Normal Plates of the Development of Squalus acanthias. 13 
Figs. 28, 28a, 28b. Embryo 20.6 mm. in length. Normal plate series No. 28. H.E.C. 1494. Lateral, 
dorsal, and ventral views. X 5. 
The fore-brain forms a prominent squarish protuberance from the ventral side of the head. The 
mid-brain forms an almost hemispherical extension from the front of the head. The angle between the mid- 
and fore-brain swellings is 130°. The roof of the hind-brain, when viewed from above, is kite-shaped and 
semitransparent. The nasal pits lie almost in the transverse plane; their anterior and lateral angles are 
deeply invaginated. The eye is almost circular, 2.0 mm. in diameter. The lens has a diameter of 1.2 mm. 
and projects as a hemisphere from the optic cup. The ganglion and ramus ophthalmicus superficialis of 
the facial nerve form a distinct ridge above the eye. The ramus mandibularis of the trigeminal nerve and 
the ramus hyomandibularis of the facial nerve form together a small ridge extending downward in front ot 
the spiracular cleft. The spiracular cleft is approximately one half as long as the second gill cleft, and 
lies at an angle of 45° to the transverse plane of the pharyngeal region. Four short gill filaments project 
form the spiracular cleft. From the remaining clefts project a large number of gill filaments, the longest 
of which are 2.5 mm. The gill flaps are prominent: their dorsal ends project backward nearly a millimeter 
from the side of the body. The mouth opening is oval in outline; its transverse diameter is I.3 mm., the 
antero-posterior diameter is 0.8 mm. There is a slight groove separating the bases of the mandibular 
arches. Both dorsal fins are prominent. The anterior fin is 2.0 mm. long at its base and 0.5 mm. high; 
the posterior 1.2 mm. long at its base and 0.5 mm. high. The pectoral fin is turned ventrally along its 
outer border; its base is 2.0 mm. in length. The pelvic fins are low rounded folds which lie on either side 
of the cloaca and meet behind it. The caudal fin is indicated by the elevation of the dorsal and ventral 
median fin folds along the posterior 4 mm. of the tail. The somites have the flattened S-shape described 
for the preceding three embryos. The lateral line placode forms a prominent band extending from the last 
gill slit to a point above the middle of the cloaca. It ends in a knob-like elevation which is due to the 
separation of the outer layer of the skin in fixing. 
This embryo shows a considerable advance in development beyond the preceding one 18.0 mm. in 
length. The chief differences in the larger embryo are: the depression between the fore- and hind-brain 
swellings and the projection caused by the epiphysis are obscured by the increase of mesenchyma in this 
region; the spiracular cleft is shorter, both absolutely and in comparison with the other clefts; the gill 
filaments are much longer; the elevations of the dorsal fins and of the caudal fin are more definite; the 
pelvic fin is present; the lateral line placode has become apparent externally. 
Fig. 29. Embryo 24.7 mm. in length. Normal plate series No. 29. H.E.C. 1492. Lateral view. X 5. 
The fore-brain protuberance is directed a little forward. The mid-brain is almost hemispherical. 
The roof of the hind-brain is semitransparent. The cerebellum appears as a thickening of the neural tube 
between the roof of the hind-brain and the mid-brain. The nasal pit no longer appears as a groove 
invaginated at one end, but as a deep crypt, the edges of which are slightly thickened. The eye is broadly 
oval in outline. Its vertical diameter (the greater) is 2.5 mm. Long thread-like filaments extend from all 
the gill clefts; the longest 4.0 mm. in length. The dorsal fins are of about equal size, the anterior one 
being a little the longer at its base. The caudal fin is of equal height dorsally and ventrally. The outer 
edge of the pectoral fin is prolonged backward beyond the base of the fin; it ends in a sharp angle 
posteriorly. The fin rays are faintly visible. The lateral line placode forms a distinct ridge between the 
epi- and hypo-axial musculature, extending from above the last gill slit to the middle of the caudal fin. 
