456 TREADWELL. [Vol. XVII. 



homologous as are these layers, and that the protoblasts of 

 homologous regions are as much homologous as are those 

 regions"; and, "We therefore reach the conclusion that, in 

 related organisms with determinate cleavage, homologies may 

 be predicted of single cells, whether they be protoblasts of the 

 nervous system, the excretory system, or the locomotor appar- 

 atus; of the ectoderm, the mesoderm, or the entoderm; of the 

 right or left, the anterior or posterior portion of the body." 



If this be true, cleavage similarities have as great a phylo- 

 genetic value as larval or adult similarities, but I shall try to 

 show that the number and completeness of these resemblances 

 have been greatly overestimated ; and that the second of the 

 above propositions by no means follows from the facts at our 

 disposal. 



The first suggestion I have been able to find along this line 

 is that of Rabl (No. 27, a) : " Ueberhaupt scheint es nach alien 

 bisherigen Beobachtungen nicht unwahrscheinlich zu sein, dass 

 jede mehr oder weniger scharf umschriebene Thiergruppe ein 

 gemeinsames fur alle Glieder dieser Gruppe giltiges Furchungs- 

 Schema besitze, und dass es daher durchaus nicht undenkbar 

 sei, dass man kiinftig einmal aus der grosseren oder geringeren 

 Uebereinstimmung im Furchungsprozesse auf eine engere oder 

 weitere Verwandtschaft zweier oder mehrerer Thierformen 

 werde schliessen konnen." Inasmuch, however, as Lillie(No.2i) 

 has shown that many of Rabl's observations were incorrect, 

 this hypothesis must be considered as a more or less fortunate 

 guess than as following logically from his results. 



At the time when Professor Wilson's work on Nereis appeared, 

 practically the only papers available for comparison were those 

 of Lang on Discocoelis (No. 19), Blochmann on Neritina (No. 

 3), Whitman on Clepsine (Nos. 33,' a and b), and von Wisting- 

 hausen on Nereis (No. 35). In addition, Wilson referred to 

 the then unpublished observations of Conklin on Crepidula. 

 While the polyclade agreed with the annelid and mollusk in 

 the most remarkable fashion in the details of its cleavage, it 

 differed very decidedly in at least one important particular, viz., 

 the origin of the mesoblast. 



These differences and the conclusions Wilson draws from 



