DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 407 
contains a cavity (fig. 16 H). These islets and their cavities 
merely indicate an irregular indented margin of the extra- 
embryonic ccelom, for as the sections are traced forwards the 
islets fuse one by one with the extra-embryonic portion of the 
caudal mesoblast. 
In the extra-embryonic area the somatic mesoblast (S M.) 
is represented by a single layer of flattened cells which covers 
the outer surface of the amnion (figs. 16 H and 16 F) and the 
inner surface of the trophoblast (fig. 19, Pl. XX VII). It is con- 
tinuous laterally with the undivided paraxial mesoblast, which 
lies at the sides of the chorda in the anterior portion of the 
embryonic area, and with the primitive streak in the posterior 
portion of the area. At the posterior end of the embryonic 
area the somatic mesoblast is continued into the rudiment of 
the solid allantois (fig. 19, Pl. XX VII, which is a semi-diagram- 
matic figure of an older embryo), and at the anterior end it 
becomes continuous with the splanchnic mesoblast of the 
extra-embryonic region, but is entirely distinct from the 
axial mesoblast of the anterior portion of the embryonic 
region, being separated from it by the bilaminar area before 
mentioned. 
The spaces in the mesoblast of the embryonic area in front 
of the hypoblastic ridge lie laterally (fig. 16 E), and are 
directly continuous behind with the cclomic spaces in the 
paraxial mesoblast, but anteriorly they disappear. 
The splanchnic mesoblast differs entirely from the somatic, 
for instead of forming a single layer of flattened cells it has 
gained new connections with the hypoblast at many irregularly 
distant points, and in these situations proliferation of the 
hypoblastic nuclei occurs, and the splanchnic mesoblast is 
thickened (figs. 16 F'and 16 A). 
In the axial line of the embryonic area the distinction 
between the primitive streak and the region in front of it is 
still well marked. A short distance behind the centre of the 
embryonic area a distinct line of demarcation, the remains of 
the neurenteric canal (NV C.,, fig. 17, Pl. XX VI), bounds the 
anterior end of the primitive streak. Immediately behind this 
