CELL. 



Fig. 109. 



L 141 J 



CELL. 



Fig. 110. 



i;  



Magnified 350 diameters. 

 Cartilage-cells from a fibrous Telrety articular car- 

 tilage of the condyle of a human femur ; all lying iu a 

 flbi-ous basis, and easily isolated, a, single cells, with 

 or without thickening "of the cell-wall, and one or two 

 nuclei: 6, secondary cells, or cells of the first genera- 

 tion, with one or two nuclei — one, two, five, and many 

 cells in the parent-cells 6'; c, cells of the second gene- 

 ration, one to three in those of the first, hh\ c/, free 

 group of secondary cells. 



mentation) -sre not unfrequently find three 

 and four nuclei in one parent-cell, so that 

 then, instead of two, a larger number of se- 

 condary celLs are formed at once, as e. g. in 

 the liver-cells of embryos. In certain ani- 

 mals ( CucuUanus, Ascaris clentata, Disfoma, 

 and the Cestoidea), instead of segmenta- 

 tion-globules, in the first stage of develop- 

 ment, nuclei only are formed in the ovum- 

 cell, which do not become surrounded by 

 cell-membranes until they have accumu- 

 lated into a large heap by successive endo- 

 genous growth. The same appears to take 

 place in the cells of the germ of the Crus- 

 tacea, in which from ten to twenty nuclei 

 frequently exist. The numerous nuclei, 

 however, in the seminal cells of most ani- 

 mals appear usually to have no connexion 



Magnified .350 diameters. 

 Ivory-cells from the tooth of a dog. 



with cell-formation, because the 

 seminal filaments are developed 

 within them; and the same applies 

 to those cells of the lower animals, 

 the numerous nuclei of which are 

 converted into urticating organs. 

 Whether in these cases the nuclei 

 ' multiply by division or endogenous 

 growth is unknown. 



Cell-formation hy division has been ob- 

 served" in the coloured blood-corpuscles of 

 the embryos of Birds and Mammalia, and 

 the earliest colourless blood-corpuscles of 

 the larvae of frogs (tadpoles) ; it also pro- 

 bably occurs in the colourless blood-cor- 

 puscles of embiyos and the chyle-cor- 

 puscles of adult Mammals. In all these 

 cases the cells first become elongated, and 

 the single nuclei appear to become divided 

 into two ; the cells are then constricted in 

 the middle and finally resolved into two, 

 each with a nucleus (PI. 49. fig. 36). 



A peculiar kind of cell-gr-owth, most 

 nearly allied to di\'ision, occurs in the cells 

 of the ivory of the teeth ; in which the nu- 

 clei, while continually elongating, enlarge 

 from time to time and become constricted, 

 so that whilst that portion next the ivory 

 ossifies, the remainder serves to a certain 

 extent as a reserve for the subsequent for- 

 mation of newly ossifying portions (fig. 110), 

 The term cell is frequently used in a 

 totally difierent sense, to denote a partially 

 closed space, or the cup-like body enclosing 



