DEVELOPMENT OF ELASMOBEANCH FISHES. 175 



hour-glass shaped section, but the roof has hardly begun to 

 thin out (PL xiv. fig. 4a and 46). 



Durinor stac^es 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 (Ph XIV. fig. 1, 

 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 histoloo-ical chancres of interest have occurred. 



During stage L an apparent rectification of the cranial flexure 

 commences, and is completed by &tage 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 PL XV. fig. la, o and 7a. 



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 thro-ugh 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 Plate XV. 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 thalamencephalon. The thalamencephalon in fig. la, 

 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 

 w^all of the mid-brain and the hind wall of the cerebral hemi- 

 spheres. It is therefore in part by the change in position of the 

 cerebral hemispheres that the angle between the trabeculse and 

 parachordals becomes increased, i.e. their flexure diminished, 

 while at the same time the flexure of the brain itself is in- 

 creased. More important perhaps in the apparent rectification 

 of the cranial flexure than any of the previously mentioned 

 points, is the appearance of a bend in the hind-brain which 

 tends to correct the original cranial flexure. The gradual growth 

 of this fresh flexure can be studied in the longitudinal sections 



