BRAIN 13 



morphological anterior limit of the head ; l and as far as this the 

 mesoblast is segmented. The great extension forwards beyond this 

 point of both the nervous system and the skull is doubtless related 

 to the great development of the special organs of sense, and of the 

 brain. Indeed, the position of the olfactory, optic, and auditory 

 organs must have been one of the chief factors in influencing not 

 only the growth of the brain, but also of the cartilaginous skull, with 

 its three pairs of protective capsules (Figs. 5 and 6). The nasal 

 capsules develop in front of the trabeculae, with which they generally 

 become continuous, if they are not so from the first. The optic 

 capsules arise independently and remain separate, partially enclos- 

 ing the optic vesicles. The auditory capsules either from the first 

 or very soon are continuous with the parachordal plates. Chondri- 

 fication extends up the sides and over the roof of the cranial brain- 

 cavity more or less completely. The skull acquires further strength 

 and rigidity for the support of the visceral skeleton and the attach- 

 ment of its muscles. 



To protect and support the brain is one of the chief functions 

 of the cranium. The brain in even the lowest known Craniate has 

 made vast advances over that of Amphioxus. Into its minute 

 histological structure it is not possible to enter in this book ; some 

 day, however, the results of a more complete knowledge of the dis- 

 position of its cells and fibres will doubtless be of the greatest 

 importance in the study of phylogeny. The gross subdivisions into 

 which the brain becomes differentiated are of no segmental signi- 

 ficance. But attempts have been made to interpret local aggrega- 

 tions of cells, and certain transverse swellings and constrictions of the 

 neural tube, which appear at a very early stage in ontogeny, as true 

 neural segments or neuromeres (Hoffmann [216], Orr, M'Clure, Locy 

 [284], von Kupffer [275], Neal [308a], Johnston [248a], and others). 

 The observations are somewhat uncertain and contradictory, and 

 these neuromeres do not appear to correspond exactly with the other 

 evidences of segmentation. The subject is too unripe for treatment 

 here ; but it may be stated that if the evidence of the neuromeres 

 is to be trusted, there would appear to be three segments in front 

 of the segmented mesoblast, of which all other trace has been lost. 



The embryonic brain of the Craniate is subdivided into primary 

 fore-brain, mid-brain, and hind-brain. 



Later on the hind-brain forms a posterior myelencephalon or 

 medulla oblongata, and an anterior metencephalon, giving rise 

 above to the cerebellum (Figs. 7, 8, 9, and 10). The hind-brain is 



1 Authors differ on this debatable point. Some place the anterior end of the 

 brain at the infundibulum, others at the edge of the neuropore. Johnston [248a] 

 believes it to lie at a point just behind the anterior commissure, and in front of the 

 optic recess and chiasma. 



