MORPHOLOGY OF CYCLOSTOMATA. 319 



on the two sides do not exactly correspond. On the right side 

 of fig. 8, the hind border of the fonrth myotome (mt.IV) is 

 cut through; the Anlage of the pronephros [a.jjn.l) presents in 

 section an oval shape, consisting of columnar cells radially ar- 

 ranged and containing a cavity of an irregular form. The 

 histological structure of the Anlage is as compared with that 

 in fig. 3, more or less loose^', and the Anlage itself is there- 

 by also distended. The lateral plate (Im.) shows, however, no 

 marked progress. The left side of this figure and the right 

 of figs. 11 and 12 represent the section through the fifth myotome 

 (mt.V) and the Anlage of the pronephros (a.pn.2) for that somite. 

 The Anlage presents almost the same development as that just 

 described. The left half of fig. 12 and the right half of fig. 

 13 shows the sixth somite {mi.VI) and the Anlage belonging to 

 it (a.pn.3). It can be inferred from the arrangement of its 

 component cells that the Anlage has been just constricted off from 

 the myotome, as is shown by the fact that the cells at the point 

 marked with x of the visceral and parietal layers are not yet 

 rearranged to form a continuous layer, — a condition which is 

 observed not infrequently in younger embryos. Fig. 14 shows 

 on the right side a section through the hind wall of the sixth 

 myotome ; the Anlage beneath it [a.pn.3) is, therefore, the hind 

 part of that represented on the right side of fig. 13 : it is 

 entirely cut off from the myotome {mt.VI), and the two layers 

 at this point have completely fused together, enclosing a com- 

 paratively wide cavity. The same condition is observed in the 



1) When tiie pronephric Anlagen are cut ofl' from the myotome, their structure is at 

 first loosened, that is, their component cells become loosely s-et together. Later the cells 

 multiply tliemselves, and are again compressed by mutual pressure; giving a compact structure to 

 the Anlage — })rohably the same condition observed by van Wyhe in Selachian embryos 

 ('89, p. 470). 



