GENERAL CONSIDERATIONS. 



573 



ditions of the blood-vascular system, and especially to the development 

 of the body-cavity which, as made known by HATSCHEK, yields the 

 key for understanding the formation of the layers in the Vertebrates. 

 It must further be pointed out that the development of AmpJiioxus 

 without doubt shows very primitive characters, a view to which 

 however, objections have been raised by DOHRN. The occurrence of 

 free-swimming ciliated larvae, nevertheless, can hardly be interpreted 

 in any other sense. 



jns 



FK;. 312. Dorsal portion of the left pharyngeal wall of Amphioxus, showing three 

 renal canals, seen from the side (diagrammatic, after BOVERI). Id, ligamentum 

 denticulatum ; m, myotome ; ms, myoseptum ; nc, openings of the renal canal into 

 the body-cavity ; nk, renal canal ; np, opening of the renal canal into the atrial 

 cavity ; 5, synapticulum ; /, primary gill-bar ; //, tongue-bar. 



Considering the agreement prevailing between Amphioxus and the 

 Vertebrates, it is important to emphasise the distinctions between the 

 two forms. Amphioxus is specially distinguished by the rostrum- 

 like prolongation of the notochord, by the secondary increase in 

 number of the gill-clefts, and by the duplication of such primary clefts 

 through division (development of the tongue-bars), this last character- 

 istic recalling the multiplication of the gill-clefts in the Ascidia. 

 With regard to the condition of the urogenital system, we must refer 

 to the accounts of BOVERI (No. 2), according to which the renal 

 canals of AmpJiioxus are to be considered as the homologue of the 

 pronephric canals of the Vertebrata, and the atrial cavity of 



