THE DIFFERENTIATION OK THE MESODERM-RUDIMENT, ETC. 213 



The ectodermal origin of the ureter ran be recognised even at a later stage 

 in its histological structure. The duel tunned as above lias been distinguished, 

 as primal \ ureter, from the secondary ureter met with in the terrestrial 

 Pulmonates. In some of these latter, the primary ureter opens into the 

 pulmonary cavity in the way above described. In others, it is continued as 

 a channel in the wall of this cavity, and in others, again, this channel partly 

 or altogether closes and, becoming finally altogether detached from the wall 

 of the respiratory cavity, yields the secondary ureter which, in the most 

 extreme cases, such as Helix />niintti<t, runs alongside of the rectum and, 

 with it. ends near the respiratory aperture (v. Jhering, No. 46; Braun, 

 No. 14). 



The origin of the ureter as a part of the pulmonary cavity which at first 



annel-like hut closes to form a tube later, gathered with some certainty 



from the study of comparative anatomy, is entirely confirmed by ontogeny 



(Braun, Behme, No. 4). The kidney, in the embryos of Helix pomatia, opens 



yn \ 



Etc 99.— Older embryo of Pdludind oivipara (after v. Erlanger). I. 

 primitive kidney ; v, velum : the rest of the lettering as in Fig. 100. 



liver; nil. 



near the primitive kidney in a depression of the body which represents the 

 rudiment of the respiratory cavity. As this cavity deepens, the glandular 

 part of the kidney and the primary ureter become differentiated. At the 

 trior part of the pulmonary cavity, the latter passes into a channel which 

 runs through the whole cavity and ends only at the respiratory aperture. 

 The channel is very broad and is distinguished by its high cylindrical 

 epithelium from the rest of the respiratory cavity which is lined with 

 flattened epithelium. The channel closes later, its edges bending together 

 and [using from behind forward, and the secondary ureter thus formed now 

 represents a closed tube which opens in the neighbourhood of the anus. 



The secondary ureter is a new acquisition within the division of the Stylom- 

 matophora, as v. Jhebing has shown. It occurs only in the so-called 

 Nephropneusta. Among these, however, in one and the same genera, forms 

 are met with possessing the secondary ureter and others exhibiting the much 



