Observations on the development of the excretory system in Turtles. 699 



the growth of the remuant of the mesonephric blastema in the oppo- 

 site direction to its original development after it has become so very 

 circumscribed, and the fact of the very short connection between the 

 two blastemata (not more than three sections at most) led me to make 

 a more careful examination, and I am now satisfied that the origin 

 of the metanephric basis is not the mesonephric blastema, although it 

 may receive a few cells from the latter. 



In examining the development of the metanephros, sagittal sec- 

 tions, Plate 50, were found very valuable. Here one finds the dense 

 blastema lying above the mesonephros in the connective tissue be- 

 tween the lining of the coelom and the vertebrae. It is sharply out- 

 lined, and separated from the mesonephros in the early stages by a 

 consideral)le layer of connective tissue. The ureter pushes further and 

 further forward, but it is always preceded by the blastema which ends 

 in a typical cluster of concentrically arranged cells. The ureter ex- 

 tends over two thirds of the length of this blastema, though it does 

 not appear so long in this drawing. 



There are no signs of tubules until the blastema is about one 

 third as long as the metanephros, and then there appear thickenings 

 in the blastema alongside the ureter but distinct from it. These very 

 closely resemble the mesonephric tubules in their early development 

 except of course that the order is from behind forward instead of 

 the reverse. They very soon come into contact with the ureter and 

 open into it. On Plate 49, Figs. 70 and 71, are drawings from an embryo 

 of stage XVI, at the posterior end of which metanephric tubules are 

 developing independently, while further forward they seem to be 

 branching from the ureter. Cross and frontal sections were also 

 examined and confirmed the independent origin of the posterior 

 tubules. With the continued growth of the metanephros, the whole 

 mass extends ventrally and presses against the upper side of the meso- 

 nephros. After some of the tubules are clearly formed, however, there 

 still remains the separating layer of connective tissue, and after the 

 mass has become quite large, the two regions are readily distinguished 

 by the difference in the size of the tubules and the character of the 

 cells, although by this time it may be difficult to follow the exact 

 line of separation in cross sections. 



Here we see an organ growing backward, contrary to the laws 

 indicated by a study of all the previous development from the time 

 the embryonic layers were formed. It is impossible to avoid con- 

 jecture as to what is the cause of such a strange proceeding. Ponder- 



