DEVELOPMENT OF THE BRAIN IN THE FCETUS. 595 



This represents the condition of the brain in the lowest 

 class of fishes. Of these ganglia, (of which those of one 

 side are well shown at A, fig. 226 taken from the work of 

 Tiedeniann,) the two lower pairs represent the medulla oblon- 

 gata ; the two next above the lobes of the cerebellum ; the two 

 middle ones the mass of the tubercula quadrigemina which 

 is of larger size than any of the rest ; the fourth the optic 

 thalami or posterior ganglia of the brain proper, and the up- 

 permost of all, the copora striata, or anterior ganglia of 

 the brain, which are even so early as this covered over on 

 each side with a thin membranous sac, filled with fluid, and 

 constituting the rudimental hemispheres of the brain. These 

 parts are all curved upon each other, so as to correspond with 

 the direction of the future cranium. 



— In the progress of development, the three lower sets of 

 ganglia speedily become united together on the median line, 

 leaving within cavities filled with fluid, continuous with each 

 other, connected behind with a canal formed in the same way in 

 the centre of the spinal marrow, and with a larger cavity in front 

 formed by the membranous sac of the hemispheres. This latter 

 cavity represents the common hollow of the ventricles, which as 

 yet, shows no mark of subdivision. 



— A further step of development is seen at B, which is a 

 representation of the brain and spinal marrow of a fcEtus 

 of twelve weeks. The olfactory, the optic, and some other 

 nerves of the brain are now found connected with it, and the 

 spinal accessory is seen at the lower part. At this period of 

 growth it may be considered the type of the same parts, as they 

 exist in their perfect state, in the amphibia. 

 — At C, we have a view of the same foetal brain from its upper 

 aspect. The lower surface of the same parts would now 

 if examined, exhibit the crura cerebelli coming off" exclusively 

 from the medulla oblongata, and the crura cerebri, extended 

 beneath the mass distinguished as that of the tubercula quad- 

 rigemina, to the thalami and corpora striata. If the membra- 

 niform hemisphere be laid open from above, we see at the bot- 

 tom of the cavity the corpora striata, which are curved like an 

 arch round the crura cerebri, the fibres of which radiating for- 



