ii NERVOUS SYSTEM 89 



on the constriction between thalamencephalon and mesencephalou 

 becomes more marked. The roof of the former remains thin and 

 membranous, forming the cushion-like dorsal sac upon which the 

 pineal body rests. The roof of the meseucephalon becomes slightly 

 thickened on each side of the mesial plane forming the tectum 

 opticum l)ii t correlated with the small size of the eyes in Lepidosiren 

 the thickening never becomes so great as to produce projecting optic 

 lobes such as are formed in most Vertebrates. 



In the hind-brain region the greater part of the roof, covering 

 in the fourth ventricle, becomes thin and membranous. Across the 

 anterior boundary of the hind- brain the roof does not undergo this 

 secondary process of thinning but persists as a transverse thickened 

 band the rudiment of the cerebellum. 



SUBSEQUENT DEVELOPMENT OF THE BRAIN REGIONS 



RHOMBENCEPHALON or Hind-Brain. The hind -brain, correlated 

 perhaps with the fact that it contains nerve-centres of supreme 

 importance to life, develops precociously and reaches a relatively 

 enormous size during early stages (Fig. 51, A, rk). The bulging 

 inwards which marks its anterior limit is doubtless to be regarded 

 as an expression of the active growth in length of its floor during 

 these early stages. 



During later stages of development it forms a conspicuous pro- 

 jecting restiforrn body on each side reaching forwards nearly to the 

 anterior limit of the mesencephalon but this becomes again less and 

 less prominent as the adult condition is approached. The cerebellum 

 retains through life its primitive condition as a simple transverse 

 thickening of the hind-brain roof. 



MESENCEPHALON. The roof, as already indicated, becomes thick- 

 ened somewhat on each side (tectum opticum) but not to such an 

 extent as to bulge outwards and form optic lobes. Close to its 

 anterior limit a conspicuous bridge of transversely -running nerve- 

 fibres makes its appearance at a late stage of development. This is 

 the posterior commissure an important brain landmark (Fig. 

 53, G, p.c). 



THALAMENCEPHALON. The side wall of the thalamencephalon 

 becomes greatly thickened to form the optic thalamus which bounds 

 on each side the slit-like third ventricle. The roof becomes for the 

 most part thin and membranous forming the dorsal sac. On either 

 side of the pineal body however it becomes greatly thickened to form 

 the habenular ganglion. As these ganglia develop a bridge of 

 transverse nerve -fibres makes its appearance uniting them the 

 superior or, better, habenular commissure. 



The pineal body as development goes on enlarges somewhat and 

 assumes a carrot shape. Its lumen becomes obliterated posteriorly 

 so that it no longer opens into the third ventricle. The anterior 

 isolated part of the cavity becomes eventually almost filled with 



