258 EMBBYOLOGY OF THE LOWER VEETEBEATES ch. 



appearance as an outgrowth which fuses secondarily with the 

 tip of the collecting tube. The Malpighian bodies begin about the 

 ninth day to develop their special characteristics in a manner 

 similar to those of the mesonephros. An important point to 

 notice is that the metanephros differentiates from behind forwards 

 instead of in the opposite direction as does the mesonephros. 



About 24 hours (the exact time varies greatly) after the first 

 appearance of the ureter the part of the mesonephric duct between it 

 and the cloaca becomes incorporated in the cloaca so that meso- 

 nephric duct and ureter come to have independent openings into 

 the cloacal cavity. 



As the metanephros goes on with its development it comes to be 

 situated in great part dorsal to the mesonephros but it will be under- 

 stood that this topographical relationship is secondary. At first it is 

 completely posterior to the mesonephros. Even the ureter is in its 

 first stage localized about segment XXXIV (Fig. 138, B) and its 

 extension forward as far as segment XXV or even farther is purely 

 secondary. It will be noticed in Fig. 138 how exactly the ureter in 

 its first beginnings resembles one of the pocket-like outgrowths of the 

 duct which in the mesonephric region develop into collecting-tubes, 

 and it seems scarcely possible to avoid the conclusion that the meta- 

 nephros of the Fowl is simply the enormously hypertrophied 

 nephridial apparatus of a single segment, the ureter being a greatly 

 elongated collecting-tube with an immense number of subsequent 

 tubules opening into it. 



OPISTHONEPHKOS IN OTHER GROUPS OF VERTEBRATES 



Crossopterygii. — Our knowledge of the early stages of develop- 

 ment is still fragmentary being based upon three specimens of 

 Polypterus (stages 32, 33 and 36) obtained by Budgett (Graham 

 Kerr, 1907). 



In the youngest of these stages a number of the opisthonephric 

 units have made their appearance in the form of rounded cell masses 

 arranged segmentally in the mesenchyme ventral to the myotomes, 

 those which are best developed possessing a distinct lumen. 



In the specimen of stage 33 these rudiments have become 

 elongated forming thick curved masses, one end of which is closely 

 applied to, or even fused with, the dorsal wall of the duct. The 

 lumen is restricted to the end farthest from the duct, which represents 

 the definitive nephrotome, while the part which extends towards 

 the duct — the tubule rudiment — is solid. 



In the 30-mm. larva described by Budgett (1902) the opistho- 

 nephros commences about 4 segments behind the pronephros and 

 stretches through about 39 segments with from two to five Mal- 

 pighian bodies and tubules in each segment. On the right side of 

 the body 18 of the Malpighian bodies— roughly one to each segment 

 — communicated with the splanchnocoele by a nearly straight peri- 



