240 Fertilisation and Hybridization 



nuclei do the paternal and the maternal pronuclei lie so 

 close together that their individual parts can exercise an 

 influence on each other. 



We have further seen that, during the life of a double 

 nucleus, throughout the successive cell-divisions, from 

 the origination of the germ to the complete formation of 

 the offspring, the contact of the pronuclei becomes grad- 

 ually more intimate. Before the first cell division they 

 are, as a rule, still visibly separated; soon afterwards the 

 border-line begins to look more indistinct, and, shortly 

 before the formation of the sexual cells, the double na- 

 ture is disclosed with certainty only in the rarest cases 

 by special structural relations. It is, therefore, clear 

 that their opportunity for mutual influence gradually in- 

 creases during somatic life. Perhaps it first occurs only 

 at the end, possibly even, only at the moment immediately 

 preceding their separation. A decision on this point has 

 not yet been reached.* But the above-mentioned vegetative 

 splittings of hybrids indicate that the process is deferred 

 as long as possible. It also seems simpler to assume that 

 it occurs only in those cells which actually lead to the 

 formation of sexual cells, because in the leaves, bark, and 

 other vegetative parts of the body, it would evidently be 

 without significance. 



We therefore imagine the mutual influence to be exer- 

 cised towards the end, or even at the very last moment 

 before the separation of the pronuclei. In the first case 



*More recent investigations indicate that the fusion of the male 

 and female chromatin elements is completed during the stage known 

 as "synapsis," which immediately precedes the reduction-division, or 

 heterotypic nuclear division, referred to above. During synapsis the 

 chromatin is aggregated into a compact mass within the nuclear 

 cavity. Tr. 



