Charles L. Edwards and Clarence W. Hahn 337 
blast, almost to the middle of the embryonic area. At the mouth it is 
funnel-shaped, being wider perhaps laterally than vertically. Its dorsal 
wall is the thick infolded epiblast destined to become the chorda. The 
epiblast of the depressed ventral wall of the blastopore becomes meso- 
blast deeper in the canal, for this mesoblast is produced by the forward 
growth of epiblast cells. Since the dorsal and ventral walls of the meso- 
blast canal contribute to the lateral mesoblast by infolding and lateral 
growth, the mesoblast sac may be regarded as having no lateral limita- 
tions, but extending between the above named mesoblast anlagen. How- 
ever, the scctions of this stage only indicate this for a short distance in 
front of the blastopore. The head process is four or five cells thick where 
it is continuous with the epiblast at the blastopore. It is continuous 
laterally at this point with the ectodermic folds that give rise on either 
side to the dorsal limbs of the mesoblastic pouches. Anteriorly it be- 
comes a wide belt of cells, three or four deep, and thin at the edges (Fig. 
3, pre. ce.) extending quite to the anterior limit of the neural plate. As 
in Fig. 3, anterior sections do not have mesoderm adjacent to the chorda 
laterally. A part of the tissue under the chorda in Fig. 3 is mesoblast 
(Fig. 3, ms’bl.“) of primary origin, but under the greater part of it is 
mesoblast (Fig. 3, ms’bl.!) which is derived from the primary mesoblast. 
In the adjacent sections the hypoblast is easily traceable to the limits of 
the embryonic area and there is no mesoblast lateral to the chorda in the 
forward end. 
In Figs. 2 and 3 the epiblast represented is not all in the same plane 
(e’bl#? and e’bl.#) ; e’bl.4 is cut in a diagonal plane and e’bl.# is cut 
vertically. In Fig. 2 (a, 6 and la. pr.) and Fig. 3 (a, b, c) aré repre- 
sented tissues which appear only at deep focus. The epiblast over the 
whole top of the embryonic-cap is four or five cells thick, but it thins out 
rapidly toward the base of the uprising wall, becoming a thin one-celled 
layer in the extra-embryonic area (Figs. 2 and 3, e’bl.‘). The hypoblast 
is uniformly one cell thick over the whole embryonic area, except under 
the chorda where it appears to be two or more cells thick, but this con- 
dition is of limited extent. Extra-embryonic mesoblast separates these 
two germ layers but a short distance from the embryonic area (Fig. 2 
ms’bl.i Fig. 3, ms’bl.###). From these observations we may conclude that 
at a time when the epiblast is in the form of a much thickened cap the 
mesoblast sac forms a canal stretching forward into the mass of tissues 
that are then accumulating at the posterior part of the embryonic cap 
and perhaps opening into the sub-germinal cavity in front of this mass 
of tissues. The mesoblast which is being produced around the blastopore, 
stretches antero-laterally from it in the forms of two hollow sacs of 
