567 



They have suffered repression as such in an antero-posterior direction, 

 the survivors at the posterior end of the series yielding the cranial 

 portion of the hypoglossal musculature. But even in the highest Cra- 

 niates more than are required are developed and disappear. Thus 

 also in the cranial region mesodermal somites are developed and dis- 

 appear during ontogeny. 



There is therefore something more than strong presumptive evidence 

 for concluding that the mesencephalon and metencephalon are derived 

 from the trunk region. Indeed the only cranial segments which are 

 pre-oral, that is to say, prostomial in position are the first two proso- 

 meres, the olfactory and optic. All the rest are oral and post- oral, 

 and at the same time parachordal. 



The first segment and the prostomial region concerned have under- 

 gone considerable development in the Craniata, and this with the se- 

 cond segment are primarily preorbital. The orbital region so far as 

 the brain segments are concerned extends from the 1st mesomere to 

 the 6th rhombomere, that is over 8 segments; and from the point of 

 view of somites, from the 2nd to the 8th, seven segments being in- 

 cluded. But these primary segmental relationships undergo a profound 

 change from that most characteristic accompaniment of brain develop- 

 ment, the mesencephalic flexure. 



C. The Mesencephalic Flexure. 



Reference has already been made to the rotation forwards of the 

 lateral portion of the hind brain and the structures which occupy that 

 region of the head. This is associated with the mesencephalic flexure, 

 which produces at the same time an opposite movement afi'ecting the 

 visceral elements. This will be evident from a consideration of Fig. 6, 

 which should be compared with the figure accompanying the paper on 

 the brain of Acanthias. 



The first result which it is desired should be noticed is that the 

 flexure brings the eye into association with the orbital mesoderm as 

 defined above. It is in this region also that the lateral sphenoid 

 plates are developed to which the muscles of the eye are attached. 

 It is quite possible that the development of the latter is associated 

 with the disappearance of the ventral roots except the most anterior 

 and the most posterior of the series, and also with the persistence of 

 the IV th in the region. 



It is evident next that the trabeculae meet and fuse with the 

 parachordals opposite the anterior end of the auditory capsule. Thus 

 the infundibulum comes to lie in a mesodermal region posterior to 

 that to which it belongs, and so does the eye. 



