Echinoderm Hybridization. 



141 



MoiRA : 



MOIRA 9 X TOXOPNEUSTES (5^: 



This cross was easily made by fertilizing the eggs immediately 

 after they were removed from the ovary. For information regarding 

 the method of obtaining the eggs I am indebted to Dr. Bartgis McGlone, 

 who has also furnished me with two drawings of the Moira pluteus, 

 which I have used in this paper. The Moira egg is beautifully trans- 

 parent and well suited for study in the living condition. A fertilization 

 membrane was formed. The copulation of the pronuclei could be 

 observed. Segmentation began 35 minutes after fertilization, as in the 

 straight fertilization. The crosses were of the intermediate type (plate 2 , 

 figs. 24, 25, 26). No trace of the maternal posterior, unpaired spine 

 appeared, although the larvse were kept alive for 7 days. 



HippoNoiJ (Tripneustes): 



The Hipponoe egg, like the Arbacia egg, is not well adapted for 

 study in the living state. The egg is small and dark in color. In com- 

 parison with the Toxopneustes egg it measures in micrometer eye-piece 

 units, Ocular 2 Objective D, Hipponoe ig X 19; Toxopneustes 26 X 25, 



Hipponoe 9 X Toxopneustes d^: 



The Hipponoe $ X Toxopneustes cJ" cross was the most successful 

 cross made with the Hipponoe egg. A series of experiraents showed 

 that the largest percentage of fertihzations was obtained when the eggs 

 were fertilized 2^ hours after their removal from the ovary. During 

 the work of 1909 the cleavage began 75 minutes after fertilization, as in 

 the normally fertilized egg. In the experiments of 19 10, although the 

 fertilization-cleavage period in the normally fertihzed eggs remained the 

 same, cleavage in the cross-fertilized eggs began in 55 minutes, a reduc- 

 tion of 20 minutes. No fertilization membrane was ever obtained, the 

 only membrane formed being the " Verbindungs-membran." In the 

 plutei of this cross (plate 2, fig. 28; plate 3, figs. 38 to 43), there is a 

 pronounced Hipponoe dominance as expressed in the skeleton of the 

 anal arm. In the experiments of 19 10 the third rod of the skeleton was 

 never developed. The plutei live well in laboratory cultures. 



Table II. — Summary of results of cross-fertilization in ordinary sea-water. 

 [Number of plutei studied, so. Temperature of water, 28.5° C] 



It will be seen from the table that 74 per cent of the plutei have 

 anal arms with rods having latticed structure. The normal sea-water 

 fertilizations of 1910 were in accord with those of 1909. A count of 50 

 plutei made in 1910 gave results approximately the same as those of 1909. 



