430 FREDERIC T. LEWIS 



is the twenty-eighth to enter the Wolffian duct on the left side; 

 three more occur below it. Of these thirty-one tubules, the 

 anterior are extensively degenerated, and lend no support to 

 MacCallum's statement that "the degeneration of tubules pro- 

 gresses from the posterior end of the organ forward." On the 

 contrary, a posterior tubule was chosen as probably typical of 

 those most highly developed. Its actual length, as calculated 

 from the model, is 1.5 mm. — nearly twice that of the preceding 

 stage. The portion C, with its glomerular end bent upward or 

 anteriorly, lies within the concavity of portion Z, which also 

 extends upward to enter the Wolffian 'duct. This disposition of 

 parts would produce the result observed by MacCallum when 

 injecting the tubules of the pig. The fluid could be seen, through 

 the thin overlying tissue, to pass from the Wolffian duct into 

 the tubules and to flow through them around the lateral surface 

 of the gland to the dorsal border. At a certain place, in tubules 

 just beneath the superficial ones, the fluid could be seen to run 

 in the opposite direction. 



Although the tubule at this stage is of a simpler pattern than 

 the double spiral, and consists of two rather than three segments, 

 the remains of the middle or U-portion can be identified. A 

 narrowing of the tubule marks the place where the U begins, 

 and it proceeds around the bend into Z, where it ends without 

 definite boundary. An interpretation of the model in terms of 

 the double spiral is shown in. figure 9, and the group of figures 

 (5 to 9) presents the evolution of a human tubule from a length 

 of 0.3 mm. to 1.5 mm. without the formation of capricious folds, 

 but according to a very simple law. 



That the Wolffian body of the pig is more highly developed 

 than in man is shown in several ways, but very clearly in the 

 greater length of its tubules. The length in a 6-mm. pig, as 

 calculated from a single model, is 1.9 mm., and at 20 mm. it has 

 increased to 13.5 mm.; that is, the tubules of the pig are found 

 to be from six to eight times longer than those of man, and more 

 extensive coiling may be anticipated. The typical double spiral 

 is formed early in the pig, as already shown in figure 4, from an 

 embryo of 4.5 mm. A somewhat later stage was modeled in 



