328 



THE MORPHOGENESIS OF THE CENTRAL- NERVOUS SYSTEM 



optic nerve. The walls of the brain show a distinct differentiation in 

 certain regions. This is especially marked 'in the myelencephalon, which 

 has a thicker ventro-lateral wall and thinner dorsal wall. 



Embryos of 10.2 mm. show the structure of the brain at the begin- 

 ning of the second month (Figs. 341 and 344). The five brain regions 

 are now sharply differentiated externally, but the boundary line between 

 the telencephalon and diencephalon is still indistinct. The telencephalon 

 consists of paired, lateral outgrowths, the anlages of the cerebral hemispheres 

 and rhinencephalon (olfactory brain). In Fig. 359 the external form of 

 the brain is seen with the origins of the cerebral nerves. It will be noted 



Diencephalon 



Pallium 



Mesencephalon 



Cephalic 

 jlcxur'e 



B 



Thai-am us 



Pallium 



Mesencephalon 



Opt: 

 cup 



Ponline flexur- 



Myelencephalon 



Meten- 

 cephalon 

 Corpus striatum 

 Optic recess 



Hypothalamus' 



Medulla oblongata 



FIG. 331. Reconstructions of the brain of a 7 mm. human embryo (His). A, Lateral view; 



B, in median sagittal section. 



that, with the exception of the first -four (the olfactory, optic, oculomotor, 

 and trochlear), the cerebral nerves take their superficial origin from the 

 myelenc ephalon . 



The cephalic flexure forms a very acute angle, and, as a result, the 

 long axis of the fore-brain is nearly parallel to that of the hind-brain 

 (Fig. 359). The oculomotor nerve takes its origin from the ventral wall 

 of the mesencephalon. Dorsally, there is a constriction, the isthmus, 

 between the mesencephalon and metencephalon, and here the fibers of 

 the trochlear nerve take their superficial origin. The dorsal wall of the 

 myelencephalon is an exceedingly thin ependymal layer which becomes 

 the tela chorioidea. The ventro-lateral walls of this same region, on the 

 other hand, are very thick. 



A median sagittal section of a brain at a somewhat later stage shows 

 the cervical, pontine, and cephalic flexures well marked (Fig. 332; the 



