78 
ANNIE MATTHEWS. 
mechanically during the ingrowth of the mesentery, or 
whether they have any embryological significance imply- 
ing a true invagination of both layers is not altogether 
certain. If the first, then the mesentery is purely endo- 
dermic, as Wilson believes ; if the second, then de Lacaze- 
Duthiers is correct. The total evidence afforded by these 
investigations seems, perhaps, to support the latter view. It 
mav be added that the rudimentary mesenteries contain many 
ectoderm cells embedded in the supporting lamella. 
11. Tentacles, Mouth and Stomoda]um. 
Eight short broad tentacles develop on the third day 
(PI. 3, fig. 5). They are simple hollow outgrowths of the 
upper surface of the young polyp, consisting, therefore, of 
ectoderm and endoderm separated by a supporting lamella 
(Text-figs. 46 and 47, T .), and arise between the mesenteries 
in a circle round the oral disc. A little later a shallow 
invagination of the latter forms the beginning of the stonio- 
daeum. This grows considerably in size and dept i, the 
exterior and only opening being the mouth (PI. 3, fig. 7, and 
PI. 4, fig. 28). The lumen of the invagination soon widens 
considerably at the base, and the floor of the cavity is 
correspondingly enlarged (Text-fig. 46). During the fourth 
day the endoderm and ectoderm of this floor degenerate, 
either at the centre or near the side or in both regions simul- 
taneously (Text-fig. 47). By this means communication is 
established between the ccelenteron and the exterior, and 
food may enter from outside. Until now all requisite nourish- 
ment has been provided by the embryonic yolk. By the fifth 
day the disintegration is complete, and the formation of the 
mouth and stomodaeum is accomplished. The ectoderm 
covering: the latter consists of taller columnar cells than else- 
where. Like the tentacles it is plentifully supplied with 
nematocysts, which, indeed, at this stage are abundant all 
over the surface of the polyp. Mucous cells and granular 
^land cells are soon formed in the stomodaeum and pour their 
