Echinoderm Hybridization. 



125 



studies (Zellen-Studien 5 and 6), and it is on the basis of this interpre- 

 tation that the more recent work of Herbst, Baltzer, and others is 

 founded. 



The beginning of a second phase of work in Echinoderm hybridiza- 

 tion was marked by the appearance of Vernon's papers. As a preparation 

 for his work on hybridization, Vernon (1895) made a careful study of 

 the effect of environment on the development of Echinoderm larvae, 

 using Strongylocepiirotus lividus as the subject of his research. As means 

 of changing the environment he used (i) Differences in temperature, 

 (2) Differences in concentration of sea-water, (3) Differences in light, 

 (4) Chemical agents. 



Strongylocentrotus eggs were placed in water of 8° or 25° C. for an 

 hour, or even for a minute, at the time of impregnation. After 8 days 

 the resulting plutei were 4.4 per cent smaller than those from eggs 



Fig. 3. — Pluteus oi Sphcerechimts 



granidaris 



(Boveri). 



Fig. 4. — Hybrid pluteus Echinus 

 microtuberculat-us (^ X Sphcere- 

 chinus granulans ? 

 (Boveri). 



fertilized at 17° to 22° C. Larvae allowed to develop in water 17° to 

 22° C. were 2 per cent or more larger than those allowed to develop at 

 temperatures above or below these limits. 



The normal breeding season of Strongylocentrotus is from December 

 to March. Larvae from fertilizations made in August were 20 per cent 

 smaller than those obtained in April, May, or October. June and July 

 larvae were intermediate in size — this due to immaturity. 



The addition of 50 c.c. distilled water to a liter of sea-water gave 

 larvae 15.6 per cent larger than larvae grown under normal conditions; 

 25 c.c. distilled water to a liter of sea-water, 9.5 per cent larger; 150 c.c. 

 distilled water to a Hter of sea-water, 4-3 per cent smaller. Larvas 

 developed in more concentrated sea-water were unchanged ; larvae grown 

 under normal conditions from impregnations made in concentrated sea- 

 w^ater were 1.6 per cent larger. Larvae grown in semi-darkness were 

 2.5 per cent larger; in darkness 1.3 per cent smaller; in blue Hght 

 (copper sulphate), 4.5 per cent smaller; in violet blue Hght (Lyons blue), 



