THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS 123 



lu 1909 Hagedoorn (9), working in Loeb's Laboratory, Pacific Grove, 

 Cal., crossed Sirongnloccntrotus ^Jiivp«'«<?is and S. franciscanus and 

 found a purely motherly dominance in the shapes of the skeletal apical 

 rods. In the following year Loeb, King, and Moore (10) repeated 

 these experiments at the same place, but reached very different results. 

 They came to the conclusion that each character was inherited 

 separately, that is, quite apart from whether it is of maternal or 

 paternal origin : that of a pair of allelomorphic characters one is 

 invariably dominant over the other in the hybrid : that the characters 

 of the Pluteus are inherited on strictly Mendelian lines. Thus, for 

 instance, they found the club-shaped ends of the skeletal rods to be 

 dominant over the arched form, the round, dome-shape of the larvae 

 to be dominant over the pyramidal, the rough spinous character of 

 the sk-eletal rods dominant over the smooth, and so on through 

 a number of characters. They made no attempt to rear their larvse 

 to metamorphosis and to follow these characters in the later stages, 

 neither did they attempt to trace, if possible, the characters in the later 

 generations, in the usual Mendelian manner. 



Lastly, Tennent (18) working at the Tortugas Laboratory, off the 

 coast of Florida, in the midst of the warm water of the Gulf Stream, 

 crossed among other forms Toxopncustes and Hijj'ponbe, always finding 

 his hybrids to resemble Hii^ponoe. He then altered the concentration 

 of the OH-ions in the seawater in which the cross was made, and 

 found by this means that the dominance was changed to the Toxo- 

 pneustcs side. Here again, as in all the previous work, it is doubtful 

 if characters sufficiently definite have been adopted as an index of 

 parental influence. For instance, the skeletal support of the post- 

 oral arm of Toxopneusics is a single rod, whereas in Hipponoe it is a 

 lattice structure. If, in his hybrids, more than one rod appeared in 

 the arm, Tennent considered it as an indication of Hipponoc influence. 

 But as previous observers have noted, and we have repeatedly found in 

 our own experiments, extra rods appear, under unfavourable conditions, 

 even in forms which do not normally possess them. 



From the above brief review of the subject, it is plain that the 

 opinions expressed by the different investigators have been most con- 

 flicting, and that the conclusions they have drawn have been, in many 

 instances, diametrically opposite to one another, although the work 

 was frequently done with the same material. This is in great part 

 due to the uncertain nature of the evidence on which these results 

 have been based, evidence which has been drawn from the early 

 development alone. No successful attempt has been made to rear the 

 hybrids, in order to follow the nature of the parental influence in the 



