mn - 
Normal Plates of the Development of Squalus acanthias. 
27 
Spinal Chord, Spinal Nerves, Sympathetic 
Eye 
Nose 
Cord much as in preceding embryo (No. 19). Ventral halves of lateral 
walls almost in contact in posterior trunk region. In anterior 
trunk region, lateral walls two to three times as thick as roof or 
floor plate. Neural crest extends backward nearly to posterior 
wall of yolk stalk. Three (?) distinct ganglionic enlargements pos- 
terior to vagus. 
Cord in greater part of trunk region broadly ovalin cross section; 
transverse diameter about three-fourths of vertical. Ovate in cross 
section in tail region. Neural crest extends posteriorly to twenty- 
sixth segment (about midway between posterior wall of yolk stalk 
and cloaca). Division of crest into separate ganglia anlagen except 
at extreme posterior end. Ventral roots present in anterior trunk 
region. Ganglia anlagen and ventral roots not joined. 
Cord oval in cross section; nearly as broad as high in trunk 
region. Middle third of walls in contact in anterior trunk region. 
Many mitoses. Twenty-eight spinal ganglia. Two occipital ganglia, 
which appear as enlargements of the ganglionic cord connecting | 
the vagus with the spinal nerve crest. Ventral roots of occipital | 
and first eighteen spinal nerves present. Dorsal and ventral roots 
not connected. Dorsal roots in region of pronephros and pancreas 
extend a little below level of dorsal surface of notochord. Neural 
crest extends posteriorly to level of cloaca. 
Cord as in preceding embryo (No. 22). Circular in cross section 
from cloaca backward. Thirty-six spinal ganglia (by sections). 
No definite ganglia beyond anterior end of cloaca. Ventral roots 
and ganglia of nerves anterior to posterior wall of yolk stalk 
united. Longest nerves (in region of pancreas) extend ventrally 
to level of aorta. 
Vertical diameter of cord in trunk region, approximately 0.25 mm. 
Transverse diameter three-fourths of vertical. Floor plate three 
times as thick as roof plate; lateral walls twice as thick as floor 
plate. Canal a narrow vertical slit, expanded ventrally and a little 
dorsally. A narrow lateral layer of ectoglia in anterior trunk 
region. Fifty-one spinal ganglia (by sections). Nerves in anterior 
trunk region extend ventrally to level of dorsal wall of aorta. Be- 
ginning of dorsal rami in this region. 
Vertical diameter of cord in trunk region, approximately 0.28 mm. 
In greater part of trunk region, transverse diameter but little less 
than vertical. Sympathetic ganglia small masses of cells on mesial 
sides of nerves of anterior trunk region at the level of dorsal 
aorta. 
In cardiac region, cord 0.35 mm. in vertical diameter. In pan- 
creatic region, 0.25 mm. in vertical diameter. Transverse diameter 
almost equal to vertical. Transverse diameter of canal approxi- 
mately one-fifth of vertical. Lateral walls in contact in pharyngeal 
and cardiac regions. Neurenteric canal still present, but con- 
stricted. Narrow peripheral layer of ectoglia in lateral walls of 
cord — thickest ventrally. Beginning of formation of cinerea in 
ventral half of cord in anterior trunk region. Interganglionic cord 
broken down in anterior trunk region; still continuous from the 
17th spinal ganglion backward. Spinal nerves of anterior trunk 
region directed somewhat posteriorly. In pancreatic region, extend 
ventrally into somatopleure of body wall. Sympathetic ganglia 
more definite than in preceding stage (No. 24). 
Ectoderm of lentic plate 30 u thick over optic vesicles, 
rather sharply marked off from skin ectoderm. Optic vesicles 
connected with fore-brain for one-fifth of their cranio-caudal 
diameter, and one-third of the dorso-ventral diameter. 
Cranial two -fifths of vesicles project forward as blind 
pouches. 
Lentic plate 50 u thick; mesial surface slightly convex. No 
lentic pit. Optic cup 0.35 mm. in diameter. Stalk one-half 
diameter of cup. Lateral wall of cup nearly twice as thick 
as mesial. 
Lentic epithelium 80 u thick, containing three rows of 
nuclei. Deep lentic pit. Optic cup 0.38 mm. in diameter. 
Cavity between mesial and lateral walls large. Lateral wall 
twice as thick as mesial. Optic stalk one-half diameter of 
optic cup. 
Lens almost spherical, 100 u. in diameter, four or five rows 
of closely packed nuclei, and many division figures. Lens 
still broadly attached to ectoderm. Lentic pit deeply V-shaped 
in cross section. Optic cup pressed against mesial surface 
of lens.. Lumen of optic vesicle a narrow cleft. Lateral 
wall go u thick; mesial wall 30 u thick; four rows of nuclei 
in lateral wall. Optic stalk nearly one-half diameter of cup. 
Lens completely cut off from skin ectoderm, although pressed 
against it. Diameter 0.3 mm. (by sections). Cavity irregular, 
often star-shaped in cross section. Division figures frequent. 
Mesial wall a little thicker than lateral. 
Lens flatly convex laterally, slightly concave mesially. 
Greatest breadth 0.38 mm., greatest thickness 0.22 mm. 
Mesial wall three times as thick as lateral, containing three 
to four rows of nuclei. Optic stalk one-third as broad as 
optic cup, nearly circular in cross section. Retinal wall of 
cup six times as thick as chorioid. 
Lens flatly oval in cross section; greatest breadth 0.46 mm., 
greatest thickness 0.3 mm. Mesial wall of lens seven times 
as thick as lateral wall. Lentic cavity elongately crescentic 
in cross section, as wide as lateral lens wall. Retinal wall 
of optic cup eight times as thick as chorioid. Many division 
figures in inner zone. Mesenchyma and hyaloid artery extend 
through chorioid fissure and just enter optic cup. 
Nasal placodes a little de-| 20 
pressed, and lie mainly an-| 
terior to optic vesicles. 
21 
Like preceding stage (No. 20). | 
| 
| 
| 
| 
| 
| 
Distinet depression of nasal| 
pits. Epithelium approxi- | 
mately three times as thick | 
as skin ectoderm. Many mi- | 
totic figures in outer border. 
22 
As in preceding embryo (No. | 
22), except that transition from | 
nasal to skin epithelium is | 
more sharply marked ven- 
trally. 
23 
Depressions a little deeper' 24 
than in preceding stage (No. 
| 23). | 
Nasal pits shallow, ventral| 
edges sunk in at right angles 
to the surface ofthehead. Epi- | 
thelium thickest and slightly | 
incurved at ventral angle of| 
pit. Epithelium approaches | 
brain, but is separated from | 
it by a little mesenchyma. 
| 
Deepest part of nasal pit “ 26 
almost semicircular in cross | 
section. Line between nasal | 
and skin epithelium sharply | 
marked ventrally, less so dor- 
sally. Olfactory nerve in con- 
tact with nasal epithelium at 
the ventral angle ofthe bottom | 
of pit. 
