850 THE PHYSIOLOGY OF REPRODUCTION. 



by the egg is a provision to necessitate sexual union, and thus 

 insure the benefits that presumably are associated with the fusion 

 of two cells originating from different individuals. 



Fertilization of the Ovum. The spermatozoon comes into 

 contact with the ovum probably at the beginning of the Fallopian 

 tubes. The meeting of the two cells is possibly not simply a matter 

 of accidental contact, although the number of spermatozoa dis- 

 charged by the male at coitus is so great that there would seem 

 to be little chance for the ovum to fail to meet some of them. Ex- 

 periments upon the reproductive elements of plants indicate, how- 

 ever, that the egg may contain substances which serve to attract 

 the spermatozoon, within a certain radius, by that force which 

 is described under the name of chemotaxis. However this may be, 

 the egg unites with a spermatozoon and under normal conditions 

 with only one. A number of the spermatozoa may penetrate 

 the zona radiata, but so soon as one has come into contact with 

 the cytoplasm of the egg a reaction ensues in the surface layer 

 which makes it impervious to other spermatozoa. The spermato- 

 zoon consists of three essential parts, the head, the middle piece, 

 and the tail. The last named is the organ of locomotion, and 

 after the spermatozoon enters the egg this portion atrophies and 

 disappears, probably by absorption. The head of the spermato- 

 zoon represents the nucleus, and contains the valuable chromatin 

 material. On entering the egg it moves toward the nucleus of the 

 latter, meanwhile enlarging and taking on the character of a nu- 

 cleus. The egg now contains two nuclei, one belonging to it origi- 

 nally, the female pronucleus; one brought into it by the sperma- 

 tozoon, the male pronucleus. The two come together and fuse, 

 superficially at least, forming the nucleus of the fertilized egg, or 

 the segmentation nucleus. The middle piece of the spermatozoon 

 also enters the egg, but its exact function and fate is still a matter 

 of some uncertainty. Boveri believes that it brings into the egg a 

 centrosome or material which induces the formation of a centro- 

 some in the ovum and is therefore of the greatest importance in 

 initiating the actual process of cell division which begins promptly 

 after the fusion of the nuclei. In the segmentation nucleus the nor- 

 mal number of chromosomes is restored, and it is believed that in 

 the subsequent divisions of the cell to form the embryo the chromo- 

 somes are so divided that each cell gets some maternal and some 

 paternal chromosomes, and thus shares the hereditary characteris- 

 tics of each parent. This view is represented in a schematic way 

 by Fig. 271, taken from Boveri, the maternal and paternal chromo- 

 somes being indicated by different colors. According to this descrip- 

 tion, both egg and spermatozoon are incomplete cells before fusion. 

 The egg contains chromatin for a nucleus, and a large cell body, 



