52 
Psyche 
[June 
In the embryo, the stomodseum begins as a shallow de- 
pression in the ectoderm at the anterior end shortly after 
the thirtieth hour, coincident with the beginning of seg- 
mentation. As it deepens, it forms a tube closed at its inner 
end by ectodermal cells (fig. 1). These cells become thin- 
ner as the end of the tube widens until at the sixty-fifth 
hour the tube is closed at its inner end by a thin membrane 
in which no nuclei are present. Between the sixty-fifth 
and the seventieth hours the end of the stomodseum flattens 
and spreads out. At the same time the stomodseal mem- 
brane increases in size and hangs loosely in the yolk. The 
stomodseum now resembles an inverted hollow mushroom, 
the membrane forming the outline of the cap (fig. 2). As 
the stomodseum continues to grow, the flattened end folds 
back on itself so that the mesodermal layer (m) surround- 
ing the stomodseum lies between two layers of ectoderm 
(fig. 3). 
The stomodseal membrane hangs loosely in the yolk and 
steadily increases in size, not as the result of cell prolifer- 
ation but apparently by secretions from differentiated cells 
located on the end of the recurved stomodseum where it 
joins the mid-intestine (fig. 2, stom m c). These cells are 
elongated, with their extremities drawn out very thin where 
the substance of the stomodseal membrane is given off. At 
the eightieth hour only a few secretory cells are to be seen. 
They increase in size and in number between the eightieth 
and ninetieth hours until they form a very conspicuous mass 
around the stomodseum (fig. 3) . In the meantime the mem- 
brane (stom m) has lengthened. The muscular wall of the 
fore-intestine, lying between the two layers of ectoderm of 
the stomodseum, is still thin. Figure three represents the 
stomodseum at ninety hours, shortly before the larva 
emerges from the egg. The wallsl of the mid-intestine at 
this time are complete. 
Between the ninetieth hour in the egg, and the time the 
hatched larva is ready to feed, the appearance of the stomo- 
dseum changes considerably (fig. 4). The sack-like stomo- 
dseal membrane becomes the tube-shaped peritrophic mem- 
