56 DEVELOPMENT OF THE FROG'S EGG [Cir. V 



The dorsal lip has passed then, in all, through 19 mm. of the 

 white area ; the ventral lip (from behind, forward) through 

 3 mm. 



If the region overgrown by the dorsal lip be compared with 

 the length of the medullary folds which soon appear in the same 

 region of the embryo, it will be found that the latter, when they 

 first appear, are somewhat longer than the region overgrown. 

 If, however, we deduct from the length of the medullary folds 

 the thickness of the anterior connective that joins the right and 

 left sides of the nerve-plate, we shall find that the remaining 

 length of the medullary folds corresponds very closely with the 

 length of the region overgrown. We must, therefore, conclude 

 that the anterior connective lies just in front of the point at 

 which the first trace of the dorsal lip of the blastopore appeared. 



We have assumed the point of injury to be a fixed point and 

 the overgrowth to be due to the progress of the dorsal lip. It 

 might equally well have been assumed that the overgrowth 

 was only apparent and was produced by the sliding forward 

 of the whole of the white area beneath the dorsal lip of the 

 blastopore. The end-result would be the same, but the process 

 different. There can be no question, however, that the move- 

 ment is really due to the progress of the dorsal lip. Other 

 experiments where two or more points of the surface are 

 injured show very conclusively that the movement is a back- 

 ward growth of the rim of the blastopore. 



Comparing the statements made above with those of Pfliiger, 

 it will be found that they differ in three unimportant respects. 

 The rapidity of the overgrowth of the very early stages, before 

 the complete establishment of the crescent, was not noted by 

 Pfliiger. The distance travelled by the dorsal lip, as just 

 described, is somewhat less than that given by Pfliiger. 

 Pfliiger thought that the dorsal lip moved over about 180 

 degrees, but added that the amount of the movement differed 

 in different individuals, and was probably between 90 and 180 

 degrees. My own results make the region of overgrowth 

 about 120 degrees. From Pfluger's figures we are led to 

 believe that the whole blastopore after the establishment of 

 its ventral lip continues to move somewhat nearer to the 

 equator of the side nearest to the ventral lip. If this really 



