326 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



alic flexure, now more marked, forms an acute angle and the pontine flexure, 

 just indicated in the previous stage, is now a prominent ventral bend in the 

 ventro-lateral walls of the hind-brain (Fig. 313 A, B). This flexure forms the 

 boundary line which subdivides the rhombencephalon into a cranial portion, the 

 metencephalon, and into a caudal portion, the myelencephalon. At a third bend 

 the whole brain is flexed ventrally at an angle with the axis of the spinal cord. 

 This bend is the cervical flexure and is the line of demarcation between the brain 

 and spinal cord. The telencephalon and diencephalon are more distinctly sub- 

 divided, and the invaginated optic vesicle forms the optic cup attached -to the 

 brain wall by a hollow stalk, which later becomes the optic nerve. The walls of 



D'lencephalon 



Cerebellum 



telencephalon 



Medulla 

 oblongata , 



Fig. 313. — Reconstructions of the brain of a 7 mm. human embryo. A, lateral view; B, in median 

 sagittal section (His). Cefh. flex., cephalic flexure. 



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 beginning of the 

 second month (Figs. 323 and 326). In Fig. 341 the external form of the brain 

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

 exception of the first four (the olfactory, optic, oculomotor and trochlear), the 

 cerebral nerves take their superficial origin from the myelencephalon. The five 

 brain regions are now sharply differentiated externally but the boundary line 

 between the telencephalon and diencephalon is still indistinct. The telencephalon 



