SCYLLIUM CANICULA 255 



concerned with the sense of smell, and this is the sense that plays a 

 large part in procuring the fish's food. Thus it is that it is relatively 

 larger than in Rana. The cerebellum, on the other hand, is the 

 great centre for the co-ordination of movement, and is also closely 

 related to the lateral line sense organs. Hence, in an active animal 

 like the dogfish, this part of the brain, too, is more developed than 

 in the frog. Tracts of nerve fibres run from the cerebrum to the 

 optic thalami, and thence through the crura cerebri to the cere- 

 bellum, medulla and spinal cord, and so we find a system of fibres 

 linking up the various sensory and motor centres of the brain and 

 allowing of a central co-ordination of activities. 



Before leaving the brain it may not be out of place to glance 

 briefly at its mode of development, since it is fairly simple in 

 Scyllium, and, just as in the adult we have a comparatively simple 

 primitive brain providing a ground plan upon which the brains of 

 the higher Chordata can be built, so we find its development pursues, 

 in general, a course followed, with but slight modification, in other 

 forms. It has previously been pointed out that the whole of the 

 central nervous system arises as a tubular structure by the closure 

 of the medullary folds. From the very beginning the anterior end 

 of the medullary plate, lying in the head region, is wider than in the 

 trunk region, and as the concrescence of the folds is proceeding 

 this region dilates to form at first two and, very shortly after, three 

 distinct sac-like enlargements separated from one another by con- 

 strictions. These are the three primary brain vesicles, and are 

 known from before backwards as the fore-brain or prosencephalon, 

 the mid-brain or mesencephalon and the hind-brain or rhombence- 

 phalon, the latter passing over gradually into the spinal cord. They 

 lie on the dorsal side of the embryo above the notochord, which 

 terminates abruptly under the mid-brain. The dorsal side of the 

 fore-brain grows much more quickly than the ventral side, with the 

 result that the prosencephalon becomes bent round over the end of 

 the notochord almost at right angles, thus producing what is known 

 as the cephalic flexure. This is characteristic of most Craniates, but 

 it disappears again in the dogfish as the adult condition is reached. 

 The folds close slowly towards the front end, so that there is left 

 for some time an anterior opening, the neuropore, which, even when 

 it eventually closes, leaves a small depression, the neuroporic recess. 

 The end of the fore-brain below the recess is termed the lamina 

 terminalis. 



Very shortly after the appearance of the three primary vesicles 

 a hollow outgrowth appears on the lateral wall of each side of the 

 fore-brain towards its ventral margin ; this is the primary optic 

 vesicle. As development proceeds this differentiates into a distinct 



