Develoiiment of the Kxcretory Or>;ai)S of HdcIIostuma .stouti Lockington. 217 



met with. Perhaps both are to be accounted for by the degenerate 

 nature of the system in this region. 



But the system here, although degenerate, acquires a great im- 

 portance when viewed in its relation to the gills. In the embryo a 

 large number of gill-slits are formed, just how many is not known, 

 but, as was stated in the preliminary papers, perhaps not fewer than 

 thirty-five. The anterior slits in the younger embryos lie as far for- 

 wards as in other vertebrates, and of course quite a distance in front 

 of the anterior end of the excretory system. Of the total number of 

 gill-slits, all disappear except the posterior ten to fourteen, and these 

 persist as the respiratory organs of the adult. In stage C there are 

 eleven gills on each side, and they open to the exterior in segments 

 nineteen to twenty-nine inclusive. From this it will be seen that in 

 the younger embryos the excretory system is present in all the seg- 

 ments where later the gills of the adult come to exist, and also in a 

 number of segments still cranialwards. The conditions in the older 

 embryo of stage B show that the excretory system disappears through 

 the greater part of this region before the gills are formed, and it is 

 probable that in no case are segmental tubules and gill-slits present 

 in the same segments at the same time; but that the excretory system 

 is at one time present throughout the greater part of the extensive 

 gill region of the embryo, is an uucontrovertable fact. 



There are twenty-seven pairs of mesonephric tubules. With one 

 exception, they no longer stand at right angles to the duct, but are 

 directed either forwards or backwards, so that in transverse sections 

 of the embryo, we get transverse sections of both duct and tubules. 

 The tubules are also in a more nearly horizontal position than in the 

 previous stages. In Fig. Va we have a section through the duct and 

 through a Malpighian corpuscle. In the latter are seen all the parts 

 of a typical Malpighian corpuscle ; the glomerulus gl^ with its covering 

 of cubical epithelium, the Bowman's capsule B.c, formed of flat epi- 

 thelium, and between covering and capsule, a slit- like space, the cavity 

 of the capsule, which, as is shown by the complete series, is continuous 

 through the tubule with the lumen of the duct. In Fig. 14 we have 

 a section through the one tubule above mentioned which stands at 

 right angles to the duct. It is the most anterior tubule on the right 

 side, and would have degenerated, no doubt, as the duct just in front 

 of it and just behind it is in the process of atrophy. It is shorter 

 than the other tubules, and a comparison with Fig. 10 from stage A, 

 and Fig. 12 from stage B, shows that it is quite a little smaller than 



