Observations on the development of the excretory system in Turtles. 68Î 



between the sixth and seventh somites, shows the relative position of 

 pronephric and mesonephric bases. We see here the middle piece on 

 the left, thickened and showing a mitotic figure. There is no sign 

 yet of the pronephric duct, excepting as the basis of the part usually 

 called the collecting duct, is involved in the ridge described. The 

 same stage is seen in Aromochelys^ II (1). 



A slightly later stage, showing the beginning of the pronephric 

 duct is seen in B, II (1) (Plate 46, Figs. 30 — 39). Here a complete 

 ridge is not formed and we can follow the separate outgrowths more 

 easily. In somite V, section 9 (Fig. 30), we see the first outgrowth, 

 extending with a peculiar enlargement on the left in somite VI, sec- 

 tion 1 (Fig. 31), to lie above the outermost cells of the second out- 

 growth in VI, 8 and 10 (Figs. 32 and 33). The second reaches over 

 somite VII to the fifth section and probably the tip fuses with the 

 third outgrowth seen in VII, 6—11 (Figs. 35—37). In somite VIII 

 (Fig. 38) the fourth outgrowth is seen and from this the duct extends, 

 the outgrowth dwindling in the ninth somite (Fig. 39). The enormous 

 number of mitotic figures in this embryo is very striking. They 

 have all been drawn in with the camera; the other cells were added 

 free hand. 



In this case, the fusion of the duct with the ectoderm is of con- 

 siderable extent and very marked. In most of the cases I have so 

 far observed it is not so easily detected. It is undoubtedly let into 

 the ectoderm, having near the end a sheath of ectoderm over it on 

 the inner side, while that seen anteriorly between ectoderm and duct 

 has disappeared. Whether there is a genetic relation here between 

 the ectoderm and the tip of the duct is not perfectly clear from the 

 preparations studied hitherto. Mitsukuri says he has proved this 

 fusion "beyond the possibility of a doubt". On Plate 45, Figs. 27 to 

 29 will be found drawings of the ends of the duct in two embryos. 



In B, II (1) (Plate 46, Figs. 30—39) the origin of the first walls 

 of the dorsal aorta from cells escaping from the sclerotome is again 

 very distinct. 



As we proceed to the later stages, however, a new factor comes 

 in which greatly modifies these conditions. Before there is more than 

 a hint of a lumen in the pronephric tubules, we see in some of the 

 same somites (from the second pronephric tubule on), as well as 

 further back, at the point where they pass into the middle plate, 

 marked thickenings. These are the bases of the mesonephric tubules. 

 See B, II, 1, somite VII (Plate 45, Figs. 15—17). They become more 



Zool. Jahrb. XIII. Abth. f. Morph. 45 



