MUSEUM OF COMPAEATIVE ZOOLOGY. 267 



('75) and of Fiirbringer ('78*, pp. 38, 39), which plainly represents the 

 Amphibian pronephros. Whether it ever persists in the adult is still a 

 matter of doubt (see "Weldon, '84) ; but in young individuals, at least, 

 the segmental duct (ureter) is prolonged anteriorly to the heart region. 

 Hei'e it gives off numerous coiled tubes, which branch and open by funnel- 

 shaped nephrostomes into the pericardial cavity. On its dorsal side, 

 the duct gives off a few tubules which terminate in glomeruli resembling 

 those of the mesonephros. This condition and the large number of 

 tubules constitute the main points of difference between the Amphibian 

 pronephros and that of Myxine. 



The pronephros of Teleosts and Ganoids appears to me to be reduci- 

 ble to a single type of structure, which can be easily derived from the 

 condition present in Amphibia and Cyclostomes (and Dipnoi 1). The 

 so-called head-kidney of Teleosts described by Hyrtl ('51, p. 29) is prob- 

 ably derived from the embryonic pronephros, though mesonephric ele- 

 ments may also be found in the adult head-kidney (see Emery, '82, 

 p. 46). 



According to Rosenberg ('67, pp. 42 et seq.) and Oellacher ('73, pp. 97- 

 100), the excretory organs arise as a pair of grooves of the somato- 

 pleure directly beneath the protovertebrae. A process of constriction, 

 which proceeds from a middle region forwards and backwards, leads to 

 the conversion of each groove into a tube, the segmental duct. The 

 anterior portion becomes wholly cut off from the body cavity, and is 

 thrown into numerous coils. The tip becomes considerably swollen, and 

 is invaginated by an outgrowth from the aorta forming a single glomer- 

 ulus on each side. 



Goette's ('75, pp. 826, 827) account of the development of the pro- 

 nephric glomerulus in Teleosts is somewhat different, and affords a better 

 basis for homologizing the pronephros of Teleosts with that of Amphibia. 

 Goette maintains that the somatopleural groove is imperfectly closed 

 in front, leaving a single nephrostome, opposite which a glomerulus 

 (glomus) is developed. Subsequently, the pronephric chamber becomes 

 separated from the rest of the body cavity, and comes to resemble a 

 Malpighian capsule with its contained glomei'ulus. While Fiirbringer 

 ('78") confirms Goette's view, Hoffmann ('86, p. 621 et seq.) has quite re- 

 cently reasserted that this Malpighian capsule is the blind infolded end 

 of the segmental duct, and the homology with the Amphibian glomus 

 and pronephric chamber, which appears to me probable, he denies. 

 Hoffmann's position does not seem to me tenable in the light of com- 

 parative studies. Even though it should be shown that the ducts 



