240 BULLETIN OF THE 



body in the case of the individual whose pronephros is represented in 

 Figure 35. The dilated chamber (Plate VI. Fig. 51) is here formed by 

 a gi'eat expansion of tiiat portion of the ascending arm of the common 

 trunk (trn. com.) which is adjacent to tlie collecting trunk (trn. dg.). 

 A similar dilated chamber occurs in the pronephros represented in 

 Figures 36 and 41 ; but in the latter case neither the dilation of the 

 lumen nor the thinning of the wall is very pronounced. In both these 

 cases the expanded chamber is present in portions of the tubular system 

 which are exactly equivalent to each other. Under these circumstances, 

 the expansion of the desrending limb of the duct occurring in the prone- 

 phros of Stage V. (Fig. 34) seems quite anomalous. The dilated 

 chamber is invariably, however, superficial in position, lying close to the 

 capsular membrane. I have been unable to reach an entirely satisfac- 

 tory opinion regarding its function. Since it is situated so near to the 

 nephrostomes, it does not seem very well adapted to serve as a reservoir 

 for the storage of fluids secreted by the gland, for by far the larger por- 

 tion of the secreting surface is situated between it and the duct. How- 

 ever, the chamber doubtless receives whatever fluids are gathered by the 

 nephrostomes or are secreted by the peritoneal tubules, and it is pos- 

 sible that the enlargement exists solely for this purpose. In following 

 the duct from the dilated region towards its outlet, a greatly contracted 

 portion is reached, and this may serve for the better retention of fluid 

 contained in the chamber. 



The capsule in these larvae is not so well marked as in those of the pre- 

 ceding stage. Between the pronephric tubules and the ectoderm there 

 has arisen a considerable quantity of mesenchyme, and the capsule now 

 appears merely as the line along which this mesenchyme comes in con- 

 tact with the pronephric tubes and blood sinuses. 



In discussing the blood supply for the preceding stage, it seemed 

 advisable to consider the vessels in older larvae as well, and I shall there- 

 fore merely refer here to the account given in that connection.* 



1 In all the larvae of this stage which I have examined, I have observed a peculiar 

 sac, of which I have been unable to find any mention in the literature. In the 

 oldest larva of this stage it consists of a capacious sinus lying in the triangular area 

 bounded by the myotomes, the somatic peritoneum, and the ectoderm. It extends 

 backwards from the niveau of the third nephrostome for a length of two or three 

 myotomes, and appears to be closed upon all sides. The sac lies in a mass of loose 

 mesenchyme, but possesses firm walls, so that any opening would naturally be easily 

 recognizable. In the interior of the sac, cells which are undistinguishable from 

 blood corpuscles are found in considerable numbers. In a younger larva the sac 

 occurs in a corresponding position, is nearly filled with blood cells, and is in open 



