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Normal Plates of the Development of Squalus acanthias. 
21 
"Notochord, Subnotochordal Rod, 
Axial Skeleton 
Ectoderm 
in early 
Stages 
Brain, and Cephalic Nerves 
Hypophysis 
and Infundi- 
bulnm 
Spinal Cord, Spinal Nerves, 
Sympathetic 
8 
Anterior end of notochord as in 
preceding stages (Nos. 9, IO and 
ır). Greatest diameter, in anterior 
trunk region, 60 ». 
Anterior tip of notochord bent 
sharply downward. In pharyngeal 
and anterior trunk region, broadly 
oval,transverse diameter being the 
greater. In tail region, the greater 
diameter is vertical. Nuclei ar- 
ranged in ring form from level of 
the first pharyngeal pouch to ninth 
segment. — Subnotochordal rod a 
flattened ridge on roof of archen- 
teron in the middle trunk region. 
Anterior tip of notochord turned 
ventrally at an angle of approxi- 
mately 130°. Shape as described 
in preceding embryo. Greatest dia- 
meter in trunk region 60 u; in 
head region 40 u. — Subnoto- 
chordal rod as described for pre- 
ceding embryo (No. 13). 
Notochord almost round in cross 
section; of the same diameter as 
in preceding stage (No. 14). — Sub- 
notochordal rod much as in pre- 
ceding stages (Nos. ı2 and 13). 
Ridge best marked at level of 
seventh segment. 
Anterior end of notochord turned 
ventrally at an angle of nearly 90°. 
Tip almost separated from preoral 
mesoderm. In head region, noto- 
chord 40 u» in diameter; in trunk 
region 70 «in diameter. Nuclei are 
arranged in ring throughout trunk 
region. — Subnotochord rod ex- 
tends from first pharyngeal pouch 
nearly to tail. Posterior to the 
second pharyngeal pouch it forms a 
ridge 15 to 20 u in height. Detached 
from roof of archenteron for a few 
sections in anterior trunk region. 
Anterior end of notochord slightly 
recurved. In anterior trunk region, 
nuclei form a dense central cluster. 
In posterior trunk region, nuclei 
arranged in central ring. In region 
ofcloaca and backward, notochord 
again cellular. Diameter of noto- 
chord in trunk region 90 y. — Sub- 
notochordal rod detached from 
gut in trunk region. 
Anteriorend ofnotochord bent ven- 
trally, not recurved. Extreme ven- 
tral end attached to dorsal surface 
ofpremandibular somite. Diameter 
ofnotochordinlargestpart(anterior 
trunk region) go u. Nuclei gathered 
in central cluster from second gill 
pouch backward to eighteenth seg- 
ment,arranged inringform from this 
point backward to cloaca. — Sub- 
notochordalrod appears as median 
ridge on roof of pharynx at level 
of Janterior edge of facial nerve, 
completely separated from gut from 
heart region backward nearly to 
cloaca. Three distinct ventral pro-. 
cesses along free portion of rod. 
Anterior end of notochord fused 
with median portion of premandi- 
bular cavity. Nuclei approximately 
as in preceding stage (No. 18). — 
Subnotochordal rod attached to 
dorsal wall of pharynx from above 
first gill slit to middle of oeso- 
phagus. In some sections cells ar- 
ranged as about a lumen. 
Medullary folds of cephalic region bent strongly mesially. They have 
met and fused for 30 . at anterior tip of head. 
Medullary folds fused at anterior (ventral) end of fore-brain. They almost 
meet over mid-brain. Mid-brain vesicle expanded a little laterally. In 
hind-brain region, edges of medullary folds turned laterally. Medullary 
canal widest ventrally in hind-brain region. — A very slight proliferation 
of cells for 130 u at the skin and neur-ectoderm junction in hind-brain 
region. Proliferation shows two to four cells in cross section. Hind- 
brain still open in this region. 
Medullary folds almost in contact throughout brain region — fused over mid- 
brain and at the anterior (ventral) end of fore-brain. Deep recessus post- 
opticus. Mid-brain a little expanded laterally. Medullary canal in hind- 
brain region widest ventrally. — Distinct neural crest extending for 300 u 
(by sections) along hind- and mid-brain. Crest extends downward over 
dorsal fourth of neural tube. Ventral border irregular. A slight division 
into middle and hind cephalic crest(?). 
Anterior (ventral) part of fore-brain region closed, but skin änd neur- 
ectoderm still in contact. Medullary folds almost meet throughout re- 
mainder of brain region, except over anterior end of hind-brain where 
they are turned a little outward. Mid-brain vesicle dilatated laterally. In | 
hind-brain region, floor and sides of medullary groove of equal thickness, 
— Neural crest extends from a little behind closed portion of neuropore 
to middle part of hind-brain, showing generally five to eight cells in 
cross section. 
Fore-, mid- and hind-brain vesicles separated by shallow grooves on ex- 
ternal surface. Small anterior neuropore; skin and neur-ectoderm fused 
at median line anterior and posterior to it. Deep recessus postopticus. 
