SECT. II.] FORMATION OF CELLS. 1 7 



cells, there arc seen forming, from the original simple nucleus, 

 to all appearance likewise by division, two ; then the cells become 

 constricted in the middle, contract more and moi'e around the 

 separated nuclei, and, lastly, break up into two, each of which 

 contains its nucleus. In the embryo-chick the blood-cells are to 

 be seen in all conceivable stages of this process ; so that ultimately 

 they are only connected by a thin filament ; and not the slightest 

 doubt can prevail concerning the real occurrence of this kind of 

 cell-multiplication. 



In other free cells, as those of the juices of glands, the process 

 of division has hitherto been observed only in one place with cer- 

 tainty ; viz. in the ova of Mermis albicans, which, according to 

 Meissner's discovery (Zeitschr.f.io. Zool.x.), become detached as 

 buds from other cells. 



In compact cell-tissues, it is much more difficult to demonstrate 

 the processes of cell-division with certainty. I assume this mode 

 of cell-formation to take place wherever, on the one hand, an in- 

 crease of cells in number has been demonstrated, and where, on 

 the other, every trace of a free cell-formation or of endogenous 

 multiplication is wanting ; and, accordingly, in all embryonal cell- 

 tissues, with the exception of cartilage, and in the adult in the en- 

 tire growth of the horny tissues, and perhaps in many other cells. 



With regard to the mode of cell-division in compact tissues, it 

 may be further remarked, that it takes place both in the longitudi- 

 nal and transverse diameter of the cells : in the former case, they 

 grow in the direction of the surface ; in the latter, in that of the 

 hickness. 



• Schwann was not aware of cell-division. The first who saw such a process 

 in the blood- corpuscles of embryos, was Reviak ; yet he subsequently re- 

 t cted his statements, to pronounce himself in favour of them again, after 

 I had confirmed and declared them to be correct. In pathological forma- 

 tions, this mode of cell-multiplication has likewise been found by Giinsburg 

 and Breuer (see Breuer, Meletem. circa Evol. ac formas Cicatricum. Vratisl, 

 1843, p. 31). I also refer the transverse and longitudinal division of the 

 protozoa to cell division ; since these animals possess the structure of 

 simple cells, and their nucleoid body participates in exactly the same way in 

 the cleavage, as the cell-nucleus does in ordinary cells. In pathological 

 formations, cell-division may perhaps be more frequently found, when once 

 attention shall have been directed to it. In the vegetable kingdom cell- 

 division is rare, and only seen in the lower organisms. 



§ 11. The Formation of Cells in others or the Endogenous Origin 

 of them presents various modifications. 



a. In the first form, which I call the endogenous cell-formation 



c 



