STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 819 



The two segmental ducts have united behind into an unpaired 

 structure in an embryo of 1 1 millims. This structure is no doubt 

 the future unpaired urinogenital chamber (Plate 39, figs. 58 A, 

 and 60, bl.}. Somewhat later, the hypoblastic cloaca becomes 

 split into two sections, the hinder one receiving the coalesced 

 segmental ducts, and the anterior remaining continuous with the 

 alimentary tract. The opening of the hinder one forms the 

 urinogenital opening, and that of the anterior the anus. 



In an older larva of about 5*5 centims. the pronephros did 

 not exhibit any marked signs of atrophy, though the duct between 

 it and the mesonephros was somewhat reduced and surrounded 

 by the trabecular tissue spoken of in connection with the adult. 

 In the region between the pronephros and the front end of the 

 fully developed part of the mesonephros very rudimentary tubules 

 had become established. 



The latest stage of the excretory system which we have studied 

 is in a young Fish of about 1 1 centims. in length. The special 

 interest of this stage depends upon the fact that the ovary is 

 already developed, and not only so, but the formation of the 

 oviducts has commenced, and their condition at this stage throws 

 considerable light on the obscure problem of their nature in the 

 Ganoids. 



Unfortunately, the head of the young Fish had been removed 

 before it was put into our hands, so that it was impossible for us 

 to determine whether the pronephros was still present ; but as we 

 shall subsequently shew, the section of the segmental duct, 

 originally present between the pronephros and the front end of 

 the permanent kidney or mesonephros, has in any case dis- 

 appeared. 



In addition to an examination of the excretory organs in 

 situ, which shewed little except the presence of the generative 

 ridges, we made a complete series of sections through the excre- 

 tory organs for their whole length (Plate 39, figs. 54 57). 



Posteriorly these sections shewed nothing worthy of note, 

 the excretory organs and their ducts differing in no important 

 particular from these organs as we have described them in the 

 adult, except in the fact that the segmental ducts are not joined 

 by the oviducts. 



Some little way in front of the point where the two segmental 



