| Normal Plates of the Development of Squalus acanthias. 47 
The mesoderm, which is represented in stipple, as in the preceding figures, extends forward into 
the head region as a flattened plate, and ends in the dilated mandibular somite (So.mand.). This structure | 
is divided somewhat into an anterior expanded position, the somite proper, and a short, thick posterior stalk. 
The hyoid somite (So.hyo.) is a distinct anterior projection of the head mesoderm, above the stalk of the 
} mandibular somite. Sixteen somites ($o.) project from the dorsal edge of the mesoderm of the trunk region. 
Q.ceph. So.mand. So.hyo. ‚So. 
I 
Rn 
SSIPDI>mmN 
Ant.neur. 
III TI > 
FRI NIÜINI N 
IN 
In.neur. 
PRESS 
No. A. 
Fig. 5: 
In only a few are there any evidences of an antero-posterior constriction of their bases, but the hases of 
all except the most posterior somites are a little constricted transversely. 
Only a little of the fore-gut (F.g.) can be seen below the ventral border of the head mesoderm. The 
rounded preoral process (P.g.) projects forward from its anterior end, between and below the optic vesicles. 
The notochord (No.) is also nearly covered by the mesoderm. It is seen partly in transverse, and 
partly in sagittal section, at the posterior end of the embryo. 
| Fig. 6. Graphic reconstruction of an embryo 3.5 mm. in length. Normal plate series No. 14. 
(H.E.C. 982.) X 50. 
This is another reconstruction of the same embryo as illustrated in Fig. 5. The plan of reconstruction 
is similar to that employed in Fig. 4 — all structures except the notochord being sectioned in the median line. 
+ m. M.c. My. 
# a 
| | 
Op. 
Rec.postop. 
P mes. P.g. Ph.Glp. Gl.p. * 
Fe 
Fig. 6. 
The anterior part of the medullary canal is closed except for the large anterior neuropore (An.neur.) 
and for a narrow slit in the hind-brain region. The roof of the mid-brain is seen in sagittal section (+). 
The left optic vesicle is seen as a deep pit extending laterally from the base of the fore-brain. Mesial to it 
is a deep median depression, the recessus postopticus or “infundibulum” (Rec.postop.). The medullary folds 
(M.f.) are fused from the hind-brain region to a point somewhat posterior to the middle of the trunk region. 
1 The anterior wall of the pharynx (P%.) is fused in part with the mass of preoral mesoderm (P.mes.) 
lying in front of it. From the base of the wall a small projection, the preoral gut, extends a little forward. 
