MUSEUM OF COMPARATIVE ZOOLOGY. 255 



In the younger embryos of this stage, the walls of the pronephric 

 tubules are all very thick ; they gradually diminish in thickness as the 

 embryo grows older. The lumen, on the other hand, is at first narrow, 

 but afterwards becomes much wider. Its size varies greatly in different 

 portions of the pronephros. For example, the lumen of the long" arm of 

 the common trunk, which forms the direct continuation of the segmental 

 duct, is usually much narrower than the average lumen of the other 

 pronephric tubules. The nephrostomal canals near their junction and 

 the adjacent portion of the common trunk usually have a wide lumen. 

 In the abnormal pronephros represented in Figure 57, however, the 

 lumen of the first nephrostomal tubule was very narrow, a circumstance 

 which, as I have already suggested, may possibly be correlated with the 

 presence of a third nephrostome. 



The lining epithelium of the tubes is composed of polygonal cells, 

 which in the younger embryos have a high columnar form, but become 

 gradually thinner as development proceeds. The nuclei when stained 

 with Czokor's cochineal show a coarsely granular or reticulate structure, 

 and are located close to the lumen of the tubule. The protoplasm takes 

 a uniform delicate tint, which is masked, however, by the deeply staining 

 yolk spherules. These are most abundant near the basal surface of the 

 cell ; they decrease in number and in size with the growth of the larva. 



In the younger embryos of this stage, the somatopleure is composed 

 of somewhat flattened cells, whose superficial dimension is approximately 

 double the thickness of the cell. The walls of the pronephric tubules 

 in these embryos have a thickness of about 37.5 /x, while the parietal 

 peritoneum has an average thickness of only about 15 fx. These two 

 epithelial layers are confluent at the nephrostomes, the wall of the tubule 

 diminishing rapidly in thickness to that of the peritoneum. The 

 nephrostomes, as well as many of the pronephric tubules, are slightly 

 pigmented on their internal surfaces ; but the pigmentation is by no 

 means so conspicuous as in Rana and Bufo. In the older larva) of this 

 stage, the peritoneum is much thinner; but since the walls of the tu- 

 bules have also diminished in thickness, nearly the same relations are to 

 be observed at the nephrostome as in the younger embryos. 



As in Piana and Bufo, the pronephric capsule in Amblystoma develops 

 in the form of a downgrowth from the somatic layer of the protover- 

 tebrre. In Amblystoma the two-layered condition of the capsule and 

 its connection with the overlying protovertebrae are maintained in the 

 oldest larva? of this stage. It seems probable, moreover, that the down- 

 growth from the protovertebra) is met by a more or less pronounced 



