150 Ww. B. SCOTTY, 
the cerebral hemispheres is a simple unpaired bud, which later 
becomes divided into lateral halves. At first only a lateral 
development of the extraordinarily small rudiment takes place, 
so that the epiphysis lies between the hemispheres and almost at 
the anterior edge of the brain. In the later stages, and especi- 
ally after the metamorphosis, the cerebrum grows longer and the 
epiphysis comes to lie behind it. The differentiation of the 
olfactory lobes takes place comparatively late in larval life. 
The epiphysis and infundibulum are diverticula of the roof and 
floor respectively of the posterior part of the fore brain. The 
pituitary body is developed as a solid cord of cells which are 
invaginated from the epiblast, together with a single median 
invagination for the olfactory pit. Only the posterior part of 
this invagination is concerned in the formation of the pituitary 
body. These cells soon lose all connection with the olfactory 
apparatus, and become divided by connective-tissue bundles into 
solid follicles; in the sexual animal the gland lies above the 
naso-palatal passage. 
In the mid-brain the lateral thickenings and thinnings of the 
dorsal median line are especially to be noticed. This division 
thus receives a bilobed roof. The brain is at first straight and 
shows no tendency to flex itself. The cranial flexure never 
attains a very great degree, about a right angle, and is partially 
corrected by an actual extension in the reverse direction. There 
is also an additional apparent correction which is caused by the 
great development of the upper lip. 
The rudiments of all the higher organs of sense appear during 
embryonic life before the epiblast has divided itself into two 
layers. The eye develops in essentials just as in the Gnatho- 
stoma and needs no especial explanation. ‘The optic vesicle is, 
however, remarkable for its length, and for the fact that only 
a part of the anterior wall of the vesicle becomes the retina. 
The lens arises as a local invagination of the single-layered epi- 
blast. The auditory vesicle develops as an invagination of 
thickened epiblast cells, which gradually deepens, becomes sphe- 
rical, and detaches itself from the skin. Its later development 
presents some details of interest; which, however, must be 
reserved for the full paper. 
The olfactory organs are, for Petyomyzon, of especial interest. 
I cannot confirm Calberla’s! result as to the paired origin of the 
olfactory pit, on the contrary, according to my observations (and 
here I am in accord with Dr. Gotte, as he informs me by letter) 
this pit is single from the very first. The first stage is a 
shallow sinking in of the epiblast at the anterior end of the head 
1 Calberla, ‘ Amt]. Bericht der 50 Versamml. d, deutschen Naturforscher, 
&c.,? Munich, 1877, p. 188. 
