THE BODY-CAVITY AND THE BLOOD-VASCULAR SYSTEM. 249 



During the differentiation of the two sections of the primitive 

 segments just described, connected layers of cells seem to be formed, 

 and these become arranged above the chain of ganglia and round 

 the intestine (Fig. 126, so and sp), at least Heathcote mentions a 

 covering of these parts which he designates (though not quite 

 correctly) as the somatic and the splanchnic mesoderm. The actual 

 body-cavity is a pseudocoele, and its condition is thus quite peculiar, 

 since, as already shown (p. 245), the largest part of the yolk lies 

 outside of the enteron and consequently in the body-cavity (Figs. 

 125 and 126 B, pp. 245 and 246). Cells still remain in the yolk, 

 and these have been considered as of great importance in the 

 development of the blood-vascular system as well as the fat-body 

 and the connective tissue (Sogkaff, Heathcote). 



As has already been mentioned, the cell -material derived from the primitive 

 segments co-operates in the formation of the tissues bounding the body-cavity 

 and especially of the musculature. It is evident, from Fig. 126 A and B, that 

 a layer of mesoderm-cells (so) becomes closely applied at first to the ventral 

 ectoderm and to the ganglionic chain, but later this layer becomes detached 

 from the outer layer; both it and the nerve-cord shift into the yolk. During 

 these processes, which are not yet satisfactorily worked out, small particles 

 of yolk seem in some way to press in between these parts and the ectoderm 

 (Fig. 126 A and B, d). On the other hand, it no doubt results that the meso- 

 dermal elements derived from the primitive segments also become distributed 

 in the yolk, and consequently the organs which develop here are not to be 

 derived solely from the cell-elements which remained in the yolk. 



In the Chilopoda, the enteric epithelium forms at the periphery of the yolk, 

 which thus conies to lie within the intestine. We should be inclined to believe 

 that in this case the mesodermal tissue would be derivable from the primitive 

 segments, and yet Sograff assumes that in the lateral parts of the embryo, 

 as well as on the back, to which the primitive segments have not extended, 

 a parenchymatous tissue consisting of star-like cells appears derived from the 

 cells which remained in the yolk.* These were said before to take part in 

 the formation of the splanchnic layer (which arose principally by the bending 

 over of the cell-plate first present) by shifting from the yolk to the periphery. 

 In the same way we must assume, according to Sograff, that wandering cells 

 come out of the yolk later, though before the enteric epithelium has formed, 

 to yield the parenchymatous tissue. From this latter the connective tissue 

 of the body-cavity, the fat-body, and the blood-corpuscles are said to be derived. 



In the Diplopoda, the connective tissue of the body-cavity, the 

 fat-body, and the blood-corpuscles arise from the cells already lying 

 with the yolk in the body-cavity. The yolk-mass in this last case 

 becomes more and more interpenetrated with cells; the yolk itself 

 seems to remain in the cavities of the pseudocoele, and here to be 



* "We believe that the above is a correct reproduction of Sograff's view, 

 although it is difficult to obtain from his Russian treatise a complete conception 

 of these somewhat complicated formative processes. 



