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Normal Plates of the Development of Squalus acanthias. 39 
Extremities 
Remarks 
Posterior and anterior dorsal fins approximately 0.25 mm. high. Median fold between 
dorsal fins almost obliterated. Posterior part of dorsal and ventral folds thickened 
to form caudal fin. Pectoral fin thickened at its base, approximately 2 mm. long. 
Muscle buds very much constricted at proximal ends. Distal ends expanded and 
bifid. Nearly all buds united by mesothelial processes. Pelvic fins represented by a 
very slight swelling of the somatopleure just anterior to cloaca; mesenchyma in- 
creased in amount and condensed; epithelium composed of two layers of cells. 
Anterior dorsal fin 1.6 mm. in length at base, nearly 0.5 mm. high; posterior edge 
rising more abruptly from back line than anterior. Posterior dorsal fin I mm. ın 
length at base, 0.5 mm. high, in shape as anterior fin. Mesenchyma of dorsal fins 
condensed. Cells proliferating into the mass from dorsal myotomes of fin region? 
Median fin fold in caudal region 0.4 mm. high. Pectoral fin 2 mm. in length, 1.2 mm 
broad, gradually rounded anteriorly, sharply rounded posteriorly; turned a little 
ventrally posteriorly. Primary muscle buds almost completely divided into dorsal 
and ventral secondary buds, which extend to respective surfaces of fins. Primary 
buds completely separated from somites. Pectoral girdle represented by a slight 
condensation of mesenchyma in body wall on either side, below anterior part of 
pectoral fin. Pelvic fins 2 mm. in length at base, posteriorly fusing into low rounded 
ridge behind the cloaca; mesenchyma much condensed. Slight division of distal end 
of muscle buds into dorsal and ventral diverticula. Primary buds connected with 
somites by slender necks which are becoming mesenchymous. 
Anterior dorsal fin 2 mm. long at base, 0.6 mm. high. Posterior fin 1.5 mm. long at 
base. Transverse diameter of fins about equal at base and distal edge. Caudal fin 
about 5 mm. long, 0.5 mm. high dorsally and ventrally. Median fin fold has entirely 
disappeared except between posterior dorsal and caudal fin. Pectoral fin extended 
outward at right angles to body, anterior edge roundly curved, posterior edge 
decidedly concave, about 2 mm. long at base. Proximal part of muscle buds be- 
coming Abrous. Anlage of basipterygium a marked condensation of the mesenchyma 
at base of anterior fin. Continuous proximally with the condensation representing 
the pectoral girdle. Lateral anlagen of pectoral girdle are fused at median line 
anterior to yolk stalk. Extend N to level of aorta. Dorsal extensions pro- 
longed posteriorly. Pelvic fin 2.0 mm. long at base, 0.75 mm. broad; anterior edge 
rounded, posterior edge right-angled. 
Median fins much as in preceding stage (No. 29); caudal fin a little broader. Pectoral 
fin of about same dimensions as in preceding stage. Slight differentiation of basi- 
pterygium condensation into an anterior part (propterygium) and a posterior part 
(meso- and metapterygium). Distal portion widely expanded, but not divided as yet 
into radials. Distal ends of muscle buds undifferentiated. Pelvic fin 2.5 mm. in length 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, digestive tract, and somites. — Mouth de- 
scribed from specimen of same stage, but not one of series. 
h Fig. 22. Digestive tract and heart of dissected specimen of same 
stage. 
Literature: MINOT, C. S., 1901, Morphology of the pineal region. Amer. 
Journ. Anat., I (Series 224). 
Reconstructed. 
Fig. 16. General structure. 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, digestive tract. — Mouth described from 
specimen of same stage, but not one of series. | 
‘ Fig. 23. Digestive tract and heart of dissected specimen of same 
stage. 
Literature: MINOT, C. S., 190I, Morphology of the pineal region. Loc. 
cit. (Series 228.) 
Woops, F. A., 1902, Origin and migration of the germ cells 
in Acanthias. Amer. Journ. Anat., II. 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, digestive tract. 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, digestive tract. 
Fig. 24. Digestive tract and heart of dissected specimen of same 
stage. 
at base. Basipterygium indicated by a very slight thickening of mesenchyma. Muscle 
buds on upper and lower surfaces of fin. 
Dorsal fins higher and more rounded than in preceding stage (No. 30). Posterior borders 
concave; base of each about 2 mm. in length. Caudal fin much expanded, point of 
greatest height on dorsal fold, posterior to that of ventral. Pectoral fin 2 mm. long 
at base, distal edge 3 mm. long, posterior edge directed sharply backward. Basale 
propterygii, mesopterygii, and metapterygü, outlined in thickened mesenchyma. Fin 
rays long radial thickenings ot mesenchyma, — about nine on distal edge of meso- 
pterygium and six on metapterygium. Pelvic girdle outlined in dense mesenchyma. 
Lateral halves fused beneath posterior end of pericardial cavity. Pelvic fin 2 mm. 
long at base, Primary basipterygium distinctly outlined, — rays from its free edge. 
Spines of anterior and posterior dorsal fins represented by small blunt projections 
from base of anterior edge of fin. General shape of pectoral and pelvic fins un- 
changed from preceding stage (No. 31). Ventral portion of pectoral girdle and basi- 
pterygii precartilaginous. Fin rays much longer and more definitely outlined. Basi- 
pterygium of pelvic fin elongated, numerous elongated rays on distal edge. 
Cleared and dissected specimen of same stage used in preparation of 
descriptions of heart, brain, digestive tract, and somites. — Mouth de- | 
scribed from specimen of same stage, but not one of series. —- External | 
appearance of lateral line system described from specimen of same stage, 
but not one of series. 
j Fig. 25. Digestive tract and heart of dissected specimen of same 
stage. 
Literature: MINOT, C.S., 1901, Morphology of the pineal region. Loc. cit. 
(Series 362). 
Cleared and dissected specimen of same stage used in preparation of 
descriptions of heart, brain, digestive tract, and somites. — Mouth de- 
scribed from specimen of same stage, but not one of series. — External ap- 
pearance of lateral line system described from specimen of same stage, 
but not one of series. 
Fig. 26. Digestive tract and heart of dissected specimen of same 
stage. 
Literature: Locy, W. A., 1905, On a newly recognized nerve, connected | 
with the fore-brain of Selachianss Anat. Anz, XXVI 
(Series 353). 
28 
29 
30 
31 
32 
