M. Ussow’s Zoologico-Embryological Investigations. 219 
Sepiola. After this preliminary remark I will now describe 
in a few words the sequence in which the ganglia make their 
appearance, their original form, and their original position in 
the Cephalopod embryo. 
I have already mentioned the time of appearance of the 
paired optic ganglia. The cells of the middle lamella, which 
are at first few, but afterwards rapidly increase in number, 
from which the two oval aggregations (the rudiments of the 
above-mentioned ganglia) separate, are observable from the 
earliest appearance of the eye-ovals. At the close of the 
second period these large rudimentary ganglia placed at the 
sides of the broad quadrangular head of the embryo have the 
form of two irregular hemispheres, the convex surface of 
which closely approaches the retina, which is already becoming 
concave, whilst the flat sides are turned towards the rudiments 
of the cerebral and visceral ganglia. The first of these, the 
cerebral ganglion, which appears on the fifth day of the second 
period, likewise originates from two compact aggregations of 
cells of the dermo-muscular layer; and these are united by a 
broad but short commissure consisting of a few layers of 
similar cells. The rudiment of the originally paired cerebral 
ganglion, which is situated dorsally at the sides of the cecal 
rudiment of the cesophagus, constantly becomes broader and 
thicker with the development of the embryo; so that towards 
the end of the third period the commissure of the two halves, 
which was originally well defined, disappears, and the ganglion 
forms a rather large compact mass. Two paired compact 
aggregations of cells of the middle lamella, observable as early 
as the fourth day of the second period, which lie behind the 
rather distant rudiments of the auditory organs, divide gra- 
dually in the first half of the third period, to form the paired 
rudiments of the pedal and visceral ganglia. The two halves 
of the former grow rather rapidly; and in the second half of 
the third period, when the cephalic lobes.approach each other, 
the united two form a crescentic ganglion, occupying the 
greater part of the anterior cephalic lobe, and lying above the 
auditory organs. Its upper part is on the same level as the 
buccal aperture, and somewhat higher than the opposite 
cerebral ganglion, which it touches with its sides. 
The visceral ganglion, lying just behind this, consisting at 
first of two subsequently coalescent halves, is developed in 
the same manner. All the three above-mentioned originally 
paired ganglia (the cerebral, visceral, and pedal ganglia) 
gradually approach each other, and unite to form an cesophageal 
nerve-mass only towards the close of embryonic life. ‘Their 
union takes place very slowly, keeping pace with the diminu- 
