DEFECTIVE PLUTEUS LARV.E. 



377 



might be expected. Finally if one of the many eggs in which 

 the cleavage fell between the two extremes, like B, was operated 

 on, either the same^result or two larvae with defective skeletons 

 might be looked for. The possibility of bilateral organization 



FIG. I. Diagrams showing three possible relations of the first cleavage plane 

 to the primary egg axis as indicated by the micropyle. The line a-a' is drawn 

 through the egg axis in each case. The shaded layer suggests the possible location 

 of the skeleton forming material. 



need not be considered for the present. In any case the results 

 of the development of isolated blastomeres, would seem to offer 

 conclusive evidence as to the differentiation in the sea-urchin 

 egg in the light of the work of Von Ubisch. 



Considered from this point of view it becomes an important 

 matter to secure data on the later development of a large number 

 of isolated blastomeres of sea-urchin's eggs. Such data must 

 record the complete results and not simply the fact that complete 

 larvae of one half size are formed from isolated blastomeres of the 

 two cell stage. It is obviously quite as important to know how 

 many such blastomeres do not form complete larvae, as to know 

 how many do. Practically all of the earlier investigators note 

 that a number of failures occur, but they are content with the 

 positive demonstration. Even under the best conditions and 

 with the greatest care in handling however, a certain number of 

 whole eggs fail to develop normally, and this is sure to be true 

 to an even greater extent when the cells are subjected to the 

 handling required by the technique of separation of the blasto- 

 meres. This is true even when the earlier stages of cleavage are 

 normal. Data of the sort suggested therefore can not be 



