ON THE EXPEEIMENTAL HYBRIDIZATION OF ECHINOIDS. 259 



forms have other characteristics, or that the differences between them are not so ereat 

 as at Naples. 



In 1895 Vernon (94) commenced a new era in this work by an investigation of the 

 effects of environment on the development of the larvae. He recorded the effects of 

 changes in light, temperature, concentration of the water, and the addition of various 

 chemical agents. Unfortunately he only considered the external shape of the larvae, 

 and (as was the case with all the early investigators) he never reared his plutei later 

 than eight days. 



In 1898 (.95), he followed this up with an investigation of the inheritance in various 

 hybrid forms. He made crosses between Sphmrechinus, Strongylocentrotus, and 

 Echinus, and also Arhacia, Keliinovardium, Dorocidaris and Echinus acutus ; but in 

 the case of tliese last, only a {av^ of the very early larvae were obtained. As indices 

 of parental influence measurements of the body and arm lengths were alone used. 

 The hybrid larvae were mostly maternal. The crosses with Stroyigylocentrotus eggs were 

 purely maternal, except when Echinus sperm was used, and even then they were 

 rather maternal than paternal. Sphcerechinus eggs, however, tended to give paternal 

 hybrids. Some genera seemed thus to have a greater potentiality for transmitting 

 tlieir characteristics than others. One interesting point brought out was that when 

 Strongi/Iocentrotus $ was crossed with Echinus $ more numerous and larger larvee 

 were obtained than by direct fertilization. As a result of these experiments Vernon 

 concluded that the characteristics of the hybrid plutei depend on the relative ripeness 

 of the sexual products. 



In the same year Driesch(25) crossed Stronyylocentrotus, Sphcerechinus, Echinus, 

 and Arhacia and obtained hybrid larvae with purely maternal characters, which he 

 suggested were inherited through the cytoplasm of the egg. He investigated the 

 rate of segmentation, the character of the blastula wall cells, the number of primitive 

 mesenchyme cells, the pigmentation, and the skeleton. He remarked on the fact that 

 some individuals of a species cross well, while others will not do so. 



At this time, too, Delage (17) confirmed the possibility of Boveri's experiments by 

 cutting a single egg of a sea-urchin in two pieces and fertilizing both the nucleated 

 and the enucleated portions. Both of them he raised to the gastrula stage. 



In 1900 Vernon (97) crossed Strongylocentrolus with SphcBrechinus and found that 

 the characters of the resulting hybrids varied with the season of the year at which the 

 experiments were made. In the spring the larvae resembled StrongyIocent7-otus, while 

 in the summer they resemljled Sphcerechinus. In the latter case, however, none of 

 the larvae were of the pure SphcBrechinus type, as he had fovnid on a previous occasion. 

 He concluded that the seasonal fluctuations in " prepotency " were due to a variation 

 in the ripeness of the sexual products ; for in the summer Stro7igylocentrotus is least 

 mature, while in spring it is most so. In the cross Strongylocentrotus $ X SphcBr- 

 echinus ? he found, contrary to the experience of O. and R. Hertwig, that the 

 keeping of the ova before fertilization did not increase the number of resulting 



2 L 2 



