NERVOUS SYSTEM OF THE VERTEBRATA. 437 



there grow in amongst them numerous vascular and connective- 

 tissue elements. The process of the infundibulum thus meta- 

 morphosed becomes inseparably connected with the true pituitary 

 body, of which it is usually described as the posterior lobe. The 

 part of the infundibulum which undergoes this change is very 

 probably homologous with the saccus vasculosus of Fishes. 



The true nature of the pituitary body has not yet been made out. It is 

 clearly a rudimentary organ in existing craniate Vertebrates, and its 

 development indicates that when functional it was probably a sense organ 

 opening into the mouth, its blind end reaching to the base of the brain. No 

 similar organ has as yet been found in Amphioxus, but it seems possible 

 perhaps to identify it with the peculiar ciliated sack placed at the opening 

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

 p. 1 8. If the suggestion is correct, the division of the body into lobes in 

 existing Vertebrata 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 DuthiersHn 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 possibility 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 

 separately 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 through- 

 out 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. 



1 " Les Ascidies simples des Cotes de France." Archives de Biologie exptr. et 

 generale^ Vol. in. 1874, p. 329. 



