CELL. 



[ 115 ] 



CELL. 



the organism continues. The parent-cells 

 then either continue their existence as such, 

 or they disappear sooner or later as histolo- 

 gically distinct formations, and become con- 

 solidated v^ith the substance connecting the 

 cells. 



The occurrence of this endogenous cell- 

 formation, which agrees essentially with the 

 formation of cells in a free blastema, is well 

 established in the case of the young cartilages 

 of all animals, and also probably occurs in 

 embryonic organs in general, in which, from 

 the period at which they consist of true 

 cells, the entire growth depends upon the 

 multiplication of the existing cells without 

 free cell-formation. It also occurs in patho- 

 logical products, as in cancer. 



In addition to this, the most common 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 yolk, 

 which must be regarded as preliminary to 

 the formation of the first embryonic cells, 

 and which, as the ovum bears the import of 

 a simple cell, falls under the type of endo- 

 genous cell-formation. The segmentation 

 consists essentially of this : after the original 

 nucleus of the ovum-cell — the germinal 

 vesicle— has disappeared in consequence of 

 impregnation, the granules of the j'olk are 

 no longer aggregated into a compact mass as 



Fig. 112. 



a 



Magnified 350 diameters. 



Three ova of an Ascaris nigrovenosa ; 1, in the first, 

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

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

 a, 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. 



before, but become distributed throughout 

 the entire cell. The first sign of commencing 

 development is then constituted by the for- 

 mation of a new nucleus, the first embryonic 

 nucleus, around a new nucleolus, which acts 

 as a centre of attraction to the yolk and 

 causes it to reunite into a globular mass — the 

 first globule of segmentation. In further 

 development two new nucleoli are formed 



from the first nucleus by endogenous growth, 

 which, as soon as they are set free by the deve- 

 lopment of the parent nucleus, become sepa- 

 rate from each other, act as new centres to 

 the yolk-granules, and thus the first globule 

 of segmentation becomes resolved into two. 

 The increase of the nuclei and the globules of 

 segmentation continues in the same way, 

 the first always preceding, until a very large 

 number of small globules are present, v.hich 

 entirely fill up the yolk-cell; sometimes, 

 but exceptionally, the globules are not re- 

 solved until the nuclei have become increased 

 to three or four, so that three or four 

 globules are formed from each, instead of 

 two. This process is termed total segmenta- 

 tion, because here the entire yolk is applied 

 to the newly-formed nuclei; partial seg- 

 mentation agrees with this in all essen- 

 tials, and only differs from it in the circum- 

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

 but a larger or smaller part of it, according 

 to the different animals, envelo])es the nuclei 

 in process of formation (figs. 110-112). 



When the process of segmentation has 

 reached a certain stage, the segmentation- 

 globules become surrounded with membranes 

 and form true cells, whence it appears justi- 

 fiable 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 

 phaenomena 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 wdth it ; secondly, the pa- 

 rent-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 subsequent genera- 

 tions. This view is moreover justified, 

 since the cells formed from the last seg- 

 mentation-globules continue for a long period 

 to multiply by endogenous production (or 

 division), and the entu'e segmentation pro- 

 cess 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 wdiich a greater or less number of 

 secondary cells are formed within persistent 

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

 lage, the supra-renal capsules the pituitary 

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



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