Charles L. Edwards and Clarence W. Hahn 339 
two and one-half to three times as long as the blastoderm of Figs. 2 and 
3. It is very nearly identical to Will’s Fig. 36, Taf. 6, which is a section 
of the embryo represented by him in Fig. 6, Taf. 1, above referred to as 
equivalent to Phrynosoma, Fig. 1. The second stage has three separate 
solid masses of mesoblast proliferation and ingrowth. ‘Two are from 
either side of the primitive plate and may be traced forward on either 
side. The cells become scattered at the edges of the growth. Some of 
these scattered cells may be seen between the thin epiblast and hypoblast 
at the anterior margin of the embryonic area, but not in the middle Tine. 
. ee Bee ™murg 
nas a la.pr bl.’ bo. marg. bl’ bo. 
Fie. 5. Dorsal view of a blastoderm older than Fig. 1. The elevation of 
the embryonic area has given place to a flat plate and the blastopore has 
changed shape. ad.em., area embryonic; bl.’po., blastopore; glb. vt., yolk 
spheres; ld. pr., primitive plate; marg. bl.’po., margin of blastopore; mur. g., 
germinal wall; * 74 diameters. 
Here the epiblast and hypoblast are closely adherent both in and outside 
of the embryonic area. The third mass of mesoblast growth is from the 
ventral wall of the notochord. The primitive plate sends forward a 
mass of cells (Fig. 4, ms’bl.%4) detached from the hypoblast and extend- 
ing almost as far as does the chorda. It is comparable to ms’bl.% of Fig. 
2. We do not regard it as a permanent source of anterior mesoblast, 
rather as evidence of a mesoblast sac. After very careful search it has 
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