ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 263 



smooth skeleton, the late development of the arms, and the nidimentary formation of 

 the middle rod. 



In 1910, Tennbnt (88), working at the Tortugas laboratory, made a large number 

 of crosses Hybrid phitei between Toxopneustes ? and Ilij)ponoe $ and the 

 reciprocal, on the evidence of the skeleton, always were found to resemble llipponoe. 



The crosses were readily effected by allowing the eggs to stand in sea-water for 

 some hours before fertilization. Larvae obtained from making the cross in sea-water 

 of increa.sed alkalinity gave evidence of an increased Hipponoe influence ; those 

 obtained from effecting the cross in sea-water of decreased alkalinity showing a 

 tendency towards Toxopyieustes. Thus there would seem to be a Hipponoe 

 dominance in water of high, and a I'oxopneustes dominance in water of low, 

 OH-ion concentration. The character of the cross can thus be changed by raising or 

 lowering the alkalinity of the sea-water by the addition of a little NaOH, or acetic 

 or hydrochloric acid. The evidence of dominance was furnished by the characters 

 of the early skeleton. This is, however, of doubtful value, as here, again, it is 

 the crossing of a single rod type of skeleton witli a lattice or multiple rod form. 

 As the multiple rod type seems to underlie the single type in many Echinoids, 

 it is not clear that mere pathological conditions may not account for the multiple 

 condition, apart from any dominance. 



It is essential to Tennent's results that he should determine with accuracy the 

 frequency with which the lattice type of skeleton appears in the pure-bred plutei of 

 Toxopneustes. Referring to the appearance of this type of skeleton in Toxopmeystes, 

 he says, in his paper, that out of the examination of seveial thousand plutei of this 

 species this type of skeleton " was found noticeably in embryos from the eggs of two 

 individuals. In one case it was found in 1 per cent., and in the other in 3 per cent. 

 of the plutei examined. In other cultures it occurred as a variation of nuich less 

 than 1 per cent. 



" The pure cultures made in 1909 as controls for the hybridization experiments 

 showed an occasional pluteus with this variation, but the number never was great 

 enough to exclude the advisability of considering the common appearance of more 

 than one rod, a Hipponoe character, as an indication of Hipponoe influence." 



The fact that the lattice type of skeleton can appear normally in the Toxopneustes 

 larva in a small percentage of cases appears to us to require that Tennen'I' should 

 make a much more extensive investigation of the conditions governing its appearance 

 than he seems to have done. Moreover, before basing any conclusions on skeletal 

 characters it should be shown that the hybrid plutei are as healthy as the pure-bred 

 plutei of the parent species ; a few of Tennent's figures show his larvse to be obviously 

 unhealthy. 



In his second paper in the 'Reports of the Tortugas Laboratory' Tennent(90) 

 gives a fuUer account of his work, with numerous figures. We are not told, however, 

 in this paper or the first, if the eggs and sperm from the sayne pair of individuals 



