THE FA 7 IDE NCR OF THE CENTRAL NERVOUS SYSTEM 45 



consequently there is no posterior fissure formed ; but, on the contrary, 

 the dorsal roof, not enclosed by the nerve-masses, remains epithelial, 

 and so forms the membranous roof of the fourth ventricle and of the 

 other ventricles of the brain (Fig. 21, B (3)). In the higher animals, 

 owing to the development of the cerebrum and cerebellum, this 

 membranous roof becomes pushed into the larger brain cavity, and 

 thus forms the choroid plexuses of the third and lateral ventricles. 

 In the lower vertebrates, as in Ammoccetes and the Dipnoi, it still 

 remains as a dorsal epithelial roof and forms a most striking 

 characteristic of such brains. 



In this part of the nervous system, then, the nervous material is 

 all grouped in its original position on the ventral side of the tube ; 

 and yet it is the same nervous material as that of the spinal cord, 

 all the elements are there, giving origin here to the segmental cranial 

 nerves just as lower down they give rise to the segmental spinal 

 nerves, connecting together the separate segments each with the other 

 and all with the higher brain-centres — the supra-infundibular centres 

 — just as they do in the spinal region. 



Why should there be this striking difference between the 

 formation of the infra-infundibular region of the brain and that of 

 the spinal cord ? Do the advocates of the origin of vertebrates from 

 Balanoglossus give the slightest reason for it ? They claim that their 

 view also provides a tubular nervous system for the vertebrate, but 

 give not the slightest sign or indication as to why the nervous 

 material should be grouped entirely on the ventral side of an 

 epithelial tube in the infra-infundibular region and yet surround 

 it in the spinal cord region. And the explanation is so natural, 

 so simple : embryology does its very best to tell us the past history 

 of the race, if only we look at it the right way. 



The infra-infundibular nervous mass is naturally confined to the 

 ventral side of the epithelial tube, because it represents the infra- 

 cesophageal ganglia, situated as they are on the ventral side of the 

 cephalic stomach, and, owing to the size of the stomach, they could 

 not enclose it by dorsal growth, as they do in the case of the forma- 

 tion of the spinal cord (Fig. 21, B (1)). Still these nervous masses 

 have grown dorsalwards, have commenced to involve the walls of 

 the cephalic stomach even in the lowest vertebrate, as is seen in 

 Ammoccetes, in which animal a ventral portion of the epithelial 

 bag has been evidently compressed and its lumen finally obliterated 



