234 NIKOLAS KLEINENBERG. 



enough to give a clear idea of the way in which the rudi- 

 ment of the cephalic ganglion is developed. In the first 

 place, it is clear that it originates in the ectoderm, and in 

 the ectoderm alone. In a narrow transverse tract, close to 

 the apex of the head, the cells of the simple layer of ecto- 

 derm divide, and group themselves into the form of a short 

 and slightly curved arch. This, increasing in thickness and 

 becoming distinctly separated from the peripheral layer of the 

 ectoderm, extends along the lateral walls of the cephalic 

 zoonite, but still more behind, where it ends on each 

 side in a conspicuous club-shaped enlargement ; it thus 

 assumes a shape which may be compared to a hernia- 

 truss with a cushion on each side, which embraces the 

 upper half of the cephalic cavity and of the oesophagus, 

 being directed obliquely from above downwards and from 

 behind forwards. From the beginning till it has reached a 

 considerable development the rudiment of the cephalic 

 ganglion is without any connection with the ganglia of the 

 ventral chain. 



I confess that I expected something different. The nature 

 of the adult organ, the mode of formation of the ventral gan- 

 gliated cord, and more general considerations, led to the 

 anticipation of a double rudiment as the first sign of the 

 central nervous apparatus of the head. But, on the other 

 hand, my observations agree with what was before known of 

 the development of the cerebral ganglion of the Hirudmea. 

 This only consists, it is true, of a short notice by Rathke for 

 Nephilis, and of a still shorter one by Leuckart for Hirudo 

 medicinalis. Rathke affirms that the rudiment of the cere- 

 bral ganglion is an arch placed on the upper side of the 

 oesophagus, without connection with the ventral germinal 

 streak.^ Leuckart also says that the formation of this organ 

 occurs, independently of the germinal streak, by the appear- 

 ance of a cellular cord, Avhich embraces the buccal aperture 

 and adapts itself to the anterior ends of the streak, without 

 at first uniting with it. He further adds that, in a subse- 

 quent stage, two lateral swellings are found united by means 

 of a pretty large commissure, both to each other and to the 

 anterior processes of the first ventral ganglion." These short 

 notices, which do not take account of the embryonic layers, 

 are not founded on investigations carried out by means of 

 sections, and are not illustrated by any figures, certainly 



1 Loc. cit., pp. 49, 50. Recently Biitscbli has upheld the truth of 

 Rathke's observations ('Zeit. fiir Wiss. Zool.,' T. xxix, ]877, p. 248. 



2 'Die menschlicheu Parasiten,' T. i, Leipzig uud Heidelberg, 1863, 

 p. 705. 



