ron 
Normal Plates of the Development of Squalus acanthias. 37 
Pharynx, Thyreoid, Thymus, 
Suprapericardial Body 
Digestive Tube 
Liver, Pancreas, Spleen 
Five anterior gill slits open, sixth slit 
still closed by athin plate ofepithelium. 
Short gill filaments from second, third, 
and fourth slits. Thyreoid 200 u in 
length (by sections). Anterior half 
attached to floor of pharynx by a 
constricted neck. 
First (spiracular) cleft one-half aslong 
‚as others. Lies at an angle of 45° to 
longitudinal axis of body. Long gill 
filaments from second, third, fourth, 
and fifth slits. Four short filaments 
spring from ventral part of anterior 
wall of first slit. Thyreoid detached 
completely from floor of pharynx, 
much mesenchyma between the two 
structures. Outer cells ofthyreoid epi- 
thelioid in arrangement, surrounded 
by a definite basement membrane. 
Suprapericardial body  represented 
by small patch of thickened epi- 
thelium of floor of pharynx on left 
side of median line at level of sixth 
gill pouch. 
Long gill filaments from all clefts. 
Thyreoid almost 0.5 mm. in length. 
Oval in cross section. Contains 
numerous small irregular cavities. 
Suprapericardial body a small deep 
thick-walled pit in floor of pharynx 
on left side near the median line 
at upper end of the sixth gill pouch. 
Extends through three sections of 
T2e% 
Spiracular slit broadly lenticular in 
outline, one-fourth length of hyoid 
slit. Epithelium on lateral side of 
dorsal tips of 2nd, 3rd, 4th, and sth 
ouches thickened (anlagen ofthymus). 
uprapericardial body expanded at 
distal end, constricted at connection 
with pharyngeal epithelium. 
Gill filaments longer (over 6.0 mm.). 
Epithelium of thymus anlagen in- 
creased in thickness. Thyreoid nearly 
1.o mm. in length. Anterior and pos- 
terior thirds of thyreoid circular in 
cross section, with no lumen. Middle 
third a flattened plate 0.45 mm. in 
width and 0.08 mm. thick, containing 
an irregular central lumen. Distal 
end of suprapericardial body ex- 
panded and directed posteriorly. Only 
a very minute lumen in stalk. 
Gill filaments and thymus anlagen 
as in preceding stage. Posterior half 
of thyreoid a flattened plate — an- 
teriorly lumen much expanded. Distal 
end of suprapericardial body ex- 
panded, forming a thin-walled cyst 
120 u in length. Stalk twisted, with 
an irregular lumen which does not 
communicate with pharyngeal cavity 
nor with cavity of distal cyst. 
Lumen of posterior two-fifths of oesophagus occluded. 
Walls actually fused for about ten sections of IO u only. 
A few cellular strands stretch across the lumen for 250 ». 
anterior to this region. Anterior three-fifths of oesophagus 
flattened dorso-ventrally; where most flattened, ventral 
diameter about one-third of transverse. Stomach about 
twice diameter of closed oesophagus, about 0.6 mm. in 
length (by sections). A slight constriction between stomach 
and duodenum. Duodenum curved a little to the left and 
downward, at an angle of 60°. 
end, of same diameter as gut. Ten turns of spiral valve. 
Lumen of gut much constricted just anterior to cloaca. 
Digitiform gland a solid bud on dorsal surface of gut, pos- 
terior to last turn of spiral valve. Cloaca dilated — floor 
in contact with ectoderm for 0.6 mm. 
Vertical diameter of anterior end of oesophagus one-seventh 
of transverse. Posterior half of oesophagus solid. Stomach 
about 0.7 mm. in length (by sections), middle third dilated. 
