138 CHANGES FROM STAGE G TO BIRTH. 



which acquires an opeuiiig to the exterior (PI. VII, fig. 23 

 e.). They become divided into a dorsal and ventral part 

 (diagram, PI. XIII, fig. 14), of which the dorsal vanishes, 

 while the ventral persists in connection with the opening above 

 mentioned, and is at first placed in the appendage (PI. VII, 

 fig. 23 e). This ventral or appendicular part is the nephridium 

 of the somite, and becomes tiie salivary gland of the adult. 

 The general form of this nephridium at the close of Stage f is 

 well shown by the diagram (PI. XI, fig. 6). It consists of a 

 tubular part (/. s. t. 3, sal. gl.), opening in front on the ventral 

 surface of the body [p. s. 3) within the lip and ending blindly 

 behind, and of a vesicle (/. s. v. 3) opening into the tubular 

 part a little in front of its termination. The structure and 

 relations of these parts to each other are illustrated by the 

 three transverse sections figured on PI. VIII, figs. 37, 38, and 

 39, and taken along the lines marked 37,38, 39 in the diagram 

 (PI. XI, fig. 6). The subsequent changes which this organ 

 undergoes are unimportant. Tliey are illustrated by the diagram 

 PL XI, fig. 7, and by figs. 8 and 9, taken from transverse 

 sections of an embryo shortly before birth, along the lines 

 8 and 9 in fig. 7. 



The tubular part has become much elongated (PI. XI, fig. 

 7, sal. gl.), so that it now extends a considerable distance 

 behind the point of communication with the internal vesicle. 

 It constitutes the salivary gland of the adult, and lies, as is 

 well known, in the lateral compartment of the body cavity 

 (lateral sinus). Tlie walls of the vesicle (/. s. v. 3) have be- 

 come much thicker. They consist (PI. XI, fig. 8) of a layer 

 of nucleated, richly vacuolated protoplasm. Finally, the por- 

 tion connecting the tubular part [1. s. t. 3) and the vesicle 

 [1. s. V. 3) has become elongated into a tube running forwards 

 from the tube to the vesicle, as shown in the diagram (PI. 

 XI, fig. 7). This communicating portion, as shown in PI. 

 XI, fig. 9, is closely applied to the dorsal side of the tubular 

 part. 



The internal vesicle, which, together with the communica- 

 ting tube, has hitherto been overlooked, persists in the adult, 



