
542 THE AMERICAN NATURALIST. [Vor. XXXVI, 
the cell bordering the cavity of the archenteron. Above the 
endoderm is the mesoderm layer (Fig. 17, M.), which varies in 
the number of its cells in different parts of the egg. Near 
the dorsal lip of the blastopore the mesoderm sheath consists 


f Fig. 8. EN.,entoderm. 
5 
PIE A n 
Other lettering as in previous figures. 
of several layers of small cells which are not pigmented. For- 
ward from this region the mesoderm gradually thins out until 
it is composed of only a single layer of rounded cells lying 
between the endoderm and ectoderm; then it gradually 
becomes thicker again, and near the ventral lip of the blas- 
topore it appears very much as it does near the dorsal lip, 
being composed of several layers of cells. 
A frontal section through the middle of an egg at the stage 
of Fig. 7 shows a single layer of mesoderm over the dorsal 
region of the embryo, which gradually becomes several layers 
thick at the sides of the archenteron (Fig. 19), and below the 
archenteron passes directly into the yolk cells of the lower 
part of the egg. Still later stages during the closure of the 
blastopore show that the mesoderm is unquestionably extended 
at the expense of the yolk cells lying just beneath the ectoderm. 
Soon after the medullary folds have appeared the lateral sheets 
of mesoderm become fused on the ventral side of the embryo; 
thus a continuous sheet of mesoderm is formed around the 
embryo, except in the mid-dorsal region where the notochord 
has by this time been cut off from it. The extension of the 