Duodenum curved a little dorsally at anterior end, directed 
ventrally at an angle of 45° posteriorly. Vitelline duct enters 
gut on right side at base of duodenal curve. Twelve and 
one-half turns of spiral valve. Lumen of intestine much 
constricted just anterior to cloaca; one-seventh diameter of 
gut at this point. Digitiform gland 150 y in length (by 
sections), lies entirely dorsal to intestine attached to intestine 
along almost its entire length. Cloaca 0.4 mm. in length; 
its floor in contact, but not fused, with skin epithelium. Mes- 
enterial fenestra, lying anterior to digitiform gland, 0.5 mm. 
in length. 
Vertical diameter of oesophagus one-sixth of transverse. 
Lumen of posterior end occluded for 0.25 mm. (by sections). 
Longitudinal furrows in epithelium at lateral angles of an- 
terior part of oesophagus. Mesenchyma about epithelium 
much condensed, outer cells having elongated form of myo- 
blasts. Stomach oval in cross section, surrounded by dense 
stroma of thickened mesenchyma. Fundus projects as a 
blind pouch Ioo u posterior to pyloric opening. Duodenum 
joins stomach almost at right angles, posterior duodenal curve 
slightly indicated. Thirteen turns of spiral valve in dissected 
specimen of same size. Gut slightly constricted posterior 
to last turn of spiral valve. Lumen occluded in this region 
for 200 u (by sections). In occluded region, external surface 
of epithelium is indented by three to six grooves, giving the 
intestine a lobulated appearance in cross section. Digitiform 
gland 0.5 mm. in length; lumen irregular, opens into gut 
through a low papilla just posterior to solid region. Cloaca 
triangular in cross section, epithelium of its ventral surface 
fused with that of skin for a few sections. Dorsal wall of 
posterior part of cloaca indented by the swelling of fused 
portion of the Wolffian ducts. 
Lumen of oesophagus occluded at posterior end for 240 u 
(by sections). Longitudinal grooves in epithelium more 
numerous than in preceding stage. Duodenal curve flatly 
S-shaped. Vitelline duct in upper part about diameter of 
duodenum. Expanded to form an irregular sack before 
joining gut. Its opening into gut 140 y. in length (by sections). 
Fourteen turns of spiral valve. Lumen of gut occluded for 
100 u. (by sections), a little posterior to last tum of spiral 
valve. Grooves in surface of gutin this region extend through 
its outer third. In a dissected specimen of 28.0 mm., digiti- 
form gland 1.2 mm. in length, its inner surface sacculated. 
Opens into gut by a constricted and convoluted duct which 
arises a little in front of posterior end of gland. 
Furrows in epithelium of oesophagus as in preceding stage 
(No. 30). Three broad longitudinal folds, involving both 
epithelium and mesenchyma in dorsal wall of oesophagus. 
Lumen of oesophagus occluded for 0.3 mm. (by sections). 
Fourteen turns of spiral valve. Intestine 1.8 mm. in diameter 
in broadest (middle) part, in a dissected specimen of 34.0 mm,, 
digitiform gland 2.0 mm. in length. 
Folds in dorsal wall of oesophagus extend posteriorly to 
occluded portion. About six smaller longitu inal folds in 
ventral wall. Solid portion of oesophagus 0.3 mm. in length 
(by sections). Mesenchyma condensed immediately below 
epithelium. Muscularis a definite band of elongated cells as 
wide as epithelial lining of gut in this region. Lumen of 
intestine occluded for 0.3 mm. (by section) posterior to last 
turn of spiral valve. Grooves in gut in this region deeper 
and wider than in preceding stage. Digitiform gland 2.5 mm. 
in length. 
Vitelline duct, at its upper | 
Gall bladder 220 u in length (by sections), expanded at 
distal end. Connected with ductus choledochus by short 
broad cystic duct, which arises from anterior fourth of its 
dorsal wall. Ductus choledochus arises from an expanded 
chamber which receives two lateral and one anterior hepatic 
duct, and cystic duct. Joins intestine on ventral surface, 
140 y. anterior to opening of vitelline duct. Pancreas con- 
nected by broad neck with intestine, near the middle of 
left side, a short distance behind opening of vitelline duct. 
