370 HeiT B. Lwoff on the Formation of the 



formed from the same elements (ectoderm cells) as those from 

 •which the nervous system is also developed. In this rudi- 

 ment the central portion (the rudiment of the notochord) 

 becomes differentiated from the lateral regions, from which 

 the mesoderm arises. But here also it may be seen that the 

 endoderm cells take part in the formation of the mesoderm, 

 since the cells which arise from the yolk-nuclei become con- 

 stricted off and join the rest of the mesoderm cells. Thus in 

 the present case, as in that of other Vertebrates, we can 

 distinguish a twofold source whence the mesoderm arises 

 (ectoblastogenous and endoblastogenonsj. While the forma- 

 tion of the notochord and mesoderm is taking place there is 

 formed upon the surface of the yolk from the yolk-nuclei the 

 continuous layer of cells from which the intestine is deve- 

 loped. In this manner in Teleosteans also the intestine does 

 not owe its origin to an invagination, but arises from the 

 derivatives of yolk-nuclei. 



While the cells of the blastoderm are o-vowiiio- round the 

 yolk, the yolk-nuclei which are in process of multiplication 

 commence to spread out over the latter, so that soon the 

 entire periphery of the yolk is provided with these nuclei. I 

 must emphatically controvert the view, which is held by 

 many investigators, that these nuclei perish without taking 

 part in the formation of the embryo. It has already been 

 mentioned that the yolk-nuclei give rise to the intestine and 

 participate in the formation of the lateral mesodermal rudi- 

 ments ; but I must add that the nuclei also which are situated 

 in the lower (ventral) portion of the yolk take a share in the 

 formation of the embryo, for in my preparations I notice 

 mitoses in this region, and it may be observed that the cells 

 which are here formed from the yolk-nuclei become con- 

 stricted off and join the blastoderm cells which have grown 

 round the yolk. I am inclined to assume that these cells are 

 homologous with the ventral mesoderm of the Amphibians. 

 In subsequent stages it may be seen that the yolk-nuclei take 

 part in the formation of the liver, since a portion of the cells 

 which arise from these nuclei is directly transformed into 

 hepatic cells. The importance of the role of the yolk-nuclei 

 cannot therefore be open to doubt. 



I will now pass on to the Selachians. 



Although at the present time Selachians are the favourite 

 subject of embryologists, the latter are nevertheless not agreed 

 as to many important developmental processes in these 

 animals, and, above all, as to the origin of the intestine. 

 While certain investigators regard the intestine as originating 

 through invagination, the rest assert that it is formed from 



