bya 
arch- 
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with 
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. dor- 
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terior 
ented 
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Sylt: 
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ento- 
0. IO). 
Normal Plates of the Development of Squalus acanthias. 17 
U 
- \ | Hypophysis 
Notochord, Subnotochordal Rod, Axial Skeleton Ectoderm in early Stages Brain, and Cephalic Nerves and In- No. 
Ba : \ y fundibulum 
— Upper cells of blastoderm form a fairly defi- — _ I 
nite epithelial layer at the edge of disc, 
not separated from lower cells centrally. | 
Average length Io cells 37 u, breadth 22 u. 
Cells loaded with yolk granules. 
_ Ectoderm slightly thickened at posterior = 2 
end of blastodisc. Here generally 2 rows 
of nuclei, elsewhere I row of nuclei. 
_ Ectoderm lying over archenteron, much = _ 3 
thickened, containing layers of elongate 
nuclei, long axes of nuclei being directed 
dorso-ventrally. Cellwalls not distinct. 
Ectoderm thins out anteriorly. Anterior 
border of disc made up of one layer of 
cells, with rounded nuclei. Here there are 
faint cell walls. 
Notochord distinguishable along entire length of archen- | Medullary plate approximately ı mm. long, == _ 4 
teron, except extreme anterior and posterior ends, but|0.6 mm. broad, rounded semicircularly an- 
in no place clearly cut off from it. Anterior fourth | teriorly. Median groove alon posterior two 
of notochord a flat plate, slighthly marked off from re- |thirds. Groove shallow and rounded an- 
mainder of archenteron roof by shallow grooves. Pos- teriorly, becoming deeper and more angular 
teriorly, plate folded dorsally until almost semicircular | posteriorly. Epithelim contains 5rows ofnuc- 
in cross section, apex of plate pressing against ectoderm |lei at sides of plate, and 2—3 rows of nuclei 
of medullary plate. In trunk region, two rows of nuclei|in groove. Clear outer zone of cytoplasm in 
in dorsal part of notochordal plate. Posteriorly, three | cells forming floor and sides of groove. 
or more layers of nuclei much more compactly placed 
than those of archenteron. | 
Notochord still attached to archenteron throughout. Medullary plate broadiy expanded anteriorly, — _ 5 
approximately ı mm. broad and ı mm. long. 
A deep median groove in posterior three 
fourths of plate, anterior fourth almost flat. 
Epithelium 60 u. thick at lateral edge of 
plate. 
Notochordal plate extends from posterior edge of head | Medullary plate more slender and longer = = 6 
expansion backward to middle of embryo, occupying |than in preceding stage (No. 5). Elongately 
median third of dorsal roof of archenteron, curved | oval in outline, 0.7 mm. wide, I.ı mm. long. 
slightly downward, nuclei gathered at dorsal surface. In | Lateral edges extend a little outward over 
the middle of trunk region notochord distinctly sepa-|body of embryo. Rounded median groove 
rated laterally from entoderm but still forming part of | along posterior four fifths, anterior fifth al- 
archenteron roof. Here flatly oval in cross section, ven- most flat. Forsubsequent history of ectoderm 
tral ends of notochordal cells directed mesially. Pos-|see tables on brain, chord and integument. 
terior to this region notochordal plate again fused late- 
rally with entoderm, and a distinct notochordal groove 
present. Notochord distinct posteriorly to within 60 u. 
of incisura neurenterica. 
Much like preceding stage (No. 6). = Medullary plate widely expanded anteriorly to = 7 
form cephalic plate. Cephalic plate broadly 
oval in outline, 1 mm. long and a little over 
5 mm. broad. Edges of plate turned a little 
ventrally,. A shallow median groove along 
posterior three fifths of the cephalic plate. 
Notochord completely cut off from archenteron, except a Cephalic plate broadest in front, narrowing _ 8 
extreme posterior and anterior ends. Roundly oval in posteriorly. Sharply marked off from cordal 
cross section in head and pharynx region. Round in folds posteriorly. Äpproximately ı mm. long, 
cross section in trunk region. Diameter in trunk region 0.7 mm. broad, shallowly concave. Anterior end 
sou. A distinct elastica externa for greater part of length. deeply indented on either side of the median 
Nuclei of middle third of notochord, when seen in cross line by developing optic vesicles. Three neuro- 
section, are arranged as a ring with cytoplasmic core meres of Locy on left side, four on right. 
and border. — Subnotochordal rod indicated by an oc- 
casional cell on dorsal surface of archenteron, just 
below notochord. 
Notochord separate from archenteron except for ex- _ Cephalic plate less definitely marked off from _ 9 
treme anterior and posterior ends. Anteriorly it bends myelon plate posteriorly, approximately ı mm. 
ventrally and is lost in cells of preoral mesoderm. broad and long. Optic vesicles form a deep 
Greatest diameter 45 u. — Subnotochordal rod forming semicircular depression in anterior part of plate, 
as a ridge on dorsal surface of archenteron in middle ending abruptly posteriorly, slightly evaginated 
trunk region. Ridge one cell thick only, nuclei turned laterally. Medullary groove broad and shallow, 
with their long axes at right angles to longitudinal extends forward to optic vesicle. Posterior to 
axes of cells. optic vesicles, lateral edges of medullary plate 
are turned ventrally. 
_ No definite line of distinction between posterior _ 10 
Anterior end of notochord as in preceding stage (No. 9). 
Greatest diameter of notochord, in pharyngeal region, 
60 u. In tail region, notochord wedge-shaped in cross 
section, the ventral edge projecting downward toward 
archenteron cavity but separated from it by a thin band 
of entodermic cytoplasm. Nuclei in mid-trunk region 
have ring-like arrangement described for No. 8. Ento- 
derm, notochord, and mesoderm fused 25 y. anterior to 
incisura neurenterica. — Subnotochordal rod as de- 
scribed for No. 9. 
Notochord as in preceding embryo (No. 10). — Subnoto- 
chordal rod much as before, forming a low broad ridge 
in the 6th, 7th and 8th segments. 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XI. 
end of cephalic plate and myelon plate, although 
indicated at the lateral edges by a shallow 
notch. Optic vesicles much expanded laterally, 
extend backward as blind pouches for three 
sections of 6 u. Lateral edges of cephalic 
plate bent ventrally. 
A slight primary head bend. Medullary folds 
of brain region very high, their edges only 
turned a little outward. In posterior brain 
region medullary groove deeply V-shaped. 
EI 
