THE ORGANS OF THE OUTER GERM-LAYER. 449 



frequently (Horse) is even in connection with the anterior cornu by 

 means of a narrow canal in the tractus olfactorius. 



The olfactory lobe (Lol -\-Tro) attains an extraordinary develop- 

 ment (fig. 257) in the Selachia, in which it, exceeds in size the 

 hetween-brain (ZH) and mid-brain (MH\ In the Selachians two 

 long hollow processes (tractus olfactorius, Tro] extend out from the 

 anterior end of the little-developed cerebrum and terminate at a 

 considerable distance from the fore-brain in two large hollow- lobes, 

 that are sometimes provided with furrows (Lol). 



B. The Development of the Peripheral Nervous System. 



Although it is easy to follow the development of the brain and 

 spinal cord, the investigation of the origin of the peripheral nervous 

 system is very difficult, for it requires the study of histological processes 

 of the most subtle nature the first appearance of non-niedullated 

 nerve -fibres and the method of their termination in embryos 

 composed of more or less undifferentiated cells. One who knows 

 how difficult it is even in the adult animal to follow non-medullated 

 nerve-fibrillae in epithelial layers or in non-striate muscle-tissue, and 

 to get a clear idea of their method of termination, will understand 

 that many, and indeed the most interesting, questions in regard 

 to the development of the peripheral nerves are not yet ripe for 

 discussion, because the observations necessary for their settlement 

 are still wanting. There is only one point which is entirely clear. 

 That concerns the development of the spinal ganglia, which His and 

 BALFOUR independently of each other were the first to recognise, the 

 one in the Chick, the other in Selachians. Since then numerous 

 investigations embracing different groups of Vertebrates have been 

 published on this subject by HENSEN, MILNES MARSHALL, KOLLIKER, 

 SAGEMEHL, VAN WIJHE, BEDOT, OSODI, BERANECK, EABL, BEARD, 

 KASTSCHENKO, and others. 



(a) The Development of the Spinal Ganglia. 



The development of the spinal ganglia in the spinal cord is very 

 easily followed. It begins just at the time the medullary groove 

 closes to form a tube (fig. 258 A and B). At this time a thin 

 ridge of cells (spy', spg) one or two layers deep grows out of the 

 neural tube on either side of the line of fusion, and, passing outward 



29 



