14 Cutler, Parthenogenesis in Animals 



At the maturation of the eggs, which will give rise to the male, 

 the two large chromosome^ pair, as do also the small ones. It 

 will be seen from the accompanying Diagram C, that two types of 

 males will be produced giving ries to two kinds of spermatozoa, 

 one containing Xx, the other Xx'. If the former fertilises the 

 eggs the resulting stem mother will be a female-producer 

 (XXxx), if the latter, there will be formed a male-producing 

 stem mother (XXxx'). 



At about the same date Morgan investigated the Aphid of 

 the Bearberry. This insect, Phyllaphis corveni Cockerelli forms 

 galls on the leaves of the plant, and in each gall there is a 

 single stem mother and ner progeny. There are males and 

 sexual females together with females which are parthenogenetic. 

 The eggs of these latter all contain six chromosomes. The 

 spermatogenesis of the male is very similar' to that of the 

 Phylloxera, but the spermatogonial cells only contain five 

 chromosomes. Of these five chromosomes four pair together at 

 synapsis, so that three chromatin clumps are formed, two of which are 

 composed of paired somatic chromosomes, the third being the 

 unpaired sex chromosome. At spermatogenesis the somatic 

 chromosomes separate, but the sex chromosome passes undivided 

 t)o one pole of the spindle. Thus two cells are formed, one with 

 three chromosomes, the other with two: this latter degenerates. 



Von Baehr and Stevens as the result of work done on Aphis 

 soliceti and seven other species confirmed the above result. 



It seems to be clear that in the Phylloxeras and Aphidse the 

 loss of one or more chromosomes from the egg is intimately 

 connected with male production. That the absence of the sex 

 chromosome is not the ultimate cause of male production is 

 obvious, since it is determined in some way that certain eggs 

 shall extrude the chromosomes and others not do so. 



This has been noticed by Don caster in his paper, " Chromo- 

 somes, Heredity and Sex," in which is (the following sentence: 

 " But this cannot logically be regarded as -a proof that the 

 presence or absence of X is not the cause of femaleness or 

 maleness ; it only means that some factor is present which 

 decides whether X shall be extruded or not." 



This factor may very well be environmental change, which 

 acts on the protoplasm of the egg in a way somewhat similar to 

 that suggested for the eggs of Hydatina senta. 



Cynipidce.- — The cytology of the eggs of Neuroterus lenti- 

 cularis has been worked out fuily by Doncaster. As I have 

 already mentioned, the life cycle of this insect exhibits a con- 

 stant alternation of bisexual and parthenogenetic generations. 

 In the Spring parthenogenetic females hatch from the Winter 



