30 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



Fertilization of the Mouse Ovum. — Normally, a single spermatozoon enters 

 the ovum six to ten hours after coitus. While the second polar body is formhag, 

 the spermatozoon penetrates the ovum and loses its tail. Its head is converted 

 into the male pronucleus (Fig. 13 D). The pronuclei, male and female, approach 

 each other and resolve themselves into a spireme stage, then into two groups of 1 2 

 chromosomes. A centrosome, possibly that of the male cell, appears between 

 them, divides into two, and soon the first segmentation spindle is formed. The 1 2 

 male and 12 female chromosomes arrange themselves in the equatorial plane of 

 the spindle, thus making the original number of 24 (Fig. 13 I). Fertilization is 

 now complete and the ovum divides in the ordinary way. The fundamental 

 results of the process of fertilization are (1) the union of the male and female 

 chromosomes, (2) the initiation of cell division or cleavage of the ovum. 



These two factors are separate and independent phenomena. It has been shown by 

 Boveri and others that fragments of sea-urchin's ova containing no part of the nucleus may be 

 fertilized by spermatozoa, segment and develop into larvae. The female chromosomes are thus 

 not essential to the process of segmentation. Loeb, on the other hand, has shown that the ova 

 of invertebrates may be made to segment by chemical and mechanical means without the 

 cooperation of the spermatozoon. It is well known that the ova of certain invertebrates develop 

 normally with or without fertilization (parthenogenesis). These facts show that the union of 

 the male and female pronuclei is not the means of initiating the development of the ova. In 

 all vertebrates it is, nevertheless, the end and aim of fertilization. 



Lillie (Science, vols. 36 and 38, pp. 527-530 and 524-528) has recently shown that the cortex 

 of sea-urchin's ova produces a substance which he terms fertilizhi. This substance he regards 

 as an amboceptor essential to fertilization with one side chain which agglutinates and attracts 

 the spermatozoa, another side chain which activates the cytoplasm and initiates the segmen- 

 tation of the ovum. 



Spermatozoa may enter the mammalian ovum at any point. If fertilization 

 is delayed and too long a period elapses after ovulation, the ovum may be weak- 

 ened and allow the entrance of several spermatozoa. This is known as poly- 

 spermy. 



The fertilization of the human ovum has not been observed, but probably takes place in 

 the uterine tube some hours after coitus. Ova maybe fertilized and start developing before they 

 enter the uterine tube. If they attach themselves to the peritoneum of the abdominal cavity, 

 they give rise to abdominal pregnancies. If the ova develop within the uterine tube tubular 

 pregnancies result. Normally, the embryo begins its development in the uterine tube, thence 

 passes into the uterus and becomes embedded in the uterine mucosa. The time required for the 

 passage of the ovum from the uterine tube to the uterus is unknown. It probably varies in 

 different cases and may occupy a week or more. The ovum may in some cases be fertilized 

 within the uterus. Fertilization is favored by the fact that the spermatozoa swim always 

 against a current. As the cilia of the uterus and uterine tube beat downward and outward the 

 sperms are directed upward and inward. They may reach the uterine tubes within two hours 

 of a normal coitus. 



