122 MULTIPLICATION OF CELLS. 



Examples of this are furnished by the ganglion-corpuscles of nerve-sub- 

 stance, and by the ovum. Kolliker, indeed, considers that all ordinary nu- 

 cleated cells should be regarded in the light of secondary or complex cells. 

 3. From the several details which have just been considered in relation 

 to the development of cells, it would appear that in the cytoblastema 

 there resides some power by which fresh cells can be continually formed 

 out of an apparently homogeneous fluid. In order that this continual 

 formation of successive crops of cells may be effected, it is essential, 

 however, that constant supplies of new formative fluid should be provided, 

 and it appears to be one of the purposes served by cells, to elaborate 

 this fresh formative material, which, when perfected, is discharged by the 

 solution of the membranous cell-walls. Out of the fresh cytoblastema 

 thus prepared and liberated, the new cells are developed in one or other of 

 the ways above pointed out. And it would seem, as stated by Schwann, 

 that, in the case of animal structures, the continued increase of cells, is 

 in most cases, effected by such fresh development in the free formative 

 fluid. But in several other cases new cells are formed within the cells of 

 a preceding generation, and by these they are surrounded until they have 

 attained a certain degree of development, when they escape, apparently 

 by rupturing the parent cell which then disappears. This endogenous 

 mode of cell- formation, (or multiplication, as it is commonly termed,) 

 although of common occurrence among vegetable structures, is, however, 

 comparatively rare in animals; the ovum, cartilage, and a few other 

 structures presenting the only known examples of it. It differs from the 

 original development of cells in the circumstance of the new cells being 

 produced more or less directly from some part of a pre-existing cell, 

 which thus acts as a kind of re-productive organ. But it is not improbable 

 that the* difference is one more apparent than real, and consists simply in 

 the circumstance of the source whence the new cells originate, being in 

 the one case retained within the parent-cell, and in the other case set free. 



The best examples of the endogenous mode of cell-multiplication have 

 been already mentioned in describing the changes which ensue in the de- 

 velopment of the ovum.* It was there shewn (in the case of the ovum of 

 Cucullanus elegans) that according to Kolliker's observation the first step 

 in the process of multiplication consists in the nucleus of the first cell 

 which is formed after the disappearance of the germinal vesicle, becoming 

 constricted in the middle, and subsequently dividing into two equal halves, 

 each of which serves as a separate nucleus, around which a new cell 

 forms ; and each new cell in its turn gives rise to two others formed in the 

 same way, and so the process goes on until the whole mass of the ovum is 

 made up of such cells. And Kolliker appears to be of opinion that in 

 most other cases of cell-multiplication the division of the nucleus is the 

 first essential step in the process. f Other cases, however, seem to occur 



* See especially p. 66 and p. 71. f Entwickelungs-gesch. der Cephalopoden, p. 150. 



