688 EMILY RAY GREGORY, 



and more distinct, until in some series we find the pronephric tubule 

 and that of this mesonephric rudiment (now as distinct as the other), 

 opening side by side into the body cavity, but further back and in older 

 embryos, we find the pronephric funnels opening into these rudiments 

 of the mesonephric funnels and through them into the coelom. These 

 conditions may be seen in PJatypeltis, II (5) and II (7). In embryo II 

 (5) in somite V, section 9, we see the rudiments of the pronephric 

 tubules (on the right apparently two) and of the mesonephric tubules 

 from the middle plate opening into the coelom, while the splanchnic 

 sheets then again bend in and push as far as they can under the 

 aorta. In somite VI, sections 12 and 13, we see that these rudiments 

 of the pronephros and mesonephros no longer have separate open- 

 ings, but fuse and have but one opening. The great irregularities 

 which occur may also be seen by a study of this series. The duct 

 is present as a solid rod. In embryo II (7) (Plate 45, Figs. 19 — 26) 

 similar conditions will be found, markedly in somite VII, sections 6 

 and 7 (Figs. 19 and 20), and in somite VIII, sections ö and 7 (Figs. 21 

 and 22). Here too, the beginning of the duct is seen. 



Thus it becomes clear that although the pronephros is distinct 

 in origin, and arises as segmental outgrowths from the somites and 

 extends over but few segments; the mesonephros, arising from the 

 middle plate, extends almost as far forward as the pronephros and 

 after the earliest stages have passed, the two are so fused that the 

 parts cannot be distinguished. It is also clear that we have here the 

 same relative positions as in the shark. It will be noticed also that, 

 although of segmental origin, even at this early stage irregularities 

 appear, and we frequently find more than one tubule in a somite. 

 Thus in II (7) the seventh somite has two tubules on each side, and 

 the eighth has two on the right side. 



Stages III— VI. 

 At this time the anterior tubules, especially those which end 

 blindly, are quite simple and straight, but the posterior ones become 

 longer and somewhat twisted and S-shaped. The walls of the tubules 

 are formed of thickly-packed, cylindrical cells, which stain very densely. 

 There is no appreciable distinction between the cells of the duct 

 where it appears and those of the tubule. Mitotic figures are also 

 sometimes seen in the tubules, but not so frequently as we should 

 expect from the great increase in size and length, which takes place 

 after this time. The enormous growth in the size of the single cells 



