MORPHOLOGY OF CYCLOSTOMATA. 345 



peritoneum still consists of a cubical or rather cylindrical epithe- 

 lium. The pronephric tubules are, in general, much prolonged 

 and begin to coil in the dorso-lateral direction, so as to cause an 

 elevation in the epiblast. The walls of the tubules consist of a 

 regular row of cylindrical cells, which passes over suddenly into 

 the thin peritoneum (figs. 77-86), except in the region of the second 

 pair of the tubules, where the parietal layer {iti.p.) of the lateral 

 plate still retains the character of the younger stages, being 

 composed of cylindrical epithelium like the tubules themselves 

 (figs. 77-79). At some regions, even a few mesenchyma-cells {inch.) 

 appear, — for instance, beneath the chorda (see figs. 80, 82, and 84), 

 in the median ventral space (see figs. 81 and 82), and also inside 

 the lateral epiblast (see figs. 77 and 80). 



Fig. 77 shows a section through the fifth somite and 

 therefore corresponds to fig. Qß, which represents the section 

 through the same plane of an embryo at a younger stage. The 

 longitudinal section of the second tubule {pt.2), together with the 

 corresponding nephrostome {nd.2), is seen on the left side of the 

 figure, greatly resembling the tubules of the same pair in the 

 younger stage (compare with fig. Q)Ç>). On the right side, the 

 nephrostome {nst.2) alone is observed ; the tubule proper is to 

 be seen in the two following sections which are represented in 

 figs. 78 and 79. 



Beneath the myotome anterior to the one just described, 

 there is found neither a tubule nor any structure that may be 

 regarded as the remnant of it. In the space between the epiblast, 

 the myotome and the lateral plate, however, a few scattered 

 cells (fig. 77, vich.) are found. I at first supposed that these 

 might be disconnected component cells of the first pair of 

 tubules ; ])ut, as free cells of quite the same character are found 



