402 E. W. MACBRIDE, 
Fic. 13.—Right view of the same larva. 
Fic. 14.—Ventral view of larva of about nine days. The arm rudiments 
form a nearly complete circle. The lobes of the water-vascular system have 
developed two pairs of accessory lobes each. Stage F. 
Fie. 15.—Right view of the same larva of about nine days. 
Fie. 16.—Left view of the same larva of about nine days. 
Fie. 17.—Oral view of just metamorphosed star-fish about ten days old. 
amb. Ambulacral ossicles. Stage G. 
Fie. 18.—Aboral view of another specimen of the same age. C. Central 
plate. JB. Basal. 7. Terminal. The curve of the arm rudiments has become 
a circle, No. V coming to be apposed to the lobe No. 1 of the water-vascular 
system. mp. Madreporic pore. Stage G. 
Fig. 19.—Aboral view of a young star-fish sixteen days old. Notice the 
anus, the additional calcareous plates, and the spines. 
PLATE 19. 
All the sections represented in this plate are magnified 80 diameters, and, 
except where otherwise stated, they have been cut parallel to the ‘larval 
plane,” i.e. they are horizontal longitudinal sections. Where several sections 
from the same series are figured, the most dorsal is in every case put first. 
The darkest shade represents the epithelium of the gut; the intermediate 
shade represents ectodermic and ccelomic epithelium, including the lining of 
the derivatives of the ccelom ; the lightest shade represents the cavity of the 
blastoccele with all its contained structures, jelly, fibres, cells, &c., and also 
the muscular tails of the epithelial cells lining the water-vascular system. In 
Fig. 27, however, a portion of the gut opening into the celom, and in Figs. 
30, 31, 34, 39, 40, and 41 the larval cesophagus, have been printed (through 
oversight) in the intermediate tint. 
Fics. 20 and 21.—Two sections of a gastrula a little older than Stage A. 
No mesenchyme has as yet appeared. 
Fic. 22.—Sagittal section of a gastrula about the same stage as Fig. 3. 
mes. Mesenchyme cells. 
Fic. 23.—Section of an embryo older than that shown in Fig. 3. It shows 
the differentiation of the archenteron into gut and cclom. 
Fics. 24 and 25.—Two sections of an embryo somewhat younger than Stage 
B, and still enclosed in the vitelline membrane. The ccelom has grown back 
at each side of the gut, forming two posterior lobes, /pe., rpc. Fig. 25 
shows, however, that these lobes do not as yet extend ventral to the gut. 
Fic. 26.—Section of larva rather older than Stage B, to show the forma- 
tion of the madreporie pore. pe. Pore-canal ending blindly in contact with 
the ectoderm. /. stom. Larval stomach. mp. Thickening of ectoderm where 
the primary madreporic pore will be formed. 
