236 RICHARD ASSHETON. 



the lower pole of the ovum through at least 170°. The ventral 

 lip according to him does not advance at all. 



Morgan and Ume Tsuda consider that the ventral lips 

 and lateral lips advance, but not to so great an extent as the 

 dorsal lips. They also notice the " first overgrowth of the 

 dorsal lip of the blastopore is more rapid than the later growth ; 

 that is, the approach to the points of injury is faster at 

 first." 



I quite agree with the latter authors that " it seems 



most probable that the blastopore does not start at the equator 

 of the egg, but some distance below that circle." 



Now my experiments do not give evidence of an overgrowth 

 by the dorsal lip of more than 60° or 70° from the moment of 

 the first commencement of the dorsal lip, and to the closure 

 of the blastopore. More probably, I think, the apparent 

 overgrowth is even less. 



According to Roux the overgrowth is at least 170° to 180°. 



If Roux is right in both his suppositions, namely, that the 

 dorsal lip moves over the white pole, and to an extent of 180°, 

 I cannot understand how the last remaining portion of the 

 blastopore to remain open should show so white a piece 

 of yolk plug. This piece of yolk plug is as white as any part 

 of the surface of the ovum of the frog ever is. There is a 

 considerable amount of variation in the pigmentation of the 

 unsegraented ovum. It is extremely rare in England to find 

 eggs in which there is deficiency of pigment over an area sub- 

 tended by so great an angle as an angle of 120°. An area 

 where there is almost an absence of pigment is much more 

 restricted. Very frequently the less pigmented area extends 

 over a much smaller arc. On Roux's supposition, the part 

 which remains longest uncovered ought to be grey, if not 

 quite black. It is, as far as I have observed it, an almost 

 invariable rule to find the yolk plug at its latest stage intensely 

 white. 



I have only once seen embryos which in the gastrula 

 stage showed a darkened blastopore, and these were from eggs 

 which in the unsegmented stage were so intensely pigmented 



