FACTORS IN ONTOGENY 253 



contrary, the process is more complex. It has been described 

 by F. M. McFarland l as follows: 



"One of the earliest results of the study of cell multiplication was 

 the discovery that division of the nucleus precedes the division of the 

 cell body. Furthermore, a careful examination of the different phases 

 of the process offers the strongest proof that the most important 

 feature of this division, an end to which all the other processes are 

 subsidiary, is the exact halving of a certain nuclear substance, the 

 chromatin, between the two daughter cells which result from the 

 division. To gain a clear conception of this process of indirect cell 

 division, called 'mitosis' or 'karyokinesis/ let us consider the changes 

 which take place in typical cell multiplication. Two parallel series 

 of changes occur nearly simultaneously, the one affecting the nucleus, 

 the other the cytoplasm. In the so-called 'resting' nucleus i. e., 

 the nucleus not in active division the chromatin, as we have seen, 

 exists usually in the form of scattered granules arranged along the 

 linin network, and does not color readily with nuclear stains. As 

 division approaches, these chromatin granules become aggregated 

 together in certain definite areas, forming usually a convoluted thread 

 or skein, which now readily takes up the nuclear stains which may be 

 used. In some nuclei this skein is in the form of a single long filament, 

 in others the chromatin is divided up from the first into a series of 

 segments, a condition which soon follows in the case of a single fila- 

 ment. By transverse fission the latter breaks up into a series of seg- 

 ments, the 'chromosomes,' the number of which is constant for each 

 species of animal or plant. Thus in the common mouse there are 

 twenty-four, in the onion sixteen, in the sea urchin eighteen, and in 

 certain sharks thirty-six. The number may be quite small, as, for 

 example, in Ascaris, a cylindrical parasitic worm inhabiting the alimen- 

 tary canal of the horse. Here the number is either two or four, 

 depending upon the variety examined. In other forms the number 

 may be so large as to render counting exceedingly difficult or im- 

 possible. In all cases, however, one fact is to be especially noted, 

 viz., the number is always an even one, a striking fact which finds its 

 explanation in the phenomena of fertilization to be discussed later on. 



"While the chromatin is collecting into the form of the chromo- 



1 Most of the discussion in the following twenty pages, whether indicated 

 by quotation marks or not, is taken from McFarland's essay on ' The 

 Physical Basis of Heredity " in Jordan's " Footnotes to Evolution" (1902). 



