Charles L. Edwards and Clarence W. Hahn 333 
blastoderm in the form of a cap with vertical margins leveled toward the 
center of the crown and having a deep recess in the posterior edge where 
the blastopore is located. The length of the area is about 4 mm. The 
width is slightly less. At the anterior margin the cap tapers to a round 
point which indicates more rapid growth in this plane. The recess at 
the posterior edge indicates either an ingrowth of the upper layer cells 
at this point or a backward movement of parts on either side of it. Sec- 
tions prove that the former is true. , 
This stage is represented as seen in longitudinal section in Figs. 2 and 
3. ‘These sections are slightly diagonal. Fig. 2 passes through the blasto- 
a 
aa.pel----- 
4 
bl’po! b. 
Fic. 1. Dorsal view of a very early stage in which probably the mesoblast 
sac has not broken through into the subgerminal space. a., anterior; aa. em., 
area embryonic; aa. pel., area pellucida; bl.’po., blastopore; p., posterior; 
x 74 diameters. 
pore, but runs to the left of the median plane in the anterior region of 
the embryonic area. The extent of this rotation may be judged from the 
fact that Figs. 2 and 3 are the 2d and 3d sections which pass through 
the embryonic area, there being 9 sections included in this area. All the 
structures of interest are to be found in these two sections. 
The elevation of the cap above the surrounding blastoderm is appar- 
ently due to the rapid increase in thickness and extent of the epiblast 
over this area. The accumulation of the mesoblast may take part in this 
elevation, but the absence of mesoblast in the greater part of the anterior 
