242 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



resemblance to the primitive communication of mesoderm segment 

 with enteron, and it is suggested that it actually is this primitive 

 communication which has remained patent while in all the other 

 segments it has disappeared. 



The two archinephric ducts open at first separately into the 

 cloaca, one on each side. In some groups of Vertebrates however 

 their terminal portions become gradually approximated and eventu- 

 ally fused together into an unpaired dorsal vesicle which may 

 undergo various modifications. In Elasmobranchs it forms the 

 urinogenital sinus which bulges forwards and on each side becomes 

 prolonged into the sperm-sac. In Lung-fishes it forms the cloacal 

 caecum : in Teleostei the urinary bladder. 



It is noteworthy that in the adult Lung-fish the communication 

 of the kidney ducts with the caecum is close to the posterior opening 

 of the latter, so that a small amount of shifting would cause these 

 ducts to open into the cloaca independently of the caecum. 



This suggests a possible evolutionary origin of the allantois. 

 It is conceivable that a caecum similar to that of Lung-fishes arose by 

 a fusion of the terminal portions of the kidney ducts ventral, instead 

 of dorsal, to the alimentary canal and that the ducts then came to 

 be emancipated from the caecum which remained as a ventral 

 diverticulum of the cloaca to form the allantois. We have no 

 definite evidence as to the evolutionary origin of the allantois and 

 it is well to bear in mind the possibility here indicated in addition 

 to the simpler and perhaps more probable hypothesis that the 

 allantois was from the beginning simply a bulging outwards of the 

 ventral cloacal wall as it is actually in ontogenetic development. 



Degeneration of the Pronephros. — The role of the pronephros 

 as the functional renal organ is usually confined to comparatively 

 early stages in development and at the end of this period, when its 

 function is being taken over by the opisthonephros, the pronephros 

 commences to undergo characteristic degenerative processes which 

 normally culminate in its almost complete disappearance. 



In the frog (Marshall and Bles, 1890) these processes become 

 apparent in the tadpole of about 20 mm. in length. The archi- 

 nephric duct becomes more or less obstructed behind the pronephros 

 and as fluid continues for a time to pass into the tubules the latter 

 become greatly distended in places, their lining cells assuming a 

 cloudy appearance, the cell boundaries becoming indistinct and their 

 inner surfaces losing their smooth outline and becoming ragged. 

 The whole organ shrinks in size, becomes invaded by leucocytes, the 

 nephrostomes close, one after the other, and by the end of the first 

 year the whole organ with the adjacent portion of the archinephric 

 duct has practically disappeared. 



M&llerian Duct. — Throughout the series of gnathostomatous 

 Vertebrates, with the exception of the teleostomatous fishes, the 

 oviducts are admittedly homologous. They — the Miillerian ducts — 

 are above all characterized by the fact that they open freely into the 



