647 



It is interesting to compare this theory with the a priori views 

 which Oppel ('06) has expressed regarding the evolution of the swim- 

 bladder. 



"1. Ursprünglich haben alle Fische ihre Schwimmblase vom Darm 

 vermittelst des Schwimmblasenganges mit Luft gefüllt und entleert. 



2. Demgegenüber steht als ein speziellen Lebensbedingungen an- 

 gepaßter, in bestimmter Richtung höchst entwickelter Typus, bei 

 welchem die Fische ihre Schwimmblase ausschließlich vom Blute aus 

 durch die Gasdrüse füllen, und vermittelst des Ovals oder analoger 

 Einrichtungen entleeren. Bei den Vertretern dieser Gruppe ist der 

 Ductus pneumaticus obliteriert. 



3. Zwischen diesen Extremen stehen Uebergangsformen, bei denen 

 Füllung und Entleerung der Schwimmblase sowohl vom Darme wie 

 vom Blute aus möglich ist, und bald eine, bald die andere der beiden 

 Möglichkeiten bevorzugt wird." 



It seems probable that the swim - bladders of the pickerel, of 

 Fundulus and of Menidia represent a deviation from the main phylo- 

 genetic course of the organ. In the anterior position of the pneumatic 

 duct, as well as in certain other particulars, the swim-bladder in these 

 forms is probably fundamentally different from the higher types of the 

 organ and does not stand in any direct genetic relation with them. 

 This, however, differs from the view of Corning ('88) who homologized 

 the oval with the rete mirabile area in the swim - bladder of Esox. 

 His theory was based chiefly on the fact that the red gland and the 

 pneumatic duct derive their blood supply from the coeliac axis while 

 the rete mirabile in the swim -bladder of Esox and in the oval are 

 supplied by branches direct from the aorta. Corning's theory, how- 

 ever, appears to be untenable, because the posterior chamber in the 

 Opsanus type of swim-bladder is supplied from the aorta, and, as has 

 been shown above, this structure is a derivative of the pneumatic duct. 

 The blood supply of these structures seems to be a secondary ad- 

 aptation, and, morphologically, not of primary importance. 



The processes involved in the transformations of the swim-bladder 

 as outlined above, may be compared in a general way to those oc- 

 curring in the course of the development of the kidney. The nephric 

 tubules were, primitively, open ducts connecting the coelom with the 

 exterior. Later, as appears in the early stages of the mesonephros, 

 a glomerulus is developed in the wall of each tubule near its entrance 

 to the coelom. In the further development of the mesonephros and in 

 the metanephros, the tubules have entirely lost their connection with 

 the coelom; they end blindly and include the glomeruli, thus forming 



