WEISMANN'S THEORY OF HEREDITY 



171 



divide each into two dissimilar halves containing different deter- 

 minants (Fig. 77, B), when the daughter chromosomes will be 

 unlike each other. In the former case the division is said to be 

 integral and in the latter differential. It is by differential division 

 that Weismann believes the histological differentiation of the 

 sorna to be brought about. 



When we consider the phenomena of maturation and fer- 

 tilization we find them in no less striking harmony with 

 Weisnaann's views. We have already pointed out, in Chapter X., 

 that .each particular species of plant or 

 animal is, as a general rule, characterized 

 by the appearance of a definite and con- 

 stant number of chromosomes in all the 

 cells of the body during the process of 

 mitosis. At some period in the life-cycle, 

 however, in typical plants during the 

 process of spore-formation and in animals 

 during the maturation of the ova and 

 spermatozoa, this number is reduced to 

 half by separation of the entire chromo- 

 somes into two groups, one of which 

 passes to each of two daughter cells. 

 Thus the mature germ cells have only 

 half the number of chromosomes charac- 

 teristic of the species (or, in the case of 

 typical plants, of the sporophyte genera- 

 tion). The full number is made up again 

 by the union of male and female gametes 

 to form the zygote or fertilized egg. 



To the combination of the maternal and paternal chromosomes 



(in the nucleus of the zygote Weismann has given the name 



ymphimixis, and he sees in this mingling of ancestral germ 



'plasms the cause of that mixture of paternal and maternal 



Jcharacters which we commonly find in animals and plants. If, for 



the sake of simplicity, we imagine that each chromosome consists 



(as appears to be sometimes the case) of only a single id or 



chromornere, and that eight of these are present in the nucleus of 



the mature germ cell, we may represent the effect of repeated 



amphimixis upon the constitution of the nucleus by means 



of the diagram (Fig. 78), which shows how the ids must become 



more and more diversified in each successive generation. In this 



FIG. 77. Diagram of 

 (A) integral and (B) 

 differential Division 

 of a Chromosome con- 

 sisting of five Ids or 

 Chromomeres. 



