MUSEUM OF COMPARATIVE ZOOLOGY. 237 



I have spoken of the expanded dorsal portion of the body cavity, into 

 which the nephrostomes open, and which contains the glomus. This 

 portion of the body cavity constitutes the so-called pronephric chamber. 

 It is not to be regarded as a closed cavity. Elsewhere the somatopleure 

 and the splanchnopleure are closely applied to each other, but there is 

 absolutely no fusion of these layers ventral to the pronephros. 



Stage VI. 



Plate III. Fig. 37. Plate IV. Figs. 38, 30, 35-38, 41. Plate VI. Fig. 51. 



The larvaj included in this stage are in general two or three days 

 older than those of the preceding stage. They are about 8 mm. long 

 from the anterior end to the tip of the tail. In this stage, the body no 

 longer tapers gradually from the branchial region to the posterior end ; 

 but a definite line of separation is established between the trunk and 

 tail regions. In the tail a distinct membranous fin has appeared, both 

 along the dorsal and the ventral median lines. The horn lips can be 

 seen surrounding the mouth, and the external gills project prominently 

 on both sides of the body. 



The pronephros of this stage has developed along lines foreshadowed 

 in the preceding stage. The general form of the organ can best be un- 

 derstood by reference to the series of diagrams (Figs. 3o-37) and the 

 reconstruction (Fig. 41) given on Plate IV. As will be evident at once, 

 the gland has reached a high degree of complexity, produced, however, 

 by a continuation of the same process of complication whicli had begun 

 in Stage V. Thus the first nephrostomal tubule ^ and the collecting 

 trunk retain throughout a nearly unmodified condition ; the third ne- 

 phrostomal tubule usually becomes slightly complicated ; the second 

 exhibits the greatest number of convolutions. The common trunk, 

 however, is the part which has been principally concerned in producing 

 the increased complexity of the gland. It is to be noted that this con- 

 tortion is not of a wholly indefinite nature ; indeed, there is consider- 

 able uniformity in the pronephridia of different individuals of the same 

 stage of development. In Figure 31, representing a pronephros of a 

 larva in Stage V., it is to be seen that there are only two bends in the 

 common trunk, which extends forward to the anterior end of the gland. 

 From this simple condition the later complications may be derived by a 

 few simple steps. In order to follow the changes it will be advisable to 



^ The same colors have been employed for corresponding parts iu both Figures 

 40 and 41. Consult explanation of Figure 41. 



