326 Charles Zeletiy. 



the earlier stages, because of the lack of the opportunity which, 

 as stated above, is afforded the earlier ones {c. f. again the ex- 

 periments on the frog's egg). The defects in the larvae have a 

 definite relation to the position of the removed part of the egg 

 because the disturbance of the protoplasm is greatest in the region 

 near the cut. The already differentiated materials may thus be 

 separated in an abnormal relation to each other and become un- 

 naturally grouped by the cell walls of the ensuing divisions. In 

 this manner, on the one hand the abnormality of the resulting 

 cleavages, and on the other the defects in the larvze developed 

 from the fragments, may be explained. The first normal cleav- 

 age, however, divides the cell into two similar parts, each of 

 which retains a relation between its differentiated materials very 

 much like that of the whole egg. The conclusion is therefore 

 reached that the relations of the materials in the isolated blasto- 

 mere of the two-cell stage are more normal (/. e., more like those 

 of the whole egg) than are those of fragments of the two-polar- 

 body stage, and therefore the capacity of regulation to form a 

 whole larva is greater in the former than in the latter. The 

 mechanism of division which is disturbed by the cut in the un- 

 segmented egg, and is capable of regulation if the cut is early 

 but is disturbed if the cut is late, is also not disturbed in the iso- 

 lated blastomeres of the two-cell stage, the cell division goes on 

 as if the other blastomere were present, and a partial (one-half) 

 cleavage results. 



It therefore seems probable that while in normal development 

 cleavage is an aid in differentiation, in development after re- 

 moval of a portion of the unsegmented egg (or segmented egg) 

 it is a distinct detriment in so far as the attainment of the normal 

 relations of the parts is concerned. For while on the one hand it 

 isolates materials and allows a more accurate differentiation, on 

 the other it restricts the power of regulation of the organism.^ 



^Yatsu has hinted at an explanation somewhat similar to the above. He 

 says that the differences between the lai'vse derived from fragments of the un- 

 segmented egg and those from isolated blastomeres of the two-cell stage may 

 be due to dififerences in the accuracy of the separation of the materials in the 

 two cases. 



