Observations on the development of the excretory system in Turtles. ß93 



Mesoiiephros. 



Owing to the fusion of the anterior part of the mesonephros with 

 the pronephros, this region of the former has already been described. 

 For the sake of clearness, however, we may repeat somewhat. The 

 basis of the mesonephros in the turtle lies in the middle plate and 

 may reach as far forward as the sixth somite, but the position of the 

 anterior end seems to be variable. The first indications of its ap- 

 pearance may be seen in the camera drawings of the embryo B II (1) 

 (Plate 46, Figs. 30— 39) already referred to. In the pronephric region, 

 the tubules of the mesonephros fused with those of the pronephros 

 or lying alongside, as before stated, open into the coelom. These 

 openings, or "funnels", as they are commonly termed, from a lateral 

 position become ventral, and often finally as close to the median line 

 as possible, the aperture at the same time decreasing in size. From 

 the tenth somite on, the tubules are entirely cut off from the coelom, 

 ending blindly near the aorta. 



It is, perhaps, impossible to be certain of the reason for this, 

 but it will be well to consider the conditions, which may be, at least 

 in a measure, causal. The pronephric welt arises at a time when the 

 embryo lies quite flat upon the yolk. The ectoderm extends only 

 just over the middle plate, and is drawn up and back again above 

 the embryo to form the amnion. The somatic layer of the mesoderm 

 follows it closely, while the splanchnic layer is equally close to the 

 entoderm. The result is that the two layers of the mesoderm thus 

 stretched apart are quite widely separated even to the middle plate, 

 where a notch marks the end of the splitting of the single meso- 

 dermal sheet. Besides there is as yet no distinctive endothelial layer, 

 so there is little hindrance and eveiy help to the opening of nephric 

 tubules into the coelom. As the embryo grows, the development of 

 the aorta and its increasing size raise it up above the yolk, and the 

 outer corner of the somites comes to point almost ventrally, bringing 

 the mesoderm with it. The folding together of the sides of the in- 

 testine draws the mesoderm still further in the median direction. 

 This growth and development early overtakes and runs ahead of the 

 nephric growth and at least favors, if it does not cause, the fusion 

 of somatic and splanchnic layers at the point where they leave the 

 middle plate. Then follows the gradual differentiation of the layer 

 next the body cavity to form a single sheet, lining it and separating 

 the mesonephric tubule basis from it. 



