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Displacement of the endoderm-area. — We shall see now 
what we find of these phenomena in the frog egg, and 
take as an example the egg of Rana fusca. At once it 
will be evident that the first of the three above-mentioned 
processes, the wandering of the endoderm area to the ventral 
(in Chordates to the dorsal) side, is here performed very 
precociously, immediately after fertilization, and conse- 
quently is already finished in the unsegmented fertilized 
egg. Thus we find, as is shown by fig. 35, that the white 
area here does not lie diametrically opposite the animal 
pole but much more to the future dorsal side which cor- 
responds to the ventral side of the Annelid. The boundary 
between the ecto- and endoderm areas probably runs parallel 
to the demarcation of the darker and lighter areas of the 
egg, as may also be concluded from the place where after- 
wards the dorsal and ventral borders of the blastopore appear 
(fig. 35). The same displacement of the endoderm area which 
in the Annelid occurs during development is evidently 
already completed in Chordates in the unsegmented egg. 
However, in Annelids also we often find such a structure 
of the egg, as may be concluded from the relative size of 
the cleavage cells. In my article on the development of 
Scoloplos (1916) 1 have tried to show that among the eggs 
of polychaetous Annelids three types are to be distinguish- 
ed. In the first place we have the small, poorly yolked, 
eggs of Polygordius, Hydroides etc, in which the cleavage 
results in a very equal coeloblastula (fig. 38a). Secondly 
we have the larger eggs of other species in which two 
types of polarity may be very early recognized, which exert 
their influence on the very determinate cleavage. In the first 
place we can distinguish the polar or radially symmetrical 
polarity, manifesting itself in the accumulation of yolk at the 
vegetative pole which again causes the endoderm cells to be 
much larger than the cells of the three quartets of ectomeres, 
so that, in fact, they deserve the name macromeres. In the 
second place the bilateral polarity which manifests itself in 
the cells of the rear side (d-side) being from the beginning 
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 
diagrams of fig 38b and c may serve to illustrate this. 
Ss a rule we see in the eggs with greater diameter both kinds 
of polarity exerting their influence on the cleavage at the 
