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polarity, which expresses itself in that the cells of the rear side 
(d-side) from the beginning are much larger than the corresponding 
cells at the anterior side (b-side), so that the entoderm area from 
the beginning does not lie diametrically opposite the animal pole. 
The scheme of fig. 11 may serve to illustrate this. As a rule we 
see at the same time both kinds of polarity in the larger eggs exerting 
their influence on the cleavage, but in one case the first predomi- 
nates, in the other the second prevails. As an example of 
the prevalence of polar polarity, | mentioned Nereis where the 
macromeres (entoderm) are especially large in regard to the ecto- 
meres, which lie over them as a little cap, while on the other side 
the bilateral polarity is only slightly expressed, the cells of the rear 
side not being much larger than those of the anterior side. In the 
reverse this last condition prevails very strongly in Scoloplos, which 
accordingly can serve as an example of the predominance of the 
bilateral polarity (fig. 10c). Especially 2d is of extraordinary size, 
while the entoderm cells are not at all remarkable for special bulk. So 
the entoderm area is displaced here from the beginning to the 
veutral side. 
Hence the eggs of Rana fusca and esculenta evidently are in the same 
relation to each other as Scoloplos and Nereis. In the first species a pre- 
cocious displacement of the entoderm area and less yolk, as appears 
e.g. from the extension of the blastopore. In Rana esculenta a later 
wandering of the entoderm area and a greater amount of yolk, at 
least a less isolecithal structure of the egg, as appears from ihe 
large blastopore together with the fact, that the foundation of the 
embryo encircles considerably less than 180° of the egg circumfe- 
rence; the belly accordingly is relatively more swollen than in 
Rana fusca. Originally the entoderm area in Rana esculenta, though 
not perfectly, yet lies much more diametrically opposite the animal 
pole than in Rana fusca, as appears from the fact, that the demar- 
cation line of the lighter and darker hemispheres of the egg and 
later the border of the blastopore make a much smaller angle with 
the egg equator than in the last mentioned species. So in Rana 
esculenta the polar or radial symmetry is originally more strongly 
pronounced, in Rana fusca the bilateral symmetry. 
In conclusion, attention may be drawn to the fact of how little this 
difference in the internal constitution of the egg influences the cleavage. 
Were things as in annelids, with their determinate cleavage, we 
might expect, that in the eight-celled stage in Rana esculenta the 
four upper cells would be relatively smaller than the four lower ones 
as compared to Rana fusca, and, reciprocally that in the latter 
