CELLS. 51 



tures, and coalesce with the substance which unites the cells as a matrix. 

 The occurrence of this endogenous cell-development, which may be 

 called cell-development around the collective contents, and which agrees 

 in all essential points with free cell-development around investing masses, 

 has been made out with certainty in the young cartilage of all animals, 

 and probably occurs in embryonic organs in general, in which, from the 

 moment when they consist of actual cells, the total growth essentially 

 depends upon a self-multiplication of the cells without free-cell develop- 

 ment. Since, however, it is as yet undecided, whether perhaps cell-de- 

 velopment by division, to which attention has been very lately drawn, 

 does not play a part in the one case or in the other, our judgment, so 

 far as regards the latter, must as yet be suspended, until more particu- 

 lar investigations have been undertaken ; and the same holds good for 

 many organs of the adult, as the horny tissues and certain glandular 

 secretions. Only, when secondary cells are observed in parent cells, as 

 especially in the pituitary body and in the supra-renal capsules, there 

 can of course be no doubt as to the existence of endogenous cell-develop- 

 ment. 



Besides these most usual forms of cell-development, there exist yet a 

 few others. 



1. In the ova of most animals a peculiar process, the so-called 

 cleavage of the yelk, occurs at the earliest period of development, 

 which is to be regarded as the introduction to the formation of the first 

 cells of the embryo; and since the ovum has the nature of a simple cell, 

 it is a case of endogenous cell-development. This cleavage takes place 

 as follows. After the original nucleus of the egg-cell, the germinal 

 vesicle, has disappeared with the occurrence of fecundation, the granules 

 of the yelk no longer form a com- 

 pact mass as before, but become 

 dispersed and fill the whole egg- 

 cell. Then, as the earliest sign 

 of commencing development, there 

 arises in the midst of the yelk, 

 around a new nucleolus, a new 

 nucleus, the primary nucleus of the embryo, which operates as a centre 

 of attraction upon the yelk and unites it again into a globular mass, the 

 first "cleavage mass" (Furchungs-kugel). In the further course of de- 

 velopment two new nuclei are formed by endogenous development from 

 the first nucleus, and these, as soon as they have become freed by the 

 solution of the parent nucleus, separate from one another for a short 

 distance, act as new centres upon the yelk, and thus break up the first 



FIG. 5. The ova ofdscaris nigovenosa, 1 from the second, 2 from the third, and 3 from 

 the fifth stage of division, with 2, 4, and 10 division-masses: a, chorion; 6, cleavage masses 

 In 1 the nucleus of the lower mass contains two nucleoli, in 2 the lowest contains two nuclei. 



