210 M. Ussow’s Zoologico-Embryological Investigations. 
verted into a tube in the manner above mentioned, consist of 
a single layer of cells * of the upper germ-lamella ; whilst in 
the oval fold (rudiment of the mantle), besides the elongated 
cylindrical cells situated at its surface, there are also two layers 
of cells of the middle germ-lamella. The first of these layers 
(dermo-muscular layer), constantly increasing with the deve- 
lopment of the fold, becomes more than one-layered under its 
margins t, and therefore also thicker; and this thickening is 
the immediate cause of the eversion of the fold over the blasto- 
derm and its constriction on the ventral side. 
Besides the above-mentioned organs, the rudiments of eye- 
ovals and of the buccal orifice make their appearance at this 
time. The buccal orifice, which can only be recognized with 
some trouble from without, appears in longitudinal sections of 
this stage as a very shallow depression of the upper germ- 
lamella. The rudiments of the eyes, which lie symmetrically 
on the sides of the dorsal surface, are developed chiefly from 
the elongated cells of the upper germ-lamella, the single 
series of which forms a longish oval convexity ¢ above the 
blastoderm. 
The Cephalopod embryo, freed from the nutritive vitellus 
in the manner already described (see p. 100, note fT), in this 
first stage of the production of the organs has the form of a 
convex disk, or rather of a hollow hemisphere, composed of 
more than one layer and more or less thickened in many 
places. The earliest and most considerable thickening corre- 
sponds to the scutiform mantle-rudiment, pointed on the dorsal 
surface, and curvilinearly bounded on the ventral side by 
the above-described rhomboidal groove, which in transverse 
* The cylindrical cells lining the bottom of the groove are rather tall, 
whilst the layer which covers the groove and subsequently grows together 
consists of small flat cells. Some agreement in the production of this 
groove and that of the intestino-glandular [epithelial] layer of certain 
animals (e. g. the Arthropoda), and the great resemblance of its cells 
underlying the upper germ-lamella to those of that layer, at first led me 
astray, and made me think that perhaps in the Cephalopoda also a portion 
of the intestinal tract is formed as in the Crustacea (see the remarkable 
Russian memoir of Bobrezky, “On the development of <Astaeus and 
Palemon”). It was only a long series of repeated observations that 
convinced me of my original error. 
+ The part of the dermo-muscular layer which is situated between the 
groove and the surface of the mantle becomes converted (in the third 
period) into the cuéis with its muscular and fibrous layer. 
-{ This mode of development of the primitive eye-ovals, which are 
soon covered by a second fold of the upper lamella and then gradually 
begin to sink, has been quite correctly observed by Metschnikoff in Seprola 
(2. c. pp. 48-49). As regards the other Cephalopoda, it is confirmed by 
my investigations ; and consequently Kolliker’s (/. c. p. 99) and Hensen’s 
(Zeitschr. fiir wiss. Zool. Bd. xv. p. 183) statements prove to be erroneous. 
