252 Edmund B. JFUsoti. 



owing to an asymmetrical distribution of material, the cleavage 

 being visibly qualitative from the beginning; and it is impor- 

 tant to note that this asymmetry of distribution is effected by the 

 process of cleavage itself, since the primary segregation-pattern, 

 as far as can be determined, is symmetrical with respect to the 

 axis. In the nemertine or sea urchin the first qualitative division 

 occurs at the third cleavage (which is also qualitative in the mol- 

 lusk)^ which for the first time separates ectoblast-stuff from en- 

 toblast-stuff. A comparison of the different forms indicates, how- 

 ever, that in respect to this cleavage they differ somewhat in de- 

 gree. In Patella the cells of the first quartet are from the first 

 completely specified, whether as a group or individually, and pro- 

 duce purely ectoblastic embryos that never show any tendency to 

 gastrulate. The same is true in Cerebratiilus, according to the 

 recent work of Zeleny ('04), which shows that if in the 8-cell 

 stage the upper and lower quartets be separated along the line 

 of the third cleavage, both quartets develop into closed swimming 

 embryos, but the upper one (although the larger) does not gas- 

 trulate, though it produces an apical organ, while the lower one 

 gastrulates but produces no apical organ." In the; sea urchin, 

 however, a small proportion of the upper cells (20-25%) are able 

 to gastrulate (Driesch, '00. '02.2) ; and this can only mean that 

 the third cleavage is less strictly qualitative or not invariably so. 



lAs was also assumed by Samassa in the case of the frog's egg, "Diese ver- 

 schiedenen Entwickelungsbedingungen konnen aber nur in verschieden Sub- 

 stanzen Hegen, die bei der qualitativ ungleiche Theilung der dritten Furchung 

 den beiden Zellarten zufallen" ('96, p. 386). 



2In the light of Zeleny's observations on the 8-cell stage, and in spite 

 of his apparent confirmation of my own preceding results on the blastula 

 stage, it seems to me very probable that the gastrulas I obtained from upper 

 fragments of blastulas in Ccrebratuhis were obtained by slightly oblique sec- 

 tion, so that a small group of entoblast cells were included in the upper frag- 

 ment. I have since observed that the entoblast-plate extends nearly to the 

 equator of the egg, so that even a slight obliquity in the plane of section 

 might give a misleading result. A similar interpretation not improbably may 

 apply to the upper fragments of echinoderm blastulae, cut in two just before 

 gastrulation, which were observed by Driesch ('95) to gastrulate; but these 

 were cut eii masse without individual orientation, and the experiments evi- 

 dently do not exclude the possibilty that the upper fragments may have con- 

 tained a part of the entoblast-region. A repetition of this work on both forms 

 by means of individual operation is much to be desired. 



