579 



was vielleicht wahrscheinlicher ist, beide sind Wirkungen einer und 

 derselben asymmetrischen Verteilung von Anlagen, die schon im Ei 

 oder in dessen Kern entschieden war". 



In my work on Crepidula (1897) I found that the spiral character 

 of the cleavage begins with the first division of the egg, which is 

 in this case always dexiotropic, Fig. SB. All subsequent divisions 

 follow in regular alternations of direction up to an advanced stage, 

 the second cleavage being laeotropic, Fig. 422, the third dexiotropic, 

 Fig. 5 R, the fourth laeotropic, Fig. 6 R etc. It is almost certain that 

 the direction of each division is dependent upon that of the preced- 

 ing cleavage, and so back to the first division of the egg which may 

 be said to 'set' the direction for all subsequent ones. But it is also 

 equally certain that the cause of the dexiotropic direction of the first 

 cleavage lies in the structure of the unsegmented egg itself. Further- 

 more the first appearance of the final asymmetry of Crepidula is 

 found in the direction of division which gives rise to the fifth quartette 

 of cells from the four stem cells or macromeres. The macromere C 

 Fig. 8i2, which occupies the right posterior region of the embryo 

 divides at right angles to the antero-posterior axis so that its fifth 

 product, 5 c, lies posterior to the stem cell C; on the other hand the 

 left posterior macromere D divides later than C and in a dexiotropic 

 direction and its product 5d is somewhat smaller than 5c and lies 

 nearer the ventral side. This cell 5d continues to move toward the 

 mid-ventral line while at the same time the cell 5 c moves toward 

 the mid- dorsal line. "This is the beginning of the final asymmetry 

 of the gasteropod. . . . The first trace of the final torsion is due to a 

 difference in the time and direction of the cleavage of these two cells", 

 (p. 155). "If the initial asymmetry is caused in Physa as it is in 

 Crepidula by the asymmetry of the cells 5 c and 5 d then it is easy 

 to see how this reversal of cleavage (in Physa) stands in a causal 

 relation to the reversed asymmetry of the adult", (p. 173). All 

 sinistral snails so far studied (Physa, Aplexa, Planorbis, Ancylus 

 rivulorum) invariably show this reversal of cleavage, while dextral 

 forms never do; there can be no doubt therefore that there is a 

 causal relation between the reversal of the cleavage and the reversal 

 of the asymmetry of the adult. The causes of the inverse symmetry 

 of the adult can be traced back step by step through the development 

 to the first cleavage of the egg, and probably to the unsegmented 

 egg itself. 



Holmes (1900) supposed that this reversal of the cleavage in 

 sinistral snails might be due to an anachronism of cleavage (Roux 



37* 



