4-00 THE O RIG IX OF VERTEBRATES 



decides, it is true, in favour of the internal vesicle, and therefore 

 considers the excretory organ to be appendicular, i.e. a coxal gland, in 

 these segments as well as in those more posterior. Still, the very 

 discussion shows that in his opinion, at all events, the external 

 vesicle might represent the end-sac of the tubule, in the absence of 

 the internal or appendicular vesicle. 



Such an arrangement as Sedgwick describes in Peripatus is the 

 very condition required to give rise to the pronephric and meso- 

 nephric tubules, as deduced by me from the consideration of the 

 vertebrate, and harmonizes and clears up the controversy about the 

 mesonephros and pronephros in the most satisfactory manner. Both 

 pronephros and mesonephros are seen to be derivatives of the original 

 annelid segmental organs, not directly from an annelid, but by way 

 of an arthropodan ancestor; the difference between the two is 

 simply that the pronephric organs were coxal glands, and indi- 

 cate, therefore, the presence of the original metasomatic appendages, 

 while the mesonephric organs were homologous organs, formed in 

 segments of later origin which had lost their appendages. For this 

 reason the pronephros is said to be formed, in part at least, from 

 a portion of the cceloin situated more ventrally than the purely 

 somatic part which gives rise to the mesonephros. For this reason 

 Sedgwick, Brauer, etc., can say that the mesonephros is strictly homo- 

 dynamous with the pronephros; while equally Kiickert, Semon, and van 

 Wijhe can say it is not homodynamous, in so far that the two organs 

 are not derived strictly from absolutely homologous parts of the coelom. 

 For this reason Semon can speak of the mesonephros as a dorsal 

 derivative of the pronephros, just as Sedgwick says that the external 

 or somatic vesicle of Peripatus is a derivative of the appendicular 

 nephric organ. For this reason the pronephros, or rather a part of it, 

 is always derived from the somatopleuric layer, for, as is clear from 

 Miss Sheldon's drawing, the part of the ccelom in Peripatus which 

 dips into the appendage is derived from the somatopleuric layer 

 alone. 



Such a ccelom as that of Peripatus, Fig. 157, would represent the 

 origin of the vertebrate ccelom, and would therefore represent the 

 proccelom of van Wijhe. In strict accordance with this, we see that it 

 separates into a dorsal part, the walls of which give origin to the 

 somatic muscles, or at all events to the great longitudinal dorsal 

 muscles of the animal, and a ventral part, which forms a nephroccele, 



