68 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



As to the final destination of these cells, my observations 

 are not yet completed. Probably a large number of them are 

 concerned in the formation of the vascular system, but I will 

 give reasons later on for believing that some of them are con- 

 cerned in the formation of the walls of the digestive canal and 

 of other parts. 



I will conclude my account of these nuclei by briefly 

 summarizing the points I have arrived at in reference to 

 them. 



A portion, or more probably the whole, of the yolk of the 

 Dog-fish consists of organized material, in which nuclei ap- 

 pear and increase either by division or by a process of in- 

 dependent formation, and a great number of these subse- 

 quently become the nuclei of cells formed around them, 

 frequently at a distance from the germ, which then travel up 

 and enter it. 



The formation of cells in the yolk, apart from the general 

 process of segmentation, has been recognised by many ob- 

 servers. Kupffer (Archiv. fur Micr. Anat., Bd. IV. 1868) and 

 Owsjannikow ('' Entwickelung der Coregonus," Bulletin der 

 Akad. St Petersburg^ Vol. XIX.) in osseous fishes 1 , Ray Lan- 

 kester (Annals and Mag. of Nat. Hist. Vol. XI. 1873, p. 81) in 

 Cephalopoda, Gotte (Archiv. fur Micr. Anat. Vol. X.) in the 

 chick, have all described a new formation of cells from the 

 so-called food-yolk. The organized nature of the whole 

 or part of this, previous to the formation of the cells from 

 it, has not, however, as a rule, been distinctly recognised. 

 In the majority of cases, as, for instance, in Loligo, the 

 nucleus is not the first thing to be formed, but a plastide is 

 first formed, in which a nucleus subsequently makes its ap- 

 pearance. 



1 Gotte, at the end of a paper on "The Development of the Layers in the Chick " 

 (Archiv. Jiir Micr. Anat., Vol. X. 1873, P- J 9 6 ). mentions that the so-called cells in 

 Osseous fishes which Oellacher states to have migrated into the yolk, and which are 

 clearly the same as those mentioned by Owsjannikow, are really not cells, but large 

 nuclei. If this statement is correct the phenomena in Osseous fishes are precisely the 

 same as those I have described in the Dog-fish. 



