907 
To my surprise, I found that the external features of the gas- 
trulation process and the behaviour of the dorsal and the ventral 
blastoporie rim in Rana fusca and esculenta differ from one another 
pretty considerably, so that a comparison of the two cases becomes 
especially interesting. Let us first consider the facts and afterwards 
look for an explanation. | 
The eight-celled stage of Rana esculenta agrees in the main with 
that of Rana fusca, as a comparison of Fig. 1 with the figure for 
a 
Fig. 1. Egg of Rana esculenta, 8 cells, from the 
side. The zone of demarcation between the darker 
and lighter area is defined by spots. 
R. fusca of my former communication shows at once. The propor- 
tion of the size of the blastomeres in both cases is nearly the same. 
Nevertheless the distribution of the pigment points to a difference 
in the internal structure: the line of demarcation of darker and 
lighter hemisphere, in both figures indicated by a dotted band, not 
only lies much nearer to the animal pole in Rana esculenta, but 
it has also a much more horizontal situation. Now this boundary- 
line does not coincide in the least with the boundary of the future 
ecto- and. entoderm, but it is apparently of importance in so, faa 
as in both frog species, as we will see, the border of the blas- 
topore shortly after its appearance nearly runs parallel to it. We 
will revert to this in due course. 
Turning to the figures 2—6, all drawn with a drawing-prism 
from the same egg, which was marked at the animal pole and at 
the point 6, we see in fig. 3, how the first indication of the blastopore 
appears as a short, transverse slit, a little beneath the equator of 
58* 
