30 



TEXT-BOOK OF EMBRYOLOGY. 



tion that the ids are arranged in linear series in the chromosomes, Weismann ventured the 

 prediction that two forms of mitosis would be found to occur. One would be a longitudinal 

 splitting so that each daughter nucleus would receive one-half the amount of each id. This 

 form of division was known at that time and is characteristic of ordinary mitosis. The 

 other form would, he suggested, be such as to give each daughter nucleus one-half the number 

 of ids. This might be brought about either by transverse division of the chromosomes 

 or by the elimination of one-half the number of chromosomes. This form of division was 

 not known at that time and the fulfillment of this second part of Weismann's predictions 

 has been one of the most remarkable discoveries in cytology, for it has been demonstrated 

 for some forms at least that transverse division of chromosomes does actually occur. 



Weismann's views form a wonderfully ingenious theory against which there is thus far 

 no structural ground for opposition. Indeed, some known facts are in its favor. Whether, 

 however, these facts possess the significance which Weismann attributed to them is still an 

 open question. 



Germinal 



epithelium 



FIG. 18. From section of human ovary, showing mature Graafian follicle ready to rupture. 



Kollmann's Atlas. 



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 

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

 two phenomena are usually associated although either may occur independ- 

 ently 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 