Roof of anterior end of hind brain composed of a single layer of columnar 
cells. Ventral and dorsal thirds of canal in hind-brain expanded laterally. 
— Cephalic crest divided into an anterior trigeminal-facial portion and 
a posterior glossopharyngeal-vagus portion. Trigeminal-facial crest ex- 
tends forward nearly to anterior neuropore. A smal process from the 
crest, posterior to optic vesicle, represents thalamic nerve? Two small 
interruptions in posterior part of trigeminal-facial crest indicate line of 
division between trigeminal and facial portions. Glossopharyngeal-vagus 
crest triangular in shape. No division into separate nerve anlagen. 
Anterior neuropore completely closed. Skin- and neur-ectoderm fused at 
median line, in region of neuropore, for I50 u. (by sections). Roof of hind- 
brain, anterior to otic plate, thin and expanded: at median line composed 
of a single layer of cells. — Trigeminal crest a .broad sheet of cells, 
attached to roof of anterior end of hind-brain by a broad dorsal process, 
urtrochlearis. Posteriorly, attached to dorsal third of hind-brain. Extends 
ventrally nearly to base of mandibular arch. Thalamic-ophthalmic process 
from roof of mid-brain to anterior border of crest. Facial acoustic crest 
extends ventrally to level of aorta. Glossopharyngeal and vagus crests 
separated; former extending over dorsal half of neural tube, latter in- 
dicated by a few cells only in 6 sections of 6 u. 
Cranio-caudal axis of fore-brain much increased as compared with preceding 
stage. Anterior neuropore completely closed. Skin and neur-ectoderm 
fused in median line for a distance of 80 u (by sections) at this point. 
Roof of hind-brain, anterior to otocyst, expanded; at median line com- 
posed of a single layer of cells. Ventricle greatly expanded dorsally; re- 
duced in middle third, by thickening of lateral walls, to a narrow slit; a 
little enlarged ventrally. In otic region, brain almost oblong in cross 
section; transverse diameter half of vertical. Dorsal and ventral expansions 
of canal of equal size. — Thalamic crest and ophthalmic portion of tri- 
geminal crest form a broad cellular band, lying dorsal to optic vesicle, 
Trigeminal and urtrochlear crests broadly connected. Acoustic portion of| 
facial acoustic crest attached to epithelium of otic pit. Facial portion 
enlarged at level of dorsal aorta, extends down dorsal third of hyoid arch. 
Glossopharyngeal crest extends nearly to base of hind-brain. Vagus crest 
completely separated from glossopharyngeal, 250 x in length (by sections). 
Extends anteriorly over dorsal two-thirds of medullary tube, gradually 
becoming narrower posteriorly. 
Skin- and neur-ectoderm fused for 48 u (by sections) along median line in 
position of neuropore. Transverse diameter of anterior end of hind-brain 
greater than vertical. Ventral expansion of ventricle from otocyst backward. 
= Thalamic and urtrochlear crests attached to trigeminal by narrow strands 
of cells. Vagus crest separated from glossopharyngeal: extends ventrally 
below dorsal wall of oesophagus. 
Differs but little from preceding | 
embryo (No. Ir). 
Medullary canal closed for 
120 u (by sections) in middle 
trunk region. Anterior to closed 
region folds are almost in con- 
tact. Posterior to closed region 
canal deeply U-shaped in cross 
section. Folds are beginning 
to extend dorsally over incisura 
neurenterica. 
Medullary folds in contact along 
most of the length of canal, 
fused over anterior two-thirds 
of it. Incisura neurenterica 
widely open. 
Cordal portion of medullary 
canal closed, except for ISO. (by 
sections) posteriorly; here there 
is still a narrow dorsal cleft. In- | 
cisura neurenterica still open, 
but very deep, its folds approach- 
ing dorsally. Cord broadly oval 
in cross section; in anterior 
part vertical diameter greater 
posteriorly, transverse diameter 
greater. Lateral walls a little 
thicker than roof or floor. 
Diameter of cord in anterior 
trunk region twice that in tail 
region. Vertical diametergreater 
in anterior trunk region, trans- 
verse diameter greater poS- 
teriorly. Neurenteric canal com- 
pletely enclosed. 
Much like 
(No. 16). 
preceding 
Vertical diameter of cord great- 
est throughout. Lumen elon- 
gately oval in anterior trunk 
region, becoming almost circu- 
lar posteriorly, larger dorsally 
than ventrally throughoutgreat- 
er part of trunk region. Late- 
ral walls thicker than roof or 
floor throughout. 
In cardiac and anterior trunk re- 
gion, transverse diameter of 
cord two-thirds of vertical. Late- 
ral walls almost in contact. In 
posterior trunk region, cord 
ovateincrosssection — broadest 
dorsally; transverse diameter 
almost equalto vertical;. ventral 
half of canal reduced to a nar- 
row slit. In tail region, cord 
almost circular. Canal almost 
circular, and dilated anterior to 
neurenteric canal. Neural crest 
extends posteriorly to thirteenth 
segment. 
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18 
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