Chap. VI.] GENERAL EMBRYOLOGY. 99 



to the mesoblast of this region. On the ventral side of the 

 blastopore, mesoblast either grows inward from the epiblast, or 

 arises by differentiation of the larger yolk-containing cells. 



Eventually all the yolk-containing cells are converted into 

 mesoblast, or hypoblast, except a certain number, which eventu- 

 ally become enclosed in the mesenteron, by the growing of 

 the hypoblast round them above the mesoblast (me. in Fig. 36, 

 iii.), so as to bring them inside the digestive cavity, where they 

 form a small internal yolk-sac, the cells of which are gradually 

 absorbed. 



The formation of the notochord, and the differentiation of 

 the mesoblastic plates into vertebral and lateral plates ; and 

 the permanent cleavage of the mesoblast in these lateral plates, 

 so as to form a splanchnic and a somatic layer, with a body- 

 cavity between them; all this takes place in a manner sub- 

 stantially similar to that described in the illustrative case. 



(2.) We have seen that in the fowl's egg segmentation con- 

 verts the germinal disc into a two-layered blastoderm, the cells of 

 which are smaller in the superficial than in the deeper stratum. 

 The central part of the blastoderm (area pellucida) rests on a 

 clear fluid contained in a shallow space (Fig. 34, iii.) ; its edges 

 (area opaca) rest on unsegmented white yolk (yk.), and appear 

 as an opaquer rim. 



The hypoblast is not, in the fowl, produced by imagination. 

 The two layers of the original blastoderm are directly converted 

 into epiblast and hypoblast. The former (epiblast) is constituted 

 from the first (i.e. before incubation) of smaller cells arranged 

 as a definite membrane. The lower layer is rapidly converted, 

 under the influence of incubation, into a definite hypoblastic 

 membrane of flattened cells. These form the roof of the shallow 

 space in the area pellucida, which may therefore be regarded as 

 the mesenteron. A segmentation cavity is only found at a very 

 early stage, and is soon obliterated by the rapid growth of the 

 lower layer cells. Between the hypoblast and epiblast there 

 remain some undifferentiated cells, but they do not form a 

 distinct intermediate layer. 



In the fowl, then, we seem to see carried further a process 



