ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 315 



the change in OH-ion concentration have any effect on the inheritance of these 

 characters. 



Plate 22, figs. 82 and 83, show typical plutei from cultures of the cross 

 E. miliaris % X E. acutus $ , which had been fertilized in waters treated respectively 

 with sodium hydrate and acetic acid. A comparison with Plate 2 1 , fig. 72, will show 

 that with regard to the presence of the green pigment and the absence of the 

 posterior ciliated epaulettes, they do not differ from larvae of this cross made in 

 ordinary sea-water. Similarly Plate 22, figs. 80 and 81, show larvae of the cross 

 E. acutus % X E. miliaris $ which have no green pigment, but have developed the 

 posterior epaulettes as usual. 



Thus the maximum and minimum concentrations of OH-ions in the sea-water, 

 which the eggs would stand, produced no effect on the inheritance of the characters 

 investigated. 



13. The Cytology of Cross-Fertiltzed Echixoderm Eggs. 



In spite of the large amount of work that has been devoted to the hybridization 

 of Echinoderms, it is only quite recently that any attention has been paid to the 

 cytology of the hybrids in the early stage of their development. 



Purely maternal larvce were described by Seeliger (1894), Morgan (1895), 

 Vernon (1898), Fischel (1906), Hagedoorn (1909), and Tennent (1910). In 1909 

 Kupelwieser(48) discovered that it was possible to stimulate the egg of a sea-urchin 

 to develop by fertilizing it with a sperm of a mollusc, although the male and female 

 pronuclei never fused. There arose, therefore, the suggestion that the "maternal" 

 nature of many hybrid larvae is, perhaps, due to the same cause, viz., that although 

 the sperm enters the eggs and causes them to develop, yet only the female pronuclei 

 are functional in development. Further, it seemed as though the discordant results 

 of various authors who had worked on the same hybrids might be explained by the 

 fact that under certain conditions the male pronucleus was functional, while under 

 other conditions it was not. 



In the former case an " intermediate " type of larvae would be expected, and in 

 the latter only those of the "maternal" type. The fact that we obtained only 

 " maternal " larvae from certain crosses during three years' work led us to investigate 

 the phenomena of fertihzation very carefully. As a result of this work we have 

 found that in every hybrid uith lohich we have ivorhed a true fusion of the male and 

 female 'pronuclei occurs in every egg which develops. A complete cytological study 

 of eggs taken from the same batches as those from which the hybrids themselves 

 were reared has shown that there is absolutely no doubt that a true zygote nucleus is 

 invariably formed. 



These results, together with those of Baltzer (who worked with the same species 

 as did earlier workers, such as Seeliger, Driesch, Vernon, etc.), show that the 



2 s 2 



