324 THE RECAPITULATION THEORY 



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 that 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 7j mm. length it becomes 

 solid and remains so until a length of about loj 

 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 duplication which is 



