WOLFFIAN DUCT AND BODY IN THE CHICK. 47 



A cause of abbreviation is so clear in this case that I heed not 

 ■waste time in stating it. 



But the whole details of the development of the secondary, 

 &c., dorsal tubules needs reworking, for, with the exception of 

 the observation of Mr. Balfour^s for Elasmobranchs, we have no 

 real knowledge of their exact method of development. The result 

 of such an investigation cannot but be exceedingly interesting 

 from a phylogenetic standpoint. 



I cannot help thinking, as before stated, that the development 

 of the external glomeruli in the chick may have some interest in 

 this relation. 



The modification of the mesonephros of the Amniota is, on 

 the above hypothesis, due to the fact that some Avian ancestor 

 possessed a larva in which the anterior part of the excretory 

 system was early developed, the development of the hinder part 

 being deferred, and consequently modified, just as we see to be 

 the case now in the Ichthyopsida, 



The still greater modification und retardation of the develop- 

 ment of the metanephros or true kidney of the Amniota, and 

 the great size which the WolfiBan body reaches in the embryo, 

 are striking facts which demand consideration in any discussion 

 of the Yertebrate excretory system. 



In my paper on the " Development of the Kidney^^ I have stated 

 my views on the relation of the Amniote kidney to the mesone- 

 phros. But one point in that paper is left untouched. 



Why does the kidney appear so late ? and also why does the 

 Wolffian body become so large and complex — so much larger 

 than the small-sized chicks, in which it is fully developed, can 

 need? 



And, further, why should this organ, apparently so well 

 adapted to serve as the excretory organ of the adult chick, 

 atrophy ? 



It may be said, in answer to the latter question, that only 

 those tubules of the mesonephros which open into the cloaca 

 independently of the Wolffian duct can function in the adult, as 

 those which have not so changed their course would interfere 

 with the function which the Wolffian duct later acquires — the 

 carriage of semen. 



It seems to me that the only answer which can be given to 

 the first of these questions is this : 



The kidney is thrown back in development for the same 

 reason that the mesonephros of the Amphibia is, viz. because 

 the ancestor of the chick underwent part of its development out 

 of the egg, at which stage of development the testis, not being 

 developed, did not interfere with the excretory functions of the 

 Wolffian tubules, or vice versa. The large size of the mesone- 



