184 THE DEVELOPMENT OF THE TADPOLE. 



portion of the medulla — at least, one-sixth of its diameter — and 

 reaches nearly to the spinal cord. 



The changes now undergone, both in form and internal 

 arrangement, of the two kinds of cells, ultimately known as white 

 and grey matter, or medullated and non-meduUated nerve-fibres, 

 of which the brain and spinal cord are composed, are so many 

 and complicated, that readers of this article would be more 

 wearied than edified were an attempt made to chronicle them all. 

 AVe will therefore merely describe some of the more salient 

 features in this organ on three days — one April 30th, when there 

 were no external signs of the hind limbs; another on May nth, 

 when the hind limbs had commenced to show, but were not fully 

 extruded : and an intermediate one of May 4th. The cerebrum 

 first takes a somewhat globular form, and in horizontal section is 

 seen to consist almost entirely of dark cells, but surrounded with 

 whiter ones. This portion of the brain, as well as the whole 

 nerve-cord, it will be remembered, has been double for a long 

 time ; but now a cavity, lined with white cells, forms in each 

 hemisphere, and the grey matter stretches across the centre, forming 

 the lamina terminalis, whilst the side cavities form the lateral 

 ventricles. The mid-brain, containing the optic lobes, is now 

 very elongated, the lobes being well separate from each other, and 

 consisting of two hemispherical masses of dark matter lined with 

 lighter cells, though still of the same general character as the 

 darker ones, and surrounded by a thick mass of most delicate 

 fibres arranged parallel with each other, and apparently similar in 

 structure to the optic nerve, which can be plainly seen branching 

 out from them and proceeding to the retina. Each fibre consists 

 of a delicate thread, which enlarges at unequal distances in its 

 course into an oval, nucleated body. Lines of these long, 

 nucleated fibres also extend around the base of the optic lobes, 

 running in a general direction at right angles to the longitudinal 

 mass. 



On May 4th, horizontal sections show the fourth ventricle to 

 perfection as a triangular cavity with the apex behind, and the 

 front bounded by the bases of the optic lobes. Within this 

 ventricle we find a chain of nerve-ganglia proceeding down its 

 centre, consisting of a delicate fibrous cord with nucleated 



