3 
Normal Plates of the Development of Squalus acanthias. 45 
Fig. 2. Graphic reconstruction from sagittal sections of an embryo of 2 mm. Normal plate series 
No. 7: (HLE:C; 1009.) X 56: 
The left side of the embryo is seen. The scheme of shading to indicate germ layers is the same 
as that employed in Fig. ı. The ectoderm is cut away a little to the left of the median line. The entoderm 
and mesoderm are seen as uncovered by the ectoderm, they are cut away along the plane where they 
extend out laterally into the blastodisc. At the posterior end of the embryo all the germ layers are cut 
in a plane a little to the left side of the median line. 
The cephalic plate (0.p.) is almost 
So.mand. 
C.p. 
flat. Posterior to the cephalic plate 
there is a distinct though shallow 
medullary groove (M.g.) which gradually 
In.neur. 
becomes flattened out posteriorly. The 
incisura neurenterica (In.nevr.) is deeper 
Fig. 2. 
than in the preceding reconstruction. 
The head mesoderm extends forward under the cephalic plate, ending in a distinct dilatation — the beginning 
of the mandibular somite (So.mand.). Posterior to the mandibular somite the mesoderm and entoderm are 
still confluent, and their juncture is indicated by a broken line. Four distinct trunk somites (S0.) are 
present. A fifth somite is probably forming anteriorly. Posteriorly the mesoderm spreads out over the 
blastodisc. Mesoderm, entoderm, and ectoderm are seen to fuse in the wall of the incisura neurenterica. 
The preoral gut (P.g.) extends forward in the head fold as a broad blunt process which is almost square 
in cross section. The walls of the archenteron flatten out rapidly laterally and posteriorly. 
Fig. 3. Graphic reconstruction of an embryo 2.7 mm. in length. Normal plate series No. 9. 
(EE.@79977)0.,850: 
The plan of reconstruction and method of indicating germ layers are the same as those employed 
in Figs. ı and 2. The ectoderm is cut away at the median line, both dorsally and ventrally. The entoderm 
and mesoderm are cut, as in Fig. 1, along the line where they flatten out to form a part of the blastoderm. 
The extreme posterior end of the embryo is represented as cut sagittally at the median line. 
Op.v. (.p. So.mand. 
In.neur. 
N 
Fig. 3. 
The cephalic plate (C.p.) is depressed, and the right optic vesicle (Op.v.) forms a deep lateral evagi- 
nation in it. The medullary groove (M.g.) is of approximately the same depth throughout its length, and 
passes over into the incisura neurenterica (In.neur.) posteriorly. The mesoderm of the embryo extends 
forward to the cephalic plate as a narrow band, and ends in a distinct expansion, the mandibular somite 
(So.mand.). There are ten well formed trunk somites (So) and an eleventh is in the process of formation 
posteriorly. The fore-gut (F.g.) extends forward to the cephalic plate. From its anterior wall, the preoral 
process (P.g.) projects anteriorly along the median line between the optic vesicles.. The notochord (No.), 
