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’ that, at an early stage in development, two 
pairs of accessory optic vesicles appear, concurrently with those 
giving rise to the retine 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 by a constriction from the 
primitive mouth sac (stomodzum) of the embryo, and the latter 
is, aS a rule, assigned genetically to the floor of the thalamen- 
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. Anzeiger, vol. ix. p. 169.] 
