Echinoderm Hybridization. 143 



I know of no analyses of sea-water which show a variation, toward 

 or away from neutrality, correlated with change of season and of temper- 

 ature. We have evidence that sea-water in different localities shows a 

 variation in reaction during the same season. It is merely a matter of 

 speculation, then, when I suggest that the artificial conditions that I 

 produced, which enabled me to control dominance at will, correspond 

 to natural conditions at different seasons of the year. 



Algae, growing in the sea-water, are credited by Loeb (1906) with 

 causing it to become alkaline. It would seem in general that with the 

 higher temperatures of the summer months there would be an increased 

 solution of phosphates and of carbonates to which the alkaline reaction 

 may be due. Whatever the explanation may be, the results of my 

 experiments show a definite effect of a change in environment on the 

 character of the embryos, an effect due to the decrease in the concen- 

 tration of OH ions, brought about by adding an acid to the sea-water. 



The two crosses made which have served as the basis for this experi- 

 mental work are HipponoeQ- X Toxopneustes^S" and the reciprocal cross 

 Toxopnensies 9 X Hipponoed^. Both of these crosses gave plutei with 

 Hipponoe characters. It will be recalled that the pure plutei differ 

 from one another in that the Hipponoe plutei have anal arm skeletons 

 of the fenestrated type and that a basket-like structure is present at the 

 posterior end of the body, while the Toxopneustes plutei have simple 

 rods as skeletons of the anal arms and no basket-like structure is present 

 in the body. 



In my presentation of the evidence of Hipponoe dominance, I shall 

 confine myself closely to the evidence afforded by a comparative study 

 of the skeletal characters of the purely bred and of the hybrid plutei. 

 From a prolonged study of pure Toxopneustes plutei (19 10), I am con- 

 vinced that characters such as the form of the larvse, in nearly similar 

 forms, the number, pigment-content, and arrangement of the chroma - 

 tophores, and comparative measurements of parts of the body, are of 

 too variable a nature to afford safe criteria upon which to base conclusions 

 of importance. 



I shall regard the presence of more than one rod in the anal arm 

 and the presence of a basket-like structure in the posterior part of the 

 body as an indication of Hipponoe influence. 



It may be urged that the intraspecific variation of the skeleton is 

 so great that I have no right to use any part of the skeleton as the basis 

 for the conclusions that I am making. We have seen that both Vernon 

 (1898) and Steinbriick (1902) observed the occurrence of multiple rods 

 in the anal arms of Strongylocentrotus plutei as a common variation. In 

 my own investigations on Toxopneustes (in which cultures from some 

 hundreds of individuals and measurements involving the careful ex- 

 amination of several thousand plutei were made) such a variation was 

 found in the embryo from the eggs of but two individuals. In one case 

 it was found in i per cent and in the other in 3 per cent of the plutei 

 examined. In other crosses it occurred as occasional variation. The 

 pure cultures made in 1909 and 19 10 as controls for the hybridization 

 experiment showed an occasional pluteus with this variation, but the 



