Parthenogenesis — Blastoderm 89 



the former invaribly develop into female, the latter 

 into male insects. On the other hand, the successive 

 broods of Plantlice produced through the summer by 

 virgin reproduction consist entirely of females. And 

 among such insects as those Gallflies of which no 

 males are known, the young produced by partheno- 

 genesis are, of course, females. The reason for these 

 cases of virgin reproduction is still a subject of con- 

 troversy. It is evident that the power must greatly 

 help the insects possessing it to increase in numbers, 

 since no female runs the risk of barrenness through 

 failure to pair. Often, as in the case of the plant- 

 lice, this power is associated with a specially abundant 

 food supply, being continued throughout the summer 

 months, but giving way to the usual method of 

 sexual reproduction in the autumn and spring (54, 57)* 



Development of the Embryo. — By division of 

 the fertilized egg-nucleus are formed the layers of cells 

 whence the various organs of the body are built up. 

 This process is complex, and its details diiFer in 

 various insects in which it has been observed (53). 

 It has been mentioned that an insect egg is relatively 

 large and contains a quantity of food-yolk. In such 

 eggs the whole mass does not divide into separate 

 cells, but a layer of cells is formed at one side of the 

 egg or around the central yolk. This method of egg 

 division is known as partial segmentation, and eggs 

 which undergo it are called merohlastic. It contrasts 

 with the complete segmentation of small eggs with- 

 out food-yolk — those of mammals and star-fishes for 

 examples, which are called holoblastlc. 



Blastoderm. — The segmentation of an insect egg 

 affords an excellent example of the importance of the 

 nucleus in cell-division. Whenever cells multiply, a 

 division of the nucleus precedes the division of the 

 cell-body. In an insect egg, segmentation begins 



