MUSEUM OF COMPARATIVE ZOOLOGY. 231 



first becomes visible close to tbe lumen. It is here also that pigment 

 makes its appearance. 



The histological character of special regions now claims our attention. 

 The pronephridia shown in Figures 31 to 33 are all histologically very 

 similar, but in the case of the gland represented in Figure 34 some 

 notable differences occur, which I shall consider later. The somato- 

 pleure covering the pronephros is at this stage very thin. Each of the 

 cells composing the membrane is thickest in its central portion, and 

 tapers rapidly towards its margins. In the more advanced larvae, the 

 cells have elongated to such an extent that the peripheral portion is 

 reduced to a thin protoplasmic plate, which is nearly devoid of yolk 

 spherules. The central mass, on the other hand, contains the nucleus, 

 and nearly all the yolk spherules. The peritoneum is continuous with 

 the columnar epithelium of the walls of the tubules at the outer rim 

 of the nephrostomes, which have the characteristic form of a funnel. 

 Before reaching the periphery of the funnel, however, the columnar layer 

 becomes slightly thinner, and at the rim it tapers rapidly, until it becomes 

 continuous with the peritoneum (compare Fig. 18 of a younger and Fig. 

 28 of an older stage). The nephrostomal funnels are always deeply pig- 

 mented. The pigment is most abundant along the incurved surface, but 

 is quite dense even up to the rim. It continues for a variable distance 

 into the tubules. In the case of the pronephros, represented in Figure 

 31, the whole system of tubes was pigmented from the nephrostomes to 

 the posterior bend near the beginning of the common trunk. The pig- 

 ment granules are always disposed in a layer along the free surface of the 

 cells. The nephrostomal tubules show in general the typical character, 

 which I have previously described. The collecting trunk appears to be 

 quite rigid, for I have never seen such a reduction of its lumen due to 

 pressure as other tubes exhibit. The calibre of this canal is usually larger 

 than that of the nephrostomal tubules. The portion of the duct which 

 lies behind the third nephrostome is nearly straight and of uniform calibre. 

 Generally the lumen is slightly wider, and the wall thinner, than in the 

 nephrostomal tubules. In the pronephridia, shown in Figures 31 and 

 33, the loop embracing the anterior bend of the common trunk seems 

 to have but little rigidity. It follows a tortuous course, and fre- 

 quently the walls are so closely pressed together that the lumen is 

 locally obliterated. 



A peculiar modification in the pronephros, represented in Figure 34, 

 has been alluded to. In this case, the common trunk, after proceeding 

 from the level of the third nephrostome for a certain distance forward 



