238 ZOOLOGY. 



and consequently of the arterial arches, is restricted to the 

 pharyngeal region. We have next to see that the embryo 

 frog has excretory tubules very different from those of 

 Amphioxus : they lie altogether behind the pharyngeal 

 region, and their metamerism corresponds to that of the 

 mesoblastic somites. 



The first rudiment of an excretory system consists in the 

 formation of a pair of longitudinal tubes in the somatic 

 mesoblast, just ventral to the mesoblastic somites (figs. 124 

 and 127). This tube is called the archinephric duct (or 

 segmental duct). At its anterior end three tubules are 

 formed in the somatic mesoblast, under the second, third 

 and fourth somites respectively ; these open into the coelom 

 at one end (nephrostome) and into the archinephric duct 

 at the other. They are called the pronephric tubules, or 

 collectively the pronephros or head-kidney (figs. 127 and 

 128). On each dorsal aorta, just opposite the nephrostomes, 

 a bunch of little blood-vessels is formed. Separated as this 

 is from the nephrostomes by the narrow coelom of this 

 region, the resemblance to the glomus of Amphioxus is 

 striking, and it may be called the glomus and regarded as 

 three glomi united in one (fig. 128) . The archinephric duct 

 and the three pronephric tubules in the embryo frog have 

 the characters which belong to coelomic ducts : they are not 

 intracellular but lined with an epithelium, they have an 

 opening into the ccelom, and they are developed in the 

 mesoblast from within outwards. They cannot therefore 

 be homologous, as was formerly supposed, with the nephri- 

 dia of Amphioxus. The Miillerian duct, which becomes the 

 oviduct is developed in the frog independently, but there is 

 evidence in the development of the dog-fish that it was 

 originally derived by splitting from the archinephric duct, 

 and as we know the posterior part of the archinephric duct 

 through the medium of the mesonephric tubules becomes 

 connected with the testis and acts as vas de/erens. The 

 excretory tubules of the frog therefore, and of all vertebrata, 

 have a close connection with the genital ducts, and were 

 probably evolved as modifications of the genital ducts. 



13. Hatching of Tadpole. We have now described the 



