THE DEVELOPMENT OF ALCYON1UM DIGITATUM. 
77- 
are constantly arising in the ectoderm of the lateral wall, and 
some are often seen lying near the inner edge of the mesentery, 
and are therefore frequently found in the groove formed at 
this point. As the mesenteries grow these grooves deepen, 
and so more interstitial cells canenter. While the mesogloea 
of the body wall is being laid down outside the original 
supporting lamella (Text-fig. 44, Mg .), it appears to flow 
between these interstitial cells, cutting them off from the 
ectoderm either singly or in small groups, just as it envelops 
and isolates interstitial cells in other parts of the body wall 
(see chapter on Mesogloea) . It is not impossible then that any 
ectoderm cells found in the mesentery are introduced by 
mechanical means, i.e. the ingrowing of the mesentery 
draws a few interstitial cells after it, which become enclosed 
in the supporting lamella, this process continuing in some 
regions until a large number are enclosed. 
(3) Transverse sections of the mesentery in Text-fig. 43a 
at level T. (indicated by a dotted line) would show a few 
cells in the supporting lamella near the body wall. At level V. . 
a similar condition would obtain. Lower still, the lamella in 
cross-section would show a very deep groove full of cells 
near the lateral wall, and a very short single sheet or none at 
all (Text-fig. 45, V. or Z . ). Towards the base of the mesen- 
tery the lamella is double and full of ectoderm cells, or in 
some cases it contains a group of cells near its inner edge 
(Text-fig. 45, IF.), cut off from the outer deep groove T. by a 
region E. } containing no cells. A vertical section of Text- 
fig. 43, A., at line R. would obviously show a double lamella 
fall of ectoderm cells and surrounded by endoderm, and this 
would appear to be the kind of section figured by de Lacaze- 
Duthiers ( 10 ). 
Considered apart from the early history of the mesentery,. 
Text-fig. 45 seems to indicate that the mesenteries are true 
invaginations .of the body wall. A solid cord of ectoderm 
cells .seems to grow in, surrounded by a tubular ingrowth of 
the supporting lamella, .and a covering of endoderm cells, but 
whether the former cells are to be regarded as drawn in. 
