454 WILSON. [Vol. VI. 



What, then, is the origin of bilateral forms of cleavage ? It 

 appears to me that they must be the result of a throwing back or 

 reflection of the adnlt bilaterality upon the early stages. In some 

 cases this influence has extended to the very beginning, as in 

 the Cephalopod or in the ascidian, or even to the unsegmented 

 ovum itself, as in some insects and other forms. In some cases, 

 of which Nereis is a beautiful example, it has not extended so 

 far ; the early stages are still dominated by the mechanical con- 

 ditions peculiar to them, and the bilateral form only appears 

 when these conditions have been in a measure overcome. 



I may call attention to the fact that there is a definite relation 

 between the law of spiral cleavage and the fact that among 

 annelids and mollusks it is invariably the left posterior macro- 

 mere that is the largest. In all cases that have been accurately 

 investigated, the first spiral cleavage of the four macromeres is 

 right-handed, the second left-handed, and the third right-handed 

 again. The subsequent origin of the mesoblast (as in Nereis, 

 Crepidula, etc.) may be regarded as belonging to another left- 

 handed spiral cleavage. Now the superior size of the left pos- 

 terior macromere, as has been pointed out, is owing to the 

 storage in it of the material for the somatoblasts, which, by the 

 second and fourth left-handed spiral divisions of the macro- 

 meres, is pushed over into the middle line, where it belongs. 

 There is, therefore, a necessary relation between the law of 

 spiral cleavage and the storage of material in the left macro- 

 mere. The exact nature of this relation, however, cannot be 

 stated, for there is no positive evidence to show which is cause 

 and which effect. The comparison with the polyclade, although 

 at first sight promising, yields no result, on account of the lack 

 of necessary data. 



One other point may briefly be considered, namely, the 

 remarkable differences in the fundamental axial relations be- 

 tween forms closely allied in the form of cleavage. The anne- 

 lids and the mollusks show exactly parallel differences in this 

 regard. In Planorbis (Rabl), Neritina (Blochmann), and some 

 other gasteropods, the first two cleavage-planes are inclined at 

 an angle of 45° to the median plane, and the representative of 

 the " large macromere " is in the median line, precisely as in the 

 polyclade (Diagram VIII, D, E). In Crepidula, on the other 

 hand, the first two cleavage-planes coincide with the transverse 



