442 TREADWELL. [Vol. XVII. 



unequal type, even though this polarity may not be expressed 

 so clearly in the form of cleavage. The alternatives to this 

 position would be the theory that differentiation came in sud- 

 denly at the 64-cell stage, or that it, so to speak, accumulated 

 very slowly, only attaining a visual importance at that stage. 

 Of the three, I believe the first is correct. It is difficult to 

 understand why closely related animals (or, for that matter, 

 animals not closely related) should vary in the amount of polar- 

 ity possessed by their ova at the beginning of development, 

 and I believe a very different principle must be invoked to 

 explain the difference. This question of differentiation in the 

 trochophore of Lepidonotus, which also has an equal cleavage, 

 has been discussed by Mead (No. 22, p. 278) ; and what he 

 has said there applies equally well to Podarke. At the time 

 Dr. Mead's paper was written there was still doubt whether 

 the trochophore of Lepidonotus had a radial or a bilateral sym- 

 metry, inasmuch as the origin of the mesoblast was not known. 

 As stated in a previous paper (No. 29, a; see also p. 436), there 

 is no doubt that Lepidonotus has a bilaterally symmetrical 

 cross and a small X1.2 cell, exactly as in Podarke; and it is 

 very probable that the history of the definitive mesoderm at 

 least is essentially the same in Lepidonotus as in Podarke. 



According to Mead, the important question to be settled by 

 a study of equal cleavage is whether one of the two cells in 

 equal is homologous with the larger in unequal cleavage. In 

 the paper just cited, I have expressed my belief that it is. 

 Later study has led me to somewhat different conclusions con- 

 cerning homologies from those held by Professor Mead (see 

 pp. 469) ; and to avoid misunderstanding it may be desirable to 

 change the terminology, although the essential meaning re- 

 mains the same. I believe that one area in the annelid with 

 equal cleavage is homologous with a corresponding area in an 

 annelid with unequal, and that there is as much differentiation 

 in the one case as in the other, though, as I shall try to show 

 later, the cells are probably not homologous. 



What is the meaning of unequal cleavage } The old explana- 

 tion was that it is due to the effects of yolk, either actually 

 present or working through heredity. Rabl (No. 27, b), in 



