STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 815 



moses form a plexus at the hilus of the testis, whose efferent 

 trunks, 13 in number, again unite into a plexus on the vertebral 

 column, which is continuous with the cardinal veins." The 

 arrangement (though not the number) of Hyrtl's vessels is 

 very similar to that of our vasa efferentia, and we cannot help 

 thinking that a confusion of the two may have taken place ; 

 which, in badly conserved specimens, not injected with semen, 

 would be very easy. 



We have, as already stated, been unable to find in our dis- 

 sections any trace of a duct homologous with the oviduct of 

 the female, and our sections through the kidney and its ducts 

 equally fail to bring to light such a duct. The kidney ducts are 

 about 19 centims. in length, measured from the genital aperture 

 to their front end. These ducts are generally similar to those 

 in the female ; they unite about 2 centims. from the genital 

 pore to form an unpaired vesicle. Their posterior parts are 

 considerably enlarged, forming what Hyrtl calls the horns of 

 the urinary bladder. In these enlarged portions, and in the 

 wall of the unpaired urinary bladder, numerous transverse 

 partitions are present, as correctly described by Hyrtl, which are 

 similar to those in the female, but more numerous. They give 

 rise to a series of pits, at the blind ends of which are placed the 

 openings of the kidney tubules. The kidney duct without doubt 

 serves as vas deferens, and we have found in it masses of yellowish 

 colour similar to the substance in the vasa efferentia identified 

 by us as remains of spermatozoa. 



II. Development. 



In the general account of the development we have already 

 called attention to the earliest stages of the excretory system. 



We may remind the reader that the first part of the system 

 to be formed is the segmental or archinephric duct (Plate 36, 

 figs. 28 and 29, sg^]. This duct arises, as in Teleostei and 

 Amphibia, by the constriction of a hollow ridge of the somatic 

 mesoblast into a canal, which is placed in contiguity with the 

 epiblast, along the line of junction between the mesoblastic 

 somites and the lateral plates of mesoblast. Anteriorly the duct 



