THE DEVELOPMENT OE ALCYONIUM DIGITATUM. 
73 
{End.), which in the early stages are long and columnar, but 
become low and broad later when the mouth has opened. The 
mesenteries arise simultaneously early on the second day of 
fixation, and usually do not appear until the flattening of the 
larva is complete. They are visible before the original 
supporting lamella of the body wall is thickened by the 
addition of fresh layers of mesogloeal substance, and therefore 
before the development of the spicules, tentacles, and mouth, 
these organs following them in the order stated. Between 
the normal eight as many as eighteen rudimentary mesenteries 
are often developed (Text-fig. 38, Sub.M.), but these either 
remain very small or eventually disappear, and are possibly 
vestiges of a more primitive condition when the mesenteries 
were very numerous (cf. Clavellina ( 9 )). In colonial forms 
the new polyps formed by budding never show these rudi- 
ments. In the early stages the mesenteries are indicated 
from outside by eight equidistant shallow vertical grooves, 
stretching up a little way from the base of the lateral wall 
(PL 3, fig. 3), while sections of rather older stages indicate 
that the mesenteries are already arranged in four pairs (Text- 
fig. 39). At this stage the mesenteries in vertical radial 
section resemble Text-fig. 12, c. During the earliest stage 
observed (Text-fig. 40, a.) the supporting lamella of each 
mesentery, consisting of a very thin sheet of mesoglcea, can 
be easily traced. Sections show it growing radially inwards 
from the supporting lamella of the body wall (Text-fig. 41, 
Mes.) between the endoderm cells lining the latter. At this 
time it is structureless, stains slightly with picro-nigrosin, and 
encloses no cells. Each mesentery grows rapidly upwards 
along the body wall and along the base of the polyp towards 
the centre (Text-figs. 40, bi, and 12, c. and d.). Meanwhile it 
increases in radial depth (Text-fig. 40, Zq), and as the supporting 
lamella deepens, the endoderm cells surrounding it grow 
inwards with it, so that each mesentery soon projects into the 
coelenteron as a shallow ridge formed by an infolding of the 
endoderm supported by a central ridge, the lamella (Text- 
fig. 42, Mes.). Figures of subsequent stages show that the 
