THE SPIRAL TYPE OF CLEAVAGE. 25 



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tion of the groups, and indeed I think it quite probable of 

 annelids and mollusks. The trochophore, reduced to its sim- 

 plest terms, doubtless arose from a larval form which occurred 

 in the life history of the common ancestor of the two groups, 

 and which I believe was probably not greatly different from the 

 polyclad larva. But the question as to whether the trochophore 

 is an ontogenetic recapitulation of an ancestral form which was 

 introduced into the ontogeny before the separation of the 

 groups must not be confused with that of its homology within 

 the groups. I do not desire to discuss this matter in detail at 

 this point, but simply to remark that even the high degree of 

 coenogenetic modification involved in the precocious appear- 

 ance of morphogenetic cleavage need not necessarily result in 

 a coenogenetic larval form. I am, however, inclined to regard 

 the original of the trochophore as distinctively a larval form and 

 not as a stage in the phylogeny of the adult forms of the groups 

 in which it belongs. 



We have seen above that the spiral period is followed by a 

 distinctly morphogenetic form of cleavage, and that this mor- 

 phogenetic period has been encroaching upon the earlier spiral 

 period. We find, moreover, that the morphogenetic period 

 shows in large part a distinct bilateral symmetry, and we now- 

 turn to a consideration of the significance of this symmetry, 

 i.e., the significance of the direction of cleavage during this 

 period. Notwithstanding some differences, a comparison of 

 various forms shows that the more important bilateral divisions 

 occur with a considerable degree of regularity and in the same 

 cell generation as a rule. Moreover, these are the first evidences 

 of bilaterality. In no case have distinctly bilateral divisions 

 been found at an earlier stage. Wilson regarded this fact as 

 one of the most striking features of the cleavage. In Crepidula 

 the change is not so abrupt, according to Conklin, though it 

 appears in almost exactly the same cell generation as in Nereis, 

 at least in certain parts of the ^g^. Wilson believed the appear- 

 ance of bilaterality was due to the reduction of the " left pos- 

 terior macromere " to the same size as its fellows as the result 

 of the formation of the first somatoblast and the mesoblast. 

 Conklin has shown, however, that this rule does not hold good 



