358 EMBRYOLOGY. 



by the ingrowth of embryonic connective tissue, and comes to lie 

 very deep (fig. 202 wd and fig. 205 ijug). This canal has acquired a 

 number of difierent names, and is cited in the literature as pro- 

 nephric, mesovephric. Wolffian, or segmental duct. The different 

 designations are explainable from the fact that the canal alters its 

 function in the course of the development of the iiephridial system 

 serving at first as an outlet for the pronephros only, afterwards for 

 the mesonephros. 



Views concerning the origin of the canal were for a time conflicting. 

 According to one supposition, which a few years ago almost all 

 investigators entertained, the longitudinal canal of the pronephros, 

 when it had been constricted off from the parietal wall of the body- 

 cavity, protruded with its posterior end as a free knob into the space 

 between outer and middle germ-layers, and gradually grew out inde- 

 pendently, by multiplication of its own cells, as far as the hind gut 

 (proctodseum). It was said, therefore, to be constricted off from 

 neither the outer nor the middle germ -layers, nor yet to derive from 

 them cell-material for its increase. 



This interpretation has recently become untenable. As is reported 

 in an entirely trustworthy manner concerning several different classes 

 of Vertebrates, — for Selachians (Wijhe, Rabl, Beaed), for Amphibia 

 (Peeenyi), for Reptiles (Mitsdkuei), and for Mammals (Hbnsen, 

 Flemming, Graf Speb), — the posterior end of the pronephric duct in' 

 process of growth is in these cases by no means an entirely isolated 

 structiu'e, but is in close union with the outer germ-layer. Attention 

 has already been called to this fact apropos of the development of the 

 pronephros. In a Selachian embryo the condition which is repre- 

 sented in fig. 197 is soon followed by a condition (fig. 198) in which, 

 in a series of cross sections, the pronephric duct now appears as a 

 ridge-Uke thickening of the outer germ-layer. By a study of various 

 older embryos it can be further established, that the ridge-like thick- 

 ening of the outer germ-layer is prolonged backwards by means of 

 cell-proliferation in that layer, while in front it is being constricted 

 off from the parent-tissue. The pronephric duct therefore grows at 

 the expense of the outer germ-layer, and moves as it were along the 

 latter, with its terminal opening behind, as far as to the hind gut. 



When Hensen, Flemming, and Graf Spee made their observations 

 on Mammals, they were thereby led to adopt the view that the 

 mesonephric duct, as well as the whole urinary system, was derivable 

 from the outer germ-layer. The union with the middle germ-layer 

 they regarded as one that had arisen secondarily. But their concep- 



