THE DEVELOPMENT OF THE CAPE OEPHALODISCUS. 211 
Increase in yolk lumen ( y . 1.), and disappearance of inner yolk columns 
from dorsal region of yolk lumen. 
PLATE 14. 
Fig. 20. — Longitudinal section showing details of formation of 
posterior body cavities, a second layer of cells forming the inner wall 
of b. c. 2 but not yet in b. c. 3 . 
Fig. 21. — Horizontal section showing the completed epithelial lining 
of b. c. 2 and b. c. 3 . 
Figs. 22-34. — Transverse sections selected from a series of 140 of 
a larva to show the positions and relations of the posterior body cavities 
(b. c. 2 and b.c. 3 ), the archenteron (arch.), the yolk (y), and the sense 
organ (s. o.). 
Fig. 22 is the 6th from the posterior end. 
23 
17th 
24 ., 
24th 
25 . 
27th 
26 .. 
29th 
27 
36th 
28 
. 40th 
29 
, 45 th 
30 . 
, 67th 
31 
95th 
32 , 
. 123rd 
33 . 
, 132nd 
34 , 
, 138th 
Fig. 35. — Transverse section from another series showing involution 
(inv. s. o.) in nervous tissue of sense organ below ectoderm. 
Fig. 36. — Transverse section showing postero-ventral thickening 
(p. v. th.), and involution ( p.v.inv .). 
Fig. 37. — Longitudinal vertical section of larva showing the anterior 
body cavity (b. c. x ). 
Fig. 38. — Longitudinal vertical section of posterior end of larva 
showing origin of anus (an.). 
VOL. 62, PART 2. NEW SERIES. 
15 
