MUSEUM OF COMPARATIVE ZOOLOGY. 259 



pronephros of a fresh specimen. In such an isolated pronephros the 

 course of the tubes can be followed with tolerable accuracy in conse- 

 quence of the pigmented areas occurring in their walls. The loss of yolk 

 spherules, to which the pronephric tubes have been subjected on reach- 

 ing the present stage, is shown in a striking manner by the transparency 

 of the gland as contrasted with tbe snow-white yolk-entoderm. 



The histological characters of the duct (Plate VII. Figs. 53, 54) re- 

 semble closely those of the pronephric tubules. Its calibre is greatest in 

 the region immediately posterior to the pronephros (Plate VII. Fig. 54), 

 becoming less as the duct passes posteriorly (Fig. 53). Throughout its 

 course it is accompanied by the posterior cardinal vein (vti. crd.) . In 

 the older larvae of this stage, the segmental duct in its passage backwards 

 to the cloaca receives a large number of mesonephric tubules, which will 

 be described in the sequel. 



The pronephros of the present stage is covered on its dorsal surface by 

 the main body of the myotomes. From the outer angle of each myotome, 

 moreover, a distinct fibrillar sheet envelops the entire lateral surface of 

 the gland. This layer is the capsule, whose origin has been discussed in 

 connection with Stage V. In the present stage, it frequently becomes 

 deeply pigmented. 



The anterior portion of the pronephros is also overlaid by a stratum 

 of smooth muscle fibres, which arises from the dorsal fascia. This mus- 

 cular sheet is continuous in front with a muscle layer which is inserted 

 upon the ventral surface of the mandible, and probably represents the 

 depressor maxillae of the adult. 



The pronephros is also covered in part by the shoulder girdle, which 

 in this stage is wholly composed of cartilage. 



The vascular sinuses enclosed within the capsule are the direct con- 

 tinuations of the posterior cardinal vein. They also receive — usually 

 about midway between the first and second nephrostomes — a blood- 

 vessel, which may be traced nearly as far back as the cloaca, and which 

 accompanies in its course the ramus lateralis of the vagus nerve (see 

 Fig. 53, just median to n. I.). I am not aware of any prior mention of 

 a vessel having this course, and am unable to state whether this vein 

 has any representative in the adult. 



The vessel emerging from the anterior end of the pronephros receives 

 a vessel from the head, and the two form the ductus Cuvieri, which pro- 

 ceeds downward and inward to join the sinus venosus. The anterior 

 branch may be traced forward into the head in the same direction as the 

 original trunk ; it accompanies in its course the ramus lateralis vagi. 



