Normal Plates of the Development of Squalus acanthias. 29 
Pharynx, Thyreoid, Thymus, Suprapericardial 
Body 
Digestive Tube 
Liver, Pancreas, Spleen 
Pharynx more elongated than in preceding stage 
(No. 19). Upper part of first gill slit open, and 
where not open the ectoderm and entoderm are 
fused. Second gill slit a minute round opening; 
ectoderm and entoderm fused for some distance 
on either side of this opening. Thyreoid a small 
shallow open pouch at postero-ventral angle of 
pharynx. 
First and second gill slits open dorsally. Ecto- 
derm fused with entoderm over third and fourth 
pouches. Thyreoid as in preceding stage. 
Dorsal halves of first and second gill slits open. 
Ectoderm and entoderm of third and fourth gill 
slits fused. Thyreoid projects backward 24 u 
(by sections) beyond the ventro-posterior angle of 
pharynx. Walls of thyreoid pouch in contact. 
Dorsal halves of first and second gill slits open. 
Dorsal third of third slit open. Ectoderm and 
entoderm fused over fourth pouch and broken 
through for one section of 8 u on either side. 
Fifth gill pouch established. Thyreoid pouch 80 u 
in length: posterior third not attached to floor of 
pharynx. 
First four gill slits open. Fifth pouch well estab- 
lished, with ectoderm and entoderm in contact 
dorsally and fused ventrally. Sixth pouch well 
formed, with ectoderm and entoderm in contact. 
Three knob-like gill filaments on posterior wall 
of second gill slit. Thyreoid projects backward 
70 u (by sections) from ventro-posterior angle of 
pharynx. 
First three gill slits widely open. Fourth gill slit 
established, but arches still in contact. Ectoderm 
and entoderm in contact over fifth and sixth 
pouches. Thyreoid pear-shaped in cross section, 
150 u long (by sections), connected with pharyngeal 
epithelium for 20 u at extreme anterior end, poste- 
riorly well separated from pharynx. 
Five anterior gill slits open. First slit very narrow 
ventrally. Ectoderm and entoderm fused and very 
thin for short distance over sixth pouch. Four 
small gill filaments on hyoid surface of second 
gill slit, two on mandibular surface. Thyreoid 
250 u in length (by sections). pointed anteriorly, 
broad and rounded posteriorly, attached to base 
of pharynx anteriorly by a constricted neck. 
deep groove in floor of pharynx just above 
thyreoid. 
Oesophagus much longer than in preceding stage 
(No. 19), tapering posteriorly, in narrowest portion 
broadly oval in outline. Lumen much reduced in 
size, about one-fifth diameter of tube. 
Oesophagus broadly oval in cross section; vertical 
diameter a little greater than transverse. Posterior 
walls much thickened and lumen reduced to one- 
seventh diameter of tube. About two-thirds of 
a complete turn of spiral valve. 
Oesophagus constricted posteriorly, almost circular 
in cross section. In narrowest part, lumen barely 
visible; less than one-tenth diameter of gut. Vitel- 
line duct approximately equal. to one-fifth length 
of gut from pharynx to neurenteric canal. One and 
one-third turns of spiral valve. Cloaca dilated: 
about twice the diameter of preceding intestine. 
Ectoderm and entoderm, on ventral surface, in 
contact for 120 u (by sections). Gut constricted 
immediately behind cloaca, but dilated anterior to 
neurenteric canal. 
Oesophagus circular or broadly oval in cross 
section. Lumen in posterior half very minute. 
Antero-posterior diameter of vitelline duct 250 u 
(about one-sixth of length of gut from pharynx to 
neurenteric canal). A distinct groove on the left 
wall of vitelline duct from liver diverticulum to 
posterior end of duct. One and two-thirds turns 
of spiral valve. Post-cloacal gut much constricted 
ln but dilated before joining neurenteric 
canal. 
Lumen in posterior part of oesophagus occluded 
for 250 u (by sections). Oesophagus in cross section 
roundly oval anterior, and triangular posterior to 
closed portion. Four complete turns ofspiral valve. 
