194 Origin of the Paired Limbs of Vertebrates 
(p) is situated quite ventral to the gill-arch (g) and that it is also situated 
outside of the blood-vessels next to the ectoderm, while the gill-arch is in- 
ternal to the blood-vessels and next to the enteron. 
In the region indicated at “x* the mesenchyme e¢ells are just beginning to 
take the first step in concentration toward the formation of the scapular part 
of the girdle. 
PLATE III. 
EMBRYONIC STRUCTURES OF CESTRACION. 
Fics. 8, 9,10. A series showing the gradual approach of the pectoral girdle 
to the gill-arches. The fourth gill-arch is included in the drawing to show the 
distance between the gill-arches as compared with that between the girdle and 
the fifth arch. Fig. 8 shows the girdle at an early stage after the formation 
of its dorsal (scapular) part. Fig. 9 is a somewhat later stage (length 35 
mm.). Fig. 10 is a still later stage (53 mm.). Somewhat later than this 
the fifth gill-arch passes partially under the girdle. 
Fic. 11. The skeleton of the anal’fin is differentiating out of the mesen- 
chyme plate as a continuous anlage (mn, neural cord; nc, notochord; ca, caudal 
artery, and cv, caudal vein). 
Fics. 12, 13. Cross-sections through the anal fin of an embryo of 40 mm. 
to illustrate that the fin skeleton arises independently of the axial skeleton. 
Fig. 12 is through the anterior end, Fig. 13 through the middle of the basale 
(bd) of the fin skeleton. 
Fic. 14. Cross-section showing origin of brachial artery (¥) from dorsal 
aorta, and the junction of the last pair of efferent branchial arteries (e€) with 
the dorsal aorta. 
PLATE IV. 
Fig. 15. Reconstruction of second dorsal fin of Cestracion (53 mm.) to 
show discrepancy of muscles (outlined in black) and skeleton (shaded) (s, 
fin spine). 
Fic. 16. Camera drawing of a Van Wijhe preparation showing skeleton of 
anal fin of a 225 mm. embryo of Chlamydoselachus. The separation from the 
axial skeleton is very marked (hs, hemal spines). 
Fig. 17. Fusion of muscle-buds (m) in the anal fin of a 35 mm. embryo of 
Cestracion. Drawn with camera. 
Fic. 18. Second dorsal fin of Cestracion, 58 mm., showing fusion of 
muscles. Camera drawing from an actual section. 
PLATE V. 
Fic. 19. Dorsal fin of Chlamydoselachus, 225 mm. Camera drawing of a 
Van Wijhe preparation. Note the wide separation from the axial skeleton 
(sp, spinous processes ). 
Fic. 20. Pelvic fin skeleton of Chlamydoselachus, 225 mm. Camera draw- 
ing of a Van Wijhe preparation. Note the flat girdle .(b) pierced by eight 
nerve foramina, and serving as a basale for about half of the radials of the fin. 
The two girdles fuse at the midline in the mesenchyme stage and the separa- 
tion at the anterior end is not yet complete. There is no indication as yet 
of the antero-median wedge-shaped element figured by Garman, 85-86, as 
lying between the girdles at their anterior ends in the adult. 
