.54 TREADWELL. [Vol. XVII. 



Conklin mit Recht als wunderbar bezeichnet, wird nun aber 

 durch mehrere Nachweise dieser meiner Arbeit noch erheblich 

 gesteigert." 



The origin of the mesoderm from a cell of the third quartette 

 is not an argument against this position, because the mesoderm 

 cells are not differentiated but are undifferentiated cleavage 

 cells, corresponding with germ cells of the lower animals, and 

 may be mixed with (beigemengt) various cleavage cells without 

 affecting the inherent quality of the latter. That the meso- 

 derm is mixed with a macromere, or a micromere, is shown by 

 the fact that the whole micromere or macromere never passes 

 into the mesoderm, but rather buds off a daughter-cell from 

 which the mesoderm arises. Inasmuch as this mixing does not 

 affect the character of the mother-cell, its homology with other 

 cells in other groups which do not happen to contain mesoderm 

 is not affected. 



I may remark, in passing, that this conception of the wan- 

 dering character of the mesoderm cells would, if accepted, 

 remove one of the strongest arguments for cell homology. The 

 constant position of the mesoblast pole cells and their origin 

 from 4d have been the strongest argument in support of the 

 theory. 



Lastly, in a short preliminary. Child (No. 4) has taken a 

 definite stand against cell homology, basing his claim on the 

 absence of a prototroch in Sternaspis, — where, however, it 

 seems to me, his criterion of homology is too severe, — and on 

 a comparison of the paratroch of Arenicola with that of Amphi- 

 trite, the origin being very different in the two cases. It is 

 only fair to say, however, that the origin of the prototroch is 

 the same in both. 



The development of Podarke not only has furnished no new 

 evidence for complete cell homology, but, on the contrary, 

 shows that many which have been assumed by previous writers 

 are not of wide application. A comparison of the various 

 details will be instructive. 



I . The Apical Rosette. — The cells composing this have the 

 same origin, and probably have the same fate, in a number of 

 different annelids and mollusks. The apical tuft of cilia is, in 



