434 ARTHUR ROBINSON. 
series of irregular masses of nucleated protoplasm. The 
former of these is called by Selenka the somatopleure, the 
latter the splanchnopleure. The former is certainly con- 
tinuous with the somatopleuric layer on the trophoblast, and 
also with that on the amnion; nevertheless it corresponds to 
the splanchnic portion of the extra-embryonic mesoblast of 
other Vertebrates, and it is naturally continuous in the region 
of the sinus terminalis with the somatic mesoblast which 
covers the trophoblast, and forms the inner wall of the 
chorion. The irregular groups of nucleated protoplasm, to 
which collectively Selenka applies the term Splanchnopleure, 
are in reality only a portion of that layer; they are the rudi- 
ments of the blood-vessels and their contents. 
The extension of the ccelom towards the proximal end of 
the cylindrical ovum is accompanied by a simultaneous but 
smaller extension towards the distal end. This portion of the 
extension is best explained by the examination of transverse 
sections of an ovum in which it is occurring. 
Fig. 16 H, Pl. XX VI, represents a transverse section through 
the posterior part of the primitive streak on the caudal side of 
the cylinder, and through the extra-embryonic area in front of 
the pro-amnion on the cephalic side. 
In the latter situation several islets of cells containing 
cavities are seen; they are merely sections of the irregular 
margins of the extra-embryonic mesoblast and celom. 
Laterally the coelomic space lies between a thin somatic and 
a comparatively thick layer of splanchnic mesoblast. It ex- 
tends from the cephalic to the caudal side, where it penetrates 
a short distance into the embryonic mesoblast. Twenty-five 
micromillimetres nearer the centre of the embryonic area the 
relations are the same, except that on the cephalic side of the 
section the pro-amnion intervenes between the two halves of 
the celom (fig. 16 G, Pl. XXVI). Six sections further back on 
the cephalic side, and further forward on the caudal side, that 
is thirty micromillimetres nearer the centre of the embryonic 
area, four sections of the coelom appear (fig. 16 #, Pl. XXV). 
Two are on the cephalic side in the peripheral margins of the 
