THE ORIENTATION OP THE FROG's EGG. 399 



anus of the frog, although apparently a new perforation, is 

 really a reopening of the temporarily closed portion of the ori- 

 ginal blastopore." Now I doubt exceedingly whether con- 

 crescence of the ventral lip of the blastopore takes place in 

 any such way as the authors' diagram indicates. The cells 

 from the sides may come later to the middle line, but not by a 

 process of apposition of the latero-posterior walls of the 

 blastopore. Rather the cells stream or migrate to the median 

 line below the surface, while the surface grows continuously 

 from behind forward. Hence we cannot speak of a reopening 

 of the blastopore, as nothing was left behind to reopen ; but 

 we must speak of a perforation at the point where the blasto- 

 poric lips first began to concresce in a sense different from that 

 used by the authors. 



IV. 



Roux and Hertwig have given accounts of embryos in which 

 the blastopore had failed to close. Roux figures an embryo 

 with a hemisphere of white exposed, and the embryo lying as 

 a thickened zone around the border line between the black and 

 white hemispheres. Hertwig has not figured such extensive 

 exposures of yolk, but describes stages with varying amount 

 of blastopore unenclosed. 



I attempted to produce such embryos artificially, and after 

 a great number of attempts that gave no favorable result found 

 at last a method that made it possible to produce such embryos 

 at will. 



Embryos in which the blastopore had just appeared were 

 put into the following solutions, with the results recorded : 



VOL. 35, PART 3. NEW SER. E E 



