i88 SOME RECENT DEVELOPMENTS 



Weismann's explanation of the process is a 

 complicated one. Regarding the nucleus as 

 specially concerned with heredity, he distinguishes 

 in the nuclear substance two kinds of matter, 

 which he names histogenic plasma and ancestral 

 plasma respectively. The former or histogenic 

 plasma he regards as specially concerned with the 

 growth, nutrition, and shaping of cells ; while the 

 second or ancestral plasma is supposed to be 

 specially concerned with reproduction and heredity. 

 He then assumes that the histogenic plasma 

 having completed its work when the egg has 

 reached maturity, it is convenient to get rid of it, 

 which is done by extruding it bodily from the egg 

 as the first polar body. The remaining half of 

 the nuclear substance is the ancestral plasma ; and 

 with regard to the second polar body, Weismann 

 argues that if the whole of the ancestral plasma of 

 the egg were retained, and were to fuse with the 

 whole of the ancestral plasma of the spermatozoon, 

 then the total amount of ancestral plasma would be 

 doubled at each generation. This he argues 

 would cause inconvenience, and so the device is 

 adopted of getting rid of half of this ancestral plasma 

 in the form of the second polar body. 



Apart from its extreme complexity, this theory 

 of Weismann's is open to grave objection. In the 

 first place the supposed difference between histo- 

 genic plasma and ancestral plasma is a pure 

 assumption, in support of which no direct evidence 

 has been advanced. Secondly, it is very difficult 

 to understand why, on Weismann's view, the 



