530 



p. E. SMITH 



less difficulty than the other two features described and so are 

 more accurately portrayed. They appear in the medullary folds 

 at an early stage, and in the consecutive observations which I have 

 made on developing eggs I have been able to trace them through 

 from the wide-open medullary plate to a stage between figures 18 



Figs. 11 to 14 Camera sketches of the open medullary plate. X 15. 



and 19, when I could trace them no longer because of their fusions 

 and absorption into the expanding prosencephalon. From the 

 drawings, it will be noticed that in the younger stages this irregu- 

 larity is partially due to a thickening of the neural folds. They 

 have the appearance of being caused by the medullary folds 

 increasing in length faster than does the base which supports 



