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Normal Plates of the Development of Squalus acanthias. 
ee — —  ———, 
35 
Spinal Cord, Spinal Nerves, Sympathetic 
Eye 
Nose 
Cord in trunk region ovoid in cross section — broadest 
ventrally. Vertical diameter from 0.25 to 0.30 mm. 
Transverse diameter five-sixths of vertical. Cord ends 
posteriorly by tapering to a point. A distinct lateral 
peripheral band of ectoglia which is broadest ventrally. 
Differentiation of nuclear zone as in preceding stage 
(No. 26). Spinal nerves extend to middle of pectoral 
fin fold, and to dorsal edge of pelvic fin fold. In 
cardiac region, dorsal rami extend nearly to dorsal 
edges of somites. 
Cord throughout trunk and cardiac region almost cir- 
cular in cross section — about 0.30 mm. in diameter. 
Ectoglia layer from 20 to 40 u in thickness in trunk 
region. Oval bundle differentiated. Slight differentiation 
of lateral longitudinal fasciculus, and division of cinerea 
into dorsal and ventral columns in cardiac region. A 
very narrow ventral commissure in anterior trunk 
region. Many giant cells in cardiac region. Sympa- 
thetic ganglia, masses of darkly staining cells lying on 
dorsal surface of cardinal veins, and connected with 
spinal nerve trunks by short ganglionic cords. 
Transverse diameter of cord from 0.4 mm. to 0.5 mm. 
In pharyngeal and cardiac regions, cord almost circular 
in cross section. In remainder of trunk, broadly oval, 
with vertical diameter approximately three-fourths of 
transverse. In anterior part of tail, transverse and 
vertical diameters about equal. Canal twice as broad 
ventrally as dorsally in anterior trunk region. Division 
of cinerea into dorso-lateral and ventral columns. 
Lateral longitudinal fasciculus a distinct oval bundle 
throughout trunk region. Narrow ventral commissure 
throughout trunk region. 
Greatest transverse diameter of cord in trunk region, 
0.4 mm. Vertical diameter four-fifths of transverse. 
Vertical diameter of canal approximately five times 
transverse. Canal slightly expanded at dorsal and 
ventral ends, and in the exact middle. Roof plate 
about half the diameter of floor-plate. Twenty-five (?) 
giant cells in cord, anterior to level of duodenum — 
none behind this region. Oval bundle and ventral 
commissure more definite than in preceding stage. 
Dorsal ganglionic commissure, connecting vagus, spino- 
occipital ganglia and first spinal ganglion, has dis- 
appeared. 
Transverse diameter of cord in anterior trunk region 
0.5 mm. Vertical diameter four-fifths of transverse. 
Läteral longitudinal fasciculus sharply marked off — 
about 40 u. in diameter in cross section. Many glia 
cells in ectoglia of ventral funiculus. 
Cord in cardiac region broader ventrally than dorsally. 
0.6 mm. to 0.7 mm. in transverse diameter — vertical 
diameter about four-fifths of transverse. In trunk region, 
cord 0.4 to 0.5 mm. diameter — vertical diameter about 
two-thirds of transverse. Beginning of dorsal closure 
of canal in cardiac region. Many glia cells in ventral 
In cardiac region, a slight separation of 
funiculus. 
dorsal column of cinerea layer into dorsal and lateral 
columns. i 
thickness. 
region. 
Ventral white commissure nearly 40 y. in 
Numerous dorsal giant cells in cardiac 
Greatest breadth of lens 0.38 mm, greatest thickness 
0.25 mm. Lateral surface moderately, mesial surface 
barely convex. Mesial wall 150 y, lateral wall 30 u 
thick. Nuclei gathered in outer two-thirds of mesial 
wall. Less lentic cavity than in preceding embryo. 
Optic cup 0.75 mm. in diameter. Pars coeca bent cen- 
trally for ı5 u. Thickness of walls as in preceding 
stage (No. 26). A few division figures in outer angle of 
cup. 
Breadth of lens 0.92 mm., thickness 0.66 mm. Curvature 
of lateral surface somewhat more than hemispherical. 
Mesial surface almost flat. Lentic cavity nearly obli- 
terated. Mesial wall forty times as thick as lateral. 
Lentic fibers decidedly converged. Greatest diameter of 
optic cup I.4 mm. Thickness of retinal wall ı5o u, of 
chorioid wall I5 u. Optic stalk 150 y in diameter. Optic 
nerve fibers in distal part of stalk, anterior to chorioid 
cleft. Hyaloid artery and mesenchyma project well into 
optic cup. 
Lateral surface of lens decidedly rounded; mesial almost 
flat. Lateral and mesial surfaces join almost at right 
angles.. Breadth of lens 1.4 mm., thickness I.I mm. 
Lentic cavity obliterated. Retinal layer of optice cup 
twelve times as thick as chorioid. A little mesenchyma 
between lower border of the pars coeca and skin. Optic 
stalk contains many fibers posteriorly. 
Lateral surface of lens fully rounded, posterior curvature 
Hatly rounded. Breadth of lens 1.4 mm., thickness 0.9 mm. 
Retinal layer of cup 215 u, chorioid layer 15 u thick. 
Differentiation between pars coeca and pars optica a 
little more marked than in preceding stage (No. 29). 
Pars coeca 230 u in length, slightly folded on mesial 
surface. A trace of pigment in outer layer. Considerable 
mesenchyma between ventral border of pars coeca and 
skin. Cavity of optic stalk still present in anterior 
portion, obliterated posteriorly by optic fibers. Vascular 
mesenchyma enters by chorioid fissure, extending into 
optic cup as far as ventral surface of lens. 
Mesial surface of lens a little more rounded than in pre- 
ceding stage. Breadth 1.7 mm., thickness I.4 mm. Pars 
coeca longer, slender, but no more corrugated than in 
preceding stage (No. 30). Pigment extends from distal 
tip of pars coeca a little past point where iris process 
goes over into retina proper. Optic stalk obliterated 
by fibers except for 20 u anteriorly. A thick band of 
mesenchyma between base of pars coeca and skin. 
Little change from preceding stage (No. 26). 27 
Nasal pit shallow caudally and mesially. In-| 28 
vaginated at cranial end forming a pouch | 
200 „ in length (by sections). End of pouch 
expanded mesially and laterally — lateral en- | 
largement is slightly the larger and extends 
farther anteriorly. 
Nasal sacs 240 u in length (by sections). Di-| 29 
vision into lateral and mesial chambers has 
proceeded much farther. Mesial chamber much. 
the larger, extending twice as far anteriorly 
(144 », by sections) as lateral. Two sets of 
Schneiderian folds, one in mesial chamber 
(three folds), one in lateral chamber (seven 
folds). Diameter of largest fold at base 45 u 
height 60 u. Both epithelium and mesenchyma 
involved in folds. 
| 
| 
Four or five Schneiderian folds in median| 
series, twelve or thirteen in lateral. Height 
of largest fold 105 u, width 30 u. Anlagen of 
nasal valves appear as slight thickenings of 
edges of nasal pit. 
30 
| 
| 
Eleven Schneiderian folds in mesial series, | 
nineteen in lateral series. Bases of valves 
more distinctly marked off from pit wall than 
in preceding stage (No. 30). Mesial flap of 
nasal valve slightly overlaps lateral. 
[97 
Mesial chamber lies almost entirely anterior, 3 
to the lateral. Little if any increase in number 
of Schneiderian folds. Largest fold (lateral| 
series) 60 u. wide and 225 u high. Nasal flaps 
slightly constricted at their bases; mesial flap 
overlaps lateral. 
