ON THE EXPEEIiMENTAL HYBRIDIZATION OF ECHINOIDS. 313 



Tennent (88), working at the Tortugas Laboratory in 1909 and 1910, made 

 experiments to find whether a change in the concentration of OH-ions in the water 

 in which cross-fertihzation was made had any influence on the transmission of 

 parental characters. He made the two reciprocal crosses between To.vopneustes and 

 Hipponoe, obtaining in both cases larvae with a dominance of Hipponoe skeletal 

 characters. The skeletal rods in the postoral arms of Toxopneustes are single, while 

 in Hipponoe they are in the form of a lattice work, and moreover, the Hipponoe 

 pluteus develops a basket-like structure at the posterior end of the body, which is 

 absent in Toxopneustes. If the hybrids developed more than one rod in the postoral 

 arms and had a basket structure at the posterior end of the body, Tennent considered 

 that they showed a dominance of the Hipponoe characters. Now, the occurrence of 

 multiple rods in the postoral arm skeletons of Strongyloce.ntrotus (which normally has 

 single rods) had been observed by Vernon (95) and Steinbruck (83), and we have 

 found that unhealthy larvse of Echinus often form multiple rods and occasionally 

 lattices in the arms Tennent, however, had previously made an investigation into 

 the variations in the skeletal characters in pure-bred Toxopneustes. He found that 

 here multiple rods were very rare, having occurred only in two experiments, and 

 then to the extent of 1 per cent, and 3 per cent, of the individuals. Hence he 

 concluded that in hybrids between Toxopneustes and Hipponoe the development of 

 more than one rod in the arm skeleton was a true indication of Hipponoe influence. 

 From his figures, however, it is seen that his hybrid larvae in most cases formed their 

 skeletons in an irregular and unsymmetrical manner. It is in just such cases of 

 comparatively unhealthy hybrid larvse, between the species of Echinus, that we have 

 found that there is the greatest tendency to the formation of multiple rods — much 

 more so than in the pure-bred forms. For this I'eason, it would not seem to be so 

 certain that multiple-rod formation in Toxopneiistes-Hipponoe hybrids is a sure test 

 of Hip)p)onoe dominance. 



Tennent altered the concentration of OH-ions in the water in which the 

 fertilizations were made, by adding small definite quantities of sodium hydrate and 

 of acetic and hydrochloric acids. Fertilization was brought about in these media, 

 and the eggs remained in them during the segmentation period. As soon as the 

 blastulse swam to the surface, they were transferred to normal sea- water. An 

 increase in the concentration of OH-ions caused a slight increase in the Hij^ponoe 

 dominance, whereas by a decrease in the concentration of the OH-ions the dominance 

 was changed from the Hipp07ioe to the Toxopneustes side. 



Whether or not these results be considered as proved, it is obviously important to 

 test them at other places, and with other material. With this object in view, 

 Tennent's experiments were repeated at Plymouth in 1911. Here we were dealing 

 with the inheritance of definite late larval characters, and the investigation should 

 give a clear result, as to whether or not the inheritance could be altered. Now, as 

 described above, in 1911 the posterior ciliated epaulettes and the green pigment 



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