SYKES: VARIATION IN EECENT MOLLUSCA. 271 



a species of Partula are isolated from one another, has hinted at the 

 effect of environment. Again, we have been told that "this abnormal 

 growth probably is caused by disturbance of the relations of the 

 embryo with its initial shell." Hartman,^ when dealing with Parhda, 

 says as follows : " We can only conjecture as to the cause of this 

 departure from the more usual conformation ; but it may be owing 

 to a reversal of the vital forces acting during the segmentation of the 

 yolk of the egg in the early stages of the formation of the embryo. 

 The eggs of the common garden slug (which are almost transparent 

 and afford good material for observation), a short time after deposition, 

 exhibit the germinal vesicle (which lies in the midst of the yolk) 

 rising to the upper part, where a distinct rotation may be seen ; after 

 which it undergoes segmentation, and the germ appears. The rotary 

 motion, which is probably due to ciliary or vital action, consists of 

 two or three turns in one direction, and the same number in a reversed 

 one ; and in this reversed vital action, during segmentation of the yolk 

 of the egg, may be the secret of sinistral or reversed shells." 



Let us now turn to recent research in biology. In 1894 Crampton 

 and Kofoid pointed out that in sinistral snails the cleavage of the egg 

 from the second division onwards is typically spiral but reversed. 

 Conklin, to whose papers'^ I am indebted for the facts here given, 

 later found that in Crepidula this spiral character commenced with 

 the first division of the egg; and in 1903 he summed up the matter 

 of inverse symmetry, stating that, in such cases, the cleavage is 

 inverse and must be preceded by inverse organization of the 

 unsegmented egg, probably arising at the time of maturation or 

 fertilization. Further, he suggests that there may be a reversal of 

 polarity after the eggs are set free, which may perhaps be due to 

 pressure on the egg-cell forcing the spindle through the egg and 

 causing the polar bodies to be formed at the opposite pole to that 

 which is usually the case. 



Space fails me to give any details of "Weldon's work from the 

 statistical standpoint and to refer to other details, but I have now 

 given some outline, brief and imperfect though it be, of variation in 

 molluscs, and I would conclude by appealing to you to give your most 

 serious attention to the subject, that the malacologist may bear his 

 full share of the work, and receive his full share of the credit, in the 

 solution of the many problems to which it is the key. 



1 Bull. Mus. Comp. Zool., vol. ix, pp. 176-177. 



- Journ. MorphoL, vol. xiii, pp. 1-226 ; Anat. Anz., vol. xxiii, pp. 577-588 ; Proc. 

 Ac. Philad., 1903, p. 753. 



