3 82 



HAROLD H. PLOUGH. 



the membranes removed were placed in similar dishes with each 



series of experiments. 



EXPERIMENTS. 



Summary of Ah Isolated Blastomeres. In Table I. a tabular 

 view of all of the results is given, regardless of whether or not 

 both members of the pair of isolated blastomeres developed. All 

 blastomeres which began cleavage normally are included. In 

 the last three columns the figures in brackets are the percentages 

 of the number which lived to the ciliated larva stage. 



TABLE I. 



The summary indicates that a large number of isolated 

 blastomeres even when apparently uninjured, do not develop 

 normally. Less than 50 per cent, of the Arbacia cells developed 

 to the gastrula stage, although the number in the sand dollar was 

 larger. Of the isolated cells which lived to become ciliated larvae, 



A B 



FIG. 2. Ciliated larvae with no ski li tuns developed from singli- blastomeres of 

 the two cell stage of Arbacia. Four days old. Measured and drawn free hand 

 from living specimens to the same scale. 



47 per cent, in Arbacia and 31 per cent, in the sand dollar never 

 developed a skeleton. Doubtless a certain number of these were 



