100 SEX INHERITANCE 



(Fig. 34). In one species, P. caryaecaulis, all the mi- 

 grants in one gall are alike in that they produce the 

 same kind of egg, i.e., in some galls all the migrants 

 contain large eggs (that produce sexual females), 

 while in other galls all of the migrants contain smaller 

 eggs (that develop into males). 



The sexual female lays one egg, that is fertilized, 

 from which the stem mother emerges the following 

 spring. The males give rise only to female-producing 

 sperm, each spermatozoon containing two sex chromo- 

 somes. The other class of sperm degenerates. Hence 

 we can understand why it is that all fertilized eggs 

 produce females only. 



The chromosomal cycle undergoes the series of 

 changes shown in Fig. 35. In P. carysecaulis there 

 are eight chromosomes, including four sex chromo- 

 somes (XxXx) . Since the history of the sex chromo- 

 somes alone furnishes certain information that makes 

 clear some of the changes in the life cycle, the other 

 chromosomes may be disregarded for the present. 



Starting at the bottom of the diagram it will be 

 seen that the sexual egg after extruding the two polar 

 bodies contains two sex chromosomes indicated by 

 X and x. Two kinds of males are indicated in the 

 diagram, one containing Xx the other Xx', and as a 

 consequence there will be two kinds of female-produc- 

 ing sperm, one kind for each male, namely, Xx and 

 Xx'. If the former fertilizes the sexual egg, the re- 

 sulting stem mother will be XxXx, if the latter, 

 the stem mother will be XxXx'. These two kinds of 

 stem mothers are indicated at the top of the diagram. 



