478 REIGHARD—PHELPS., [VoL. XIX. 
c. The head cavity. In this stage the mesoblast of the head 
is mostly mesenchymatous, but extending forward from the 
anterior end of the recessus lateralis on each side, beneath the 
optic vesicle to its anterior border is a plate of more compact, 
delaminated mesoblast about 70 microns broad and 30 microns 
thick. It is connected across the middle line with the plate of 
the other side by a solid cord of cells which lies in the saddle 
cleft just behind the infundibulum and in contact with the front 
end of the notochord. ‘These cell plates are surrounded by 
loose mesenchyme except at the ends. The anterior end of 
each continues into a slender cord of more compact mesoblast 
which connects it with the germinal wall at the sides of the 
adhesive organ. The posterior end of each is continuous with 
a similar cell cord which extends backward to the recessus 
lateralis and beyond it. Each of these plates contains a well 
defined cavity whose form is the same as that of the plate 
itself and whose extent is nearly as great. The cavities do 
-not extend into the connecting cell cord. The dorsal wall of 
each cavity is formed of columnar cells which are highest 
toward its caudal end; the cells of the ventral wall are more 
flattened. 
The third stage; a. The Adhesive Organ. In this stage 
the embryo, measured from the tip of the forebrain to the 
end of the tail (Plate, Fig. 3), encloses about two hundred 
and twenty degrees of the circumference of the yolk. The 
tail is protuberant, but so slightly that the re-entrant angle be- 
tween its ventral surface and the surface of the yolk is obtuse. 
The midbrain is larger and the recessus laterales extend further 
forward than in the second stage. The lines indicating the 
gill pouches have changed their relative position so that they 
lie close to the recessus laterales, and the anterior pair is less 
marked than formerly. 
As seen externally the adhesive organ has changed its forna. 
In place of the two protuberances of the last stage we find two 
kidney-shaped ridges, the concavities of which face each 
other, and the median plane (Plate, Fig. 3). This gives 
the adhesive organ as a whole the form of a broad ring 
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