394 T. H. MORGAN AND UM^ TSUDA. 



of the anus of Rusconi develops the anlage of the central 

 nervous system. 



Pfliiger adds, '^In order to avoid a misunderstanding I 

 must say that I do not by any means believe that the whole 

 anlage of the central nervous system is a derivative of the 

 white hemisphere. Since the lighter substance of the white 

 hemisphere is directly continuous with the lighter substance 

 of the black, it is possible that the anterior portion of the 

 medullary plate corresponding to the brain, and even to the 

 upper portion of the neck, may form in the black hemi- 

 sphere." 



There are two statements only in the foregoing account 

 from which I should dissent. In the first place it seems rea- 

 sonably certain that the blastopore does not originate in the 

 equator of the egg, but at some distance below it. In the 

 second place Pfliiger believes that the blastopore, as it en- 

 croaches on the yolk-plug, moves as a whole further along the 

 meridian of migration. This means that after the ventral in- 

 vagination of the blastoporic rim has formed, the ventral lip 

 still moves upwards towards its nearest equatorial point. 

 This migration of the whole blastopore is stated in the text, 

 and is definitely shown in the series of diagrams drawn to 

 illustrate the process of overgrowth. 



I have attempted to show that the overgrowth of the dorsal 

 lip itself is sufficient to account for the length of the medullary 

 folds ; also that the posterior lip of the blastopore, after its 

 formation by invagination, closes by a forward growth. There 

 is, therefore, no evidence for such a migration as Pfliiger sup- 

 poses, and if the circular blastopore after its formation does 

 move further upwards it must be due to a slight rotation of 

 the egg as a whole in this direction. But the statement that 

 it does continue to move must be re-examined in living 



eggs. 



It is difficult to give any adequate summary of Roux's 

 results. In his later papers he is not always consistent with 

 his earlier views. Schultze's damaging criticism of some of 

 B-oux's earlier conclusions Roux has not answered, although 



