THE DEVELOPMENT OF ALCYONIUM BIG L TATUM. 
75- 
mesentery is free along its inner edge but attached elsewhere 
to the body wall and base of the polyp. By the end of the 
second day the mesenteries nearly meet in the centre of the 
oral and aboral surfaces of the polyp, but they are still fairly 
shallow (PL 3, fig. 5, and Text-fig. 12, d.). After the forma- 
tion of the tentacles the stomodaeum develops as an invagina- 
tion of the oral surface, in the space encircled by the upper 
edges of the mesenteries, and as it grows inwards the upper 
edges of the mesenteries are carried down with it, so that all 
eight become attached along the entire length of the stomo- 
daeum (Text-fig. 13,/. and S.). There has been some discussion 
as to whether a mesentery is purely endodermic in origin (the 
view taken by Wilson and Kowalevsky), or whether both the 
endoderm and ectoderm contribute to its formation (the view 
held by de Lacaze-Duthiers), and the following account may. 
Fig. 38. Aboral view of polyp, some hours older than PI. 3, fig. 4. 
The eight permanent mesenteries are distinguished from the 
subsidiary ones by their greater development. Spicules just 
appearing (x 27). Fig. 39. Transverse section of polyp about 
stage drawn in Text-fig. 12, showing paired arrangement of 
mesenteries. Fig. 40. Vertical section of polyp with very 
young mesenteries, early on second day of fixation. The 
vertical dotted line divides the diagram into halves, the right 
showing a more advanced stage than the left. At a. a very 
young mesentery is drawn, which does not yet project beyond 
the endoderm. At b x a rather older mesentery is shown (at 
a stage similar to the mesenteries in Text-fig. 42). Fig. 41. 
Diagram of a transverse section of a young polyp near its 
base. The mesentery is similar to that shown in Text-fig. 40 
(a). Hence section cuts through a solid layer of endoderm 
(the multilayered endoderm at the base of the young settled 
polyp), and through the supporting lamella of eight young 
mesenteries ( Mes .). Fig. 42. Transverse section of a rather 
older polyp, at stage drawn in Text-fig. 40 (&/ some distance 
higher up than the level of Text-fig. 41. The mesenteric 
ridges have deepened radially, and are covered by columnar 
endoderm cells. Fig. 43. Transverse section of polyp some- 
what older than Text-fig. 42, showing how the lamella of the 
lateral wall becomes pulled in during the inward growth 
of the mesenteries. Fig. 43a. Lamella of young mesentery, 
reconstructed from sections. Fig. 44. Here the mesoglcea is 
being rapidly thickened. It is seen penetrating between the 
ectodermic cells which have been drawn into the root of the 
mesentery, and isolating them (Mg. and I. C. S.). Fig. 45. 
Transverse section of polyp, showing mesenteries cut across- 
near base, i.e. at level M. B. in Text-fig. 43a. 
