256 BULLETIN OF THE 



upgrowth from the somatopleure immediately ventral to the pronephros. 

 The thickness of the capsular sheath gradually diminishes in the course 

 of the development of the larvae, but it is in general approximately equal 

 to that of the peritoneum in the same individual. In the older larvae, 

 moreover, the pronephros, and especially the segmental duct, become 

 partially covered by a downward extension of the myotome. In such 

 larvae the anterior limb bud is prominently developed at this stage, and 

 its cells cover in part the posterior ventral portion of the pronephros. 



The sinuses within the capsule are bounded by mesenchymatic cells 

 and contain scattered blood corpuscles ; they are continuous posteriorly 

 with the posterior cardinal veins, so that the venous blood in passing 

 forward from the hinder portions of the body bathes the pronephric 

 tubules on every side. 



The vessel emerging from the anterior end of the pronephros receives 

 a large vessel from the head, and from the point of union the ductus Cu- 

 vieri leads to the sinus venosus. The former vessel is one of the jugu- 

 lar veins. The distribution of this vein and its probable representative 

 in the adult will be considered in connection with the following stage. 



The first trace of the glomus appears in embryos of this stage. It 

 consists, as in Rana and Bufo (compare Plate I. Figs. 8, ( J, and Plate VI. 

 Fig. 47), of a horizontal fold of splanchnopleure lying close to the dorsal 

 angle of the body cavity. This fold extends, when fully formed, from 

 the first nephrostome backwards to the second. The outer layer of the 

 organ consists, as shown by its development, of splanchnic peritoneum, 

 which is usually bounded within by a sharp contour. I am of opinion 

 that the latter is in reality a thin structureless basement membrane. 

 The interior mass of the glomus consists of several different elements. 

 In the young stages embryonic blood cells form a prominent constituent. 

 Other cells are present, which have an elongated form and arc evidently 

 connective-tissue elements, and there appear to be still other cells which 

 are of a less modified character and in which nuclear mitoses occur. 

 Many of the latter may well represent young stages in the development 

 of blood corpuscles, for I have observed mitotic division of blood cells 

 even in certain older larvae of Stage VI. In addition to the classes of 

 cells just mentioned, there are a few large cells whose nature is to 

 me quite obscure. These cells measure 60 /x or more in diameter, and 

 contain large yolk spherules, which are closely packed together and 

 make up almost the entire substance of the cell. The histological 

 characters of these cells ally them most closely with those of the ento- 

 derm, and in the youngest stages in which I have been able to identify 



