216 FERTILIZATION 



somes in the life history was formulated. E. B. Wilson led this work, 

 and the theory that he formulated became the assured basis of cyto- 

 logical and genetical sex studies constituting one of the greatest present 

 day advances in zoology. "McClung's anticipation of this theory is a 

 striking example of scientific imagination applied to painstaking obser- 

 vation" (Lillie, F. R., '40). 



Not only were the sex chromosomes studied, but other chromo- 

 somes as well, and an intense series of genetical investigations were 

 initiated by Morgan and his students and others which succeeded in 

 tying a large number of hereditary traits to individual chromosomes 

 and also to definite areas or parts of the chromosomes. Thus the 

 assumptions of Roux and Boveri were amply demonstrated. More- 

 over, these observations established experimental proof for the con- 

 cept that in the gametic fusion which occurs during fertilization, the 

 chromosomes pass from one generation to the next as individual 

 entities, carrying the hereditary substances from the parents to the 

 offspring. The heredity of the individual was in this way demonstrated 

 to be intimately associated with the reunion of the haploid groups of 

 chromosomes in the fertilization process. 



C. Types of Egg Activation 



1. Natural Activation of the Egg 



Natural parthenogenesis, i.e., the development of the egg spontaneously 

 without fertilization was suggested by Goedart, in 1667, for the moth, Orgyia 

 gnastigma, and by Bonnet, in 1745, in his study of reproduction in the aphid. 

 (See Morgan, '27, p. 538.) Since this discovery by Goedart and Bonnet, many 

 observations and cytological studies have shown that there are two kinds of 

 eggs which are capable of natural parthenogenesis: 



(1 ) That which occurs in the so-called, non-sexual egg, i.e., the egg which 

 has not undergone the maturation divisions and, hence, has the diploid 

 number of chromosomes; and 



(2) that which results in the sexual egg, i.e., the egg which has experi- 

 enced meiosis (Chap. 3) and thus has the reduced or haploid number 

 of chromosomes (Sharp, '34, pp. 409, 410). 



Parthenogenesis from a non-sexual egg is found in daphnids, aphids, flat- 

 worms, and certain orthopterans. In the case of the sexual egg, parthenogenesis 

 normally occurs in bees, wasps, ants, some true bugs, grasshoppers, and 

 arachnids. In this type of egg, development may result with or without fer- 

 tilization. For example, in the honeybee. Apis mellifica, haploid males arise 

 from eggs which are not fertilized, workers and queens from fertilized eggs. 



Extensive studies of the animal kingdom as a whole have demonstrated, 

 however, that the majority of oocytes or eggs depend upon the fertilization 



