HEAD-FOLD TO TWELVE SOMITES 115 



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 j^late. 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 Coelome. The coelome 

 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 (1-3 s stage). By 

 successive fusion of these cavities and their extension centrally 

 and laterally, there arises a continuous cavity, the coelome, 

 which extends from the nephrotome to the margin of the vascular 

 area (Fig. 68), and which becomes the pleuro peritoneal and per- 

 icardial cavities in the embryo, and the extra-embryonic body- 

 cavity beyond the boundaries of the embryo. 



Of the two lavers of the lateral mesoblast thus established, 

 the external is known as the somatic and the internal as the 

 spla7icknic layer. In the course of development the somatic 

 laver becomes closelv 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 



