Echinoderm Hybridization. 



137 



Toxopneustes is therefore of especial value. The favorable nature of 

 the Toxopneustes egg for the observations of nuclear phenomena during 

 the living stage has been mentioned by other investigators. It has 

 been possible to see, in this egg, the fusion of the pronuclei and to con- 

 vince myself that the processes of fertilization were being completed. 



TOXOPNEUSTES $ X HIPPONOE cT: 



This and the reciprocal cross Hipponoe 9 X Toxopneustes J* are the 

 ones that have made the experimental portion of my work possible. 

 The Toxopneustes eggs were fertilized 6 hours- after their removal from 

 the ovary, a series of fertilizations showing that the highest percentage 

 of fertilizations was obtained if the eggs were allowed to remain in sea- 

 water for this length of time. A fertilization membrane was formed. 

 Cleavage began 40 minutes after fertilization, this being the period 

 characteristic of the egg. 



The character of the hybrids resulting from this cross is shown in 

 plate 2, figs. 27, 29 to 31, plate 3, figs. 32 to 37, and plate 5, figs. 75 to 76. 

 With the exception of the individual shown in plate 3, fig. 32, a pro- 

 nounced Hipponoe dominance is shown. This dominance is expressed 

 in the great number of fenestrated anal arm rods, in the number of 

 multiple rods, and in the general form of the body skeleton. In the 

 fertilizations made in 1910 the Hipponoe dominance was further expressed 

 in the occurrence of the basket-like structure at the posterior end of the 

 body. The best idea of the amount of inclination to the paternal type 

 is shown in table I. 



TABLE I.- Summary of results of cross-fertilization in ordinary sea-water. 



[Number of plutei studied, 50. Temperature of water, 28.5 C.] 



EXPLANATIONS OF HEADINGS OF THE COLUMNS. 



" Plutei with lattice structure " are plutei which have parallel rods connected 

 by crossbars in one or both anal arms. 



"Anal arm rods with lattice structure" designates the total number of anal 

 arms of the plutei considered in the preceding columns, which have a skeleton 

 composed of parallel rods connected by crossbars. 



"Arms more than one rod" indicates an anal arm skeleton of more than one 

 straight rod. 



"Perfect Hipponoe rods" are anal arm rods which are as perfect as those 

 found in purely bred Hipponoe plutei. 



"Perfect Toxopneustes rods" indicates a single straight rod with thorn-like 

 protuberances . 



"Perfect Toxopneustes plutei" and "Perfect Hipponoe plutei" indicate 

 respectively plutei of the normal Toxopneustes and Hipponoe type. 



"Basket" indicates the basket-like structures present in the posterior part 

 of the body of purely bred Hipponoe plutei. 



