6 WILLIAM MORTON WHEELER, 



described by W. Müller. The pronephric duct opens into the cloaca 

 in embryos 5,5 mm long. The reduction of the pronephros takes 

 place as described by W. Müller except that in larvic 18 cm long 

 remnants of the tubules could still be detected. 



Schneider (1879) corroborated the observations of W. Müller 

 on the pronephros of Ammocœtes and emphasized the fact that the 

 pronephric funnels open into the pericardial cavity, a condition prob- 

 ably observed but not expressly mentioned by W. Müller, 



Scott (1880) in his preliminary paper on the development of 

 Petromyzon was the first to describe the origin of the pronephros. 

 The pronephric duct arises as a solid strand in the parietal plate of 

 the mesoderm, and soon acquires a lumen and an opening anteriorly 

 into the body-cavity. At the anterior end of the duct, near the heart, 

 a series of ciliated funnels is developed. These open on the one hand 

 into the body-cavity, on the other into the duct. A glomus arises as 

 in the Amphibia. Scott believes that the funnels are ditferentiated 

 from the duct. The two ducts open by means of separate orifices 

 into the cloaca. 



Scott (1881) in a second paper claims that the Anlage of the 

 pronephros appears in embryos fourteen days old. The idea of the 

 preceding paper, i. e. the origin of the pronephric tubules from the 

 anterior end of the duct, takes on a more difinite form : "Die beiden 

 Gänge zeigen (fig. 33, tab. 10) eine Anzahl symmetrisch vertheilter 

 Erweiterungen , welche später in die Leibeshöhle einmünden und zu 

 wimpernden Trichtern werden. Die Trichter entstehen also aus den 

 Gängen, und nicht durch Einstülpungen der peritonealen Schicht, wie 

 die Tubuli der Urnieren." The pronephric duct, according to Scott, 

 does not grow backward independently, but is split off" in situ in a cranio- 

 caudal direction. The funnels of the pronephros are metameric. Their 

 number at the time of hatching is not constant; usually there are 

 three or four; at no time were more than five observed. 



Scott (1887), in a third paper, referring to his preceding publi- 

 cations says: "my account of the formation of the mesoblast by splitting 

 from the yolk hypoblast, of the development of the pronephros from 

 diverticula of the segmental duct, of the formation of the anus and 

 neurenteric canal, are incorrect." 



V. KuPFFER (1888) sums up the results of his study of the pro- 

 nephros in P. planeri in the following sentence: "Der Vornierengang 

 ist exodermaler Herkunft, die drei Vornierencanäle entstehen succes- 

 sive als kegelförmige Erhebungen des Parietalblattes gegen das 



