THE CHROMOSOME THEORY OF HEREDITY 297 



ovum then undergoes changes which normally prevent the entrance 

 of other sperms. 



The head of the spermatozoon then undergoes changes which 

 transform it into a t7iale pronucleus, resembhng a resting nucleus, 

 which is similar to the female pronucleus already present. A 

 centrosome appears, divides, and forms a mitotic spindle. The 

 source of this centrosome is uncertain. It was once thought 

 that it was introduced by the male cell, and this may sometimes 

 be the case, but there is evidence to show that it may arise from 

 the cytoplasm of the oviuu. After the formation of the mitotic 

 spindle the pronuclei undergo changes similar to those which 

 occur in the prophase of an ordinary mitotic division and the 

 resulting chromosomes group themselves together in the equa- 

 torial plate. From this point the process is similar to the 

 three final stages of mitosis, viz., the metaphase, anaphase 

 and telophase, and its completion is the formation of two 

 daughter cells. This is the first cleavage of embryonic develop- 

 ment. 



The significant fact of fertilization is the combination in one 

 cell of two similar groups of chromosomes derived from different 

 parents. The diploid complex is maintained in the new individual 

 by mitosis. By referring to Figure 169F and G, in which the 

 chromosomes derived from the female parent are outlined and 

 those from the male are black, the possible results may be seen. 

 There is apparently no definite association between the different 

 kinds of chromosomes, hence the reduction division in the indi- 

 vidual derived from this cell may place some maternal and some 

 paternal chromosomes in each resulting germ cell. Such a reas- 

 sortment of parental chromosomes is of the greatest importance. 

 The number of combinations possible is obviously hmited by the 

 number of chromosomes present. 



Sex Chromosomes. The foregoing account omits consideration 

 of a common contrast in the chromosome complex of males and 

 females of the same species. Sometimes a difference is discernible 

 between two synaptic mates when the number is constant, and 

 in other cases there is a numerical difference, one sex having an 

 odd number of chromosomes and the other an even number due 

 to the presence of a synaptic mate for the odd chromosome. These 

 phenomena have given rise to a theory of chromosomal determina- 

 tion of sex which appears to be largely sound, although recent 



