THE BRAIN 



3 2 7 



In embryos of 3.2 mm., estimated age four weeks, three important 

 changes have taken place (Fig. 330): (i) the neural tube is bent sharply 

 in the mid-brain region (the cephalic flexure) , so that the axis of the fore- 

 brain now forms a right angle with the axis of the hind-brain; (2) the 

 fore-brain shows indication dor sally of a fold, the mar go thalamicus 

 which subdivides it into the telencephalon and the diencephalon; (3) the 

 lateral walls of the fore-brain show distinct evaginations, the optic vesicles, 

 which project laterad and caudad. A ventral bulging of the wall of the 

 hind-brain indicates -the position of the future pontine flexure. 



A nterior 



ncuropore Pallium of telencephalon 



,Diencephalon 



Pallium 



Corpus slrialum 



A nterior 

 neuropore 



Future pontine _$ E^ 

 flexure 



Mesencephalon Optic 



v recess 



Isthmus Future pontine 

 flexure 



Rhombcncephalon 



Mesencephalon 



Cephalic flexure 



: - Rliombenceplialot: 



A B 



FIG. 330. Reconstructions of the brain of a 3.2 mm. human embryo (after His). X about 35. 



A, Lateral surface; B, in median sagittal section. 



In embryos of 7 mm. (five weeks) the neuropores have closed (Fig. 

 331). The cephalic flexure, now more marked, forms an acute angle, 

 and the pontine flexure, just indicated in the previous stage, is now a 

 prominent ventral band in the ventro-lateral walls of the hind-brain. 

 This flexure forms the boundary line which subdivides the rhomben- 

 cephalon 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, the cervical 

 flexure, is the line of demarcation between the brain and spinal cord 

 (cf. Fig. 333 A). The telencephalon and diencephalon are more dis- 

 tinctly subdivided, and the evaginated optic vesicle forms the optic cup, 

 attached to the brain wall by a hollow stalk, in which later grows the 



