448 WILSON. [Vol. VI. 



of the blastomeres, and hence is without phylogenetic significajice. 

 The truth of this proposition is at once apparent upon compar- 

 ing the spiral cleavage of the annelid with that of the polyclade. 

 The form of cleavage is identical in the two, the products ex- 

 tremely different, as has been pointed out at p. 441. Unless, 

 therefore, we are prepared to maintain 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 enibryological origin is not a proof of homology, as 

 far, at least, as the cleavage-stages are concerned, Balfour, 

 long ago, pointed out the fact that similarities in the general 

 form of cleavage have no necessary relation to adult relation- 

 ships, but it is a very surprising fact that a resemblance so close 

 as that between the polyclade and annelidan ovum should be 

 without morphological meaning. This is the more surprising 

 because the comparative study of the annelids shows that 

 within the limits of this group adult homologies are represented 

 by accurate cell-homologies in the cleavage-stages (p. 436). We 

 must conclude, however, that precisely similar modes of cleav- 

 age may arise quite independently of the nature of the mate- 

 rials, upon which the cleavage operates. 



If, then, the spiral form of cleavage has no phylogenetic 

 meaning, and at the same time has no direct relation to the 

 adult form *(as has been sufficiently pointed out), it must be 

 due to mechanical conditions peculiar to the earlier stages 

 of embryonic life. Before attempting to consider the nature 

 of these conditions, I must call attention to a fact, which 

 has been fully and clearly set forth in a profoundly interesting 

 paper of Rauber's (No. 19), namely, that the mechanical laws 

 of cell-division have been far more thoroughly investigated 

 in plants than in animals ; and it is to botany that we must 

 look for a clue to the significance of cleavage-forms among ani- 

 mals. I need not review in extenso the brilliant researches of 

 Sachs and others in this field, since their principal results may be 

 found in his well-known Pfianzeitphysiologie (Vorlesung XXIV) 

 as well as in Rauber's paper. The essential points may be thus 

 briefly summarized. The form of cell-division is the rcsidt and 

 not the catise of the form of the dividing mass ; the form of the 



