398 DEVELOPMENT OF ELASMOBRANCH FISHES. 



by a divarication of the two sides of the hind-brain, and the 

 simultaneous introduction of a fresh structure in the form of a 

 thin sheet of cells connecting dorsally the diverging lateral halves 

 of this part of the brain. By stage I, the hind-brain in Pristiurus 

 also acquires an hour-glass shaped section, but the roof has 

 hardly begun to thin out (PL 15, figs. 4^ and 4^). 



During stages I and K the cranial flexure becomes more and 

 more pronounced, and causes the mid-brain definitely to form 

 the termination of the long axis of the embryo (PI. 15, figs. I, 2, 

 etc.), and before the close of stage K a thin coating of white 

 matter has appeared on the exterior of the whole brain, but no 

 other histological changes of interest have occurred. 



During stage L an apparent rectification of the cranial flexure 

 commences, and is completed by stage Q. The changes involved 

 in this process may be advantageously studied by comparing 

 the longitudinal sections of the brain during stages L, P, and Q, 

 represented in PI. 16, figs, la, 5 and /. 



It will be seen, first of all, that so far from the flexure of the 

 brain itself being diminished, it is increased, and in P (fig. 5) 

 the angle in the floor of the mid-brain becomes very acute 

 indeed ; in other words, the anterior part of the brain has been 

 bent upon the posterior through nearly two right angles, and the 

 infundibulum, or primitive front end of the brain, now points 

 nearly directly backwards. At the same time the cerebral hemi- 

 spheres have grown directly forwards, and if figures la and 

 5 in PI. 1 6 be compared it will be seen that in the older brain of 

 the two the cerebral hemispheres have assumed a position which 

 might be looked on as the result of their having been pushed 

 dorsalwards and forwards against the mid-brain, and Having in 

 the process pressed in and nearly obliterated the original thala- 

 mencephalon. The thalamencephalon in fig. \a, belonging to 

 stage L, is relatively large, but in fig. 5, belonging to stage P, it 

 only occupies a very small space between the front wall of the 

 mid-brain and the hind wall of the cerebral hemispheres. It is 

 therefore in part by the change in position of the cerebral hemi- 

 spheres that the angle between the trabeculae and parachordals 

 becomes increased, i.e. their flexure diminished, while at the 

 same time the flexure of the brain itself is increased. More 

 important perhaps in the apparent rectification of the cranial 



