458 • TREADWELL. [Vol. XVI !.■ 



this is an indispensable datum for the study of comparative 

 embryonic anatomy. The fjindamcntal forms of cleavage are pri- 

 marily due to mechanical conditiotis, and are only significant 

 morphologically in so far as they have been secondarily retnod- 

 elled by processes of precocious segregation. (The italics are in the 

 original.) Since, however, the facts seemed to him to show that 

 this process has taken place to a greater or less extent, Wilson 

 closes his paper with a plea for the study of cell lineage as the 

 best means by which we may hope to obtain a firm basis for a 

 comparison of the germ layers. 



In his paper on Unio (No. 21), written two years later, Lillie, 

 while recognizing the fact that difficulties exist in the way of the 

 assumption, argues for an homology of cells, basing this argu- 

 ment on the origin of the mesoblast from a corresponding cell 

 in a wide range of annelids and mollusks, and on the similar 

 origin and fate of three quartettes of ectomeres in all these 

 forms. Lillie thus argues for a cell homology. His definition 

 of this term, as I understand it, is that two cells whose prod- 

 ucts are homologous must be themselves homologous, even 

 though they have a different origin in the cleavage. Lillie 

 calls attention to one important difference between Nereis and 

 Unio, viz., that a cell which in Nereis is a stomatoblast, in Unio 

 gives rise to the larval mesoblast. His explanation of this 

 difference is that the form of cleavage being constant, when- 

 ever a new organ appears, it must appear in some cell already 

 present, the formation of a new cell being an impossibility. 

 Hence this larval mesoblast, being an ectodermal structure, 

 must make use of a cell which in other cases remains in the 

 outer germ layer. 



In his lecture on " The Embryological Criterion of Homol- 

 ogy " (No. 34, e), Wilson, while recognizing the " marvelous 

 agreements " in the cytogeny of related forms, argues that as 

 we extend the comparison " the contradictions reach a climax." 

 The facts of regeneration, also, show important differences 

 from embryological processes, differences which seem to invali- 

 date the whole germ-layer theory. The conclusion which 

 Wilson reached was that, for the study of homologies, adult 

 structures are of much more value than embryological, since 



