410 WILSON. [Vo. III. 
VII. Tue ENTOoBLAST. 
Just before the invagination of the gastrula the entoblast 
forms a flat plate of prismatic cells covered dorsally by the 
mesoblast and the thinned ectoblast, and at the sides by the 
mesoblastic bands and the thickened areas of the ectoblast. 
The nuclei lie near the lower ends of the cells (Figs. 34, 45). 
During the invagination the anterior edge of the entoblastic 
plate remains stationary, while the lateral margins bend down- 
wards and the posterior margin grows forward. The superjacent 
ectoblast takes part in this change, but the thickened areas 
overlying the mesoblastic bands are left behind, so that the 
archenteron is covered below with very thin large polygonal 
cells (Figs. 36, 41, 42). At the completion of the invagina- 
tion the archenteron has a nearly spherical form (Fig. 38). Its 
cells are clear, columnar, with their nuclei near the inner ends. 
As soon as the embryo begins to swallow the albumen, which is 
done almost immediately after the narrowing of the blastopore, 
the whole body becomes greatly distended and enlarged, so that 
the archenteric wall becomes much thinner and the cells undergo 
a marked alteration in form. Their cell substance also changes 
in character, the protoplasm now containing numerous slightly 
staining globules of various sizes. I am unable to explain by 
what process absorption takes place ; the appearances indicate 
that the albumen is taken bodily into the cells, amceba-fashion. 
The ingrowth of the stomodzeum and the concomitant increase 
of the mesoblast in the anterior region cause still further changes 
in the form of the archenteron. The stomodzum grows back- 
wards and downwards in the median line (see p. 25), pushing 
the archenteric wall before it, while the pressure within the 
archenteron causes it to bulge forward into the head-cavity, 
which is thus often almost or quite obliterated. In early stages 
this forward bulging is often so extreme as to cause the mouth 
to face directly downwards. 
The later changes may be very briefly treated. The archen- 
teron continually elongates with the growth of the body, its 
hinder extremity remaining in contact with the ectoblast just 
behind the primary mesoblasts, at which point the stomodzeum 
is afterwards formed ; in front the archenteron becomes widely 
separated from the extremity of the body, owing to the elongation 
