76 
ANNIE MATTHEWS. 
serve to show how support for both views could be obtained 
in the case of Alcyonium according to the part of the 
•mesentery studied : 
(1) Sections show that by the time the mesenteries reach 
up to the oral surface of the polyp, the supporting lamella 
contains a great many ectoderm cells which lie very near the 
body wall. Text-fig. 43a is reconstructed from a series of 
sections of a polyp with mesenteries at this stage. It is a 
diagram of the supporting lamella of one mesentery, while 
the covering endoderm is not represented. Many small 
round cells are seen embedded in it towards the outer edge 
and at the base (Ect. C.). These are nematocysts and young 
scleroblasts which have entered from the ectoderm in a 
manner to be discussed later. Therefore both ectoderm and 
endoderm elements occur in the mesentery at this stage. 
However, it is seen that the greater part of the lamella con- 
tains no ectoderm cells, while as the mesentery grows larger 
the proportion of the lamella which contains ectoderm elements 
grow necessarily less and less, so that in the adult the 
mesentery is almost wholly endodermic in structure. This 
.seems to support the view that the function of the ectoderm 
cells in the mesentery may be unimportant, and that the 
endodermic covering can provide for all further increase in 
the size of the lamella. 1 
(2) When the mesentery first develops, its supporting 
lamella grows radially inwards from the lamella of the body 
wall and is directly continuous with it (Text-figs. 41 and 42, 
Mes.). A little later, as the mesentery continues to grow 
inwards, the lamella of the lateral wall often appears slightly 
pulled in where the mesentery joins (Text-fig. 43, L. /.), and 
these two cases may appear in one section. Interstitial cells 
1 The fate of the nematocysts is uncertain: possibly they migrate 
to some definite place for future use. It is known that they occur 
in the six ventral mesenteric filaments, but probably all of these are 
derived from the ectoderm of the stomodseum, as described later. 
The spicules secreted by the scleroblasts found in the lamella would 
•certainly help to stiffen it at the base and near the body wall. 
