52 GENERAL ANATOMY OF THE TISSUES. 



cleavage mass into two. In this way the multiplication of nuclei and 

 of cleavage masses proceeds, the former always taking the lead, until 

 a very great number of small globules are produced which fill the whole 

 cavity of the yelk-cell ; it is only in exceptional cases that the cleavage- 

 masses break up after the development of three or four nuclei within 

 them ; so that then, instead of two, three or four cleavage-masses imme- 

 diately proceed from one. This process is called total cleavage, because 

 here the whole yelk is disposed around the newly-developed nuclei : 

 partial cleavage is essentially similar, differing only in the circumstance 

 that it is not the whole yelk, but 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 themselves with membranes, 

 and become actual cells, whence we are justified in considering this to 

 be a process of endogenous 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 (Muller 

 saw a division of it occur in the Molluscs which are developed within 

 Synapta digitata*) 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 endogenous development ; and the whole 

 process of division may be regarded as a kind of endogenous cell-deve- 

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

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

 tions 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 continued multiplication, other generations, which 

 sometimes lie quite free, and sometimes are wholly or partially included 

 in the parent cells of the second generation ; or a more free development 

 of a secondary cell within a parent cell occurs, from which cell-develop- 

 ment then takes place in one mode or in the other. 



In connection with the process of endogenous cell-development, we may 

 very properly speak of the formation of a great number of nuclei within 

 cells, a process which is frequently the precursor of cell-development, 



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

 -TBS.] 



