256 
munication by means of a slight break between the cells uniting their 
walls (fig. 6, left; the plane of the section is not quite true). Succes- 
sive sections here resemble closely those figured by KUPFFER (loc. cit.) 
from the anterior region of the head of Petromyzon, and I think the 
true inter pretation here, as there, to 
be the reduction by accelerated deve- 
lopment of lateral pockets from the 
alimentary canal, homologous with those 
found in Amphioxus. 
Fig. 6. Cross section to show the relation 
of the alimentary canal to the mandibular cavity. 
ect ectoderm. 67 brain. Ch. an chorda Anlage. 
2 mandibular cavities. ali alimentary canal. 
The most anterior mesoderm of the head does not take its origin 
from the mesodermic plates, but from the dorsal wall of the alimen- 
tary canal. The mesodermic plates end with the mandibular cavities. 
However, the connection between the walls of the anterior portion of the 
mandibular cavity and the endoderm, bridges the transition from mandi- 
bular to praemandibular mesoderm, and gives the impression of conti- 
nuous development. An impression essentially true if we may be allowed 
to draw an homology between the development of the mesoderm in Elas- 
mobranchs and in Amphioxus. 
After the compression of the walls of the adimentary canal at 
the anterior extremity of the embryo, coincident with the downward 
growth of the floor of the brain, the mass of tissue which constitutes 
the Anlage of the anterior head mesoderm comes to occupy a pe- 
culiar position. The posterior portion of this tissue, namely that con- 
tinuous with the walls of the mandibular cavity (m. 2, fig. 5) and that 
constituting the Anlage of the praemandibular cavity (m. 7) now 
extends in a line at right angles to the long axis of the embryo, 
while the anterior portion of the tissue, which lies in front of the 
Anlage of the infundibulum, maintains its original direction, parallel 
to the long axis of the embryo. From this tissue, both posterior and 
anterior to the infundibulum, cells are now proliferated laterally. Those 
posterior to the infundibulum form the walls of the praemandibular 
cavities. Those anterior to the infundibulum form the walls of an 
anterior pair of head cavities, not described in other Elasmobranchs. 
Fig. 7 is a diagram of the anterior mesoderm, projected upon 
a horizontal plane, at a stage previous to the appearance of the two 
anterior pairs of head cavities at a@ and 7. At inf the floor of the 
