220 M. Ussow'd Zoulogico-Emhryoloyical Investigations. 



tion of the nutritive vitcllus which lies between them in the 

 head and in the so-called neck. In the second half of the 

 third period tlie paired rudiments of the superior and inferior 

 buccal (janglla make their ap])earanee on each side of the 

 pharynx, composed of small spherical comjjact aggregations of 

 cells of the middle lamella. At the same time and in the 

 same manner originate the ganglia stellata, in tlie position in 

 which they are found in adult Cephalopoda — and also the 

 large S])herieal ganglion splanchnicuvi, which is situated 

 between the stomach and the two halves of the liver. 



The internal structiu'C 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 roimded homogeneous cells of tlie middle lamella, 

 appears a dark, finely granular mass (" Punktsubstanz "), con- 

 sisting of very line 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 tiie other ganglia, 

 Ave 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 peri])heral nerves 

 of the skin are developed towards the close of embryonic life 

 independently of the ganglia, at the points 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 

 fio-ures, 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 

 • Loc. cit. p. 37. 



