1, 
Normal Plates of the Development of Squalus acanthias. 67 
wall has been removed and the dissection has been carried forward to the base of the mandibular arches. 
The thyreoid gland (T%R.) is a small tongue-like body, the base of which is attached to the pharynx. 
The heart still possesses its primitive “S”-shape, and in the view presented only the 
ventricle and bulbus cordis are seen, these structures obscuring the dorsally placed 
atrium. Although the septum transversum is formed along the median line a, 
this stage, it is obscured in this view by the overlapping liver. The liver (L.) 
consists of two lateral lobes and an anterior median and connecting portion. The 
right lobe is somewhat larger than the left. The vitelline duct opens slightly 
to the right side into a somewhat dilated portion of the gut. To the left of 
the vitelline duct is seen the projecting ventral edge of the pancreas (Panc.). The 
spiral valve (Sp.v.) makes eight turns. The great obliquity of the upper turns 
as compared with the lower is noticeable. The gut narrows rapidiy behind the 
last turn of the spiral valve, as it approaches the cloaca. The cloacal plate 
(Cl.p.) is elongately heart-shaped in outline and its central portion is slightly 
depressed. 
Fig. 22. Dissection of an embryo I8 mm. in length, corresponding with normal plate series 
N02 2726: 
The plan of this dissection is the same as that shown in the preceding 
figure. The thyreoid (Th.) is now attached to the pharynx by a slender anterior 
stalk. The bulbus cordis (B.cor.) is somewhat dilated as compared with the 
same structure in the preceding figure and the ventricle is a little narrower and 
longer. The somewhat dilated atrium (Af.) is seen below the anterior edge of 
the ventricle. The right, left and median lobes of the liver are similar to those 
of the preceding dissection, and in this specimen also the right lobe is somewhat 
larger than the left. The vitelline duct is cut off somewhat closer to the intestine 
in this dissection than in the preceding one. The projecting ventral edge of the 
pancreas (Panc.) is again seen to the left of the vitelline duct. There are eleven 
turns of the spiral valve ($p.v.). Dorsally and to the right of the gut immediately 
behind the last turn of the spiral valve there projects a small pouch, the anlage of 
the digitiform or rectal gland (Dig.gl.). The cloacal plate (Cl.p.) is now bounded by 
a pair of rounded arches which meet in front but are separated posteriorly by a 
narrow cleft. Fig. 22. 
Fig. 23. Dissection of an embryo 21.0 mm. in length, corresponding to normal plate series 
N082702%06: 
In this embryo the lateral and ventral chambers of the nasal pits are sharply marked off, and 
the beginning of the canalis medialis (On.m.) of the lateral line system is just visable. The mouth is 
broadly oval in outline and slender gill filaments project from all of the gill slits. No attempt has been 
made to expose the thyreoid in this dissection. The heart is somewhat shorter and broader than in 
the preceding stages. The bulbus cordis (B.cor.) shows a marked bulging which is characteristic of 
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