DEFECTIVE PLUTEUS LARV/E. 



385 



pluteus stage, it seems possible to attain to a fair degree of cer- 

 tainty by limiting our consideration to those eggs only of which 

 both blastomeres developed to the stage of a ciliated larva. 

 These data I have tabulated in Table II. 



TABLE II. 



SHOWING TYPES OF LARY/E RESULTING FROM THE Two BLASTOMERES OF THE 

 SAME EGG, WHEN BOTH LIVE TO THE CILIATED STAGE. 



Table II. is largely self-explanatory. The total number ot eggs 

 of which both blastomeres developed is only 7 per cent, of the 

 total number of blastomeres followed in Arbacia, but is 54 per 

 cent, in the sand dollar. When good eggs of the sand dollar can 

 be secured it is obviously a much more favorable species for such 

 work. The same fact appears in column (2), since in 44 per cent, 

 of the eggs of A rbacia which gave rise to two larvae no skeleton 

 developed in either, while only one out of nine, or 1 1 per cent, 

 gave this result in the sand dollar. This fact introduces a source 

 of error in the interpretation of columns (3) and (4), for it is not 

 certain that some of the larvae with no skeleton here might not 

 have possessed skeleton forming material. This possible error is 

 not serious, however, as an examination of the remaining data 

 will show. 



