110 M. Ussow’s Zoologico-Embryological Investigations. 
ened inner layer of the area opaca, which consists of rounded, 
scattered, spontaneously dividing cells* (of the second germ- 
lamella), ’ forms, at the boundary of the central part of the 
germinal disk, a wall which penetrates more or less into the 
nutritive vitellus. In consequence of this pressure, the nutri- 
tive vitellus on its part penetrates into the slightly aye 
central part of the germinal disk (like the “ Dotterpfropfe ” 
the frog’s egg). At the same time a very narrow ee 
is formed from the cells separated by constriction from the 
segments ; this les between the first rmg and the segments. 
On the sixth and seventh days this new ring exactly sur- 
rounds the equator of the vitellus. Its four- or five-angled 
cells, which are rather large, lie in consecutive series. Indeed, 
in general, all the cells, both of the inner and outer germ- 
lamella, arrange themselves in such consecutive series ; the 
latter, during their division (constriction), when they are for a 
short time free, move upon the surface of the nutritive vitel- 
lus by means of their contractile protoplasm and longer or 
shorter pseudopodia. 
At the end of the seventh day the cells of the central coni- 
cal part of the upper germ-lamella multiply very rapidly by 
longitudinal division (Sepiola, Loligo, Argonauta). By this 
means is produced a thickening, which, however, by no means 
occupies the whole central part of the germinal disk, but only 
forms at its margin an oval fold, which, spreading in ’ the polar 
direction, begins gradually to conceal the central part. Simul- 
taneously with the formation of this fold, the part of the 
germinal disk circumscribed by the fold sinks a little and 
forms a furrow broader and deeper in the middle, having the 
shape of an extended rhomboid. In the rhomboid the ger- 
minal disk consists of a single layer of cells of the upper germ- 
lamella. But beneath the oval fold the cell-layer of the 
second germ-lamella which is there thickened begins to 
double itself by transverse division, and thus forms two 
layers—the upper the dermo-muscular layer, and the lower 
the intestino-fibrous layer. These two layers may be most 
clearly observed at the boundary of the former area opaca 
and the central part of the germinal disk, and, indeed, on 
the future ventral surface of the embryo, whilst at first they 
gradually coalesce towards both the equator and the pole, so 
as not to be distinguishable. The further splitting of the 
second germ-lamella into two superposed layers takes place 
* The cells divide both in a longitudinal and transverse direction, by 
which means their layer becomes thicker and broader towards the prince 
pole. 
