FROM TWELVE TO THIRTY-SIX SOMITES 155 



and shows indication of branching. Subsequently it becomes 

 much branched and quite massive and unites with the infun- 

 dibulum to form the pituitary body. (See Chap. VIII.) 



The Mesencephalon. This portion of the brain comes to 

 occupy the summit of the cranial flexure, which indeed owes its 

 origin largely to the rapid growth in extent of the roof of the 

 mesencephalon. In longitudinal section it thus appears wedge- 

 shaped, with short floor and long arched roof (Figs. 87 and 

 88). Its walls remain of practically uniform thickness up to 

 the seventy-second hour. The lateral walls expand more rapidly 

 than the roof and thus form the optic lobes. But these are 

 barely indicated at the 36 s stage. 



Isthmus. The great expansion of the mesencephalon does 

 not involve the portion immediately adjacent to the hind-brain, 

 which is henceforth knowm as the isthmus (Figs. 87, 88). 



The Rhombencephalon {Primary Hind-brain) . Two divisions 

 of the embryonic brain arise from the rhombencephalon, viz., 

 the metencephalon and the myelencephalon; the former becomes 

 the region of the cerebellum and pons of the adult brain, 

 and the latter the medulla oblongata. The metencephalon is a 

 relatively short section of the original rhombencephalon, and 

 includes only the most anterior neuromere of the rhomben- 

 cephalon or the sixth of the series (Fig. 83 D, E). It may be 

 distinguished at the beginning of the period under consideration 

 by the fact that its roof remains as thick as that of the mesen- 

 cephalon. At the end of this time, i.e., seventy-two hours, the 

 roof in sagittal sections appears to rise sharply from the isthmus 

 and thins towards the summit, where it passes into the thin epi- 

 thelial roof of the myelencephalon (Figs. 87 and 88). The rudi- 

 ment of the cerebellum is slightJy thicker on each side of the 

 middle line at seventy-two hours. 



The myelencephalon becomes sharply characterized by the 

 thinness of its roof and thickening of ventral lateral zones and 

 floor. The epithelial roof has a triangular form, the base resting 

 against the metencephalon. The neuromeres remain very distinct 

 (Figs. 83, 89), but change their form. Up to about tw^enty-three 

 somites they still form external expansions, but as the wall 

 thickens the external surface becomes smooth, and the neuro- 

 meres may now be recognized as a series of concavities in the 

 lateral wall, with intervening projections (Fig. 89). The arrange- 



