DEVELOPMENT OF ELASMOBEAXCH FISHES. Ill 



It is during the stage intermediate between I and K that 

 the first changes become visible which indicate a distinction 

 between 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. x. fig. 8). Towards the end of stage K, however, 

 there appears between the two a network of mesoblastic cells 

 connecting them together. In the rudimentary outgrow^th 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. X. fig. 8), and by the close of this period become concentrically 

 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 (PL xi. fig. 9 c, 9 d). 



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 splanch- 

 nopleure 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. AVithout 

 entering into any full description of these it may be pointed 

 out that both the true body-cavity and its continuations into 

 the protovertebrae appear later in Torpedo than in Pristiurus 



