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The somites and the lateral plate are not in immediate contact 
but are separated by a short stretch of cells continuous with 
both, known as the nephrotome or intermediate cell-mass or 
middle plate. The intersegmental furrows do not extend into 
the intermediate cell-mass, and the latter therefore remains 
unsegmented like the lateral plate. It consists fundamentally 
of two layers of cells, dorsal and ventral, of which the former 
is continuous with the dorsal wall of the somite and the somatic 
layer of the lateral plate, and the latter with the ventral wall 
-of the somite and the splanchnic layer of the lateral plate (Fig. 
68 B). Thus if the two layers of the intermediate cell-mass 
were separated the space between them would be continuous 
with the coelome that arises secondarily in the lateral plate. This 
condition actually exists in some of the Anamnia (Selachii, for 
instance) in which the intermediate cell-mass is also segmented. 
The Lateral Plate. This name is given to the lateral meso- 
blast within which the body-cavity arises. It is separated from 
the somite by the nephrotome and its lateral extension coincides 
with the margin of the vascular area. 
Development of the Body-cavity or Celome. The cclome 
or body-cavity arises within the lateral plate as a series of sep- 
arated small cavities, distributed throughout its whole extent, 
which appear first in the anterior portion (1-38 s stage). By 
successive fusion of these cavities and their extension centrally 
and laterally, there arises a continuous cavity, the ccelome, 
which extends from the nephrotome to the margin of the vascular 
area (Fig. 68), and which becomes the pleuroperitoneal and per- 
icardial cavities in the embryo, and the extra-embryonic body- 
cavity beyond the boundaries of the embryo. 
Of the two layers of the lateral mesoblast thus established, 
the external is known as the somatic and the internal as the 
splanchnic layer. In the course of development the somatic 
layer becomes closely bound to the ectoderm, thus constituting 
the somatopleure, and the splanchnic layer becomes similarly 
united to the entoderm, thus establishing the splanchnopleure. 
The somatopleure is destined to form the body-wall and the 
extra-embryonic membranes known as the amnion and chorion; 
from the splanchnopleure is derived the alimentary canal with 
all its appendages, and the yolk-sac. As described in detail in the 
next chapter, this splitting of the mesoblast progresses with 
