214 FERTILIZATION 



illumination of the details of sperm and egg behavior during fertilization really 

 began with the studies of O. Hertwig in 1875. The more important studies 

 which have shed light upon the problems involved in gametic fusion are pre- 

 sented below: 



(1) O. Hertwig, 1875, 1877, in the former paper, described the fusion of 

 the egg and sperm pronuclei in the Mediterranean sea urchin, Toxo- 

 pneustes lividus. One aspect of the work published in 1877 was con- 

 cerned with the formation of the polar bodies in Haemopis and 

 Nephelis. In a part of the latter publication O. Hertwig presented 

 descriptions of sperm migration from the periphery of the egg and 

 the ultimate association of the sperm and egg pronuclei during the 

 fertilization in the frog, Rana temporaria (fig. 1191, J). 



(2) Fol, 1879, contributed detailed information relative to the actual en- 

 trance of the sperm into the sea-urchin egg and showed that in the 

 eggs of various animal species only one sperm normally enters. He 

 also described the formation of the fertilization membrane in the egg 

 of the sea urchin, Toxopneustes lividus. 



(3) Mark, 1881, made important contributions relative to the formation 

 of the polar bodies in the slug, Deroceras laeve (Limax campestris). 

 He also presented information which showed that the egg and sperm 

 pronuclei, although associated near the center of the egg during fer- 

 tilization, do not actually form a fusion nucleus in this species as de- 

 scribed for the sea urchin by O. Hertwig. This is an important contri- 

 bution to the fertilization problem, as fusion nuclei are not formed 

 in all animal species. 



(4) Van Beneden, 1883, in his studies on maturation of the egg and fer- 

 tilization in Ascaris megalocephala, demonstrated that half of the 

 chromatin material of the egg nucleus was discharged in the matura- 

 tion divisions. (He erroneously thought, however, that the female 

 ejected the male chromosomes at this time, and in the male, the reverse 

 process occurred.) (See fig. 133C, D.) He demonstrated also that the 

 two pronuclei in Ascaris do not join to form a fusion nucleus at fer- 

 tilization. His work revealed further that the male and female pronuclei 

 each contributes the haploid or half the normal number of chromo- 

 somes at fertilization and that each haploid group of chromosomes 

 enters the equatorial plate of the first cleavage spindle as an inde- 

 pendent unit (fig. 133F-I). Upon the equatorial plate each chromo- 

 some divides and contributes one chromosome to each of the two 

 daughter nuclei resulting from the first cleavage division. This contri- 

 bution of the haploid number (half the typical, somatic number) of 

 chromosomes from each parent Van Beneden assumed to be a funda- 

 mental principle of the fertilization process. This principle was defi- 



