Echinoderm Hybridization. 139 



The greater number of hybrids of this cross were of the intermediate 

 maternal type (plate i, figs. 13 to 17; plate 2, figs. 18 to 20) having 

 multiple rods in the anal arms. All of the plutei obtained in 1907 were 

 of this character. From two crosses made in 1908, a small percentage, 

 about I per cent, of hybrids of a purely maternal form were obtained 

 (plate 2, figs. 18 to 20 and text fig. 5). These showed no trace of their 

 hybrid origin, their perfect form suggesting that they were pure Toxop- 

 neustes larvae which had arisen from chance fertilizations. This possi- 

 bility, I believe, is excluded by the extraordinary care that was taken 

 in making the fertilizations and by the care that was exercised in avoid- 

 ing contamination while the larvas were being reared. Herbst and 

 Vernon also obtained similar larvas of a striking maternal form. 



The pluteus shown in plate 2, fig. 20, resembles in all respects a purely 

 bred pluteus of the same age, the epaulets, Echinoderm rudiment, and 

 the first pedicellariae being well developed. I w^as not able to obtain 

 adults from any of these plutei, although during the same season I 

 carried laboratory fertilized Toxopneustes embryos through their meta- 

 morphosis. At the end of 45 days in one instance with the crosses I 

 had 9 and in another 6 plutei in good condition. Upon trying to find 

 them on the succeeding day nothing could be made out, although a 

 careful search of the diatom mud in the bottom of the culture dish was 

 made. 



Toxopneustes 9 X Echinasterc?: 



The result of this cross was not especially noteworthy. No fertiliza- 

 tions were obtained after the usual method of allowing the Toxopneustes 

 eggs to stand 5 hours before treatment with sperm. In a second attempt 

 the eggs were exposed to the action of CO^ sea -water for from i^ to 10 

 minutes, and later fertilized after the method that I described five years 

 ago (Tennent 1906). The most successful lot was that treated with 

 CO2 for 4 minutes. Inasmuch as segmentation did not begin until 2 

 hours after fertilization, I believe that the fertiHzation was ineffective 

 and that the segmentation was parthenogenetic. A third attempt at 

 this cross, when I again used the CO2 treatment, exposing the eggs to 

 the action of COg sea-water for 4 minutes and fertilizing with Echinaster 

 sperm 10 minutes after the egg had been transferred to sea-water, 

 resulted in the occurrence of segmentation 55 minutes after fertilization. 

 Cleavage was irregular and the "embryos" obtained were formless, 

 ciliated clumps of cells. 



Toxopneustes 9 X Holothuria floridanac?: 



(i) The Toxopneustes eggs were allowed to stand for 2 hours and 

 were then fertilized with Holothuria sperm. A fertilization membrane 

 was formed at once. A small percentage of segmentation was obtained. 



(2) Toxopneustes eggs were treated with MgClj (i c.c. to 100 c.c. 

 sea-water) for 3 minutes. Segmentation began 45 minutes after fertiliza- 

 tion with Holothuria sperm. 



(3) Toxopneustes eggs treated with CO2 for 4 minutes and fertilized 

 with Holothuria sperm 50 minutes after transference to sea-water. 



