72 ANATOMY OF AMPHIOXUS. 



body-cavity persists as the nephrostome. (Cf. Figs. 33 

 and 35 B.) 



The pronephric duct, therefore, becomes secondarily 

 employed in the surface of the mesonephros. So that, 

 while the mesonephros and its future duct form two dis- 

 tinct morphological structures, the pronephros and the 

 same duct form one inseparable whole. 



From the above considerations we may conclude that 

 the pronephros represents the primitive and ancestral 

 excretory organ of the craniate Vertebrates. Just as the 

 notochord has been largely replaced first by cartilage and 

 then by bone, so the pronephros has been replaced first 

 by the mesonephros and then by the metanephros. 



Returning now to Amphioxus, we have to note in the 

 first place the absence of a common matrix surrounding 

 the excretory tubules, and, secondly, the absence of a com- 

 mon duct. Since in the higher Vertebrates the interstitial 

 growth of connective tissue among the tubules, binding 

 them together into a compact organ, is a secondary phe- 

 nomenon, the absence of such a matrix in Amphioxus 

 need not detain us. 



Judging from the analogy of the other systems of or- 

 gans in Amphioxus, it will be at once concluded that the 

 excretory tubules of the latter represent the pronephric 

 system of the embryos of the craniate Vertebrates. And 

 this, in fact, is Boveri's contention. 



As we have seen, the excretory tubules of Amphioxus 

 open separately into the atrial cavity. While they do not, 

 therefore, open directly to the exterior at the ectodermic 

 surface of the body, they do actually open at an ecto- 

 dermic surface, since the atrial cavity is a space enclosed 

 from the outside, and so is lined by ectoderm. The pri- 

 mary fusion of the pronephric tubules with the ectoderm, 



