M. Ussow's Zoologico-Emhryological Investigations. 211) 



Sepiola. After this preliminary remark I will now dcscrilje 

 in a tew words tlie se([uencc in wliicli the ganglia make their 

 a|)pearance, tlieir original form, and their original position in 

 the Cephalo|)od embryo. 



I liave already mentioned the time of appearance of the 

 paired optic ganglia. The cells of the middle lamelhi, 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 tlie broad quadrangular head of the embryo have the 

 form of two iiTCgular 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 ea3cal 

 rudiment of the assophagus^ 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 ai)proach each other, 

 the united two form a erescentic 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 coaleseent 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 oesophageal 

 nerve-mass only towards the close of embryonic life. Their 

 union takes place very slowly, keeping pace with the dimiuu- 