A distinct posterior diverticulum of this neck at juncture 
with gut. Spleen a slight thickening on left side of mes- 
entery, just posterior to stomach. 
Lateral lobes of liver extend backward to between first and 
second turns of spiral valve. Hepatic tubules from 40 to 50 u. 
in diameter. Eight to ten cells in cross section. Lumen about 
one-sixth diameter oftubule. Gall bladder elongated, expanded 
at ventral end. Cystic duct elongated as compared with pre- 
ceding stage; its long axis in vertical plane of body. Ductus 
choledochus not dilated at distal extremity. Arises from union 
of cystic and three hepatic ducts. In anterior half of course is 
arched dorsally. Joins intestine on left side, just behind open- 
ine of vitelline duct. — Body of pancreas lies above and to 
Ich of first turn of spiral valve. Made up of short irregular 
tubules opening into dilated distal extremity of pancreatic 
duct. Pancreatic duct joins intestine on left side of ventral 
surface, 120 u (by sections) posterior to entrance of ductus 
choledochus. A considerable mass of pancreatic tissue ventral 
and to left side of duct at its juncture with intestine. Spleen 
a projecting mass of thickened mesenchyma from left side of 
mesogastrium and mesentery, 370 4 in length (by sections). 
Liver approximately the same size as in preceding stage 
(No. 28). Tubules similar to those in preceding stage. 
Ductus choledochus joins intestine on ventral surface, just 
osterior to entrance of vitelline duct. Pancreatic duct 
joins intestine on ventral surface, 0.3 mm. (by sections) 
posterior to ductus choledochus. Spleen 0.5 mm. in length 
(by sections), extends posteriorly nearly to level of second 
turn of spiral valve; broadest posteriorly. Epithelium higher 
than that of mesentery. Mesenchyma of spleen compact 
and vascular. 
Lateral lobes of liver approximately 2.5 mm. in length, ex- 
tending backward to level of third turn of spiral valve. 
Average diameter of hepatic tubules 75 v. Lumen one- 
seventh diameter of tubule: bounded by six to eight cells 
in cross section. Ductus choledochus joins intestine on 
ventral surface, I.o mm. (by sections) posterior to entrance 
of vitelline duct. Pancreatic duct joins intestine on left 
side 0.2 mm. posterior to entrance of ductus choledochus. 
Lateral lobes of liver 7.3 mm. in length in dissected specimen 
of 34.0 mm. Extend backward to middle of body cavity. 
Lumina approximately one-tenth diameter of hepatic tubules. | 
Ductus choledochus joins intestine on right side, about 
0.6 mm (by sections) posterior to entrance of vitelline duct. 
Tubules of pancreas broadly separated by septa of mes- 
enchyma. Pancreas shows adult divisions of head, isthmus, 
and body. Pancreatic duct joins intestine on ventral sur- 
face at base of first turn of spiral valve, 0.25 mm. (by 
sections) posterior to entrance of ductus choledochus. Spleen 
1.5 mm. in length (by sections). Posteriorly attached to 
mesentery only by ventral border. 
Lateral lobes of liver 12.0 mm. in length in a dissected 
specimen of 37.0 mm; extend backwards through anterior 
three-fifths of body cavity. Generally six or seven cells in 
cross section of hepatic tubule. Lumen from one-twelfth 
to one-fifteenth diameter of tubule. Gall bladder 0.7 mm. 
in length (by sections), 0.2 mm. in diameter. Ductus chole- 
dochus 0.075 mm. in diameter, joined by four hepatie ducts 
in upper part of course, Joins intestine on the upper part 
of its right side, 0.6 mm. (by sections) posterior to entrance 
of vitelline duct. Epithelium of both gall bladder and ductus 
choledochus surrounded by dense layer of flattened mes- 
enchymal cells. Lower part of pancreatic duct lies in groove 
on floor. ofintestine. Joins withintestine on right side of ventral 
surface, 0.5 mm. posterior to entrance of ductus choledochus. 
27 
28 
29 
30 
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