166 THE URINOGENITAL ORGANS OF VERTEBRATES. 



the permanent kidneys is developed in nearly the same way 

 as the supposed homologous duct in Selachians, the suggested 

 identity gains further support. The only difficulty is the fact 

 that in Selachians the tubules of the part of the kidneys under 

 comparison develope as segmental involutions in point of time 

 anteriorly to their duct, while in birds they develope in a manner 

 not hitherto certainly made out but apparently in point of time 

 posteriorly to their duct. But when the immense modifications 

 in development which the whole of the gland of the excretory 

 organ has undergone in the bird are considered, I do not think 

 that the fact I have mentioned can be brought forward as a 

 serious diffiulty. 



The further points of comparison between the Selachian and 

 the bird are very simple. The Miillerian duct in its later 

 stages behaves in the higher vertebrates precisely as in the 

 lower. It becomes in fact the oviduct in the female and 

 atrophies in the male. The behaviour of the Wolffian duct is 

 also exactly that of the duct which I have called the Wolffian 

 duct in Ichthyopsida, and in the tubules of the Wolffian body 

 uniting with the tubuli seminiferi we have represented the 

 junction of the segmental tubes with the testis in Selachians 

 and Amphibians. It is probably this junction of two inde- 

 pendent organs which led Waldeyer to the erroneous view that 

 the tubuli seminiferi were developed from the tubules of the 

 Wolffian body. 



With the bird I conclude the history of the origin of the 

 urinogenital system of vertebrates. I have attempted, and 

 I hope succeeded, in tracing out by the aid of comparative 

 anatomy and embryology the steps by which a series of inde- 

 pendent and simple segmental organs like those of Annelids 

 have become converted into the complicated series of glands 

 and ducts which constitute the urinogenital system of the 

 higher vertebrates. There are no doubt some points which 

 require further elucidation amongst the Ganoid and Osseous 

 fishes. The most important points which appear to me still 

 to need further research, both embryological and anatomi- 

 cal, are the abdominal pores of fishes, the generative ducts of 

 Ganoids, especially Lepidosteus, and the generative ducts of 

 Osseous fishes. 



