ic 
ds 
S, 
15 
1 
Median fold approximately as in preceding stage (No. 19), its free edge a little 
- line, epithelium two-layered. Posteriorly, the fold is as in preceding stage. Mes- 
Normal Plates of the Development of Squalus acanthias. 31 
Extremities 
Remarks 
broader than base. Ventral fold higher than dorsal, extends anteriorly to cloaca. 
Dorsal median fold 20 u high in trunk region, 40 u high in cloacal region. A little 
mesenchyma between dorsal surface of cord and skin ectoderm in anterior and 
middle trunk region. 
Ectoderm of the sides of median fold separated a little anteriorly. Mesenchyma 
between dorsal surface of cord and skin ectoderm in anterior half of tail region. 
Posteriorly as in preceding stage. Fold highest over and of tail. 
Much as preceding stage (No. 22). In tail region, dorsal fold 40 u high, ventral 
fold 80 u high. 
Ventral fold two to three times as high as dorsal. Anterior part of dorsal fold 
broadly V-shaped in cross section. Mesenchyma forms a slightly condensed mass 
between the ridge and dorsal wall of neural tube. On either side of the median 
enchyma at base of ventral median ridge, condensed and increased in amount. Pectoral 
fin fold appears externally as a rounded horizontal ridge about 1.o mm. in length, 
lyingljust posterior to branchial plate. Epithelium two-layered in this region. Con- 
siderable mesenchyma lying between it and body cavity mesothelium. Ventral ends 
of somites extend to dorsal end of fold. Somites enlarged at this point, with small 
ateral diverticula which represent beginning of primary muscle buds. 
Pectoral fin fold approximately I mm. long, the posterior end directed a little 
ventrally. Middle three-fifths of the distal edge bent sharply ventrally. Epithelium 
much thickened over this edge. Mesenchyma increased in amount and much con- 
densed. Ventral ends of somites extend into dorsal third of fin fold. 
Anlagen of anterior and posterior dorsal fins as elevations of median fin fold. 
Between elevations, median fold much flattened. Pectoral fin approximately 1.5 mm. 
in length, its posterior end best defined and curved a little ventrally. Distal border 
increased in thickness. Distal edge turned sharply ventrally along most of its 
length. Epithelium two layered, inner layer of high columnar cells. Mesenchyma 
much thickened distally. Muscle buds expanded distally; anterior ones joined together 
by constricted mesothelial processes. Beginning of division of primary buds into 
secondary dorsal and ventral buds. 
Reconstructed, j 
Fig. 9. General structure of head and pharyngeal region 
Fig. ı9. Head somites. 
Reconstructed. 
Fig. ı0. General structure. 
Cleared specimens of same stage used in preparation of descriptions of 
heart and brain. 
Reconstructed. 
Fig. II. General structure of anterior half of embryo. 
Fig. 20. Head somites. 
Cleared specimens of same stage used in preparation of descriptions of 
heart, brain, and somites. 
Reconstructed. 
Fig. 12. General structure of anterior half of embryo. 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, digestive tract and somites. — Mouth described 
from specimen of same stage, but not of series. 
Literature: MINOT, C. S., IgoI, Morphology of the pineal region, based 
upon its development in Acanthias. Amer. Journ. Anat., 
I, 81—98, 14 figs. 
Woops, F. A., I902, Origin and migration of the germ cells 
in Acanthias Amer. Journ. Anat., I, 307—320. 
Cleared and dissected specimens of same stage used in preparation of| 
descriptions of heart, brain, and digestive tract. | 
Reconstructed. 
Fig. 13. General structure. 
Fig. 14. Aortic arches. 
Cleared and dissected specimens of same stage used in preparation of 
descriptions of heart, brain, and digestive tract. — Mouth described trom 
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. | 
Fig. 21. Digestive tract and heart of dissected specimen of same 
stage. 
Literature: MınoT, C. S., 1901, Morphology of the pineal region. 
Journ. Anat. 
Woops, F. A., 1902, Origin and migration of the germ cells 
in Acanthias. Loc. cit. 
Amer. 
20 
21 
22 
23 
24 
25 
26 
