12 Cutler, Parthenogenesis in Animals 



on fertilisation, eggs from, which female- producing females hatch, 

 the other male-producing females. 



Aphidce. — This group of insects has been worked upon a 

 great deal by American cytologists. Morgan has investigated 

 with much care the Phylloxera and a fairly complete account 

 of the life history from a cytological standpoint is known. 



In P. fallax there is a single stem mother which lays par- 

 thenogenetic eggs, and from these develop wingless female in- 

 sects. These females produce large eggs from which sexual 

 females appear, and small eggs producing males. Fertilisation 

 occurs, and from the eggs laid the stem mother of the follow- 

 ing Spring hatches. 



The stem mother has twelve chromosomes, four of which 

 are sex ones. The parthenogenetic eggs which this female lays 

 also differ in their mode of maturation. From the sexual female- 

 producing eggs one polar body is extruded and there is no 

 reduction in chromosomes; the male-producing eggs also form 

 one polar body, but the four seix chromosomes pair and two 

 pass into the polar body to [be) thrust out of the egg, leaving 

 ten chromosomes (8 -f- 2X) in the female pronucleus. The males 

 thus possess ten chromosomes and the sexual females twelve 

 (8 -\- 4X). At spermatogenesis the eight somatic chromosomes 

 pair and four pass to each pole 1 of the spindle, the 2X chromo- 

 somes also pair but do mot separate, and both pass to one pole. 

 In this way two classes of spermatids are produced, one with six 

 chromosomes (4 -f- 2X), the other with four; this latter degen- 

 erates, so that all spermatozoa contain six chromosomes. 



The maturation of the eggjs of; the sexual females is of the 

 normal type, so that when fertilisation takes place the original 

 chromosome number is restored to the egg, which gives rise to 

 the next stem mother. (Diagram B.) 



A second species, P. caryascaulis, is of interest because it has 

 been demonstrated that there are two types of stem mothers ; 

 one kind which produces nothing but sexual females, and the 

 other from whose eggs only males develop. 



In this species there are eight chromosomes, including four 

 sex ones. The general scheme of the chromosome cycle in the 

 several generations is like that for P. fallax; there are, how- 

 ever, slight differences in the sex chromosomes, two of them 

 being large and two small. In the males also the two 

 small chromosomes slightly differ one from the other. Adopt- 

 ing Morgan's notation, we may designate the large chromosomes 

 by X and the small ones by x; the differences in the male being 

 denoted by priming one of the xfs. 



