M. Ussow’s Zoologico-Embryological Investigations. 109 
interspaces, such as I have seen in no species investigated by 
me. The mode of formation of the embryonal cells is also, 
as follows from the preceding statements, quite erroneously 
described by Kdlliker. 
Ill. The formation of the Germ-lamelle. 
The above-mentioned concluding stage of the process of 
segmentation (7. ¢. the appearance of the germinal disk, or the 
one-layed germ, consisting of the upper germ-lamella, which 
appears at the upper pointed part of the nutritive vitellus and 
covers a twelfth part of it) occurs in most of the Cephalo- 
poda observed by me on the second day after the commence- 
ment of development*. The important moment of the 
appearance of the second germ-lamella falls in the beginning 
of the third day (Sepia, Loligo, Ommastrephes). The original 
separation of the second germ-lamella takes place in the fol- 
lowing manner :—In the middle part of the above-mentioned 
one-layered ring, situated immediately below the centre of the 
germ (now very like the area opaca), the cells, which are 
continually undergoing further division in a longitudinal di- 
rection, begin also to divide gradually in a transverse direc- 
tion, the division commencing at the lower periphery and 
advancing towards the centre. The nucleus of each cell of 
the one-layered upper germ-lamella becomes elongated; and 
at the same time the protoplasm is also elongated, like a drop, 
downwards ; and then a new cell is constricted off from the 
mother cell. As the result of this transverse division a second 
germ-lamella is produced, at first only in the median ring of 
the gerrainal disk, but afterwards also in the central part and 
in the segment-part. At the spots where it has been formed, 
the germinal disk soon becomes quite opaque, and appears dull 
white by direct light. E 
In the following days (about to the fourth or fifth) the 
above-described process of growth is continued, and now in all 
parts of the germinal disk, by which means, 1, the diameter of 
its still one-layered central part increases considerably ; 2, the 
middle two- or more-layered thick part (area opaca) spreads 
more and more towards the inferior pole; and, 3, the region 
of the segments dividing up into cell-groups which follows 
directly on the ring now commences at the equator of the 
vitellus (and therefore much lower than before). The thick- 
* In Argonauta the germinal disk is formed as early as the seventh or 
eighth hour from the commencement of segmentation. 
