THE NERVOUS SYSTEM 135 



growths from the roof of the brain, of which one, the pineal 

 organ, can with certainty be regarded as originally a sense organ. 



[Locy, from the study of young shark embryos, has adduced 

 reason for believing 1 that, at an early stage in development, two 

 pairs of accessory optic vesicles appear, concurrently with those 

 giving rise to the retinae of the paired eyes. The ultimate fate 

 of the former has yet to be fully worked out, and nothing is 

 as yet known concerning the post-embryonic development of the 

 paraphysis. There is, however, reason for thinking that the 

 latter probably takes part in the formation of the choroid plexus ; 

 but whether this is the case or not, Locy's observation seems to 

 indicate that the pineal organ at least may have been originally 

 paired.] 



At the under surface of the thalamencephalon, and connected 

 with the infundibulum, there lies an appendage of the brain called 

 the hypophysis or pituitary body. 



Two distinct structures enter into the formation of this 

 organ, one glandular and the other nervous. The former arises 

 in Man and the higher Vertebrates bylT constriction from the 

 primitive mouth sac (stomodseum) of the embryo, and the latter 

 is, as a rule, assigned genetically to the floor of the thalamen- 

 1 cephalon. Future research must show how far this is the primary 

 origin of at least the glandular portion of the organ, and this is 

 the more desirable since some very interesting results recently 

 obtained by von Kupffer, from the study of Lamprey and Sturgeon 

 embryos, have given new zest to the inquiry into the primitive 

 history of this enigmatical structure. The subject cannot be 

 dealt with in detail here, but mention, may be made of at least 

 a few of the chief points concerning it. 



According to von Kupffer, the hypophysis arises in the 

 above-named Fishes in the manner described by Scott for the 

 Amphibia (Amblystoma). At a very early embryonic stage an 

 ectodermal cell-strand grows in from the anterior region of the 

 head. This cell-strand in the Sturgeon consists of two closely 

 applied epithelial plates which form a fold, and at the point 

 at which it arises the antero- dorsal border of the fore-brain 

 is connected with a thickened portion of the ectoderm by an 

 originally hollow and subsequently solid tract. This ectodermal 

 thickening is termed by von Kupffer the median olfactory 

 plate, and the corresponding cerebral outgrowth the lobus 

 olfactorius impar : in fact, according to this author, the Sturgeon, 

 1 [Anat. Anzeiyer, vol. ix. p. 169.] 



