360 



THE CEREBRAL HEMISPHERES. 



of the pharynx in the Tunicata, the development of which was described 

 at p. 15. If the suggestion is correct, the division of the body into lobes 

 in existing Yertebrata must be regarded as a step towards a retrogressive 

 metamorphosis. 



Another possible view is to regard the pituitary body as a glandular 

 structure which originally opened into the mouth in the lower Chordata, 

 but which has in all existing forms ceased to be functional. The intimate 

 relation of the organ to the brain appears to me opposed to this view of 

 its nature, while on the other hand its permanent structure is more easily 

 explained on this view than on that previously stated. In the Ascidians 

 a glandular organ has been described by Lacaze Duthiers ' in juxtaposition 

 to the ciliated sack, and it is possible that this organ as well as the 

 ciliated sack may be related to the pituitary body. In view of this pos- 

 sibility further investigations ought to be carried out in order to determine 

 whether the whole pituitary body is derived from the oral involution, or 

 whether there may not be a nervous part and a glandular part of the organ. 



The Cerebral Hemispheres. It will be convenient to treat sepa- 

 rately the development of the cerebral hemispheres proper, and that 

 of the olfactory lobes. 



Although the cerebral hemispheres vary more than any other part 

 of the brain, they are nevertheless developed from the unpaired 

 cerebral rudiment in a nearly similar manner throughout the series 

 of Vertebrata. 



In the cerebral rudiment two parts may be distinguished, viz. the 

 floor and the roof. The former gives rise to the ganglia at the base of 

 the hemispheres — corpora striata, etc. — the latter to the hemispheres 

 proper. 



The first change which takes place consists in the roof growing 

 out into two lobes, between which a shallow median constriction makes 

 its appearance (fig. 257). The two lobes thus formed are the rudi- 

 ments of the two hemispheres. 

 /r- 



/^^ 



x>.: 



V-~ 



CO 



The cavity of each of them opens 

 by a widish aperture into the 

 vestibule at the base of the 

 cerebral rudiment, which again 

 opens directly into the cavity of 

 the third ventricle (3 v). The 

 Y-shaped aperture thus formed, 

 which leads from the cerebral 

 hemispheres into the third ven- 

 tricle, is the foramen of Munro. 

 The cavity (Iv) in each of the 

 rudimentary hemispheres is a 

 lateral ventricle. The part of 

 the cerebrum which lies between 

 the two hemispheres, and passes 

 forv/ards from the roof of the 



^ "Les Ascidies simples des C6tes de France." Archives de Biologic exper. et g4niraU, 

 Vol. III. 1874, p. 329. 



I op.t/i 



Fig. 257. Diageammatic longitudinal 

 hokizontal section through the fobe- 



BEAIN. 



3.r. third ventricle ; Iv. lateral ventricle ; 

 It. lamina terminalis ; ce. cerebral hemi- 

 sphere ; op.th. optic thalamus. 



