32 TEXT-BOOK OF EMBRYOLOGY. 



grate, neither kind remaining alive as a rule for more than a few hours. 

 Among these animals the medium in which fertilization occurs is necessarily 

 water, and since it takes place outside of the animal body it is called external 

 fertilization. 



In reptiles, birds and mammals the spermatozoa enter the genital tract 

 of the female and there come in contact with and enter the ova. This is 

 internal fertilization, but the medium in which it occurs is fluid the secre- 

 tions of the female genital tract. A fluid medium is essential because the 

 progress of the sperm depends upon its flagellate activity. In reptiles and 

 birds the spermatozoa move through the genital passages to the ovarian 

 portion of the oviduct where they enter the ova before the secondary egg- 

 membranes, the albumen and the shell, are deposited. After fertilization 

 development begins at once and, in birds at least, continues until the egg is 

 laid and exposed to the lower external temperature. If it has been fertil- 

 ized, the egg at the breakfast table has undergone a considerabled degree of 

 development, the small white disk on the surface of the yolk attesting this 

 phenomenon. 



In mammals the bulk of evidence shows that fertilization occurs as a rule 

 in the upper third of the oviduct, that is, the third nearest the ovary, the 

 spermatozoa having advanced from the vagina through the uterus and lower 

 portion of the oviduct against the current created by the action of the cilia 

 on the epithelial lining of these structures. Development begins at once 

 and while it is in progress the ovum (as it is still named even after develop- 

 ment has set in) is carried down the oviduct and into the uterus where it 

 becomes attached to or embedded in the mucous membrane and continues 

 its transformation into an embryo. In the human also fertilization probably 

 takes place in the great majority of cases in the upper (outer) third of the 

 oviduct (Fallopian tube) . The time required by the spermatozoa to reach 

 this region after insemination has not been determined with accuracy. It is 

 supposed that they advance into the oviducts within a few hours after 

 insemination. If ovulation has occurred prior to this and a mature ovum is 

 moving through either oviduct, fertilization may take place soon after 

 cohabitation. 



That fertilization in the human may and sometimes does occur elsewhere 

 than in the upper third of the oviduct is attested by the position of the grow- 

 ing embryo. Occasionally an embryo develops in the abdominal cavity, 

 which probably shows that spermatozoa have passed all the way through 

 either oviduct. In rarer instances development of the ovum sets in on the 

 surface of the ovary or even within a Graafian follicle. It has been stated 

 that fertilization may occur in the uterus, but there is little evidence to 

 support this conclusion. 



