330 F. M. BALFOUR. 



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 concerned 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 

 independent 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. fiir Micr. Anat.,' Bd. iv, 1868) and 

 Owsjannikow (" Entwickelung der Coregonus," ' Bulletin der 

 Akad. St. Petersburgh,' vol. xix) in osseous fishes,^ Ray Lan- 

 kester (' Annals and Mag. of Nat. History,' vol. xi, 1873, 

 p. 81) in Cephalopoda, Gotte {' Archiv fiir 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 

 appearance. 



Formation of the Layers. 



Leaving these nuclei, I will now pass on to the formation 

 of the layers. 



At the close of segmentation the surface of the blastoderm 

 is composed of cells of a uniform size, which, however, are 

 too small to be seen by the aid of the simple microscope. 



The cells of this uppermost layer are somewhat columnar, 



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

 Chick" (' Archiv. fiir Micr. Anat.,' vol. x, 187H, p. 196), mentions that the 

 so-called cells in Osseous fishes which (Ellacher states to have migrated 

 into the yolk, and which are clearly the same as those mentioned by 

 Owsjannikow, are really not cells, hvii 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, 



