Some Phases of the Gastrulation of the Horned Toad 
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closely packed cells, one on each side, which act as feeders to surrounding 
parts, there being an extension of it along each lateral margin to a posi- 
tion well in front of the embryonic cap. The cavities of these sacs are 
in communication with the blastopore. The epiblast turns in at the 
blastopore and gives rise to the chorda and mesoblast. The latter also 
receives accessions below from the primitive plate region, in fact from 
epiblast cells of the floor of the mesoblast sac as we shall endeavor to 
prove later. 
Fic. 4. Longitudinal section of an embryo in which the cap-like condition 
is less marked but in which the mesoblast sac has not yet fused with, or 
broken through, the hypoblast. bl.’po., blastopore; cd.d., chorda dorsalis; 
e.’bl., epiblast; h.’bl., hypoblast; la pr., primitive plate; ms.’bl., mesoblast; 
x 16674 diameters. 
The next oldest blastoderm in the collection was sectioned lengthwise. 
In the stage just described we have not been able to affirm positively that 
the mesoblast sac does not communicate with the subgerminal space, but 
in the second stage (Fig. 4) the hypoblast is unbroken. It differs from 
the younger stage in the absence of the cap-like elevation, in the more 
compact and mature condition of the primitive plate, and in the absence 
of the mescblastic pouches so characteristic of the early stage. They are 
already flattened and their cavity all but obliterated. The chorda is 
relatively shorter than in the embryo represented in Figs. 2 and 3, but it 
is actually of about the same length while the whole blastoderm is about 
