24 



GENERAL ANATOMY OF THE TISSUES. 



the original 



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 develop- 

 ment,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 

 nucleus of the egg-cell, the germinal vesicle, 



has disappeared with the 

 occurrence of fecundation, 

 the granules of the yelk 

 no longer form a compact 

 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 development 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 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 pro- 

 duced, 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, dif- 



Fig. 5. The ova of Ascaris nigrovenosa, — 1 from the second, 2 from the third, and 

 3 from the fifth stage of division, with 2,4, and 16 division-masses: a, chorion: 

 b, cleavage-masses. In 1 the nucleus of the lower mass contains two nucleoli, 

 in 2 the lowest contains two nuclei. 



