Charles L. Edwards and Clarence W. Hahn 343 
servations do not confirm the hypothesis, it seems probable that along 
with the process just described, the space from the median hypoblast in 
front of the primitive plate to the point where the chorda is in contact 
with the hypoblast anteriorly is due at first to the degeneration of the 
hypoblast and lower layer of the chorda invagination which Will, 93, 
found in the Gecko and represents in Figs. 57 and 58, Pl. 9. Then the 
rapid forward growth of the chorda carries its anterior contact still 
farther from the primitive plate hypoblast. In Fig. 7 one may see the 
anterior reach of the lateral mesoblast which has established the five or 
six mesoblast cells that are to be seen in the middle line in front of the 
embryonic area in Fig. 6. 
Longitudinal sections of a stage which is a little younger than this 
are represented in Figs. 8 and 9. The cap-like form still persists. Other 
conditions are essentially the same as in the embryo just described. The 
head process is fused with the hypoblast and extends to the germinal 
wall, but the histological differentiation has not taken place in the an- 
terior third of its length. In Fig. 9 the continuity of the epiblast and 
primitive streak mesoblast is not different from conditions described in 
the older embryo of this stage. The extension of the primitive plate cells 
under the chorda verifies the assertion previously made that the primitive 
plate mesoblast proliferates or grows laterally and forward as the ventral 
wall of the mesoblast sac. By so doing it is the counterpart of the epi- 
blast on the dorsal wall in a single invagination. In this stage the net- 
work of mesoblast cells to be seen under the primitive plate in Fig. 2, 
has been replaced by the solid tissue. The condition persists, however, 
posterior to this region. Three or four sections from the median line 
this embryo has the same appearance as in the older specimen represented 
in Fig. 7. Cross sections of this stage are reproduced in Figs. 10-15 
Fig. 10 passes through the blastopore where the V-shaped depressions 
mentioned in connection with Fig. 5 are prominent. Here the epiblast 
may be seen turning into these depressions and fusing in this section 
with the epiblast of the floor of the depression in the formation of a 
lateral growth of mesoblast along the mesoblast sac. The appearance of 
the nuclei of the epiblast and of the floor of the mesoblast sac is identical, 
and in mesoblast cells, which are continuous laterally with each of these 
tissues, there are here and there small dark nuclei, similar to the epi- 
blast nuclei. These nuclei in mesoblast cells are not met with except in 
the immediate vicinity of the epiblast and the floor of the mesoblast sac. 
Under the blastoporic epiblast is a layer of mesoblast of limited lateral 
extent, which is sharply marked off from the overlying epiblast. This 
condition prevails in sections back of this point, but three sections an- 
