520 THE HUMAN EMBKYO. 



almost smooth, the primary sulci having almost completely dis- 

 appeared, and the secondary having not yet appeared. During 

 the latter part of the sixth month, and during the seventh month, 

 most of the principal secondary or permanent sulci appear ; 

 but the majority of the minor or accessory sulci, to which the 

 complex appearance of the adult brain is so largely due, are not 

 formed till after birth. 



The secondary sulci vary considerably in different indi- 

 viduals, and on the two sides of the same brain. The purpose 

 of their formation appears to be to maintain the proportion of 

 the superficial grey matter, relatively to the more deeply placed 

 mass of white matter, in the hemispheres. 



c. The Thalamencephalon. 



The side walls of the thalamencephalon thicken very early, 

 and form the optic thalami, the outer surfaces of which subse- 

 quently fuse extensively with the corpora striata, in the manner 

 noticed above. 



The roof of the thalamencephalon is thin, almost from the 

 first ; it remains flat up to the end of the fourth week, when it 

 becomes folded to form a longitudinal, externally projecting 

 ridge. During the third month this ridge becomes inflected 

 into the ventricle, and vascular folds of connective tissue, growing 

 in between its two layers, give rise to the choroid plexus of the 

 third ventricle. The pineal body does not appear until the end 

 of the fifth, or beginning of the sixth week. It at first projects 

 forwards, but later on becomes directed backwards, and its 

 cavity gradually becomes blocked up by calcareous deposits. 



The floor of the thalamencephalon is separated from the 

 Sylvian aqueduct of the mid-brain by a strong overhanging 

 crest. The floor is at first thin along its whole length, but 

 becomes thickened in front by the optic chiasma, and behind by 

 the corpus mammillare. The infundibulum is a prominent, 

 ventrally directed depression of the floor, which early comes into 

 close relation with the pituitary diverticulum of the stomato- 

 dasum. 



d. The Mid-brain. 



The mid-brain of the human embryo remains small through- 

 out the whole period of development. The roof thickens, but 



