288 MEAD. [Vol. XIII. 



cleavage. The next furrow always divides this cell equally into 

 a right and a left mesodenn cell. 



Clepsine, as described by Whitman,i° fails to accord with 

 the above in that the original mesoderm cell is differentiated 

 at the ideal i6-cell stage instead of at the 64-cell stage, but Dr. 

 Whitman tells me that certain small cells are budded off before 

 the bilateral division of the mesoblast. It is possible, there- 

 fore, that the origin of the mesoblast cell in Clepsine is not 

 exceptional. 



I fully concur with Wilson in his criticism of the origin of 

 the mesoblast in Nereis Dtcmmerillii, as recorded by Goette and 

 von Wistinghausen. Goette's^^ account of the cleavage is 

 incomplete, and von Wistinghausen ^ probably overlooked a cell, 

 and thus threw the responsibility of mesoblast formation upon 

 the wrong one (cf. Wilson,^ p. 435). 



According to Lang ^^ the origin of the mesoderm in Disco- 

 cceltts differs from that in the annelids and molluscs in at least 

 two important respects: it arises not from one, but from all 

 four quadrants, and from cells of two generations, both earlier 

 than that giving rise to mesoderm in the annelids, etc. 



In annelids these two generations of cells form a part of 

 the ectoderm of the subumbrella, including a portion of the 

 prototroch. Granting, for a moment, the correctness of Lang's 

 observations, we find ourselves in a dilemma well stated by 

 Wilson,^ p. 448 : " Unless, therefore, we are prepared to main- 

 tain the absurd proposition that the mesoblast of the polyclade 

 is homologous, not with the mesoblast of the annelid, but with 

 the ectoblast of the lower hemisphere (including, of course, the 

 ventral plate with the ventral nerve cord), we cannot escape 

 the conclusion that exact equivalence of embryological origin 

 is not a proof of homology, so far at least as the cleavage stages 

 are concerned." Again, in his lecture on " The Embryological 

 Criterion of Homology,"^ Wilson says: "We find (in the 

 polyclade) a cleavage very closely resembling the annelid type 

 in form, yet the individual blastomeres have from the very start 

 an entirely different morphological value." Lillie^ (p. 37) men- 

 tions the mesoblast of Discoccslis as affording another instance 

 of cells of different origin which have the same destiny. 



