274 ZOOLOGY. 



Here we need only mention the development of the dorsal 

 and ventral fissures of the spinal cord. The dorsal fissure 

 appears to represent, as in Amphioxus, the dorsal portion 

 of the original central canal that becomes obliterated partly 

 by mere opposition of its side- walls, and partly by the 

 growth across of decussating nerve-fibres. Fig. 137 shows 

 how large the central canal is at first, and how the sides of 

 its dorsal portion tend to come in contact. The way the 

 dorsal fissure seems to open out into the fourth ventricle is 

 also in accordance with this view of its origin. The ventral 

 fissure, on the other hand, is the space left between the 

 right and left masses of white matter that are formed 

 ventral to the original neural tube. 



9. Ganglia. The origin of the ganglia is very interesting. 

 About the time the neural folds are closing in, in the chick, a ridge 

 of epiblast-cells appears alongside the dorsal end of each fold. When 

 the folds close these ridges unite as the neural crest (fig. 120 : it 

 should have been shown also in figs. 130 and 137, in a similar position). 

 It is from the neural crest that all the nerve-cells in the ganglia 

 spinal and sympathetic are formed. The neural crest soon leaves 

 its position on the dorsal side of the neural tube, and moves to the 

 side, being at the same time metamerically segmented each segment 

 corresponding to a mesoblastic somite. Next, each of these masses of 

 nerve-cells divides into two one remaining where it was, to become 

 a spinal ganglion; the other moving to a more ventral and more 

 median position, where it becomes a sympathetic ganglion. The 

 cells of the former ganglion become bipolar, and send out one 

 axis-cylinder-process into the spinal cord, the other in the opposite 

 direction thus the dorsal root is established. The two axis-cylinders 

 at first arise from opposite ends of the bipolar cell, but eventually 

 they join together and the characteristic unipolar cell is produced. 

 The motor roots of the spinal nerves grow out from the spinal cord 

 at a later stage. 



In the brain-region, the origin of the nerves is less clear, owing to 

 the obscured metamerism of this region. It seems certain, however, 

 that all the ganglia in the head are also derived fro'm the neural 

 crest, and from these ganglia are derived all the sensory fibres of 

 mixed nerves and sensory nerves, except the optic the olfactory 

 being doubtful, and the auditory ganglion being incorporated in the 

 brain. The relation of the cranial nerves to the obscured meta- 

 merism of the head, and to the nerves of Amphioxus where the 

 metamerism is not obscured, are subjects of very great interest, but 

 beyond the scope of this work. 



It should be noted that the neurilemma of all nerve-fibres is 

 derived from mesoblast. 



