276 BULLETIN OF THE 



sible to draw a rigid line between pronephros and mesonephros. Indeed, 

 such is a part of the conclusion which I think we shall finally be able to 

 draw from the entire review. 



The numerous accounts which have been recently given of the pro- 

 nephros in the higher Amniota may be conveniently treated under three 

 heads : — 



(1.) According to Balfour and Sedgwick ('78, '79), the Miillerian duct 

 in the Chick first appears in a region somewhat behind the front end of 

 the Wolffian duct as three slender invaginations of the peritoneum which 

 covers the Wolffian body. These invaginations later fuse at their distal 

 extremities, and the most posterior involution grows backwards in con- 

 nection with the Miillerian duct. There is thus formed a longitudinal 

 canal with three peritoneal funnels, the whole structure being comparable 

 to the pronephros of Amphibia. Slightly in front of the nephrostomes 

 there is attached to the radix mesenterii avascular body which resembles 

 the Amphibian glomus. It receives blood-vessels from the aorta, and 

 projects into the body cavity enclosed in a distinct sac of peritoneum. 

 Gasser ('74, pp. 58, 59) had previously observed somewhat similar condi- 

 tions in the anterior end of the Miillerian duct ; and, by renewed inves- 

 tigation, Gasser and Siemerling were able to confirm the occasional 

 occurrence of the phenomenon, though a single invagination appeared 

 to be the rule. Multiple invaginations have also been mentioned by 

 Kollmann ('82 b , p. 20), Siemerling ('82, p. 29), Janosik ('85, p. 43), and 

 Mihalkovics ('85, p. 295) ; but Braun (79) and Renson ('83, p. 37) were 

 unable to find any evidence of such a condition. Braun also opposed 

 Balfour and Sedgwick in their view respecting the nature of the vascular 

 body, and Sedgwick ('80 b ) later came to the conclusion that this struc- 

 ture was really a series of greatly modified mesonephric glomeruli. 

 This interpretation was adopted by Balfour ('81 a , p. 590). 



(2.) The second view is set forth in the recent account of Felix ('90), 

 who describes in a chick embryo with eight protovertebrae a series of 

 outgrowths, which, emerging from the lower hinder portions of protover- 

 tebrae IV.-VIIL, extend backward and outward toward the ectoderm. 

 The latter layer occasionally presents local thickenings in this region, and 

 in some cases a connection between the mesodermal outgrowths and the 

 ectodermal thickenings can be observed. In older embryos no trace of 

 the structures can be found. As was the case with the evaginations 

 found by Hoffmann ('89) in Reptiles, no fusion of their distal extremities 

 is recorded. This condition makes them at once unlike the Selachian 

 pronephros described by Riickert, and the early stages of the Amphibian 



