DEVELOPMENT OF PETROMYZON FLUVIATrLIS. 185 



plate or " Kastchen" of Stannius occupies the horizontal 

 space between two myotomes, and that they lie one on another, 

 so that in a horizontal section we see only one, in a transverse 

 or vertical section we see one lying on another like sheets of 

 paper. Each " muscle-plate" contains several nuclei, which 

 stain more deeply than the muscle-substance. It is trans- 

 versely striated, and faint longitudinal striae can also be de- 

 tected; these correspond with fibrillse, into which the muscle- 

 substance easily breaks up. These latter are especially large, 

 and can be easily recognised in transverse sections near the 

 most external part of the " muscle-plate." 



The development of these muscle- plates is as follows: — 

 The outermost layer of cells forming the mesoblastic somite 

 does not appear to be converted into muscles. For a long time 

 it persists as a definite layer of cubical cells with large nuclei 

 lying between the skin and the myomere; this is the case till 

 long after the other cells of the mesoblastic somite have deve- 

 loped into muscles. Finally, this layer seems to disappear, 

 but remains of it can still be distinguished lying just within 

 the skin, even when the myomere has assumed the appearance 

 characteristic of the full-grown Ammocoete. This view that 

 the somatic layer does not take part in the formation of the 

 myomeres, is not in agreement with what Balfour has de- 

 scribed in the Elasmobranchs, where both the inner and 

 outer layer become muscular; but, on the other hand, the 

 muscles of the myomeres in Amphioxus appear to be de- 

 rived from the splanchnic layer only, and the same view is 

 supported by Gdtte and the Hertwigs. 



The remaining cells of the mesoblastic somite begin to 

 grow in between one another, and between each neighbouring 

 somite an intermuscular septum is deposited. The process of 

 growing in between one another is carried on until each cell 

 occupies the whole length from one myotome to the next, and 

 at the same time, each cell becomes somewhat flattened, so 

 that their transverse section, which was at first round, become 

 oval (fig. 24). At the same time longitudinal thickenmgs 

 occur in the cortical part of the cell, the medullary portion 



14 



