406 ARTHUR ROBINSON. 
embryonic area; it lies free between epiblast and hypoblast, 
its thickened internal extremity abuts against the hypoblastic 
ridge, and its external extremity contains a small cavity, a 
portion of the celom. The other portion is more extensive ; 
it also lies free between epiblast and hypoblast, except in the 
middle line on the caudal side, where it is continuous with the 
epiblast in the primitive streak. It belongs, therefore, to the 
caudal section of the embryonic area. In its peripheral 
margin a small section of the ccelomic cavity is visible. 
In the anterior portion of the embryonic area, in front of 
the hypoblastic ridge, the mesoblast extends across the middle 
line, and it presents peculiarities which are worthy of notice. 
In mesial longitudinal sections of a mouse embryo at this 
period the hypoblastic ridge, recognisable by the regularity of 
arrangement and the oval shape of its nuclei (fig. 17 A, Pl. 
XXVI), terminates abruptly just at the point where the super- 
jacent epiblast begins to thin. In front of it, beneath the 
thin epiblast of the anterior portion of the embryonic area, 
the hypoblast is irregularly thickened, and it contains many 
nuclei rounded in form, but varying in size. 
At this period transverse sections of the most anterior part 
of the embryonic area show that the mesoblast on the cephalic 
side of the cylinder becomes gradually reduced in amount 
from behind forwards (compare fig. 16 D, 60, behind the 
anterior extremity of the area, with fig. 16 H, 15 behind the 
same point). Still further forward the reduction continues, 
until this portion of the mesoblast entirely disappears (fig. 16 G, 
P]. XX VI) at the edge of the embryonic area, whilst at the 
same time the caudal mesoblast encroaches towards the cephalic 
side; but the two extremities of the caudal mesoblast do not 
reach the middle line on the cephalic side simultaneously with 
the disappearance of the cephalic mesoblast. In front of the 
point where the cephalic mesoblast ends for a distance of 20 u 
the epiblast of the extra-embryonic area is in contact with the 
hypoblast (fig. 16 G, Pl.XXVI). Immediately in front of this 
bilaminar area the extra-embryonic mesoblast appears between 
the epiblast and hypoblast as a series of islets, each of which 
