340 Some Phases of the Gastrulation of the Horned Toad 
been impossible to discover unmistakable mesoblast cells outside the im- 
mediate vicinity of that from the embryonic area. On all sides are 
crowded hypoblast cells derived from the germinal wall, to which the thin 
epiblast is closely apphed. In several instances mesoblast cells have 
been seen some distance from the embryonic mesoblast yet so similar in 
position to isolated mesoblast cells that are undoubtedly migrants from 
the embryonic area, that it seems safe to assume that they, too, have 
wandered. All cases of this character have been in the vicinity of iso- 
lated masses of wandering mesoblast. There is no evidence that up to 
the age of these two embryos, mesoblast arises elsewhere than around the 
blastopore. In subsequent stages all mesoblast found in the extra em- 
bryonic area must be suspected of having its origin in the detached cells 
which we have already called attention to some distance from the cell 
mass to which they originally belonged. 
The caplike elevation of the blastoderm of Phrynosoma gives place to 
a flat embryonic area. Fig. 5 is a dorsal view of this stage. In this 
blastoderm the embryonic area was .95 mm. long and .89 mm. wide. A 
few large yolk spherules can be seen through the blastoderm in front of 
the blastopore, whence they extend forward over an ever widening area 
and may be seen around the rim of the embryonic area, except on the 
posterior side. They may possibly owe their size and form to the action 
of reagents, but their pecular distribution is, without doubt, due to some 
structural condition before fixation. It is a very common thing to find 
in sections, large yolk laden cells engrossed by the hypoblast adjacent to 
the germinal wall, and beneath this hypoblast, large yolk masses. Hence 
it seems natural to regard the yolk masses in Fig. 5 as evidence of the 
rapid addition of yolk-laden formative. cells to the hypoblast from the 
adjacent yolk mass. From the surface view there is evidence of greater 
thickness of the blastoderm back of the blastopore than is the case in 
the youngest stage. This is due rather to the spread of mesoblast than 
to thickening of the epiblast. The blastopore is less V-shaped than 
earlier ana more lke a half-moon. The depression between the primi- 
tive plate cells within the blastopore and the epiblast wall which rises 
in front of it represents the posterior end of the mesoblast sac. It is 
V-shaped in section (Fig. 10) opening postero-medianly. The inturned 
edges of the epiblast along the entire length of the blastopore imply a 
process of ectodermic invagination along the whole margin. The surface 
view scarcely suggests the part the primitive plate is taking in this 
process. 
This same embryo was subsequently sectioned in a longitudinal plane. 
