CELLS. 25 



fering only in the circumstance that it is not the whole yelk, 

 hut a greater or lesser portion, according to the animal, which 

 invests the nascent nuclei. • 



When the process has attained a certain stage, the cleavage- 

 masses all together, or in successive layers, surround them- 

 selves with membranes, and become actual cells, whence we 

 are justified in considering this to be a process of endoge- 

 nous cell-development. In fact, it is nothing else than an 

 introduction to cell-development in the egg-cell, and differs 

 from the ordinary phenomena of that class only in this, that 

 firstly, the nucleus of the parent cell or the germinal vesicle, 

 in most cases (Miiller saw a division of it occur in the Mol- 

 luscs which are developed within Synapta digitata 1 ) has nothing 

 to do with it ; secondly, that the parent cell itself persists ; 

 and thirdly, that the investing globules developed by the 

 successive multiplication of nuclei become cells only in the 

 latest generations. This view is the more justifiable, as the 

 cells which have arisen in consequence of the metamorphosis 

 of the last cleavage-masses long continue to multiply by endo- 

 genous development; and the whole process of division may 

 be regarded as a kind of endogenous cell-development, in 

 which, on account of the rapidity with which the nuclei mul- 

 tiply, no formation of cell-membrane takes place in the early 

 generations of "cleavage-masses.""' 



2. Closely allied, in some respects, to the cleavage process, 

 are those forms of endogenous cell- development, in which a 

 greater or smaller number of secondary cells are developed 

 within persistent parent cells, as we see here and there in the 

 cartilages, in the supra-renal capsules, and in the pituitary 

 body. In this case there either arise in the ordinary manner, 

 in a cell, two secondary cells, which wholly or partially fill it, 

 and from these, by a continued multiplication, other generations, 



1 [Dr. Nelson (Phil. Trans., 1852, p. 580), has observed the same thing in 

 Ascaris mystax. — Eds.] 



2 [We must altogether demur to the notion that the " nuclei" of the dividing yelk 

 exercise any attraction upon the yelk substance. The careful observations of lteichert 

 (Der Furchungs prozess und die sogenannte Zellenbildung um Inhaltsportionen, 

 Midler's Archiv, 1846), of Remak (Ueber den Furchungs-prozess im Froschen-Eie, 

 Mull. Archiv, 1851), and of Nelson, 1. c, appear to furnish demonstrative evidence 

 that no such attraction exists.— Eds.] 



