CELL. 



[ 140 ] 



CELL. 



kind of endogenous cell-formation, there are 

 others, viz. : 



a. In the ova of most animals at the earliest 

 period of development, a peculiar process 

 occurs called the segmentation of the yelk, 

 which must be regarded as preliminary to 

 the formation of the first embryonic cells; 

 and which, as the ovum hears the import of 

 a simple cell, falls under the type of endo- 

 genous cell-formation. The essential fea- 

 tures of the segmentation are as follows. 

 After the original nucleus of the ovum-cell 

 the germinal vesicle has disappeared in 

 consequence of impregnation, the granules 

 of the yelk no longer remain aggregated 

 into a compact mass as before, but become 

 distributed throughout the entire cell. The 

 first sign of commencing development is 

 then constituted by the formation of a new 

 nucleus the first embryonic nucleus,around 

 a new nucleolus, which acts as a centre of 

 attraction to the yelk, and causes it to re- 

 unite into a globular mass the first glo- 

 bule of segmentation. In further develop- 

 ment, two new nucleoli are formed from the 

 first nucleus by endogenous growth, which, 

 as soon as they are set free by the develop- 

 ment of the parent nucleus, become separate 

 from each other, act as new centres to the 

 yelk-granules, and thus the first globule of 

 segmentation becomes resolved into two. 

 The increase of the nuclei and of the glo- 

 bules of segmentation continues in the same 

 way, the first always preceding, until a very 

 large number of small globules are present, 

 which entirely fill up the yelk-cell ; some- 

 times, but exceptionally, the globules are 

 not resolved until the nuclei have become 



Fig. 106. 



Fig. 108. 



Magnified 350 diameters. 



Three ova of an Ascaris nigrovenosa : 1, in the first' 

 2, in the second, and 3, in the third stage of segmenta- 

 tion, with two, four and sixteen segmentation-globules. 

 c, outer coat of the ovum ; b, segmentation-globules. 

 In 1, the nucleus of the lowest globule contains two 

 nucleoli ; in 2, the lowest globule two nuclei. 



increased to three or four, so that three or 

 four globules are formed from each, instead 

 of two. This process is termed total seg- 



mentation, because here the entire yelk is 

 applied to the newly-formed nuclei : partial 

 segmentation agrees with this in all essen- 

 tials, and only differs from it in the circum- 

 stance that in it, not the whole of the yelk, 

 but a larger or smaller part of it, varying in 

 different animals, envelopes the nuclei in 

 process of formation (figs. 106-108). 



When the process of segmentation has 

 reached a certain stage, the segmentation- 

 globules become surrounded with mem- 

 branes and form true cells, whence it ap- 

 pears j ustifiable to arrange this process with 

 endogenous cell-formation. In fact it is 

 nothing more than a preliminary to cell- 

 formation in the ovum-cell, and only differs 

 from the ordinary phenomena of this kind 

 in the circumstance that first, the nucleus 

 of the parent-cell or the germinal vesicle in 

 most cases has nothing to do with it ; se- 

 condly, the parent-cell itself persists ; and, 

 thirdly, the portions of the contents formed 

 in it by the successive increase of nuclei do 

 not assume the form of cells until subse- 

 quent generations. This view is moreover 

 justified, since the cells formed from the 

 last segmentation-globules continue for a 

 long period to multiply by endogenous pro- 

 duction (or division) ; and the entire seg- 

 mentation-process may be regarded as a 

 kind of endogenous cell-formation, in which, 

 on account of the rapidity with which the 

 nuclei increase, in the first generation of 

 globules it does not come to the formation 

 of membranes (see OVUM). 



b. In some respects allied to segmentation, 

 are those forms of endogenous cell-forma- 

 tion in which a greater or less number of 

 secondary cells are formed within persistent 

 parent-cells, as seen here and there in carti- 

 lage, the suprarenal capsules, the pituitary 

 body, &c. In this case, either two second- 

 ary cells are formed in the usual way in a 

 cell, almost or entirely filling it, and from 

 these other generations, either free or all or 

 individual ones enclosed in parent-cells of 

 the second and subsequent generations, or 

 only one secondary cell is formed in a cell, 

 whence cell-formation then proceeds in 

 either manner (fig. 109), or the secondary 

 cell is formed in a bud-like protrusion of the 

 parent-cell (see ECHINOCOCCUS). 



The formation of a larger number of 

 nuclei within cells, which frequently pre- 

 cedes cell- formation, but may also exist 

 alone, may be well arranged under endo- 

 genous cell-growth. Even in ordinary en- 

 dogenous cell-formation (and also in seg- 



