PARTHENOGENESIS 



271 



to which the parents be- 

 longed. From here on the 

 cells cleave or split again 

 and again by mitosis until 

 enough cells are formed to 

 take on the shape of the 

 plant or animal embryo. 

 The yolk in the egg sup- 

 plies food for the rapidly 

 dividing cells. Fertiliza- 

 tion produces a variety of 

 characters by combining 

 chromosomes from differ- 

 ent individuals. This re- 



The parthenogenetic frogs produced experimentally 

 by Jacques Loeb resembled normal frogs in their ap- 

 pearance. 



Spring and summer aphid s or plant 

 lice are wingless females which pro- 

 duce young, parthenogenetically, e very 

 lo to 20 days. In the autumn, males 

 are produced, mating occurs, and fer- 

 tilized eggs are laid which last over the 

 winter and hatch into females the fol- 

 lowing spring. 



suits in the possibility of slight or great 

 differences among offspring. 



Parthenogenesis. It has been stated 

 that the entrance of a sperm into an 

 egg stimulates a chemical change in the 

 membrane which seals the micropyle, 

 and stimulates the egg to go through 

 repeated divisions. The late Jacques 

 Loeb, formerly at the Rockefeller Insti- 

 tute, carried on a series of experiments 

 to investigate the nature of the fertili- 

 zation process. He pricked the eggs 

 of sea urchins with an electric needle 

 and found that this brought about a 

 membrane activity and the egg began 

 to cleave. He next found that certain 

 chemicals caused the same results. 

 This process of the development of the 

 egg without the entrance of a sperm 



