70 WILLIAM MORTON WHEELER, 



V, subintestinalis bin ich noch nicht zu sicherer Entscheidung gelangt, 

 doch glaube ich, dass beide gleichfalls in loco entstehen." Thus the 

 view which I hold is intermediate between the well-known view of 

 His and that of Felix : in Tetromyzon the endothelia, blood-corpuscles 

 and plasma have wandered in from the entoderm, but the muscular 

 and connective tissue elements of the vascular walls are derived in 

 situ from the mesoderm, and are laid down before the immigration 

 of the corpuscles takes place. The conditions in Petromyzon appear 

 to me to be so clear and unequivocal that I believe the origin of the 

 blood and endothelia from the entoderm must stand, no matter what 

 view is adopted in regard to the origin of these structures in other 

 Vertebrates. In this matter the "Amphibienbrille" has not distorted 

 the observations of Goette. I am also convinced that the lumina of 

 the blood-vessels in Petromyson are derivatives of the primitive body 

 cavity (blastocœle) and not of the coelom. These cavities may appear, 

 as in the case of the subintestinal vein, in a region on the lower sur- 

 face of the entoderm not yet reached by the down-growing mesoderm^ 

 so that a coelomic origin is necessarily excluded. 



d) Some Relations between the Renal Organs of 

 Vertebrates and Invertebrates. 



The epoch-making work of Rückert on Selachians, together with 

 the studies of other recent authors on the nephric systems of Verte- 

 brates, establishes the fact that the pro- and mesonephros are not 

 serially homologous portions of a primitive homonomous series of 

 nephridia, but more or less complete and, in some forms, even over- 

 lapping portions of two different series of metameric nephridia, arising 

 from different regions of the mesodermal walls of the body cavity. 

 The recent work of Maas, together with the observations embodied 

 in the present paper tends still further to confirm this conclusion. It 

 seems obvious, moreover, that the second and more dorsal series of 

 nephridia, constituting the mesonephros, made its appearance in the 

 ancestral Craniote, since they are not represented in existing Acrauia. 

 That the single series of nephridia existing in the latter group is the 

 homologue of the Craniote pronephros plus the pronephric duct is 

 established by the researches of Boveri and Maas, and it will be 

 conceded that if any homologues of the nephridia of Amphioxus exist 

 among Invertebrates, they are probably the nephridia of Annelids and 

 not those of Plathelminths, as eminent authority once maintained. 



