Normal Plates of the Development of Squalus acanthias. 
19 
Archenteron 
Pharynz, Thyreoid, 
Supra- 
Thymus, 
pericardial Body 
Digestive 
Tube 
Liver, 
Pancreas, 
Spleen 
Urogenital 
System 
Heart, Peri- 
cardium, 
Blood Vessels 
Extremities 
Remarks 
No. 
No invagination. 
Invagination of archenteron forms a very shallow bay 
in posterior edge of blastoderm. Epithelium of the roof 
contains four of five rows of nuclei. No definite anterior 
end of archenteron, the entodermal cells there pass 
gradually off into the indifferent cells of the blastoderm. 
Archenteron a broad shallow invagination about 0.75 mm. 
in width, extending anteriorly from the posterior border 
of the blastodisc for En one tenth its length 
(i. e. 0.2 mm.). Epithelium of roof contains 3—5 rows 
of nuclei. 
Archenteron 0.75 mm. in length (by sections). A few 
cells from anterior end and from. anterior part of lateral 
walls extend ventrally to form floor of archenteron in 
that region. 
Archenteron 0.7 mm. in length. A slight fold in an- 
terior wall indicates beginning of formation of preoral 
process. Cells in floor of archenteron have increased 
in nımber. The nuclei of the anterior part of the roof 
lie at base of cells, leaving a clear outer border. 
Preoral gut projects forward as a distinct pouch for 
6 sections of 6 u. Posterior to preoral archenteron 
almost square in cross section. Extreme anterior part 
of fore gut floored by entoderm. Posterior to this, 
en cells form narrow shelves on lateral edges 
of floor. 
Preoral gut 92 u in length, ending bluntly anteriorly. 
Archenteron floored by entodermal cells for 120 u pos- 
terior to preoral gut. Small pocket in floor of arch- 
enteron just behind foregut. 
Preoral gut 240 y. in length (by sections), pointed an- 
teriorly, broadly oval in cross section posteriorly. An- 
terior third of archenteron floored with entoderm. Late- 
ral floor shelves of entoderm extend along greater part 
of length of archenteron. 
Preoral gut widely expanded laterally. Posterior to 
foregut, archenteron triangular in cross section. Ento- 
dermal cells form floor of anterior third of archenteron, 
broad lateral floor shelves posteriorly. 
Preoral gut extends through 45 sections of 6 x. For 
further history of archenteron see tables on pharynx 
and digestive tube. 
Reconstructed. 
Fig. 1. General structure. 
Reconstructed. 
Fig, 2. General structure. 
Reconstructed. 
Figs. 3 and 4. 
structure. 
General 
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