HEREDITY IN HETEROGENEOUS HYBRIDS 3 



plutei are purely maternal. It should, however, be borne in mind, 

 that the objection might be raised that the presence of the skel- 

 eton in the sea-urchin pluteus might be dominant over its absence 

 in the starfish larva. I have thus far vainly tried to produce a 

 starfish larva with a pluteus skeleton by the hybridization of the 

 two species. 



We are therefore compelled to state that the hybrids between 

 the sea-urchin egg and the starfish sperm represent more closely 

 the purely maternal form than do the hybrids between two sea- 

 urchins, which always show paternal characters. 



2. It is well known that Herbst and Tennent have made 

 experiments in which the paternal influence in the hybrid embryo 

 was diminished. Tennent states that in the cross between Hip- 

 ponoe and Toxopneustes, Hipponoe characters become dominant 

 in sea water of a high OH concentration and Toxopneustes char- 

 acters in sea water of a low OH concentration.^ The amount of 

 acid or alkali Tennent needed to accomplish his result was very 

 small; namely about 2 cc. ^ acetic or hydrochloric acid to 500 

 cc. of sea water. A. R. Moore, W. R. King and myself made a 

 number of experiments in which the hybrids between S. purpura- 

 tus and S. franciscanus were raised in sea water to which varying 

 quantities of HCl or acetic acid or NaHO were added (from 

 to 0.4 cc. T^ acid or NaHO to 50 cc. of sea water). We were 

 able to retard or accelerate the rate of development, but the char- 

 acter of the hybrid remained absolutely unaltered. 



I wish to call attention to the necessity of sterilizing the pipettes 

 by boiling them after each experiment, instead of sterilizing them 

 by rinsing in distilled or fresh water as is often done. 



3. Moenkhaus measured the rate of segmentation in hybrid fish 

 eggs and found that the rate for the first five cleavages is deter- 

 mined by the egg.-* The egg of Ctenolabrus segments about forty 

 minutes after impregnation with sperm of its own kind, while 

 the egg of Batrachus tau, if fertilized with the sperm of the same 

 species, segments after about eight hours. If the egg of Batra- 

 chus be fertilized with the sperm of Ctenolabrus it also does not 



' Tennent, Publication 132, Carnegie Institution, 1910. 

 * Moenkhaus, Am. Jour, of Anat., vol. 3, p. 29, 1904. 



