Il8 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



This backward growth of the pronephric duct is apparently (at 

 least in most forms) the result of growth of the duct itself, 

 without any cellular additions from other sources ; although a 

 few years ago the duct was described in most vertebrates as 

 being wholly of ectodermal origin, a view arising from the fact 

 that the duct in its progress fuses with that layer. 



A second pronephric element is the glomus. The dorsal 

 arterial blood-vessel (aorta) gives off an arterial twig on either 

 side opposite each nephrostome. Each artery breaks up into a 

 vascular network just beneath the dorsal splanchnic epithelium 

 of the hypomere (Fig. 126, A), and pushes out so that the struc- 

 ture projects into the dorsal portion of the body cavity. This 

 vascular outgrowth is the glomus. In most vertebrates it is 

 unsegmented, but forms a continuous rete mirabile, and projects 

 freely into the coelom. In certain forms, however (e.g., Ichthy- 

 opJiis), the glomus becomes distinctly segmented, while the 

 dorsal portion of the body cavity becomes cut off from the rest, 

 forming a separate envelope (Bowman's capsule) around each 

 glomar segment, so that here we have a series of vascular cap- 

 sules almost exactly comparable to the Malpighian bodies to be 

 described in connection with the mesonephros. 



There is considerable evidence to show that the pronephros 

 originally had a much greater extent than in most existing 

 forms ; and indeed the structure may have extended nearly to 

 the vent, as is apparently the case in Bdellostoma, if we may 

 judge by recent studies. Its fate in all vertebrates except this 

 cyclostome will be better understood after a history of the meso- 

 nephros. ' 



The mesonephros or Wolffian body is usually confined to 

 segments behind the pronephros, and is often spoken of as a 

 later generation of excretory structures. The fact, however, 

 that pro- and mesonephric tubules can occur together in the 

 same segment tends to show that the two structures are dis- 

 tinct. 



The mesonephric tubules are formed in a manner similar to 

 the pronephric tubules, except that they arise from the more 

 dorsal portion of the nephrotome. They grow outwards, and 

 finally connect with the pronephric duct, although they do not 



