THE BRAIN 



327 



THE BRABM 



We have seen that in embryos of 2 to 2.5 mm. the neural tube is nearly 

 straight, but that its cranial end is enlarged to form the anlage of the brain (Fig. 

 324). The appearance of two constrictions in the wall of the anlage subdivides 

 it into the three primary brain vesicles — the fore-brain or prosencephalon, mid-brain 

 or mesencephalon, and hind-brain or rhombencephalon. 



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

 have taken place (Fig. 330) : (1) 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 



Corpus slrhdum 



Anterior 



neuropore Pallium of telencephalon 



/Diencephalon 



Pallium 



A nterior 

 neuropore 



Future pontine [> 

 flexure 



Mesencephalon Oplu 



rcrcs 



Isthmus Future poulin. 

 flexure 



Rhombencephalon 



Mesencephalon 

 •I 



■Cephalic flexure 



~- Rhombencephalon 



A B 



Fig. 330. — Reconstructions of the brain of a 3.2 mm. human embryo (after His). X about 35. ^, Lateral 

 surface; B, sectioned in the median sagittal plane. 



angle with the axis of the hind-brain; (2) the fore-brain shows indication dorsally 

 of a fold, the margo 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. 



In embryos of 7 mm. (five weeks) the neuropores have closed (Fig. 331). 

 The cephaUc 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. This flexure forms the boundary 

 line which subdivides the rhombencephalon into a cranial portion, the meten- 



