124 ISABELLA M. DRUMMOND. 



logical facts, by poiutiug out how the auus must have lain at 

 all stages in the palleal groove^ and then proceeds to build 

 up his own theory of unequal growth, and the resulting 

 gradual approximation of mouth and palleal complex on the 

 right side. That such an approximation does take place in 

 the ontogenetic history is, of course, well known, and the 

 manner in which it is brought about seemed to Butschli 

 equally obvious. According to him, at a time when the anus 

 lies in the middle line posteriorly, a narrow zone on the 

 right side of the animal ceases to grow altogether, while the 

 correspondiug zone on the left grows with great vigour, and 

 thereby the anus appears to be pushed up the right side of 

 the body, while in reality the distance between it and the 

 mouth remains always the same. Meanwhile the foot on the 

 one hand, and the mantle on the other, continue to grow 

 symmetrically. This process cannot by itself, however, bring- 

 about the crossing of the visceral connectives. For this 

 Butschli has to invoke the aid of the mantle cavity, which, 

 he says, is formed rapidly at a time when the anus lies far 

 forward on the right side of the body, and, by its growth 

 backwards and to the left, carries the organs of the original 

 right side of the body back with it and over the mid-dorsal 

 line. All this he puts forward as ontogenetic fact, and 

 therefore probable phylogenetic theory. 



Biitschli's views have been adopted with more or less slight 

 modification by many authors, and have recently been brought 

 forward again with some additions by Plate (15). The great 

 dithculty, to which no one could find a fully satisfactory 

 solution, was the absence of any known cause of asymmetrical 

 o-rowth in a perfectly symmetrical body. Plate seeks an 

 explanation in the asymmetry of the liver. Starting from 

 the nearly symmetrical liver of the Chitons, and comparing 

 it with the asymmetrical organ in the Gasteropoda, he 

 describes how, in the primitive form, a rapid growth of the 

 left liver must have taken place at the expense of the right, 

 which would result in the formation of a hernia posteriorly 

 on the left side. Thu6 the first rudiment of a coil is formed, 



