344 s. HATTA : 



region, the neural cord {71.) is still solid ; the mesoblast {vhs.) is 

 many-cell-layered and its metameric segmentation is still going on. 

 On the right side of the figure, the section passes through the 

 mid-plane of the myotome showing no sign of its separation 

 from the rest of the mesoblast, while on the left, which shows the 

 intersomitic portion, the process of separation {/nt. and Im.) is going 

 on. On both sides, however, there is no structure that can be 

 recognised as the Anlage of the segmental duct. 



T/ir six pairs of prone-phiHc tubules observed in this stage 

 are the maximum number for Petromyzon ; this stage ought, there- 

 fore, to be regarded ecs the highest p)oint of development toith reference 

 to the pronephros. Even in the present stage, the foremost tubules 

 show a tendency to degenerate. 



Period 3. 



The embryos of Stage iv, which have about 35 mesoblastic 

 somites, present a remarkable progress. The head-fold is much 

 prohjuged ; in older embryos of this stage, it begins to twist 

 ('1)7, fig. 1, D). Figs. 77-91 i-epresent sections through one of 

 these embryos. In some myotomes, the sclerotomic fold goes 

 deeper between the muscle-layer and the chorda. The parietal 

 layer of the lateral plate i;m-p.) is much lessened in thickness, 

 so that it is reduced, in the dorsal region (the posterior two thirds 

 of the pronephric extent), into a thin epithelial lining of the 

 body wall (see figs. 79-85). The coelomic projection is likewise 

 reduced into a thin plate (figs. 82-86, c.j)) except in the anterior 

 two segments of the pronephic region, in wdiich it still keeps the 

 characters of the younger stages (figs. 77-81, cj).), only folding in 

 deeper than in the foregoing stages. The visceral layer {m.v.) of the 



