318 M. Ussow’s Zoologico-Embryological Investigations. 
or cylindrical cells (more elevated at the upper, pointed pole 
of the egg) forming the blastoderm or upper germ-lamella 
(horn-lamella, sensory lamella), on the second (Argonauta) 
or third (Loligo, Sepiola) day of development a second lamella 
originates in the middle part (area opaca) of the germinal 
disk, by transverse division of the upper germ-lamella ; and this, 
during the period of the appearance of the organs, plays the 
part of the middle germ-lamella of the Vertebrata, Annulosa, 
Mollusca, &c., and like that lamella soon divides, in some of 
the animals mentioned, into two layers, the dermo-muscular 
and the intestino-fibrous layers. 
From the lamella thus dividing, and indeed from the upper 
first lamella which becomes inverted on the two opposite 
(ventral and dorsal) sides of the embryo, the young Cepha- 
lopod is developed upon the broad hemispherical germinal 
spot or disk, extending as far as the equator of the egg, in 
from 25 (Argonauta) to 40 days (Loligo). The lower part of 
the germ, which in most of the species mentioned closes over 
the obtuse pole of the egg at the end of the first period, 
becomes the yelk-sac, composed of the upper lamella and the 
dermo-muscular layer. 
Development commences in the central part of the germinal 
disk, and, indeed, by the appearance on the future dorsal sur- 
face of the animal of an at first insignificant furrow, which 
rather quickly acquires the form of a groove and subsequently 
becomes converted into a perfectly closed tube. 
Simultaneously with the primitive groove appears the rudi- 
mentary mantle, surrounding it and gradually growing together 
over it, which separates by constriction at first from the ventral 
side, but afterwards and more slowly also from the dorsal side. 
Then there appear one after the other the eye-ovals, the 
rudiment of the anterior part of the intestinal tract, the paired 
rudiments of the branchiz, funnel, arms, and auditory organs, 
and in the original solid anal tubercle the pit-like depression, 
which is afterwards converted into the ink-bag and the hinder 
part of the intestinal canal (rectum). 
At a later period than the above-mentioned organs, the 
central organs of circulation (auricles, ventricles, &c.) and 
those of the nervous system (the paired ganglia optica, cere- 
bralia, pedalia, visceralia, buccalia, and stellata, and the un- 
paired ganglion splanchnicum) make their appearance. 
All the organs appearing in the sequence just indicated are 
developed from three different germ-lamellz in one of two 
ways :—either as a local thickening (excrescences and internal 
thickenings) sometimes of the upper, sometimes of one or 
other layer of the middle lamella, or as an invagination or 
