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Normal Plates of the Development of Squalus acanthias. 23 
Liver, Urogenital 
Digestive Tube ee Sem Heart, Pericardium, Blood Vessels Extremities Remarks|No. 
pleen 
= — — = = — 12 
Gut anteriorly triangular in cross — — _ — — 13 
section, twice as high as broad; 
walls much thickened dorsally 
(45 u). Posteriorly, gut rapidly flatt- | 
tens as it approaches rim of blasto- 
disc. Walls of equal thickness 
(30 u), dorsally and ventrally. 
Gut anteriorly broadly triangular — — — = Recon- | 14 
in cross section; posteriorly, walls structed 
spread out until almost flat. Figs. 5 | 
and 6. 
General 
structure. 
| 
Anteriorly, gut higher and nar- = = _ _ — |B 
rower than in preceding stage. 
Floored with entoderm an- 
teriorly for 200 4 (by sections). | 
H 
Oesophagus approximately 0.Imm. = er Endothelial tube of heart indicated by two strands of cells lying | Ectoderm formsa | Recon- | 16 
in length (by sections). Nearly on either side of ventral part of pharynx. In some sections vas- |narrow thickened | structed | 
three times as high as broad. An- cular cells arranged as about a lumen. Ventral ends of lateral| plate along the | Fig. 7. | 
tero-posterior diameter of yolk plates of mesoderm turned mesially, and enclose in part the |median dorsal line General | 
stalk 2 mm. Hind gut 0.25 mm. vascular cells of this region. — Irregular, often interrupted, cords | of the trunk and |structure., 
in length. Twice as high as broad; of vascular cells represent first aortic arch and aortaein pharyngeal tail. In middle | Fig. 17. 
expanded dorsally. region. A few cells on either side of subnotal rod in middle trunk | trunk region it is Hd 
region represent a second segment of aortae. Anlage of ventral| elevated, forming | somites. 
aorta not connected with that of first aortic arch? Vitelline veins | a median fold 23 u | 
small, irregular, often collapsed. Can be traced backward onlya;, high. No mes- | 
few sections from heart, Left vitelline vein larger and more |enchyma between | 
distinct than right. dorsal surface of | 
neural tube and | 
ectoderm. | 
Oesophagus two thirds as broad Ei Three solid pro- | Anterior part of primitive coelom somewhat dilated to form peri- Median fold ex- — 4 
as high. Lateral walls 60 u in nephric anlagen | Cardial cavity. Endothelial tube of heart bifurcated behind, simple| tends over a | 
thickness. Antero-posterior dia- projecting from straight tube anteriorly. Mesothelial layer of heart connected with | greather length of | 
meter of yolk stalk 2 mm. Hind 7th, 8th and gth splanchnic mesoderm above by a broad dorsal mesocardium, below | trunk region than | 
gut 0.3 mm. in length; lateral segments. Wolffian |by a narrow ventral mesocardium which is interrupted for three |in preceding em- | 
walls and roof muck thicker than duct a solid cord | or four sections in middle heart region. — Ventral aortae, first, bryo. Greatest | 
floor. of cells attached to |aortic arch, dorsal aortae, and anterior ends of vitelline veins re- height3o,. Median 
first, second, and presented by cords of vascular cells, which are generally continuous thickening extends 
anterior part of and in some places arranged as though about a lumen. Cells of to end of tail. Ecto- 
third pronephritic dorsal aortae extend posteriorly to middle of trunk. derm thickened 
anlagen; does not jalong median ven- 
extend posterior to tral line of tail. | 
gth segment. 
Oesophagus very much flattened — Pronephric an- |Heart 0.25 mm. in length (by sections). Ventral mesocardium en- Median fold ex- | Recon- | 18 
laterally ; in narrowest portion lagen solid, present tirely broken down. Dorsal mesocardium continuous length of tends around tail | structed. 
twice as high as broad; lateral on 7th, 8th and gth heart, but very thin over middle of heart tube. Posteriorly, heart and for I2o u. (by, Fig. 8. 
walls almost in contact. Gut ex- segments, and just distinctly dilated on the right side. Endothelial tube bifurcated | sections) along | General 
panded laterally just in front of forming on Iöth in posterior part of heart. — Ventral aortae and first aortic arch ventral line. |[structure.| 
cloacal region. Ectoderm and ento- segment on left |thin-walled tubes. Dorsal aortae extremely irregular in calibre, Fig. 18. 
derm in contact for 30 u (by side. Wolffian duct |apparently solid in places, extend along the anterior two thirds Head 
sections) on ventral surface of solid,sickle-shaped of trunk. Fused to form a single vessel over anterior part of yolk somites. 
cloaca. Neurenteric canal widely in cross section an- stalk. A small enlargement and division of dorsal aorta on left | 
open. teriorly, circular |side, over second gill pouch, represents beginning of second | 
posteriorly; ex- |aortic arch? — Vena capitis medialis represented by a small | 
tends through 26 median vessel from anterior end of aorta. Right and left vitelline 
sections of 6 u. | veins can be traced from heart to blastoderm. Left vitelline vein 
Fused with ecto- |the larger. Scattered vascular cells, representing subintestinal 
derm forlasttwo or | veins, extend backward on either side of gut nearly to cloaca. 
three sections. Continuous with vitelline veins anteriorly ? 
Oesophagus approximately twice |Liver a shal- | Solid pronephric | Heart somewhat expanded ascompared with preceding stage (No. 18). Like preceding == 19 
as high as broad. Antero-posterior lowpouch,s6,.J anlagen from 7th | Middle half of dorsal mesocardium broken through. Posterior fourth | stage (No. 18). | 
diameter of vitelline duct I30 x (by [in length (by | to Iıth segments | ofendothelial tube of heart bifurcated. — Dorsal aorta extends back- | 
sections). Intestine approximately | sections) in inclusive. Wolffian | ward nearly to cloaca. Bifurcated at its extreme posterior end and 
0.5 mm. in length (by sections) from |ventral wall of duct solid, but cells | from the middle of the oesophagus to above first gill pouch. Three 
posterior wall of vitelline duct to| gut, where |of anterior end are | diverticula of aorta on right side represent’pronephric arteries. First 
the cloaca. A broad shallow groove |joining yolk | arranged as though and second aortic arches completely formed. Ventral aortae 
on right side of upper part of in- |stalk. Shallow about alumen. Ex- |separate throughout. — Vena capitis medialis broadly connected 
testine, may represent beginning of lateral tends posteriorly | with anterior end of aorta. Extends forward from this connection 
spiral valve. Lumen ofintestinecon- |grooves on ex-| for 47 sections of |to optic cup, and backward to posterior edge of trigeminal crest. 
stricted dorsally. Vertical diameter |ternal surface Su. Right and left vitelline veins extend to blastoderm. Left vitelline 
of cloaca two and one half times | of gut mark veinthelarger. Right and left subintestinal veins,! which communi- 1 
transverse. Ectoderm and ento- |dorsal boun- cate with respective vitelline veins. Subintestinal veins anastomose 
derm in contact on ventral surface. | daries of liver below gut, just posterior to vitelline duct. Left subintestinal vein 
larger than right. Both can be traced backward to cloaca. 
Neurenteric canal widely open. 
pouch. 
