Normal Plates of the Development of Squalus acanthias. 11 
The yolk stalk has an anterio-posterior diameter of 1.o mm. The back line is a little convex. The 
primary head bend is of 65°, the mesencephalic bend is of 45°. The fore-brain vesicle is expanded laterally 
and dorsally. In the median line, cranial to the fore-brain swelling, there is a small rounded projection 
caused by the epiphysis. The mid-brain forms a broad rounded swelling which is a little constricted at its 
base. A faint groove marks the boundary between mid-brain and hind-brain. The roof of the hind-brain is 
translucent, — through it can be seen the floor, indented by a deep median groove and five neuromeres. 
The nasal pits are a pair of grooves placed below and mesial to the eye. They are deeply invaginated at 
their cranial ends. The optic cup is almost circular. The optic cleft (fissura cupulae, FRORIEP) is faintly 
visible. The lens is almost circular in outline. The otocyst is a prominent oval swelling lying over the 
hyoid clef. The ductus endolymphaticus can be seen extending upward and backward from its dorsal 
surface. The premandibular somite appears as a low swelling lying in the angle between the floors of the 
hind- and fore-brain vesicles. Behind the premandibular somite is seen the fan-shaped expansion of the 
mandibular somite lying between the roof of the hind-brain and the eye, and extending down into the 
mandibular arch. Five gill slits are present. The first two are a little expanded at their dorsal ends. 
Three short gill filaments project from the posterior surface of the hyoid arch. The pharyngeal plate is 
sharply outlined above and below. The anterior part of the mouth, when seen in direct ventral view, is 
covered by the projection of the head. The mouth opening is diamond-shaped in outline. The mandibular 
arches are bent mesially at about their middle, at a angle of 135%. The two arches meet at the base of 
the mouth at an angle of 75°. The pericardial cavity is dilated and its walls are semitransparent. The 
pelvic fins appear as rounded ridges, approximately 2.0 mm. in length, lying immediately behind the branchial 
plate. They extend outward 0.25 mm. from the body. The median ridge is decidedly elevated along the 
middle of the back and along the ventral median line posterior to the cloaca. In the latter region it is 
2.0 mm. high. Seventy-five somites can be counted externally. The anterior somites have the shape of a 
flattened V, with the apex pointed anteriorly. The dorsal ends of the last twelve somites are directed 
decidedly anteriorly. 
There are many important differences between this and the preceding embryo of 9.0 mm. Among 
them are: the greater expansion of the fore-brain vesicle; the breaking through of the fourth gill cleft; the 
distinct boundaries of the branchial plate; the enlargement of the pericardial cavity; the widening of the 
mouth opening, and bending of the mandibular arches; the appearance of the pectoral fin folds and increased 
height of the median fin fold; the change in shape of the anterior somites, and increase in number of 
somites by nearly a fifth. 
Fig. 25. Embryo 13.0 mm. in length. Normal plate series No. 25. H.E.C. 223, 224, 226. Lateral 
VIEWED 
The yolk stalk is almost circular in cross section and nearly 0.1 mm. in diameter. The back line 
of the embryo is a little convex. The mid-brain projects forward almost as a hemisphere from the front of 
the head. The fore-brain is expanded ventrally. The caudal border of the head meets the base of the 
pharyngeal region at right angles. The epiphysis causes a slight median swelling between the fore-brain 
and mid-brain protuberances. There are five gill slits, the first four of which open to the pharynx. The 
spiracular cleft is somewhat the longer, and lies at angle of 30° to the longitudinal axis of the body. The 
pectoral fins are rounded ridges, approximately 1.o mm. in length, lying above the yolk stalk. The cloaca 
forms a distinct protuberance from the ventral surface of the body. It is impossible to count the somites. 
They have the shape of a much flattened BZ the apices of which point tailwards. A broad shallow groove 
2* 
