412 INTRODUCTION TO CYTOLOGY 



without fusing with the egg nucleus. In one species the sperm is 

 necessary for activation, although it does not participate further in 

 development.^^ 



Similar phenomena are frequently encountered in the course of 

 hybridization experiments and in pathological material. The degenera- 

 tion of the sperm nucleus in the egg cytoplasm has been observed in a 

 cross between a sea urchin and a mollusk.-^ In certain other crosses the 

 paternal nuclear matter is extruded from the fusion nucleus or during the 

 early cleavage mitoses. ^^ Such elimination during cleavage results 

 largely from the lagging of the paternal chromosomes in mitosis; in fish 

 hybrids this appears to be related to the viscosity of the egg cytoplasm 

 (Pinney). Embryos formed by gynogenesis are maternal in character. 

 Gynogenesis has been induced in amphibians and echinoderms by treating 

 the sperm with radium.-^ 



B. Androgenesis. — The development of embryos with the paternal 

 chromosomes alone has been induced by incapacitating the egg nucleus 

 with radium before syngamy.^" In Chcetopterus it has been shown that 

 such larvse have the monoploid chromosome number, 9. Whether such 

 larvae may develop into adults or not is unknown. 



C. Merogony. — The development of an enucleate egg fragment 

 which has been entered by a normal sperm may be induced more or less 

 readily in a number of animals. Such enucleate fragments are obtained 

 by shaking the eggs^^ or by cutting them in two with a knife or hair 

 noose. ^'- The development of merogonic individuals usually does not 

 continue beyond the larval stage. 



Many such experiments have been made with the idea of testing the 

 nuclear theory of heredity. The results have varied with the species 

 used, the methods, and the direction of the crosses, but, when all things 

 are considered, they have tended to support the nuclear theory. The 

 evidence in such cases is often inconclusive because larval characters, 

 which must be chiefly depended upon, are not widely different in the 

 species crossed and may begin their differentiation in the egg cytoplasm 

 before syngamy (p. 419).^^ 



26 Kriiger (1913), P. Hertwig (1920a). See Wilson (1925, p. 460). 

 -' Echinus 9 X Mylilus d" (Kupelwieser, 1912). 



28 Godlewski (1911), Baltzer (1910), Kupelwieser (1912), Tennent (1912), Pinney 

 (1918, 1922). 



2»0. Hertwig (1910 et seq.), G. Hertwig (1911 et seq.), P. Hertwig (1911 et seq.). 

 ^^ G. Hertwig (1911, 1913; on amphibians), Packard (1918; on Chcetopterus). 



31 O. and R. Hertwig (1887), Boveri (1889 et seq.), and Godlewski (1906) on 

 echinoderms. 



32 Wilson (1903), Yatsu (1904, 1910), Zeleny (1904), Spemann (1914, 1919; hair 

 noose) and Baltzer (1921) on nemertines, mollusks, and amphibians. 



33 For more extensive accounts of experiments of the kind cited in this section, see 

 Morgan (1924o-ff), Conklin (1924), G. Hertwig (1920a6), P. Hertwig (1923), Stomps 

 (1923), Wilson (1925), and Brachet (1927). 



