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. 1 8 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 larvae were being reared. Herbst and 

 Vernon also obtained similar larvae 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 pedicellaria? being well developed. I was 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 ECHINASTER J 1 : 



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 

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

 hours after fertilization, I believe that the fertilization was ineffective 

 and that the segmentation was parthenogenetic. A third attempt at 

 this cross, when I again used the C0 2 treatment, exposing the eggs to 

 the action of CO 2 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 ? X HOLOTHURIA FLORIDANA cJ 1 : 



(1) 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 MgCl, (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 CO 2 for 4 minutes and fertilized 

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



