THE EVIDENCE 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-infaundibular 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 
Ammoceetes, in which animal a ventral portion of the epithelial 
bag has been evidently compressed and its lumen finally obliterated 
