390 HAROLD H. PLOUGH. 



because of the difficulty of being certain of all the cells when rapid 

 cleavage was going on, and when a number ot eggs are under 

 observation at the same time, I have accurate records on only 

 about twenty-lour blastomeres of this type. Fifteen ot these 

 developed as stated above tor Echinus, namely giving off two 

 micromeres at the third cleavage (normal fourth). The others 

 appeared to have formed no micromeres at all rather eight 

 equal cells though two may have had one only. Only eight of 

 the total number developed to the pluteus stage, a number far too 

 small from which to draw positive conclusions. From half eggs 

 showing two Tnicromeres there developed larvae both with and 

 without a skeleton, and in two cases a larva with a skeleton 

 developed from blastomeres in the cleavage ot which no micro- 

 meres were observed. So far as they go the data suggest that 

 there is no essential relation between the presence of micromeres 

 in artificially divided eggs and the development of a skeleton. 

 Yet Von Ubisch has shown that the skeletal material is normally 

 passed out into the micromeres, and Harnly's work apparently 

 indicates that when the vegetative portion of the undivided egg 

 is cut off, micromeres do not develop. On the other hand 

 Tennent and Taylor, working on another egg cut before fertiliza- 

 tion, did get noimal larvae from fragments which formed no 

 micromeres. It seems probable that normally the pattern of 

 cleavage is determined by the first cleavage plane, and the 

 skeleton torming material goes as usual into the micromeres. 

 When the first two blastomeres are severed this pattern is ordin- 

 arily continued, but it may sometimes be disturbed. Whether 

 it is or not apparently has no influence on the development of a 

 skeleton, since that in Arbacia at least appears to depend on the 

 presence or absence of skeleton forming material in either or both 

 of the separated blastomeres. 



Since this paper was written the more complete account of 

 i he work on development of fragments of the unfertilized egg of 

 Lyt echinus by Taylor, Tennent and Whitaker (1926) has appeared. 

 Their work convincingly proves that in this egg micromere form- 

 nig substance is not differentiated before fertilization. Whether 

 their disagreement with the findings of Harnly is due to differ- 

 ences in behavior between Lytechinus and Arbacia, future work 



