TITE DKVRf.OPMRNT OF PAT.ITDTNA VIYTPARA. 181 



fellow on the other side of the body. Now, as the former 

 was orio'inally on the rif>"ht side of the body, it would fall 

 into the zone where Biitschli supposes that growth ceases, 

 while the snbiiitestinal gang-lion, on the other hand, would 

 be originally in a zone of very active growth. This, Aniaudrut 

 argues, should lead to the snbiiitestinal ganglion being- 

 pushed the further back of the two, while pi-ecisely the 

 opposite is actually the cnse. If, however, we suppose that 

 instead of the left it was at first the right side which grew 

 most actively the existing condition of affairs would be 

 obtained ; and as, after torsion, the zone of active growth 

 would be transferred from the right to the left, we probabl}' 

 find an explanation of Biitschli^s error in his having exa- 

 mined too late stages of development. The nrgnment 

 concerning the position of the ganglia seems sound so far 

 as it goes, but the alleged reason of Biitschli's mistake is 

 not so easy to accept, seeing that he starts from a form 

 in which the anus lies in the middle line behind, and in 

 which even the ventral flexure has apparently not yet 

 begun. 



Biitschli himself relies for his evidence of unequal growth 

 upon having demonstrated that in different stages of develop- 

 ment the mouth and anus do, as a fact, remain exactly the 

 same distance apart while the body is increasing in size and 

 torsion is taking place. Now, it seems to me that the value 

 of this argument depends largely upon the view we take of 

 the development of the mantle cavity. If we regard it as 

 von Erlanger did, and as I believe we must regard it, as 

 the result of an outgrowth of the mantle rather than as an 

 invagination of the surface of the body, Biitschli's argument 

 is entirely destroyed, for then it is not the mouth and anus 

 that we must compare so much as the mouth and the back of 

 the mantle cavity; and the fact that the anus lies near the 

 outer edge of the mantle cavity shows only that rapid growth 

 of the rectum has been taking place in this i-egion, and has 

 caused a closer approximation of the anus to the mouth than 

 would otherwise have been the case. Whichever view we 



