i8 



HUMAN ANATOMY. 



the chromatin, so that the mature egg, after the completion of maturation, represents 

 the third generation, and is, therefore, morphologically equivalent to a spermatozoon. 

 Attention has already been directed to the important fact that the cells of a 

 given species contain a fixed, definite, and even number of chromosomes (page 

 12); hence, in their primary condition, each germ-cell contains the full complement 

 of chromatin segments. vSince, however, the new being arises from the elements 

 derived from the segmentation of a cell to the nucleus of which both parents con- 

 tribute an equal number of chromosomes, it follows that, unless some provision be 

 made whereby the number of chromosomes in each germ-cell be reduced to one- 



Primordial germ-cell 



Fig. 14. 



Primordial germ-cell 



Male 



Spermatogonia 



Primary sperma- 

 tocyte 



Secondary sper- 

 matocytes 



Spermatid? 

 Spermatozoa 



Division-period ' 



Growth-period 



Maturation-period 



Femalh 



— Oogonia 



Primary oocyte 

 (ovarian egg) 



Secondary oocytes 

 (egg and first 

 polar body) 

 Mature egg and 

 polar bodies 



a • • . » y 



Diagram illustrating the genesis of the male and female germ-cells. {After Boveri.) 



half the full number, the elements of the new being would be provided with double 

 the number required to satisfy the normal complement for the particular species. 

 In fact, such reduction of the chromosomes of the germ-cells does take place during 

 the development of these elements, in consequence of which the ovum and the 

 spermatozoon each contribute only one-half the number of chromosomes, the nor- 

 mal quota being restored to the segmentation nucleus, and subsequently to the cells 

 of the new being, by the sum of the contributions of both parents. 



Interpreted in the light of these considerations, maturation may be regarded as 

 the means by which correspondence between the sexual cells is secured, and, further, 

 the polar bodies may be considered as abortive ova. 



Fertilization of the Ovum. — Impregnation, or fertilization of the ovum, 

 includes the meeting of the male and female elements, the penetration into the sub- 

 stance of the latter by the former, and the changes immediately induced by the 

 presence of the spermatozoon within the egg. 



Coincidendy with the rupture of the distended Graafian follicle, the surface of 

 the ovary is embraced by the expanded fimbriated extremity of the oviduct, along the 

 plications of which the liberated matured ovum is guided into the tube. It is highly 

 probable that not an inconsiderable number of the ova discharged from the ovary fail 

 to reach the oviduct and are lost in the abdominal cavity. 



Recent investigations have shown that both germ-cells contain particular acces- 

 sory chrojiiosomes, which are probably important factors in the determination of the 

 sex of the new being. 



The spermatozoa overcome the obstacles offered within the narrow channels 

 by the mucus and the opposed ciliary currents of the uterine and tubal mucous 

 membranes by virtue of their long actively vibrating tails, and advance at a rate 

 estimated at from 1.5 to 2.5 millimetres per minute ; it is therefore probable that 

 the seminal cells accomplish the journey from the mouth of the uterus to the ovum 

 in from eight to ten hours. Spermatozoa retain their vitality and fertilizing pow- 

 ers for many days within the normal female genital tract ; repeated observation on 

 the human subject has shown that this period may extend throughout an entire 



