42 



GASTKULA THEOKY 



The gastrula theory and the mammalian ovum. It is impossible here to deal 

 at length with the gastrula theory, because the evidence can only be satisfactorily presented 

 by extensive comparative treatment. A short statement must therefore suffice. 



not.pl. 



p.s. 



all 



FlG. 62. SUBFACE VIEW OF A 

 BLASTODEBM OF CEBCOCEBUS 



CYNOMOLGUS. (After Selenka.) 



n.c., neurenteric canal ; p.s., 

 primitive streak. 



FIG. 63. SUBFACE VIEW OF A BLASTODEBM OF 

 HYLOBATES CONCOLOB. (After Selenka.) 



The amnion has been opened to expose the 

 germinal disc. 



am, cut edge of amnion ; y.s., yolk-sac ; not.pl., 

 notochord-plate ; n.c., neurenteric canal ; 

 2>.s., primitive streak ; all, allantoic diverti- 

 culum in connecting stalk. 



It will be observed that in the mammal the two primary layers of the blastoderm, at least 

 their principal part, are formed by a separation into two strata of the cells of the inner granular 

 mass, which occupies the interior of the ovum after segmentation. The bilaminar condition 



n.f. 



ent not.pl. ent 



FIG. 64. TBANSVEBSE SECTION THBOUGH BLASTODEBM OF DOG. (After Bonnet.) 

 n.p., neural plate ; n.f. neural folds; not.pl., notochordal plate ; ent, entoderm ; mes, mesoderm. 



may therefore be said to result from a process of delamination in an originally simple 

 mass or stratum. But in Amphioxus amongst vertebrates, and in many invertebrates with 

 holoblastic (alecithal) ova, the bilaminar blastoderm is produced not by delamination, but by 



