DEVELOPMENT OF THE HYPOPHYSIS 407 
The position of the adult hypophysis has been described by 
Sterzi (’09) and others. The long tongue-like anterior lobe lies on 
the median ventral wall of the inferior lobes of the brain. Its ante- 
rior end extends forward almost to the optic chiasma. The supe- 
rior part is placed posterior to this and ina more dorsal plane. It 
extends asfar caudally as the saccus vasculosus. The inferior 
sacs of Squalus do not extend as far ventrally as has been de- 
seribed for other selachians. But they are ventral to the superior 
lobe and extend farther caudalward (fig. 10). From their middle 
connection a slender canal joins them to the ventral side of the 
caudal end of the anterior lobe. 
2. Early development of the hypophysis 
Recent work on the development of the hypophysis in elasmo- 
branchs shows that it arises at an earlier period than was formerly 
believed. Hoffmann (’96) stated that the position of the future 
hypophysis is well marked in Acanthias embryos of 15 somites 
but that there is no indication of an evagination even in 8 mm. 
(50 somites) and 10 mm. embryos. Haller (’96) began his de- 
scription of the hypophysis in Mustelus in embryos 22 mm. long. 
At that time, the hypophysis is already a distinct outpouching. 
More recently Johnston (’09) briefly described the earliest for- 
mation of the hypophysis in Acanthias. In an embryo of 24 
somites the ectoderm from which the hypophysis develops is 
readily recognized. He stated that a short anterior lobe, de- 
veloping later, extends toward the optic chiasma. Also, that the 
posterior part crowds between the brain and the median mass 
connecting the premandibular somites. Scammon (711) men- 
tioned a thickened hypophyseal plate in a 5.2 mm. embryo and 
a beginning evagination in a 6.2 mm. embryo (50-51 somites). 
A median sagittal section of an Acanthias embryo 8 mm. in 
length? is shown in figure 2. The anterior superior end is par- 
tially insinuated between the brain and the median mass con- 
necting the premandibular somites, as had been noted by Johns- 
ton (09) in about the same stage. During the time that the 
3In the description of the figures ‘H. E. C.’ has reference to the embryos of 
the Harvard Embryological Collection used for this study. 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 3 
