514 MR GREGG WILSON ON THE DEVELOPMENT 



nephrostome is accordingly very much contracted in breadth ; but it extends distinctly 

 for several sections anterior to the opening and for about a score of sections posterior 

 to it. The cells are brick-shaped, and extend so far laterally as to meet the outgrowth 

 of the body-wall that projects to meet the radix mesenterii. The secondary attachment 

 of the alimentary canal ceases behind the sixth section posterior to the first nephro- 

 stomial opening ; but the lateral outgrowth continues for other fourteen sections (fig. 6). 



Four or five sections posterior to the termination of the first patch of thickened 

 epithelium the plate that surrounds the second nephrostome begins. It extends across 

 the pronephros for a few sections, and then becomes much reduced. Yet it can be 

 followed as a narrow thread of cells through the eight sections succeeding the close of 

 the second nephrostome ; in the ninth section, which occurs just where the segmental 

 duct makes a spiral turn before running straight backwards, a remarkable thickening 

 occurs, and the epithelium forms a ciliated depression (p, fig. 7). I have found no 

 similar appearance in any other specimen. 



For five or six sections j)osterior to the depression the thickening of the epithelium 

 is noticeable, and then ceases. It lies outside the segmental duct. 



The segmental duct passing back from the pronephros shows a more prolonged 

 spiral than in " A," with the result that at short intervals appearances such as are 

 indicated in figs. 8-11, and which might in many cases be taken as indicating a 

 budding, are found on different sides of the duct. 



The glomerulus begins on one side in the seventh section behind the first 

 nephrostome, and ends just opposite the second nephrostome. It fuses with the 

 peritoneal epithelium of the pronephros a few sections before its posterior end. On 

 the other side the glomerulus begins two sections anterior to the opening of the 

 first nephrostome. 



A specimen (C) 20 mm. in length shows in the section that meets the first nephro- 

 stome only three divisions of the ccelom. The fusion of the lungs with the lateral walls 

 is in this region complete, so that there is now no middle diverticulum. The attach- 

 ment of the alimentary canal to the body- wall, on the other hand, is just as in "B," 

 — the diverticulum persists to receive the anterior nephrostome, but it begins only 

 three sections anterior to the opening. 



On both sides, the glomerulus begins one or two sections in advance of the first 

 nephrostome. On one side it continues to one, on the other side to the fourth section 

 posterior to the pronephros. 



Between the two nephrostomies there is a continuous plate or band of heightened 

 epithelium, which continues for a few sections posterior to the second nephrostome, but 

 without forming a ciliated depression as in " B." It is immediately posterior to the 

 second nephrostome that the lateral connection of the gut with the body-wall ceases, 

 and here the heightened epithelium runs far round the ventral wall. 



In specimen " D" (32 mm.) the relations in the anterior region of the ccelom are 

 very similar to those described in " C." Where the funnel of the first nephrostome 