Cloaca dilated — vertical diameter three times that of 
gut preceding it. 
Oesophagus closed for 170 u. (by sections). Trans- 
verse diameter of anterior part three times vertical. 
Solid portion circular or oval in cross section. Im- 
mediately behind the solid oesophagus (i. e. region 
of future stomach) gut dilated, broadly oval in cross 
section. Six and one-half turns of spiral valve. 
Behind cloaca gut reduced to a slender solid cord, 
which acquires a lumen and is a little dilated before 
passing over into neurenteric canal. 
Oesophagus solid for 150 u (by sections). Stomach 
enlargement distinet — nearly twice diameter of 
solid oesophagus. Curved a little ventrally. Vitel- 
line duct approximately one-tenth the length of gut 
from pharynx to cloaca. Gut folded over to the 
left ot vitelline duct along the anterior two-thirds 
of their connection. Eight turns of spiral valve. 
Gut much constricted just anterior to cloaca. Lumen 
equal to one-sixth diameter of gut in this region. 
Cloaca ends abruptly posteriorly; no post-cloacal gut. 
Much as preceding embryo (No. 19). Anterior wall of 
pouch better defined. 
Liver pouch expanded laterally. Walls almost twice 
as thick as those of gut. Pancreas a small elongated 
diverticulum from dorsal wall of gut over yolk stalk. 
Directed Dem and expanded laterally at its pos- 
terior end. 
Liver an elongated ventral pouch constricted a little 
transversely at its connection with gut. Distinct lateral 
outpouchings from posterior end of ventral pouch. An- 
lage of gall bladder, a depression in posterior end of 
floor of ventral pouch, — continuous with anterior wall 
of vitelline duct posteriorly. Posterior end of pancreas 
projects backward over dorsal wall of gut for 25 u (by 
sections). 
Liver evagination four times as broad as gut connected | 
with it. In its widest part, four times as broad as high. | 
Connection between gut and liver slightly constricted | 
laterally. Gall bladder, a ventral pouch from liver in- 
vagination, about 130 u in length, sharply marked off 
anteriorly but broadly connected with liver above and 
merging with yolk sac posteriorly. — Pancreas a little 
larger than in preceding stage (No. 22). Twice as broad 
as gut with which it is connected. Connected with 
dorsal wall of gut for almost entire length of pouch. 
Epithelium of pouch twice as thick as that of gut. 
Lateral pouches of liver extend upward nearly to dorsal 
wall of gut, and are produced posteriorly. Connected 
with median portion along base of their anterior two- 
thirds. Several secondary outpocketings from anterior 
part oflateral pouches. Median chamber of liver evagin- 
ation broadiy connected with gut above it. all 
bladder a deep ventral pouch from median chamber. 
Pancreas projects backward over dorsal wall of gut for 
Ioo u (by sections). Attached part approximately the 
same length as free posterior part. 
Median chamber (ductus choledochus) of liver evagin- 
ation separated from gut anteriorly. 
Numerous secondary outpouchings from lateral pouches. 
Gall bladder expanded ventrally, attached to median 
chamber of liver along entire length by a transversely 
constricted neck. Pancreas attached to dorsal wall of 
gut by its anterior four-fifths. 
Lateral pouches | 
connected with median chamber by short wide ductes. | 
Outlines of original lateral pouches of liver obscured by | 
the very numerous tubules formed from them. Lateral 
pouches communicate with median chamber (ductus | 
choledochus) by narrow lateral openings 180 u in length 
(by sections). Ductus choledochus, flattened at its distal 
end, extends Ioo u posterior to liver; roundly triangular 
in cross section in this part; approximately same calibre 
as gut. Opens into ventral wall of gut 200 u (by 
sections) anterior to vitelline duct. Gall bladder 250 u. 
in length (by sections), attached along its middle 
three-fifths to ventral wall of ductus choledochus by a 
laterally constricted neck. Pancreas 300 u in length 
(by sections), connected by its middle third with dorsal 
wall of gut. Formation of tubules on right side and 
roof. 
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