CEEKI1KAL HEMISPHERES 115 



develop in the same manner as the lateral, showing at first as internal diverticula 

 and outer prominences on the sides of the fore-brain. In the displacements 

 resulting from the backward growth of the hemisphere they become pushed back 

 so as to be seen on the surface of the mid-brain. 



The floor of the fore-brain early shows a deep depression behind the optic recess, 

 which becomes the infundibulum, and gives origin to the cerebral lobe of the 

 pituitary body. This is at first an open diverticulum (fig. 152), but later it 

 becomes cut off from the cavity by the obliteration of the lumen of the stalk. The 

 vesicle thus formed comes into intimate relation with the epithelial portion of the 

 body, to the posterior aspect of which it is applied, the two becoming bound 

 together by vascular connective tissue. The epithelial portion is formed as a 

 diverticulum of buccal ectoderm from the roof of the stomodceum. The diver- 

 ticulum extends backwards as a flattened cleft (fig. 176, p. 134) which divides 

 into two horns embracing the infundibulum. Towards the end of the second month 



mesencephalon / I cerebral 



cerebellum 



FIG. 158. MODEL OF THE BKAIN OF A FCETUS OF 53 MM. (ELEVENTH WEEK). (His.) 



The right hemisphere has been cut away to show the mesial surface of the left hemisphere ; 

 the corpus striatum is seen arching round the stalk of the hemisphere. 



the walls become folded and epithelial sprouts grow to form a small mass of 

 tubules, the lumen of which is afterwards obliterated. The stalk is separated 

 from the buccal epithelium and becomes absorbed. 



The mamillary bodies are developed from a mesial recess of the floor of the 

 fore-brain between the infundibulum and the anterior basal angle of the floor of 

 the mid-brain. 



Cerebral hemispheres. The cerebral-hemisphere rudiments first appear as 

 shallow bays in the fore-part of the alar laminae (fig. 149). In the roof and 

 anterior wall of the tube the walls of the bays run directly into one another, so that 

 the two rudiments appear as a single anterior swelling of the fore-brain (fig. 148). 



This, with the fore-part of the anterior extremity of the basal part of the fore-brain, is often 

 termed the telencepkalon, while the remainder is called the diencephalon. Opinions differ 

 as to whether the hemispheres are to be looked on as separate lateral diverticula, or as two 

 lobes of a single rudiment. 



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