THE NERVOUS SYSTEM. 477 



ventral wall of the anterior portion (Figs. 416 and 472). In the space thus 

 enclosed the dorsum sellae is subsequently formed. Associated with this 

 increase of the cephalic flexure is an increased prominence of the mid-brain 

 roof. The pontine flexure has begun, there being now a bending of the whole 

 tube in the pons region, the concavity of the bend being dorsal. At the same 

 time there is a corresponding tendency for the roof of the rhombencephalon to 

 become shorter and wider. There is also a further thinning of the above 

 mentioned expanded portion of the roof plate in this region, and associated 

 with this a thrusting of the thick lateral walls outward at the top so that they 

 come to lie almost flat instead of vertically as in the cord. From the cord 

 to the place of greatest width above mentioned, this dorsal thrusting apart 



FIG. 416. Profile view of a model of the brain of a human embryo during the third week. His. \ 

 A, Optic vesicle; A.v., auditory vesicle; Br, pons region; H, pallium; Hh. cerebellum; J, isthmus; 

 M, mid-brain; N and /?/, medulla; XK, cervical flexure; Pm, mammillary region; 2>, in- 

 fundibulum; Z, inter-brain or diencephalon. 



of the lateral rhombic walls obviously becomes more and more pronounced. 

 In front of this region of greatest width, the roof plate becomes narrower and 

 the dorsal parts of the walls (alar plates) form the rudiment of the cerebellum, 

 the rest of the rhombic brain forming the medulla oblongata. Each lateral 

 wall of the rhombic brain is now divided into a dorsal longitudinal zone or 

 plate (alar plate) and a ventral zone or plate (basal plate) by a longitudinal 

 furrow along its inner surface, the sulcus limitans. A study of the external 

 appearances and transverse sections of this part of the brain tube will make 

 these relations clear (Figs. 456, 436 to 439 and 427). Neuromeres are also 

 present at this stage (see p. 489). In the meantime the neural tube has also 

 become bent ventrally at the junction of the brain and cord, forming the cervical 



