MUSEUM OF COMPARATIVE ZOOLOGY. 239 



the same in this stage as in the foregoing. The lateral plate is now 

 wholly cut off from the myotomes ; but a study of serial sections shows 

 that each nephrostome lies beneath a myotome. These myotomes corre- 

 spond to somites II., III., and IV. 



The course of the duct in this stage is the same as in Stage V. The 

 openings into the cloaca (Plate III. Fig. 27, dt. sg.) are now situated at 

 the bottom of a depression in the dorsal wall of the cloaca (clc). While 

 the excretory products enter the main cloacal chamber by a single aper- 

 ture, a glance at the histological characters of the short median unpaired 

 trunk shows that it is lined with entoderraal cells, and is therefore really 

 a diverticulum of the roof of the cloaca. The ducts of the two sides, 

 therefore, are not to be regarded as uniting into a common trunk, but 

 as opening separately into a dorsal diverticulum of the cloaca. 



The histological characters of the pronephi-ic system have not under- 

 gone any great changes since the preceding stage. Figure 28 (Plate IV.) 

 shows a cross section of the left pronephros of the larva, whose right 

 pronephros is diagrammatically represented in Figure 37. The plane 

 of the section passes through the first nephrostome, and the transition 

 from the pavement cells of the peritoneum to the columnar epithelium 

 of the tubules is clearly shown. This section also shows, besides the 

 first nephrostomal tubule, the anterior ends of two loops which belong 

 to the transverse portion of the common trunk. The walls of all the 

 tubules are thinner than in the preceding stage, and since the nuclei re- 

 main of about the same size as heretofore, they now occupy a far larger 

 proportion of the cell, and in the case of the thinnest-walled tubules are 

 frequently almost in contact with both the outer and inner surfaces of 

 the cell. The amount of yolk in the cells is considerably lessened, 

 especially in those parts which exhibit the greatest number of convolu- 

 tions. In some cells, a single large spherule is the sole remnant of the 

 formerly abundant yolk. Pigment is present as scattered grains in the 

 walls of all the tubules; it also shows a tendency, as in the -previous 

 stage, to accumulate along the free surfaces of the cells. The nephro- 

 stomes, however, are densely pigmented on the surface that is directed 

 towards the body cavity and the lumen of the tubule. The duct pos- 

 terior to the pronephros (Fig. 30) offers no features worthy of special 

 mention. It is accompanied throughout by the cardinal vein {vn. crd.), 

 on the median side of which the earliest fundaments of the mesonephric 

 tubules are visible. 



I have described a special enlarged region of the convoluted duct in 

 a larva of Stage V. A similar condition is apparent on both sides of the 



