INTERNAL ANATOMY. 7 1 



although they occur mostly behind the region of the pro- 

 nephros, yet rudiments of them appear in the same seg- 

 ments occupied by the latter. Unlike the pronephric 

 tubules, they arise, not as evaginations from the base of 

 the somites, but in such a way that an adjacent portion 

 of the somite, lying dorsal to the pronephric tract, loses 



ttf.C/ 



-?nn 



Fig. 34. Three diagrams illustrating the hypothetical phylogenetic develop- 

 ment of the excretory organs in Selachians. (After RUCKERT.) 



s. Somites, pn. Pronephric tubules fused with ec, the ectoderm in A ; collected 

 into a common duct w.d, the Wolffian or pronephric duct in B; and finally 

 aborted in C, with the exception of one, which persists as the ostium abdominale. 

 inn. Mesonephric tubules, w.d. Pronephric duct in B; mesonephric duct in C. 

 cl. Cloaca, p. Posterior region. 



its primary connection with the rest of the somite, which 

 consists of the myotome proper, and becomes bodily con- 

 verted into a mesonephric tubule whose blind end curves 

 round the pronephric duct and eventually opens into it ; 

 while its point of communication with the unsegmented 



