116 DEVELOPMENT OF CELLS. 



loped into vesicles, which then enlarge, apparently at the expense of 

 the nucleus which disappears when these vesicles have attained a cer- 

 tain size, the vesicles themselves likewise disappearing soon afterwards. 

 But in other cases, as will be presently shewn, the nucleolus, instead of 

 disappearing, becomes constricted in the middle, and subsequently divided 

 into two equal portions, around each of which a new cellular body or 

 nucleus is then developed.* In all those cases, however, in which the 

 nucleus or cell undergoes transformation into a higher tissue, the nucleolus 

 disappears.! But this ' ls certainly not invariably the case, for in the per- 

 sistent nuclei of capillary blood-vessels, of the sarcolemma, and of several 

 other tissues, a small dark particle, apparently identical with the nucleolus, 

 may usually be observed. 



2. In considering the various modes in which the development of cells 

 in the formative fluid or cytoblastema is effected, it must be remarked, in 

 the first place, that it appears immaterial to the process in what part the 

 formative fluid is situated. The same succession of changes in the forma- 

 tion of cells seems to be pursued whether the process occurs in the cyto- 

 blastema of the early ovum, in the secondary cytoblastema from which the 

 several embryonic tissues are produced, or in the organisable material 

 effused from the blood-vessels into the interstices of the various parts of 

 the growing or adult body. The fluid in each case appears to possess the 

 same formative properties, and the chief or only difference observed in the 

 process relates to the mode in which the cells are ultimately disposed of. 

 In the increase, also, of cells, by endogenous multiplication, the formative 

 fluid out of which the young cells grow, so far as concerns its power of 

 producing new cells, appears to be essentially the same as the cytoblas- 

 tema elsewhere : and the differences in the mode of growth are probably 

 more apparent than real, the developing cells in the one case lying free in 

 the interstices of parts, in the other case being enclosed within a membra- 

 nous envelope or parent-cell. 



In the opinion of Schwann the development of cells pursues an almost 

 exactly similar course in every case ; and he believes that the subsequent 

 multiplication of animal cells, is usually effected by the same series of 

 changes as are undergone in their original development, the endogenous 

 mode of origin so common in vegetable structures being rarely pursued in 

 animal tissues. But, as will be presently shewn, the result of more recent 

 investigations have made it probable that this mode of origin, or rather 

 of multiplication, is of more frequent occurrence than Schwann supposed. 

 The plan of cell development recognised by Schwann, is detailed in Pro- 

 fessor Miiller's Elements of Physiology. J In addition to this, the ordinary 

 mode of development, Schwann also suggested the probable occurrence of 

 a variety, or modification of it in some cases. For having observed that 

 occasionally the nucleus of a cell contained two nucleoli, he thought that 

 * Kolliker, op. cit. p. 150. f Op. cit. p. 151. J Pp. 390 and 1641. 



