404, ARTHUR ROBINSON. 
(fig. 15 F’) the third layer has already arrived at the margins 
of the cephalic surface where it is not yet separated from the 
hypoblast. It ceases as a distinct layer at the same distance 
from the middle line through twenty-five sections which pass 
through a gradually expanding cylinder, but after the circum- 
ference of the cylinder becomes uniform the mesoblast advances 
nearer to the middle line on the cephalic side, and imme- 
diately beyond the extra-embryonic area the mesoblastic plates 
of opposite sides meet and fuse with each other. When the 
mesoblastic plates first meet and fuse in the middle line on the 
cephalic side, they are united with the hypoblast, which pro- 
jects as a thickened ridge (figs. 15 and 15 C, Pl. XXIV) against 
the epiblast, and thus produces a portion of the cephalic amnion 
fold. Immediately to the distal side of the ridge a small 
portion of both the epiblast and hypoblast is carried inwards 
towards the interior of the cylinder, and in this very short 
double layer is the rudiment of the true pro-amniotic fold. 
Almost immediately afterwards the apex of the ridge becomes 
separated from the hypoblast, and thus in the middle line on 
the cephalic side a continuous layer of mesoblast is produced 
which intervenes between the hypoblast and epiblast imme- 
diately in front of the embryonic area (see fig. 15 H), which 
represents an oblique section along the line H (fig. 15 B). 
In front of the advancing margins of the mesoblastic plates 
the hypoblast is thickened, and contains the nuclei irregularly 
arranged in more than one row (figs. 15 Fand 15G). That 
the same conditions occur wherever the mesoblast is about to 
separate from the hypoblast is shown not only by transverse, 
but also by lateral longitudinal sections (fig. 15 #, Pl. XXIV). 
The next succeeding stages of mesoblast formation are 
represented in the transverse sections of a rat ovum depicted 
in figs. 16 to 16 F, Pl. XXV, and Pl. XXVI, fig. 16H. From 
a mere glance at the series it appears that the diameter of the 
cylindrical ovum is very much increased, and that the increase 
in size of the cylinder is not due to distension accompanied 
by thinning of all the wall, for the hypoblast is not thinner 
than in preceding stages, and the mesoblast is evidently thicker, 
