MATURATION. 



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several vertebrates such as the rat, fowl, guinea-pig, and even man. In many 

 cases the "accessory" chromosome of the male germ cell has a mate which 

 differs, however, in some way (size, appearance, etc.) and is designated the Y- 

 chromosome. An ovum fertilized by a spermatozoon containing the Y-chro- 

 mosome will give rise to a male ; if fertilized by one containing the X-chromosome 

 the egg will develop into a female. 



There are many cases, particularly among parthenogenetic forms, where 

 sex cycles arise, which cannot be explained by chromosomal behavior. In 

 these cases nutrition seems to play an important part in determining the sex of 

 the individual. But as to the great majority of forms investigated, the weight 

 of evidence supports the view that the chromosomes are the chief agents in sex 

 determination. 



OVULATION AND MENSTRUATION. 



By ovulation is meant the periodic discharge of the ovum from the Graafian 

 follicle and ovary. By menstruation is meant the periodic discharge of blood 



FIG. 19. Showing ovary opened by longitudinal incision. The ovum has escaped through the 

 tear in the surface of the ovary. The cavity of the follicle is filled with a clot of blood (corpus hacmor- 

 rhagicum) and irregular projections composed of lutein cells. Kollmann's Atlas. 



from the uterus associated with structural changes in the uterine mucosa. The 

 two phenomena are usually associated although either may occur independently 

 of the other. They normally occur every twenty-eight days. That ovulation 

 and menstruation are not necessarily dependent upon each other and that either 

 may occur without the other has been proved by a number of observations; 

 thus the occurrence of fertilization during lactation when the menstrual func- 

 tion is in abeyance; the occurrence of impregnation in young girls before the 



