THE DEVELOPMENT OF ALCYONIUM DIGIT ATUM. 
93 
outgrowth over the free edge of the dorsal mesentery, which forms 
the dorsal mesenteric filament, D. 0. The dorsal mesentery is only 
partly shown at D. M. x 133. 
Fig. 27. — Part of ectoderm and endoderm of swimming planula. 
Multilayered and adjacent one-layered endoderm both shown. Granular 
ectoderm cells expanded at base rest on the separating membrane. 
Round interstitial ectoderm cells seen. Endoderm still full of yolk 
globules. X 760. (Ectoderm and endoderm of the pre-planula in the 
latest stage are very similar in detail.) 
Fig. 28. — Vertical section of polyp showing stomodseal invagination. 
There is as yet no communication with the exterior. Yolky detritus 
is still present, and mesoglea is flowing between the ectoderm cells of 
the attached base, and round the inner ectoderm cells of the lateral 
walls. X 133. (The stomodseal invagination is cut across laterally, 
and so is very narrow.) 
Fig. 29. — Transverse section of polyp settled from thirteen to fifteen 
days, cut below stomodseum ; the six ventral and two dorsal mesenteric 
filaments are shown. X 133. 
Fig. 30. — Section through the mesogloea of a fairly old polyp after 
dissolving the calcareous part of the spicules by staining with picro- 
nigrosin. The nuclei and organic remains of the spicules are shown, 
and it can be seen that the cavities occupied in the mesoglea by the 
spicules are replicas of the latter, x 245. 
Fig. 31. — Young spicules in scleroblasts, from solitary polyp. X 760. 
Fig. 32. — Young scleroblasts before secretion of spicule. X 760. 
Fig. 33. — Vertical section through the stomodseum of a polyp five 
days fixed. The remains of the degenerate base of the stomodseum 
are still visible, and the ectoderm of the stomodseum is growing down 
over a mesentery as a strap-like process (E. S. 0 .) ; the section is 
beyond the actual mouth opening. The rest of the mesentery ( Mes .) 
was seen in subsequent sections. X 450. 
Fig. 34. — Transverse section of a polyp settled from thirteen to 
fifteen days, cutting through the siplionoglyph. x 133. 
PLATE 5. 
Figs. 35, 36, 37 and 39. — Vertical radial sections of part of wall of 
young settled polyp, showing ectoderm, endoderm and origin of 
mesoglcea : 
Fig. 35. — Mesogloea secreted by endoderm, beginning to stream 
between the interstitial cells at the base of the ectoderm, at the time 
of the early formation of the mesenteries. X 380. 
