CELL. 



CELL. 



The formation of cells in a free blastema 

 is not a general process ; the only instances 

 of its occurrence in man and the higher 

 animals, are in the formation of the chyle 

 and lymph corpuscles, the cells of certain 

 glandular secretions (seminal cells, ova), and 

 glandular organs (the closed follicles of the 

 intestine, the lymphatic glands, the splenic 

 corpuscles with the splenic pulp, and the 

 thymus) ; lastly, of the cellular elements in 

 the impregnated uterus, in the corpus 

 luteum, the marrow of foetal bones, and in 

 the soft ossifying blastemata. In the case of 

 the chyle and the spleen, at the commencement 

 of cell-formation, we find roundish, appa- 

 rently homogeneous bodies of 1-11000 to 

 1-5600" diameter, which, increasing in size, 

 some appear distinctly as vesicles (fig. 108), 



Fig. 108. 



Magnified 350 diameters. 



Contents of a Malpighian body from the spleen of an 

 ox. a, small, b, larger cells ; c, free nuclei. 



and on the addition of water, exhibit an 

 internal large body resembling a nucleolus, 

 as also several granules. The minute details 

 of this stage of the process of formation, are 

 not accurately known. As soon as the nuclei 

 are formed, cell-membranes are formed 

 around them, but not always in the same 

 manner. Sometimes the cell-wall is deposited 

 directly around the nucleus, so that it is but 

 little larger than the latter; at others the 

 nucleus becomes surrounded by a larger or 

 smaller quantity of blastema which becomes 

 more solid, and around which the cell-mem- 

 brane is subsequently deposited. The latter 

 occurrence has hitherto only been satisfac- 

 torily observed in the case of the ovum, in 

 which the germinal vesicle, i. e. the nucleus 

 of the ovum-cell, which is formed first, 

 becomes surrounded by a quantity of yolk, 

 before the vitelline membrane is formed.* On 

 the other hand, the formation of the cell- 

 wall directly around the nucleus, takes place 

 in all the other localities mentioned above, 

 and is especially shown by the occurrence of 

 free nuclei and larger cells, together with 

 very small cells closely surrounding the 

 nuclei, or separated from them by a slight 

 interval only. It is possible that in this 

 instance also, the cell-membrane, even at its 



first formation, may be separated from the 

 nuclei by a quantity of blastema too minute 

 to be detected. 



This free formation of cells is very general 

 in pathological productions, as in pus, exu- 

 dations, &c.; the cell-membrane being depo- 

 sited directly around the nucleus, the inter- 

 mediate layer of blastema rarely being 

 present. 



This extra-cellular formation of cells is 

 unknown in plants. 



The endogenous method, or the formation 

 of cells within others, is very common, and may 

 be readily observed in the tissues of embryos. 

 In the most ordinary form of this kind of cell- 

 formation an original or parent-cell produces 

 within it two secondary cells, which from 

 the first completely fill it. The first phseno- 

 menon observed in the parent-cell is the 

 increase of the nucleus, which acquires two 

 nucleoli, becomes elongated and resolved 

 into two nuclei. After this the nuclei 

 separate from each other, and a partition is 

 formed between the cells, dividing the parent- 

 cell into two perfectly distinct spaces, each 

 of which encloses a nucleus and half of the 

 contents (fig. 104). The exact manner in 



Fig. 109. 



Magnified 350 diameters. 



An elongated nucleus, and one containing two secondary 

 nuclei, from the ovum of an Ascaris dentata. 



which the increase of the nucleus occurs is 

 not certain, but it appears that the nucleoli 

 always become resolved into two by subdivi- 

 sion and then separate from each other. In 

 the nuclei, which at the same time become 

 elongated, the first trace of division is then 

 usually a median partition, which in favour- 

 able instances appears to arise from the 

 presence of two secondary cells in close 

 contact by plane surfaces and entirely filling 

 up the parent nucleus. Very frequently 

 nothing is seen but first an elongated nucleus 

 with a partition and two nucleoli, and then 

 two hemispherical nuclei in contact by their 

 plane surfaces (fig. 109), no endogenous 

 nucleus-formation being perceptible ; in this 

 case, division of the nucleus has taken place, 

 the parent-nucleus containing two nucleoli 

 becoming finally resolved into two by a 

 deeper and deeper constriction. This mode 

 of cell-formation is often continuously re- 

 peated, frequently so long as the growth of 



