480 REIGHARD—PHELPS. [Vov. XIX. 
b. The Mesoblast and the Head Cavities. The walls of the 
head cavities are but little altered. The cavities themselves 
are of about the same extent in a transverse direction as be- 
fore, but have increased about three times in dorso-ventral 
extent. The dorsal wall of each is still composed of colum- 
nar cells, the ventral wall of flattened cells. In one specimen 
of this stage, although the cavity was continuous on the left 
side, it consisted on the right side of three isolated cavities 
contained in the common sheet of mesoblast. The posterior 
of these was the largest. It is probable that several small 
isolated cavities are formed on each side and that these subse- 
quently fuse into the single head cavity. 
The head cavities of the two sides now extend for some 
distance on each side into the connecting cell cord, but the 
middle part of the cord is still solid. The walls of the head 
cavities are separated from the structures anterior to them 
by loose mesoblast; that is the compact cell cord connecting 
them with the germinal wall in front is being transformed 
into mesenchyme. Posteriorly they are still continuous with 
the compact mesoblast which extends along the sides of the 
neutral tube backward to the auditory vesicle. 
The fourth stage is one in which the embryos agree closely 
in external appearance with those of stage three (Plate, 
Fig. 3). The tail is more protuberant than in stage three, 
and the re-entrant angle beneath it acute. The only differ- 
ence noted in the brain is in the greater length of the re- 
cessus laterales. The hyoid arch has become prominent and 
causes the second pair of gill pouch lines to be less marked. 
The pre-hyoidean pouches are not visible. The adhesive organ 
shows no external change, except that the plane of the two 
diverticula is nearly at right angles to the frontal plane 
instead of being at an obtuse angle as in stage three. 
The further step in the development of the organ which 
has been taken since the last stage is one which is observable 
only in section (Fig. E). The lumen of each diverticulum 
is no longer of uniform diameter but shows a series of dila- 
tations connected by narrower portions. These give to the 
