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MANUAL OF HISTOLOGY. 



must be produced independently of those originally present "by a species 

 of spontaneous generation in the tissues themselves. 



That animal cells really possess the property in question, is indicated 

 by the processes of segmentation which have been long known to take 

 place in them, to which attention was directed many years ago by Remtik, 

 and which have since then been observed over and over again. 



Division of cells appears invariably to depend upon the vital contrac- 

 tility of their protoplasm, and to be an impossibility so soon as the cell- 

 body becomes transformed into other substances. It is, therefore, essenti- 

 ally a vital property of young cells. The process of segmentation may 

 either take place in cells destitute of membranes, or in those contained 

 within capsules. Owing to this the occurrence is variously modified. 

 When the division takes place in membraneless elements, the whole 

 structure is constricted until separation into two halves occurs; while in 

 those which possess an envelope or capsule, the latter remains unaffected 

 by the process which divides the cell within it. The last variety is 

 known as " endogenous multiplication or cell growth." 



1. Division of naked cells, or, as it may be called, free segmentation, 

 can be observed accurately in the white corpuscles of the blood of young 

 mammalian animals and embryonic birds. In 

 the first (fig. 86) we usually find a round nucleus 

 (a) in a spheroidal cell. When segmentation is 

 about to take place, the former becomes some- 

 what elongated, and is shortly after observed to 

 be marked by a slight transverse constriction, 

 the whole cell assuming at the same time an 

 oval form (&). This transverse furrow on the 

 nucleus then deepens more and more until the 

 latter is finally divided into two pieces (c), 

 Avhich at first lie very close together, indicating 

 their origin, but soon begin to separate from 

 one another (c?). On this the body of the cell 

 undergoes the same constriction (commencing 

 either at one side, or regularly all round), which 

 causes the cell to assume a form likened at times 

 to a double loaf (e). Later on there only exists a narrow band of con- 

 nection between the two portions of the body (/), which is finally com- 

 pletely severed, giving rise to the formation of two cells. By subsequent 

 growth these rapidly attain the typical dimensions of such elements. In 

 the embryonic chick, an easily obtainable object, the nucleoli may be 

 first seen to undergo the process of division within the 

 nucleus. This fact has been improperly denied by some 

 in connection with embryonic blood (Bittroth). 



But the process of segmentation does not always occur 

 with the regularity of the example just cited. Thus, in 

 the frog division is described by Remak as occurring in 

 such a manner as to produce from a single corpuscle 

 three, four, or six cells. In other respects, however, and 

 as regards nucleus and body, the process is similar to that of simple cleaving 

 (fig- 87). 



Those very remarkable formations known as "giant cells" (Kiesenzel- 

 len) (fig. 64, p. 72) are produced by multiplication of the nuclei with- 

 out corresponding segmentations of the body. 



Fig. 86. Blood-corpuscles from a 

 young deer embryo, a a a, 

 spheroidal cells ; b-f, segmenta- 

 tion of the same. 



Fig. 87. A cell from 

 the frog undergo- 

 ing segmentation 

 into three portions 

 (after Remak), 



