1118 THE REPRODUCTIVE ORGANS 



be compared with the rejuvenation effected in "senile" protozoon by the method 

 of conjugation. A somewhat different explanation is made possible by the sugges- 

 tions of Trivianus, Brooks, and Weismann, that fertilization is essentially a process 

 by means of which variations are produced in consequence of the acquisition of second- 

 ary elements, insuring a constant mingling and repeated renewal. 



The Fertilization of the Ovum. The physiological principle 

 underlying sexual reproduction, is the process of fertilization effected 

 by the fusion of the two germ-cells, one of which is of maternal and 

 the other of paternal origin. In most cases, this union takes place 

 within the body of the mother, but may also be accomplished in an 

 outside medium which is accessible to both the female and male germ- 

 cells. The manner in which these elements are brought together 

 differs greatly in different animals, and hence, the subsequent discus- 

 sion pertaining to the mechanics of sexual reproduction, must nec- 

 essarily be restricted to an enumeration of the functions of the 

 different sexual organs of the mammals. The minute changes, how- 

 ever, are usually studied in the eggs of the lower forms, for example, 

 in those of the sea-urchin and the thread-worm. 



Subsequent to the discovery of the spermatozoon by Hamm 

 (1677), Leeuwenhoek expressed the idea that this element must pene- 

 trate the egg, an assumption which was later on confirmed by Spal- 

 lanzani (1786), Newport (1854), and Pringsheim (1855). It seems, 

 however, that only the head of the spermatozoon actually takes part 

 in the fertilization, because in some animals, such as the echinoderms, 

 the tail remains entirely outside the egg. But it is also true that 

 the eggs of the molluscs, insects, nematodes and some annelids fre- 

 quently display the tail of the spermatozoon within their cytoplasm, 

 forming here a delicate coiled up structure. At the time of contact 

 between the male and female elements, the ovum produces two minute 

 globular masses at its upper extremity which are known as the polar 

 bodies. Since these projections take no part in the subsequent changes 

 but degenerate and may make their appearance even before the en- 

 trance of the spermatozoon, it seems that they merely indicate that 

 the egg has reached its mature state and is ready to receive the male 

 sperm-cell. In all probability, a place of least resistance is formed by 

 this means, through which the spermatozoon first arriving in this 

 vicinity, is enabled to enter. Immediately upon conception, a tough 

 envelope, the vitalline membrane, is developed around the ovum, 

 thereby preventing the entrance of those spermatozoa which may have 

 reached their destination during the interim. As soon as the head of 

 the successful spermatozoon has been lodged in the cytoplasm, the 

 tail atrophies and disappears. Now follows a gradual enlargement of 

 the former and the breaking up of its chromatin material into a thread- 

 like formation and its characteristic number of chromosomes. The 

 egg then embraces two nuclei (Hertwig, 1875), one of which is of pater- 

 nal and the other of maternal origin. This is the crucial point of 

 fertilization, because these male and female pronuclei, containing 



