378 HAROLD H. PLOUGH. 



conclusive, for one can never be certain how many of the failures 

 are due to deficiency of materials in the initial cell and how many 

 to abnormal development resulting from the handling. 



This difficulty in interpreting the results can be eliminated to a 

 very large degree if the development of both members of the 

 pair of blastomeres from one egg are followed, and the results 

 recorded for single pairs rather than for groups of cells. When 

 one or both of the blastomeres fail to develop to the stage of the 

 ciliated larva, this pair need not be considered. When, under the 

 same conditions and with identical handling, both blastomeres do 

 develop to this stage the result may be considered significant 

 evidence of organization of the undivided egg. It seems an 

 extraordinary fact that in all of the published work on isolated 

 blastomeres of sea-urchins there is so little in which the develop- 

 mental history of each of the blastomeres derived from a single 

 egg is known. In all of his earlier work Driesch was content to 

 show that whole larvae of one half size were formed from isolated 

 one half blastomeres, and no attempt was made to prove that 

 both blastomeres from one egg did so, nor even to indicate how 

 many out of a large group failed to differentiate in this way. In 

 1906 Driesch published his first study of the "Physiology of 

 Bilaterality," in which he did take up this question. He followed 

 the development of both blastomeres of the two cell stage, usually 

 two eggs at a time, in several series ot experiments on Echinus 

 multituberculatus and Sp<zrechinus. Ihe eggs were observed for 

 two days only, and failures were not recorded. From this data 

 I find that out of 9 eggs divided, only 2 gave normal one half size 

 plutei from both blastomeres, 4 gave two "prismen" larvae too 

 young to determine whether the skeleton was normal 1 2 from 

 which one pluteus was complete and the other apparently in- 

 complete, and I in which no skeleton appeared in either. These 

 data are insufficient for any conclusion as to the organization of 

 the egg along the lines indicated, yet so far as they go they 

 suggest that both blastomeres do not necessarily develop into 

 whole larvae. In all of Driesch's remaining experiments, includ- 

 ing several series reported in his 1908 paper on the same subject, 

 the blastomeres w T ere observed for one day only that is to the 

 gastrula stage and it was assumed that they would develop into 



