THE DEVELOl'MENT OF PALUDINA VJVIPARA. 133 



therefore it cannot form an ontogenetic cause for the 

 forward movement of the palleal complex. Phylogeneticaliy 

 tlio evidence is only negative^ but at least embryology gives 

 no support to this part of Plate's theory. 



Putting aside, however, the question of the coiling of the 

 visceral hump, if we follow the progress of torsion from stage 

 to stage we can, I think, in no way regard this as dependent 

 upon the growth of the liver in each stage. Comparing 

 (Stages C and D, for instance, both as complete embryos and 

 in sections (cf. especially figs. 11 and 12), we find the slight 

 growth of the liver more than counterbalanced by the great 

 development of the pericardium and mantle cavity, so that 

 whereas in fig. 11a line joining the junction of the stomach 

 and liver with the ridge x, which, for our present purposes, 

 may be taken to represent the position of the rectum, divides 

 the section into very nearly equal portions, a sin)ilar line in 

 tig. 12 makes that portion which contains the liver considerably 

 smaller than the other, which contains the great original 

 right extensions of the pericardium and mantle cavity. In 

 the next stage (Hg. 13) this has been partly rectified by rapid 

 growth of the liver, but in Stage F the inequality is again 

 very marked, and, in fact, from Stages E to G the growth of 

 the liver is very slight, while torsion is rapid. 



While entirely repudiating the idea, however, of the liver 

 acting, as it were, as the propelling force throughout ontogeny, 

 it may yet be possible to agree with Plate in regarding it as 

 the original disturber of symmetry; and some support is lent 

 to this view not only by the very early asymmetry of this 

 oi'gan, but also by the fact that it is not present, or is only 

 very slightly developed, in the symmetrical monstrosities. 

 But it should, at the same time, be noticed that the develop- 

 ment of Paludina gives no more support to this part being 

 acted by the liver than by the mantle cavity. The latter 

 organ also is asymmetrical from the time of its first formation 

 in normal forms ; while in Monstrosity III, which was slightly 

 twisted towards the left, the mantle cavity is considerably 

 developed in a manner to correspond to the torsion, while the 



