DEFECTIVE PLUTEUS LARVAE. 375 



lend support to Boveri's conception that the egg is stratified at 

 right angles to the initial egg axis. With such a distribution of 

 differentiated substances and the first cleavage cutting at any 

 angle, one would expect that staining one blastomere of the two 

 cell stage would give larvae with the blue area in every possible 

 sector. This Von Ubisch found. Such a result obviously is not 

 at variance with Boveri's view, but it can hardly be called evi- 

 dence in favor of it for the same might be true if there were no 

 organization of the egg whatever. The only disproof of the latter 

 idea would consist of the demonstration of the same sort of rela- 

 tion in larvae actually developed from isolated one half blasto- 

 meres. This evidence I have secured for Arbacia and Echinarach- 

 nius as will be indicated below. 



In addition to the problem of the organization of the undivided 

 egg of the sea urchin with respect to stratification of differentiated 

 substances, there is the added question as to a possible bilateral 

 organization of such materials. Boveri obviously held that the 

 right and left sides of the body of the larva were determined by 

 the plane ot the first cleavage, but Von Ubisch's work disproves 

 this. The same criticism applies to the conclusions of Schaxel 

 (1914) and of others who worked with isolated blastomeres. If 

 bilaterality bears no relation to the first cleavage plane, then it 

 must be determined in the egg before the first cleavage or at a 

 much later stage in development. To establish the first relation, 

 since the first cleavage cuts the egg at any angle, it would be 

 necessary to show that isolated blastomeres formed only certain 

 definite parts of some obviously bilateral structure like the 

 skeleton. It seems extremely unlikely, and yet curiously enough 

 there is good evidence for exactly this situation in at least one 

 sea-urchin, Paracentrotus the same egg, by the way, in which 

 appears the stratified pigment layer described by Boveri. J. 

 Runnstrom (1914) described the development of isolated blasto- 

 meres of this egg, and for some reason his results seem to have 

 been largely overlooked in discussions of the subject. Quite 

 contrary to the results of Driesch on Echinus and Sphaer -echinus 

 he found that the blastomeres of the two cell stage did not develop 

 as whole larvae even after the blastula stage. Instead they gave 

 rise to partial larvae, showing the skeleton ot either the right or 



