THE BRAIN 



331 



spinal cord, cranially into the cerebral aqueduct. The increase in the flexures of 

 the brain and the relative growth of its different regions may be seen by comparing 

 the brains of embryos of four, five, and seven weeks (Fig. 333) . 



Diencephalon 



I 



Mesencephalon Isthmus 



Metencephalon 



Telencephalon 



Okc vesicle 



Myelencephalon 



Cervical flexure 



Mesencephalon Isthmus Metencephalon 



, i 



Dienccpluilon- 



'%, "Myelencephalon 



Telemcphalon — \ 



Mesencephalon 



Diencephalon 



Telencephalon- 



o pf b 



Fig. 333. — Brains of human embryos, from reconstructions by His: A, 4.2 mm. embryo (X 20); 

 B,6.9 mm. embryo (X 16); C, 18.5 mm. embryo (X 4). 0, Optic vesicle; in, infundibulum; m, mam- 

 millary body; pf, pontine flexure; ol, olfactory lobe; 6, basilar artery; p, Rathke's pouch (American 

 Text-Book of Obstetrics). 



In the table on page 332 are given the primitive subdivisions of the neural 

 tube and the parts derived from them: 



