs (Plate I., figs. 6 and 7). As in the embryonal 

 body the two middle germ-layers separate from each other and allow 

 the body-cavity to appear between them, so, too, at a later stage 

 the same process occurs in the extra-embryonic area. Throughout 

 the region of the middle germ-layer there is formed a narrow 

 fissure, for which the name " extra-embryonic body-cavity," or 

 blastospheric coelom (cavity of the blastoderm, KOLLIKER), would be 

 most suitable. It separates the envelope of the yolk into two layers, 

 of which the inner is the immediate continuation of the intestinal 

 wall (splanchnopleure), the outer, on the contrary, that of the body- 

 wall (somatopleure). Therefore, to be exact, we have before us a 

 double sac formed around the yolk, which we can distinguish as 



Pig. 123. Advanced embryo of a Shark 



BALFOUR. 

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



v itellina ; vv, vena vitellina. 



