172 



A. R. Moore 



(Temp. 16 18C) the skeletons began to be deposited and soon the 

 larvae assumed the characteristic pluteus form (Figs. 1, 2, and 3). 



LOEB, KING, and MOORE have shown that the spininess of the 

 skeleton, early development of arms, cross bars and dome shaped 

 body, all S. franciscanus characters, are dominant over the smooth 

 skeleton, lack of cross bars, short arms and pyramidal form of the 

 S. purpuratus. My plan was carefully to record the development 

 of the larvae from day to day and determine whether the dominant 



Fig. 4. 



Fig. 5. 



Fig. 6, 



characters appeared in the hybrid plutei as soon as in the 

 pure culture. I have followed through five sets of experiments. 

 Each experiment contained thousands of larvae. I find, in all cases, 

 that the development of the dominant characters such as spininess, 

 length of arms, and body shape - - is retarded in the heterozygote. 

 I had at first supposed that the amount of cross-bar formed might 

 be a reliable index to the rate of development of the dominant 

 characters. This, however, is not true, because in some cases I found 

 the cross bar fused in the hybrid plutei on the third day while in 

 other cases it had entirely failed to fuse on the fifth. This is due 

 to the fact that the enlargement of the gut pushes the skeleton 



