PRESIDENTIAL ADDRESS. 23 



mechanically by flattening of the granular cells through pressure 

 against this rigid wall of spicules. 



Embryology supplies us with many unsolved problems, and it is not 

 to be wondered at that this should be the case. Some of these may 

 fairly be spoken of as mere curiosities of development, while others are 

 clearly of greater moment. I do not propose to catalogue these, but 

 will merely mention two or three which I happen to have recently run 

 my head against and remember vividly. 



The solid condition of the oesophagus, in Elasmobranch embryos, 

 first noticed by Balfour, is a very curious point. The oesophagus has 

 at first a well-developed lumen, like the rest of the alimentary canal ; 

 but at an early period, stage K of Balfour's nomenclature, the part of 

 the oesophagus overlying the heart, and immediately behind the 

 branchial region, becomes solid, and remains solid for a long time, the 

 exact date of reappearance of the lumen not being yet ascertained. 



Mr. Bles and myself have recently noticed a similar solidification of 

 the oesophagus occurs in tadpoles of the common frog. In young free 

 swimming tadpoles the oesophagus is perforate, but in tadpoles of about 

 7| mm. length it becomes solid and remains so until a length of about 

 10| mm. has been attained. The solidification occurs at a stage closely 

 corresponding with that in which it first appears in the dogfish, and a 

 curious point about it is that in the frog the oesophagus becomes solid 

 just before the mouth opening is formed, and remains solid for some 

 little time after this important event. 



This closing of the oesophagus clearly cannot be recapitulation, but 

 the fact that it occurs at corresponding periods in the frog and dogfish 

 suggests that it may possibly, as Balfour hinted, ' turn out to have 

 some unsuspected morphological bearing.' 



Another developmental curiosity is the duplication of the gill slits 

 by growth downwards of tongues from their dorsal margins ; a dupli- 

 cation which is described as occuring in Amphioxus and in Balanoglossus, 

 but in no other animal ; and the occurrence of which, in apparently 

 closely similar fashion, is one of the strongest arguments in favour of a 

 real affinity between these two forms. It is hardly possible that such 

 a modification should have been acquired independently twice over. 



A much more litigious question is the significance of the neurenteric 

 canal of vertebrates, that curious tubular communication between the 



