Charles L. Edwards and Clarence W. Hahn 341 
Fig. 6 is a semi-diagrammatic drawing of a sagittal section, and Fig. 7 
of the next section to one side. In the latter the continuity of the epi- 
blast and primitive plate mesoblast with the anterior mesoblast confirms 
what has been seen in the surface view with regard to the infolding epi- 
blast and establishes the lateral growth of the primitive plate mesoblast 
as well. Hence, both the posterior and anterior lips of the blastopore 
are undergoing a process of invagination. The yolk masses can be seen 
adhering to the hypoblast and anteriorly it is receiving additions of large 
yolk-laden cells from the germinal wall. At this stage the mesoblast 
bre.ce. can.nent. 
Fig. 7. 
ld 
Fics. 6 anp 7. Longitudinal sections of the embryo represented in Fig. 5. 
Fig. 6 is sagittal and Fig. 7 is the third section to the right of Fig. 6. 
can. n/ent., canalis neurentericus; e.’bl., epiblast; glb. vt., yolk spheres; 
h.’bl., hypoblast; la. pr., primitive plate; ms.’bl., mesoblast; prc. ce., head 
process; X 109 diameters. 
canal is an open passage into the subgerminal cavity, the canalis neuren- 
tericus, (cf. Hertwig, 03, pp. 832-4). The head process has become the 
lower layer along the middle line. This is due, no doubt, to the fact 
that its elongation has kept pace with the increase in length of the epi- 
blast, hence after fusing with the hypoblast in a position near the middle 
of the embryonic area at the time, this point is carried forward relatively 
by the rapid elongation of the head process. The hypoblast has not yet 
grown together under the head process as it does later. While our ob- 
