156 CYTOLOGY CHAP. 



The statement made above, that inheritance from male and female 

 parents is on the average of approximately equal intensity, besides being 

 a matter of common knowledge, is established beyond question experi- 

 mentally. Contrary opinions have, however, been expressed by workers 

 in one special field of biology namely, the study of hybrids (especially 

 of Echinoderms) in embryonic or early larval stages, where it has been 

 held that the influence of the female parent is predominant. From this 

 the conclusion has been drawn that the cytoplasm also contains 

 idioplasm. 



The Echinodermata have shown themselves to be very adaptable to 

 these experiments, crosses being comparatively easily effected between 

 species, genera or even classes, and an efficient technique has been 

 evolved for dealing with this work, which has attracted a large number 

 of investigators. Many of these have found that the hybrid embryos 

 and larvae from certain crosses exhibit purely maternal characters. 

 Thus Vernon (1898), making reciprocal crosses between seven different 

 genera of sea-urchins, found that, as a rule, the hybrid larvae (plutei) 

 were of the maternal type. The results varied considerably in this respect, 

 however, in different seasons. Thus the cross Sphaerechinus $ x Strongylo- 

 centrotus $ gave in May- July mostly maternal larvae, while in December- 

 January the larvae from the same cross were all paternal in type. Shearer, 

 de Morgan and Fuchs have also shown (1913) how the prepotency of 

 one or other species may vary at different times. Their crosses were 

 made between Echinus esculentus or acutus and miliaris. The pluteus 

 larvae of the former two species exhibit, in the older stages, posterior 

 ciliated epaulettes but no masses of green pigment, while the larvae of 

 miliaris at the same stage have masses of green pigment but no posterior 

 epaulettes. In 1910-11 crosses between esculentus or acutus and miliaris, 

 in whichever direction they were made, produced larvae resembling the 

 species used as female parent (in regard to these two features). In 1912, 

 however, similar crosses gave a different result, miliaris appearing 

 practically incapable of transmitting its characters at all, so that when 

 this species was used as the female parent the larvae resembled the 

 male parent species, This result appeared to be due to something un- 

 favourable to miliaris in the environment, since pure cultures of this 

 species, which in 1910-11 were easy to rear, proved very difficult to bring 

 up in 1912. 



Godlewski (1906) succeeded in fertilizing Echinus eggs by the sperm 

 of Antedon, which belongs to a very different class of the Echinodermata. 

 Union between male and female gamete nuclei took place, and the two 

 sets of chromosomes, though of such different origins, appeared to har- 

 monize on the spindle and to co-operate at telophase to form the 

 resting nucleus as in normal fertilization and cleavage. The hybrids 



