AS THE ELEMENTAL GERM OF AN ORGANISM 351 



would not be room for them in a spermatozoid. It is, therefore, 

 unavoidable, that in bisexual reproduction, the union of the 

 parental idioplasmic bodies must take place without causing a 

 corresponding and permanent increase of their substance." 



Nageli has attempted to overcome this difficulty by assuming, 

 that idioplasm consists of strands, which are fused together in 

 such a peculiar way, that the transverse section of the product of 

 fusion remains the same as that of the simple thread, whilst the 

 length of the whole is increased (IX. 20, p. 224). 



Weismann (IX. 32-34) has investigated this subject most care- 

 fully, and has attempted to demonstrate, that a summation of the 

 hereditary mass is prevented by means of a process of reduction, 

 it being halved before each act of fertilisation. He considers that 

 theoretically it is so absolutely necessary for reduction to take 

 place in each generation, "that the processes by which it is brought 

 about must be discoverable, even if they are not to be deduced 

 from the facts already mentioned." 



Weismann has been led to these conclusions by considering the 

 nature of idioplasm ; however, his views do not agree with the 

 ones I have mentioned above. He groups them under the common 

 name of "ancestral plasma theory," to the essential points of which 

 I will refer later. 



The enquiry into the processes of fertilisation and of nuclear 

 division proves logically, on the one hand, that the two hereditary 

 masses must fuse, and must subsequently be re-distributed amongst 

 the cells, and on the other that a summation of the nuclear sub- 

 stance of the hereditary mass must be avoided. The unanimity of 

 opinion as regards the assumption, that the nuclear substance is 

 the hereditary mass sought for, may certainly be taken as evidence 

 in its favour, especially if, during the fusion of the nuclei, pro- 

 cesses can be demonstrated, which correspond in every respect 

 to the necessary conditions. 



A priori, there are only two possible means of preventing the sum 

 of the equal quantities from being greater than either of the added 

 parts. Either the quantities, which are to be added together, must 

 be halved beforehand, or their sum must be halved subsequently. 

 Both methods appear to have been adopted during the process of 

 fertilisation. 



The one course occurs in phanerogamous plants and in animals. 

 When the male and female sexual products are mature, the nuclear 

 mass of both the egg and sperm mother cell, as was described at 



