HeiT B. Lwoff on the Formation of the 



genous and endoblastogenous mesoderm cells. In this brief 

 communication, however, devoid as it is of figures, I cannot 

 enter more closely into these details, and must postpone an 

 account of them until the publication of the longer paper 

 which is to follow. 



In passing on to the description of the corresponding deve- 

 lopmental processes in the meroblastic ova of Selachians and 

 Teleostean fishes, I must at the outset join issue with those 

 embryologists who, following the example of Hwckel, would 

 regard the yolk merely as a store-room from which the germ 

 derives nutritive matter, and who would deny to the vitelline 

 elements all participation in the formation of the embryo. 

 Altliough this view must now be regarded as an anachronism, 

 there are still some investigators even at the present time 

 who assume that the yolk in Teleosteans and Selachians 

 takes no share in segmentation. In contradiction to these 

 embryologists I have to state that, according to the results at 

 which I have arrived, the entire endoderm (the definitive 

 intestine and the endoblastogenous mesoderm) in Teleostean 

 fishes as well as in Selachians owes its origin to the vitelline 

 elements. 



I will commence the description of my investigations with 

 the Teleostean fishes, which, owing to the fact that their ova 

 contain less yolk, approach the Amphibians more closely in 

 these processes than do the Selachians, in which the same 

 developmental processes are more modified, probably in con- 

 sequence of the larger quantity of yolk. In the case of all 

 the Teleosteans studied by me {Lahrax, Julis, and Gohius), 

 no sharp boundary can be distinguished between the blasto- 

 derm cells and the yolk. The lower blastoderm cells are so 

 intimately connected with the subjacent yolk that no dividing- 

 line is to be seen. These cells divide in an equatorial direc- 

 tion, and, moreover, in such a way that the uppermost cell 

 vv^hich is thus produced becomes constricted off and joins the 

 blastoderm cells, while the lower one, on the contrary, remains 

 in connexion with the yolk. I have observed no trace of a 

 segm.entation cavity in any of the Teleosteans which I have 

 investigated. After the blastoderm has been formed and the 

 blastoderm cells commence to grow round the yolk we may 

 observe upon the surface of the latter a continuous layer of 

 protoplasm with nuclei, around which the outlines of cells 

 are sometimes to be seen. This is the intermediate layer of 

 authors, the nuclei of which, which are direct descendants of 



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