STAGES G TO K. THE MESOBLAST. 329 



an epithelium (endothelium) lining the body-cavity and the 

 connective tissue adjoining this. 



There are at first but very few connective-tissue cells between 

 the epithelium of the somatic layer of the mesoblast and the 

 epiblast, but a connection between them is established by peculiar 

 protoplasmic processes which pass from the one to the other 

 (PI. 11, fig. 8). Towards the end of stage K, however, there 

 appears between the two a network of mesoblastic cells connect- 

 ing them together. In the rudimentary outgrowth to form the 

 limbs the mesoblast cells of the somatic layer are crowded in an 

 especially dense manner. 



From the first the connective-tissue cells around the hypo- 

 blastic epithelium of the alimentary tract are fairly numerous 

 (PI. 1 1, fig. 8), and by the close of this period become concentric- 

 ally arranged round the intestinal epithelium, though not divided 

 into distinct layers. A special aggregation of them is present in 

 the hollow of the rudimentary spiral valve. 



Behind the anal region the two layers of the mesoblast 

 (somatic and splanchnic) completely fuse during stage K, and 

 form a mass of stellate cells in which no distinction into two 

 layers can be detected (PI. 11, figs, gc, gd}. 



The alimentary canal, which at first lies close below the aorta, 

 becomes during this period gradually carried further and further 

 from this, remaining however attached to the roof of the body- 

 cavity by a thin layer of the mesoblast of the splanchnopleure 

 formed of an epithelium on each side, and a few interposed 

 connective-tissue cells. This is the mesentery, which by the 

 close of stage K is of considerable length in the region of the 

 stomach, though shorter elsewhere. 



The above account of the protovertebrae and body-cavity ap- 

 plies solely to the genera Pristiurus and Scyllium. The changes 

 of these parts in Torpedo only differ from those of Pristiurus in 

 unimportant though fairly noticeable details. Without entering 

 into any full description of these it may be pointed out that 

 both the true body-cavity and its continuations into the proto- 

 vertebrae appear later in Torpedo than in Pristiurus and Scyllium. 

 In some cases even the muscle-plates become definitely separated 

 and independent before the true body-cavity has appeared. As 

 B. 22 



