No: 2:] THE ADHESIVE ORGAN OF AMIA. 473 
about the lateral and anterior border of the crescent ento- 
blast there is a continuous germinal wall by which the ento- 
blast passes into the yolk. 
In an embryo of the stage represented in Fig. 1 this ento- 
blast of the anterior head region has separated into two layers. 
Of these the dorsal and thicker layer is composed of loosely 
aggregated mesoblast cells, while the ventral layer is a colum- 
nar entoblast, which forms the dorsal wall of the archenteron. 
The mesoblast begins at a point just in front of the optic 
vesicle, as a thick sheet of cells still connected with the ento- 
blast near the median line, but free laterally. As the sheet 
is traced backward it becomes broader. It retains its con- 
nection with the entoblast near the median line and extends 
thence laterally to the limit of the archentron, where it be- 
comes continuous with the germinal wall. It is without a 
cavity. The mesoblast of the anterior head region is thus 
clearly formed by delamination from the entoblast. No meso- 
blast is formed from the colwmnar entoblast of the crescent. 
The second stage: a. The Adhesive Organ. The next 
stage of the adhesive organ to be described is found in embryos 
which show the following external features: The embryo 
(Plate, Fig. 2) is still flat on the yolk; the midbrain is now 
visible externally in addition to the forebrain and_ hind- 
brain; the tip of the forebrain extends beyond the optic 
vesicles for a short distance as in the preceding stage; the 
recessus laterales are distinguishable in the hindbrain, so that 
the cavity of the hindbrain may be described as triangular 
in outline, the anterior lateral angles of the triangle forming 
the cavities of the recessus laterales. Two oblique lines are 
seen on each side within the branchial area. These are 
gill slits. The first is the pre-hyoidean or spiracular slit, 
which subsequently disappears. The arch behind it is the 
hyoid arch. At its dorsal end the auditory vesicle is visible. 
The crescent of the preceding stage has given place to 
three hemispherical protuberances which are nearly as promi- 
nent a feature of the head as the optic vesicles in front 
of which they lie. The two lateral of these protuberances 
