ANIMAL PEDIGREES 229 



the animal to the surrounding environment remain 

 unchanged, while internally preparations for the 

 later stages are in progress. 



Cinderella and the princess are equally possible 

 entities, each being well adapted to her environ- 

 ment. The exigencies of the situation do not 

 permit however of a gradual change from one to 

 the other : the transformation, at least as regards 

 external appearance, must be abrupt. 



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 

 one which I happen to have recently run my head 

 against, and remember vividly. 



The solid condition of the oesophagus in dogfish 

 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 ascer- 

 tained. 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 inch length it becomes solid 

 and remains so until a length of about J inch has 

 been attained. The solidification occurs at a stage 



