230 



THE ELASMOBRANCH FISHES 



cells (fig. 208b) which are modifications of the cells bounding the central canal 

 or neurocoele. The processes from these cells often reach entirely across to 

 the external margin of the cord. The second type of supporting cell is the 

 neurogleal cell which maj^ take the form shown in figure 208a, or its processes 

 ma^- be long. 



Development of Central Nervous System 



In its origin the nervous system is laid down as a flattened, horseshoe-shaped 

 plate of cells, of ectodermal origin, which later becomes spatulate (fig. 209). 

 The broad end of the spatula, which will later form the brain, differs consider- 

 ably from the narrower handle which is the 

 , '' rudiment of the spinal cord. The margins of the 



plate (i?./., fig. 209) fold up and the whole 

 l)late then sinks down along the middorsal line. 

 In the region of the brain, where the plate 

 widens, the li]« of the plate close slowl.y; but in 

 the region of the cord the closure goes on with 

 more rapidity. Even before the closure of the 

 plate, however, outpocketings to form the primi- 

 tive optic vesicles (op.r.) appear. The plate 

 then closes into a tube, the point at which 

 closure last takes place remaining as the neuro- 

 pore (np., fig. 212). 



The tube thus formed is divided into the 

 ])rimitive forebrain, the midbrain, and the 

 hindbrain. By further development the fore- 

 brain and the hindbrain are separated into four 

 of the permanent segments of the adult brain, 

 while the mesence^^halon does not divide 

 further. The prosencephalon or forebrain becomes the telencephalon and 

 diencephalon and the hindbrain or rhombencephalon becomes the metenceph- 

 alon and myelencephalon. Thus the five divisions of the adult brain which we 

 have described for Hepfanchus are formed. 



Fig. 209. L'tvelojiment of medul- 

 lary plate, Acanfhias. (From 

 Locy.) 



«./., neural fold ; op.v., optic 

 vesicle. 



GEOSS FORM OF BRAIN 



Superficially, different Elasmobranch ])rains present very different appear- 

 ances. This difference is due largely to the condition of the olfactory tracts and 

 their appended olfactory bulbs. The bulbs, arising as outgrovv^ths from the 

 prosencephalon, may still be practically in contact with it in the adult. In 

 Scijmvus (fig. 213a) they may extend but a short distance out from the pros- 

 ence})halon. In other types, however, as in Hepfanchus, they may be drawn 

 far forward so as to be remotely removed from their place of origin (Squatina, 

 Laemargus horeolis, and especially Hexanchus) . In addition the olfactory 

 tracts may be widelj' divergent as in Raja (fig. 211b) and Zygaena, or they 



