Normal Plates of the Development of Squalus acanthias. 15 
there extends forward a short broad rostral process. This is separated from the main part of the head 
above by a shallow groove. The eye is broadly oval in outline, the vertical diameter being equal to about 
two thirds of the antero-posterior. The edges of the nasal pits are thickened and both anterior and 
posterior flaps of the nasal valves are prominent. The jaw ridges appear as prominent V-shaped elevations 
bounding the angles of the mouth. Placed laterally to each ridge is a triangular depression — the 
labial pocket. 
The main divisions of the lateral line system of the embryo can be traced by the rows of ampullae 
associated with the canals. The supra-orbital canals extend forward from above the gill slits, over and 
around the front of the orbit; there they become continuous with the ethmoid canals. Each ethmoid canal 
makes an abrupt median bend in front of the nasal pit and, extending backward, joins the postorbital canal 
lateral to the angle of the mouth. Although not shown in the figure, in another specimen of the same 
stage the angular canal arose from this juncture and extended inward around the angle of the mouth, and 
a short distance along the lower jaw. The preoral canals arise from the ethmoid canals, about midway 
between the angle of the mouth and the nasal pits; and pass mesially, making an abrupt anterior and a 
posterior bend. They nearly meet at the median line in front of the mouth. From this point, they extend 
forward, diverging laterally, and terminate midway between the nasal pits and the anterior end of the 
rostral process. The postorbital canals extend upward from their juncture with the ethmoids, and end 
by joining the supra-orbital canals behind the eye. This dorsal termination of the supra-orbital canals is 
not shown in the figure. Although no occipital canal is shown in the figure, it was present in another 
embryo of the same stage. There it formed a connection across the median line, between the supra-orbital 
canals, at the level of the third gill slit. The mandibular group of ampullae appear as a cluster of about 
twelve pits a short distance behind the angles of the mouth. The posterior infra-orbital ampullae lie in 
front of the angle of the mouth, and are bounded anteriorly by the preoral, and laterally by the ethmoid 
canals. The infra-rostral group of ampullae is represented by a few scattered pores lying below the orbit 
and lateral to the ethmoid canal. The anterior infra-orbital ampullae lie in front of the transverse limb of 
the preoral canal and posterior to the nasal pit. The hyoid ampullae occupy the upper part of the surface 
of the hyoid arch, between the spiracle and the upper part of the second gill slit. 
The anterior and posterior dorsal fins are thickened and rounded, and a small rounded knob, the 
spine, projects from the anterior end of the base of each. The caudal fin is spatulate in outline, and its 
dorsal and ventral folds are of equal height. Both pectoral and pelvic fins project backward some distance 
from their attachment to the body. The cloacal plate is elongately oval in outline, and is bounded on 
either side by a rounded ridge. These ridges fuse together in front of the cloaca, but are separated 
posteriorly by a narrow median cleft. 
As compared with the preceding stage, 34.0 mm. in length, the present specimen shows the following 
changes. The increase of mesenchyma in the head region and the development of the rostral process have 
obscured the outline of the brain, which has up to this stage determined the modeling of the head; the 
spines of the anterior and posterior dorsal fins have appeared; the lateral line system is somewhat farther 
developed. 
