THE NERVOUS SYSTEM. .'J( ).'i 



Hi' two parts connected together by a commissure which at first is 

 broad but narrows later; these are the two halves of the cerebral 



ganglion, which become detached from the superficial ectodermal 

 layer and, in sagittal section (Fig. 146 />', eg), appear as spindle- 

 shaped bodies. The ectoderm, at the formative centre of the 

 cerebral ganglia (above the mouth) is now again a thin layer (Fig. 

 14li A' and 0). In the median section (C), the cerebral commissure 

 (c) can here be recognised. 



The paired rudiment of the cerebral ganglion was early observed 

 (by the above-named zoologists as well as by Geenachee). The 

 two ganglia greatly increase in size and finally fuse together. In 

 consequence of processes of growth, the relative position of the 

 cerebral ganglion and the mouth is modified in a striking manner 

 (Fig. 133, p. 283). 



The optic ganglia also form as massive thickenings of the ectoderm 

 in the cephalic region. They are directly connected with the ecto- 

 dermal thickenings which become the cerebral ganglia and soon press 

 in below the optic vesicles, a process which evidently gave rise to the 

 statement that the optic ganglion arises on the posterior side of the 

 optic vesicle, from the mesodermal cell-elements there present. The 

 optic ganglion soon shows a close connection with the optic vesicle, 

 a connection which leads to the formation of the optic nerve. 



The pedal and the pleuro-visceral ganglia, like the other ganglia, 

 are said to originate through differentiation of the cell-masses lying 

 between the ectoderm and the alimentary canal. In reality they are 

 due to ectodermal thickenings in the ventral part of the embryo. 

 The pleuro-visceral and pedal ganglia, like the cerebral ganglion, are 

 each composed of two parts of distinct origin. The two cell-masses 

 which yield the pleuro-visceral ganglion lie behind the otocyst, and 

 the masses that produce the pedal ganglion in front of that vesicle 

 (Bobretzky, Ussow, Fig. 133, p. 283). 



During the course of development, the three ganglia just mentioned 

 gradually shift nearer one another and, as the yolk-sac disappears, 

 move towards the stomodaeum, where they finally fuse, the pleuro- 

 visceral and pedal ganglia uniting to form the sub-oesophageal mass 

 which is connected with the cerebral ganglion by two short com- 

 missures, a broad posterior and a narrow anterior commissure. 



The anterior section of the sub-oesophageal mass, which is, to a 

 certain extent, marked off from the rest, and into which the narrow 

 commissure enters, is distinguished as the brachial ganglion. This 

 part becomes differentiated from the pedal ganglion at a time when 



