M. Ussow’s Zoologico-Embryological Investigations. 111 
at the time when the nutritive vitellus is entirely surrounded * 
at the inferior pole by the cells of the upper germ-lamella 
formed by terminal division of the segments, and by the 
upper layer of elongated fusiform cells of the second germ- 
lamella. 
On the seventh and eighth days the germ enclosing the 
nutritive vitellus gradually changes its form from oval to 
perfectly spherical. In Loligo, Sepiola, and Ommastrephes the 
surfaces of most of the cells of the upper germ-lamella (sphe- 
rical embryo) (those on the part where the eye-ovals will be 
formed and some others excepted) become covered with cilia, 
which, in the species above enumerated, cause the rotation of 
the embryo by their continual movement. In Sepia and 
Argonauta the embryo does not rotate, either in this or the 
following stage of development. ‘The period of formation of 
the blastoderm (including the process of segmentation) lasts 
from four (Argonauta) to nine (Loligo, Sepiola) and more days 
(? Sepia). 
Thus at the commencement of the rotation, with which the 
second period of development (that of the production of the 
organs) begins, the germ covers the whole of the nutritive 
vitellus, and consists of two germ-lamelle here and there 
composed of several layers, namely :—1. The blastoderm or 
upper germ-lamella ({ornblatt). 'The thickness of this lamella, 
which is still one-layered, increases somewhat as we approach 
the upper pole of the nutritive vitellus}, and, indeed, at the 
point where the oval fold covering the rhomboidal part of 
the germ, situated on the dorsal surface of the embryo, is 
formed. The rhomboidal centre of the germ, which was 
at first round, and the oval, broader or narrower annular fold 
originate from the considerably grown central part of the ger- 
minal disk, situated at the acute pole and bordering upon the 
so-called area opaca; but this part itself has originated from 
the fourteen primitive segmentation-spheres, which rapidly 
increased in number and appeared at different times. The 
middle portion of the germ, which now covers nearly half 
the surface of the nutritive vitellus (from the margin of 
the above-mentioned fold to the equator) and attains its 
ereatest breadth upon the dorsal surface, represents the con- 
siderably widened middle ring of the germinal disk, which 
originated from the multiplication of the cells chiefly con- 
stricted off from the segments by the meridional furrow. Here 
* In Loligo, Sepiola, aud Argonauta; in Sepia the blastoderm, as already 
remarked, only closes in the second period of development. 
+ By transverse division of its cells, which become cylindrical and 
generally contain two sharply defined nuclei. 
