Charles L. Edwards and Clarence W. Hahn 345 
terior to Fig. 10 the separation cannot be made out (Fig. 12). In the 
third and fourth section forward (Figs. 12 and 13) several epiblast 
nuclei are distinguishable in the upper region of this mesoblast layer, 
otherwise its epiblast character has given place to that of mesoblast. The 
straggling nuclei indicate that the mesoblast underlying the mesoblast sac 
is mesoblast derived from the epiblast. Near the anterior end of the 
neurenteric canal this layer lies on each side of it, Fig. 14. While pos- 
teriorly the ventral mesoblast is separate from the epiblast of the floor 
of the mesoblast sac, it is continuous farther forward with lateral meso- 
blast which is plainly derived from the epiblast, as shown by the isolated 
epiblast nuclei above referred to. Hence, we are led to conclude that the 
lateral is derived mesoblast from two ectodermal sources in the sense just 
as explained. In Fig. 13 the tissue derived from the upper wall of the 
mesoblast sac in the middle line is the chorda, that from the upper wall 
on either side of the middle line is the upper layer of mesoblast (ms’bl.'), 
that from the lower wall of the mesoblast sac is the lower layer of meso- 
blast (ms’bl.”). In Fig. 14 where the mesoblast sac approaches the 
hypoblast, it divides the mesoblast from the lower wall of the mesoblast 
sac so that from here forward there is a lateral layer of lower mesoblast 
(ms’bl.#) on each side. This condition can be traced forward for sev- 
eral sections. Ectoderm nuclei may be found here and there in the 
lateral mesoblast also, Figs. 10, 11, 12. In Fig. 11 is seen the line of 
juncture of chorda and epiblast just in front of the anterior hp of the 
blastopore. The chorda is well differentiated from the lateral mesoblast 
immediately in front of its place of origin (Fig. 12), but it becomes 
less sharply separated from it as one passes forward (Figs. 13, 14), and 
but a short distance forward it is quite indistinguishable from the lateral 
mesoblast. No doubt the differentiation of cells proceeds as a process 
from behind forward. There is no evidence that the chorda becomes 
separated from the lateral mesoblast by the fusion of the upper meso- 
blastic layer with the lower mesoblastic layer as in the Chelonia, Mit- 
sukuri, 92, (Figs. 14, 15, 16). The significance of these two layers of 
mesoblast can be no other than that they are the upper and lower wall 
of a much compressed hollow pouch such as that described in the young- 
est embryo here figured (Figs. 1, 2 and 3). One must conclude that 
from the V-shaped depression seen at the entrance to the mesoblast sac 
in surface view (Fig. 5), and in section Fig. 10, a pouch highly com- 
pressed laterally has grown forward and laterally in a diagonal direction, 
that the upper wall of this pouch is derived from the epiblast at the 
sides and in front of the mesoblast sac and that the lower wall is formed 
by the spreading of a small knot of epiblast tissue in the floor of the meso- 
