20 THE GERM CELLS: MITOSIS, MATURATION AND PERTILIZATION 



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

 into the male pronucleus (Fig. 14 D). The pronuclei, male and female, approach 

 each other and resolve themselves first into a spireme stage, then into two groups 

 of 20 chromosomes. A centrosome, possibly that of the male cell, appears be- 

 tween them, divides into two, and soon the first segmentation spindle is formed 

 {F-H). The 20 male and 20 female chromosomes arrange themselves in the 

 equatorial plane of the spindle, thus making the original number of 40 (/). Fer- 

 •tilization is now complete and the ovum divides in the ordinary way, the daughter 

 cells each receiving equal numbers of maternal and paternal chromosomes. The 

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

 female chromosomes to form the cleavage nucleus of the fertilized ovum, (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 

 fertihzed 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 develop by chemical and mechanical means without 

 the cooperation of the spermatozoon {artificial parthenogenesis) . Even adult frogs have been 

 reared from mechanically stimulated eggs. It is well known that the ova of certain inver- 

 tebrates develop normally without fertilization, that is, parthenogenetically. 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 fertiliza- 

 tion. 



Lillie {Science, vols. 36 and 38; 1912, 1913) has recently shown that the cortex of sea 

 urchin's ova produces a substance which he terms fertilizin. This substance he regards as an 

 amboceptor essential to fertilization, with one side chain which agglutinates and attracts the 

 spermatozoa, and another side chain which activates the cytoplasm and initiates the cleavage 

 of the ovum. According to Loeb, the spermatozoon activates the ovum to develop by in- 

 creasing its oxidations and by rendering it immune to the toxic effects of oxidation. 



Spermatozoa may enter the mammab'an 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. In such cases, however, only one spermatozoon unites with the female 

 pronucleus. 



Fertilization of the Human Ovum. — This has not been observed, but prob- 

 ably takes place in the uterine tube some hours after coitus. Ova may be fertil- 

 ized and start developing before they enter the uterine tube. If they attach them- 

 selves to the peritoneum of the abdominal cavity, they give rise to abdominal 

 pregnancies. If the ova develop within the uterine tube tubal pregnancies result. 

 Ovarian pregnancies are known also. Normally, the embryo begins its develop- 



