234 REPRODUCTION 



of the ovum are occurring in cases like Toxopneustes where the 

 polar-body formation normally takes place at the time of fertiliza- 

 tion. When this maturation has been completed, the centriole 

 of the ovum disappears, and the egg-nucleus, with its haploid 

 number of chromosomes, moves to a position beside the sperm- 

 nucleus, while the centriole from the middle-piece of the sperma- 

 matozoon divides and the mitotic spindle of the zygote is formed. 



As a result of fertilization, the male and female pronuclei, 

 therefore, come to occupy a position on either side of the first- 

 division spindle and their chromosomes are distributed equally 

 to each cell of the two-cell stage. If this process were continued 

 throughout the development, the cells of the adult body would 

 contain paired chromosomes arising in such a manner that one 

 member of each pair would be descended from the corresponding 

 chromosome of the ovum and the other member from that of the 

 spermatozoon. As a matter of fact, such a paired relationship of 

 the chromosomes can be observed in many animals, not only in 

 the first division of the zygote but also in many subsequent cell 

 divisions. Moreover, in the cells of the adult body the chromo- 

 somes are found to occur in pairs wherever their diversity of 

 size and appearance renders such a distinction possible. These 

 facts, together with the theoretical conclusions regarding the 

 chromosomes that are drawn from their behavior in correlation 

 with the inheritance of adult characters, are the basis for the gen- 

 erally accepted conclusion that the chromosomes, as they are 

 found in their diploid condition in the cells of an adult animal, 

 have descended one-half from the pronucleus of the ovum and the 

 other half from the pronucleus of the spermatozoon. If this is 

 correct, one can hardly escape the conclusion that the chromosomes 

 furnish a mechanism for the inheritance that is observed in sexual 

 reproduction. 



In addition to this union of two lines of descent in the produc- 

 tion of a new individual, the process of fertilization also involves 

 the stimulus which causes the ovum to develop into the adult 

 animal after its union with the spermatozoon. The term fertili- 

 zation is sometimes restricted to this developmental stimulus, 

 which may be called the activation of the egg, since the word 

 fertilization suggests the act of making an egg fertile and hence 

 a stimulation to development. In this discussion the authors 

 have chosen to use the term fertilization to include both the 



