1 88 CONK LIN. [Vol. XIII. 



Physa. And the fact that the asymmetry of the adult body is 

 reversed in those gasteropods (Physa, Planorbis) which show- 

 reversed cleavage makes it probable that the direction of the 

 spirals influences not only the cleavage stages but also the 

 entire development. 



The general significance of radial cleavages, both orthoradial 

 and spiral, may be considered here, since it has much to do 

 with the interpretation of cleavage in general. The mere 

 alternation of divisions explains many fundamental features of 

 radial cleavage, but it by no means touches upon its most 

 interesting and remarkable characters. These characters are 

 particularly well shown, not only in the radial symmetry mani- 

 fested in the direction of division, but especially in the size 

 and shape of the blastomeres. Unequal cleavages in them- 

 selves, as I have argued elsewhere, must signify more than 

 extrinsic forces ; they can be explained only by assuming cer- 

 tain intrinsic causes, and when we have these unequal cleavages 

 minutely repeated in the different quadrants, even though the 

 mechanical environment in those quadrants may be different, 

 we have to reckon with causes which are still more complex 

 and obscure. 



As striking illustrations of this radial symmetry in the posi- 

 tion, size, and shape of cells may be mentioned the following : 

 the formation of four macromeres, frequently equal in size ; 

 the formation of at least three, usually four, quartettes of 

 micromeres ; the radial symmetry manifested in the history 

 of each quartette, unless modified by the early appearance of 

 definitive structures ; the radial symmetry of embryonic or 

 larval structures, such as the trochoblasts, the apical cells, the 

 terminal and basal cells of the cross, the reversed cleavage 

 in each of the latter, and the long-continued resemblances 

 between the right, the left, and the anterior arms of the 

 cross ; the origin of the mesoblast from the second quartette 

 in quadrants A, B, and C, and from the fourth quartette in 

 quadrant D. 



In several cases these radial structures seem to belons: to the 

 same category as the radial structures of the trochophore larva. 



