THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 137 



seen in E. miliaris: and that the clubbed condition, like that of 

 E. miliaris, is dominant over the arched form, as that of E. csculentus. 

 This inheritance is stated to take place irrespective of whether the 

 characters are paternal or maternal. This we find to be the case in 

 our crosses, but we also find that the female has a stronger influence 

 on the character of the hybrid skeleton tlian the male. As an illustra- 

 tion of the dominance of the spinous condition over the smooth, it will 

 be seen in Fig. 5 that in the cross E. acutus S x -E. miliaris ? and its 

 reciprocal the rough condition appears. Again, with regard to the 

 clubbed and arched conditions, in the cross E. csculentus c? x E. miliaris $ 

 and its reverse tlie clubbed form appears in the hybrids. It is 

 also clear from the figure that maternal influence is stronger than the 

 paternal. 



VI. THE CHEMICAL CONTROL OF INHEEITANCE. 



In 1910 Tennent (18), working at Tortugas, made the cross Toxo- 

 pneustcs S x Hipponoe $ and its reciprocal, producing hybrid larva? which 

 had, in both cases, the characteristics of Hippome. The skeleton was 

 used as an index of parental influence, and w^e have already discussed 

 the doubtful value of this evidence. He altered the concentration of 

 the OH-ions in the water in which the fertilizations were made, by 

 adding small definite quantities of Sodium hydrate flnd of Acetic and 

 Hydrochloric Acid. My tliis means he claims to have altered the 

 dominance, so that the hybrid skeleton now resembled that of Toxo. 

 pncustcs. While, from a close examination of his figures, it is clear 

 that a considerable percentage of his Plutei showed skeletal abnor- 

 malities, his main result may or may not be considered as proved. If 

 it is true, the conclusion is obviously a very important one, and one 

 which should be tested with other material and at other places. For 

 this reason, although the inheritance of the late larval characters of the 

 hybrids at Plymouth is strictly maternal, and is not determined by any 

 particular species, yet these characters are of such a definite nature, 

 that we thought it to be a good opportunity for repeating tliis work. 



With this object the crosses E. escidcntus S x t]- miliaris $ and 

 E. acutus 3" X E. milians ? , and their reciprocals were made. The eggs 

 were placed in sea-water, the OH-ion concentration of which had 

 been altered by the addition of .25 to 1 cc. N/10 NaOH in the one case, 

 and HCl or Acetic Acid, in the other, per 200 cc. of sea-water. As 

 soon as the blastulae swam to the surface, they were transferred 

 to " outside " or to " Berkefeld sea- water." At the same time, cultures 

 of the same crosses were made, which were fertilized and raised from 

 the commencement in sea-water of the normal OH-ion concentration. 



NEW SEIilES.— VOL. IX. No. 2. OCTOr.Bi:, 1911. K 



