108 M. Ussow’s Zoologico-Embryological Investigations. 
(‘ germinal spot,” Kélliker) is produced. In this, as regards 
the size and form of its constituent cells, and also their dis- 
tribution, the following two divisions may be distinguished : 
—1, the centre of the germinal disk, which presents the form 
of a convex circle, and has been formed by the multiplication 
of the high cylindrical primitive segmentation-cells (see the 
stage of eight segments) ; and, 2, the originally very narrow, 
but gradually widening ring, which immediately follows the 
above-mentioned disk : the somewhat broader, but flatter, pen- 
tagonal or hexagonal cells of which have been formed chietly 
from the apices of the segments constricted off by the meri- 
dional furrow (see the stage of the meridional furrow). 
Directly united with this ring is the inferior part, which 
extends to the inferior pole of the nutritive vitellus and 
encloses the latter. This part consists of the apices of seg- 
ments* slowly advancing in their division, and of the 
segments themselves, which are here (at the inferior pole) 
not sharply separated, but often even mutually coa- 
lescent. Their number remains as before (thirty-two). 
Their finely granular protoplasm covers with a very thin 
layer the whole mass of the nutritive vitellus, which in this 
way is enclosed as in an envelope from the very commence- 
ment of the segmentation in the so-called formative vitellus, 
or, to be more exact, in the protoplasm of the primitive ovi- 
cell, lying uniformly on its surface except at the superior pole, 
where it is perceptibly thickened. The so-called disappear- 
ance of the segments in reality never occurs. Earlier or later 
they all divide, as we shall see, and furnish a certain number 
of the cells forming the one-layered blastoderm f. 
From the actual course of the process of segmentation of the 
Cephalopod ovum here described, and which I have traced in 
all its details, we may easily convince ourselves of the inac- 
curacy of the opinion expressed by Kélliker upon this ques- 
tion. And, in fact, I have perfectly convinced myself, by a 
series of frequently repeated investigations, that he observed 
stages in the development of the ova of Sepia which were 
quite independent of each other, and that his researches were 
carried on under abnormal conditions, in which the union of 
the segments and the segmentation-apices was already much 
injured. Thus, for example, Kolliker indicates in the centre 
of union of the segmentation-spheres indefinite and irregular 
* In the last stage of segmentation the apex of every segment divides 
into groups of cells, which arrange themselves in parallel rows on the 
equator. ‘ 
_t In Sepia the blastoderm closes at the inferior pole of the nutritive 
vitellus only in the second period, as, indeed, KGlliker has described. 
