220 M. Ussow’s Zoologico-Embryological Investigations. 
tion of the nutritive vitellus which lies between them in the 
head and in the so-called neck. In the second half of the 
third period the paired rudiments of the superior and inferior 
buccal ganglia make their appearance on each side of the 
- pharynx, composed of small spherical compact aggregations of 
cells of the middle lamella. At the same time and in the 
same manner originate the ganglia stellata, in the position in 
which they are found in adult Cephalopoda—and also the 
large spherical ganglion splanchnicum, which is_ situated 
between the stomach and the two halves of the liver. 
The internal structure of all the above-mentioned ganglia 
begins to become differentiated soon after they make their 
appearance. In the central part of the ganglia, which at first 
consist of rounded homogeneous cells of the middle lamella, 
appears a dark, finely granular mass (“ Punktsubstanz”’), con- 
sisting of very fine variously intercrossed fibrillar threads— 
fine processes of the original cells of the middle lamella, now 
gradually being converted into small brown nerve-cells. As 
early as the close of the first half of the third period, especially 
in the peripheral part of the optic ganglia, in various parts of the 
cerebral ganglia, and subsequently also in all the other ganglia, 
we may distinctly observe the production both of the inner 
thin nerve-bundles serving as commissures to the different 
parts of the ganglia and of those running outwards (e. g. the 
broad but short optic nerves which unite the peripheral part 
of the optic ganglia with the retina). ‘The peripheral nerves 
of the skin are developed towards the close of embryonic life 
independently of the ganglia, at the poimts which they after- 
wards occupy, from the elongated cells of the dermo-muscular 
layer, which unite with each other. 
I have obtained all these briefly reported results chiefly by 
the comparative study of different sections belonging to different 
stages of development, a more or less accurate examination of 
the nervous system in living embryos being almost impossible 
on account of their opacity. As it is rather difficult without 
fizures, to describe the various changes in the form and posi- 
tion of all the parts of the nervous system, I here conclude my 
description of that system, keeping the details for a more 
complete memoir with plates, which will soon appear. 
In all the Cephalopoda investigated by me it is not alone 
the upper germ-lamella, as Metschnikoff thinks *, but also, 
and, indeed, chiefly, the dermo-muscular layer of the middle 
lamella that is implicated in the formation of the different 
dermal layers. The skin begins to be differentiated in the 
first days of the third period (in Loligo and Sepiola approxi- 
mately on the nineteenth, in Argonauta on the fourteenth or 
* Doc. cit. p. 37. 
