30 BIOLOGICAL LECTURES. 



Hertwig, Boveri and Weismann, as well as a number of other 

 well-known investigators, assert that the hereditary substance, 

 by means of which heritable qualities are transmitted from one 

 generation to another, is contained wholly within the nucleus, 

 and, speaking more strictly still, within that part of the nucleus 

 called the chromatin. Since, however, many of the characters 

 of the cytoplasm are heritable, and in fact are generally the 

 only characters which are known by observation to be herit- 

 able, these authorities are compelled to assume that the control 

 of the cell, if not the actual genesis of all its constituents, is 

 located wholly within the chromatin. As a matter of fact, it 

 must be acknowledged that this assumption that the chromatin 

 makes the cell-body just what it is in point of structure, func- 

 tion, shape, position and size, does not rest upon observation 

 but upon supposed theoretical necessities. All these necessi- 

 ties find their source in the assumption that the nucleus, and 

 especially the chromatin, is the only bearer of heredity, an 

 assumption which was wholly justified so long as it was sup- 

 posed that fertilization consisted merely in the fusion of the 

 two pronuclei ; but now since it is known that the cytoplasm, 

 and especially the male and female asters, take a very import- 

 ant part in the process of fertilization, it seems to me that 

 such an assumption is wholly unwarrantable. 



The spermatozoon does not cease to be a cell the moment it 

 has entered the ovum. Both its cytoplasm (in the form of an 

 aster) and its nucleus are represented, and they grow just as a 

 cleavage or tissue cell does by the assimilation of food material, 

 which in this case is contained within the egg cell. Although 

 within the ovum the spermatozoon must be considered a dis- 

 tinct cell, preserving all its fundamental peculiarities, until that 

 time when its various constituents lose their identity by fusion 

 with corresponding parts of the egg cell. While it is known 

 in several cases that the spermatozoon introduces a nucleus 

 and an aster, which then fuse with similar parts of the ovum, 

 it is not known, with one or two exceptions, that any portion 

 of the general cytoplasm of the male cell is carried into the 

 ovum. In the case of Ascaris, both Van Beneden and Boveri 

 agree that a certain part of the cytoplasm of the spermatozoon 



