198 WILSON. [Vot. II. 
the direct metamorphosis of the ends of ectoderm or endoderm 
cells. As tothe much-discussed question of which layer secretes 
the membranes; in early stage, such as Fig. 5, it is impossible 
to decide whether the secretion is the peculiar property of either 
layer. But in later stages, it is found in places where it can 
only be endodermic, and in others where it is evidently ecto- 
dermic. I shall return to this point further on. The various 
steps in the formation of the permanent layers can often be 
observed, with but few gaps, in a single section. Fig. 5 is such 
a section, with a few additions from another. 
The permanent endoderm is formed from the larval endoderm 
by a differentiation of a peripheral layer from the central por- 
tion, Fig. 5. The peripheral layer is not formed continuously 
over the whole surface, but in spots here and there. It is at 
first marked off from the central portion only by its somewhat 
greater density, but it gradually breaks its connection with the 
latter by the acquisition of a smooth limiting surface, Figs. 5 
and 10. Cell boundaries soon appear. The permanent endo- 
derm as thus formed is a single layer of vacuolated cubical cells, 
except in the immediate neighborhood of the cesophagus, round 
the sides of which the cells are massed so as to fill up, more or 
less completely, the space between it and the body wall. This 
is especially noticeable on the left side of Fig. 5. 
The cesophageal invagination as seen in Fig. 5 is much 
dilated at its base. Here the ectoderm has preserved its 
intimate connection with the endoderm, the ectoderm cells not 
even having acquired a smooth bounding surface. In Fig. 6 
this even surface has been acquired, but at no time is a support- 
ing lamella secreted over this area. The absence of the lamella 
clearly facilitates the absorption by the endoderm (or yolk) of 
the base of the invagination. The details of the absorption are 
as follows. The dilated bottom of the invagination, shown in 
transverse section in Fig. 10, is broken through at one point. 
Through this breach the yoke passes into the lumen, Figs. 8-10, 
and the base of the cesophagus thus enclosed above and below 
by yolk is absorbed. It is, I think, possible to recognize the 
base after it has passed into the yolk, though of course the 
histological structure is gone. In Fig. 8 is seen a curved mass 
of tissue extending from the left side of the cesophagus nearly 
to the right. Though in structure like the yolk, it stains a much 
