THE NERVOUS SYSTEM. 501 



The Diencephalon. 



The stage of development of the diencephalon at four weeks has already 

 been mentioned (p. 448). (Figs. 423, 433 and 434.) In the lateral walls the 

 principal feature is the presence of a furrow, the sulcus hypothalamicus, which 

 beg : ns ventrally as an extension of the optic recess and extends dorsally and 

 caudally toward the mid-brain. A branch of it extends to the posterior part 

 of the foramen of Monro. This is the sulcus Monroi. The sulcus hypothala- 

 micus is sometimes regarded as the representative in this region of the sulcus 

 limitans. It is doubtful whether it has the same morphological value as the 

 latter. Such a comparison is seen a priori to be difficult when it is considered 

 that this region is in the most highly modified part of the brain tube, lacking 



Ma. 



FIG. 423^ Transverse section through the diencephalon of a 5 weeks' human embryo. Dp., Roof 

 plate; Ma., mammillary recess; P.s. hypothalamus; S.M., sulcus hypothalamicus; Th., 

 thalamus. His. 



motor peripheral apparatus, and that it is also the end region of the tube where 

 all longitudinal divisions would naturally merge. The sulcus deepens till the 

 end of the second month (Fig. 429). Later it becomes shallower, but appears 

 to persist till adult life. The region of the diencephalon ventral to the sulcus, 

 as already mentioned, is termed the pars subthalamica or hypothalamus. The 

 ventral part of the optic stalk forms a transverse groove in the floor, the pre- 

 optic recess, caudal to which is a ridge or fold, the chiasma swelling, in which the 

 fibers of the optic chiasma later appear.* Caudal to this is the recess or invagi- 

 nation of the floor, representing the postoptic recess and the beginning of the 

 infundibulum (Figs. 424 and 425) . Its extremity later becomes extended into the 

 infundibular process, the posterior part of which in the fifth week comes into 

 contact with the hypophyseal (Rathke's) pouch. This is a structure formed 



* According to Johnston, the chiasma is formed in front of the optic recess which would then be 

 represented by the postoptic recess. In this case the chiasma would be regarded as falling in the 

 region of the telencephalon instead of forming the optic part of the hypothalamus (comp. Figs. 364 

 and 433). 



