MUSEUM OF COMPARATIVE ZOOLOGY. 309 



tion of the ccelom. It is very probable that this arrangement represents 

 the earliest differentiation of a special excretory surface of which evi- 

 dence is preserved in the ontogeny of Vertebrates. 



The next step in the specialization of the urinary organs is the es- 

 tablishment of definite conduits for the purpose of conveying the excreted 

 products to the outside. It is possible that simple apertures, such as 

 the abdominal pores, at first served this end ; or, if the enterocoelous 

 condition represent a phylogenetic stage, communications with the 

 intestinal tube may have afforded an outlet to the excreted fluids. Be 

 this as it may, it is evident that the ancestors of our present Vertebrates 

 early acquired specialized tubes subserving this purpose. 



In the account of the development of the Amphibian pronephros and 

 duct given in the first section of the present paper, emphasis was laid 

 upon the fact that these structures are differentiated from a solid so- 

 matopleural thickening, and do not arise as a fold of the peritoneum. 

 Manifestly the former condition is coenogenetic ; such a solid thickening 

 could in no wise function as an excretory conduit. On the other hand, 

 it must not be rashly assumed that the somatopleural thickening is a 

 disguised fold of that layer. On the contrary, the pronephros, on canal- 

 ization, shows itself to be already composed of a series of metameric 

 evaginations of the ccelom, and it is perfectly conceivable that the pro- 

 nephric thickening is a modification from a condition where the separate 

 evaginations had their independent means of communication with the 

 exterior, the several diverticula being fused into a solid mass. Either 

 interpretation would be physiologically intelligible. In the first case, a 

 certain region of the peritoneum would first sink as a groove into the 

 parietes of the coelom. This channel might, like the nephrostomes, be 

 provided with vibratile cilia, and might thus serve to carry the fluids 

 lodged in it back to a single pair of orifices situated near the posterior 

 end of the coelom. As a further differentiation, it is to be conceived 

 that this groove became at intervals constricted off from the coelom, 

 forming a retroperitoneal duct with a series of nephrostomal tubules. 



According to the second alternative, it is necessary to suppose that 

 the several evaginations communicated distally either directly with the 

 exterior or with an independent longitudinal duct. The nephridia of 

 Heteromastus and Capitella (Eisig, '88, pp. 242, 272), in which no ex- 

 ternal opening is present, show us that the gradational steps in the 

 formation of such outgrowths may be conceived to be functional. 



In judging between the two views to which allusion has just been 

 made, it is important to consider whether the ontogeny of other groups 



