348 GEORGE A. BATES 



It will be seen that these statements present the two phases of 

 the problem, i.e., the origin of the duct from the mesoderm and 

 the contribution of the ectoderm to its development. The 

 phylogenetic theoretical consideration is also suggested. 



Among the many observers who have discussed the develop- 

 ment of the pronephric duct may be mentioned Semper ('74, 75), 

 Balfour (78), Spee ('84), Beard ('87), Haddon ('87), Ruckert 

 ('88), Van Wijhe ('86, '88, '89), Mitsukuri ('88), Rabl ('96), 

 Gregory ( '98, '00). Of these Ruckert, Van Wijhe and Rabl have 

 entered actively into the controversy, the two former being advo- 

 cates of the theory of ectodermic origin or contribution to the 

 duct, and the latter directly and radically opposed and an ardent 

 supporter of the theory of its exclusive mesodermic origin. 



Riickert ('88) seems to have taken the initiative in his very 

 radical position, stating ''that the duct is developed wholly from 

 the ectoderm. " In his figure 36, plate 16, he shows three mitotic 

 figures at the point of union of the ectoderm and anlage of the 

 duct. He states that two layers, ectoderm and anlagal, are fused 

 together at the point of contact, making the thickness of the wall 

 much greater, and this, in conjunction with the presence of the 

 numerous mitotic figures in this region, gives much reason for his 

 contention. He says: "Die auffallend zahlreichen Mitosen, 

 welche sich stets in diezer Region des Ectoblast finden, geben 

 einen weiteren Beleg fiir diese Auffassung. Ein in dieser Hin- 

 sicht ziemlich pragnanter Schnitt (Horizontal-schnitt) durch das 

 hintere Ende des Vornierenganges ist in Fig. 36 dargestellt." 



Van Wijhe claims that the ectoderm contributes to the struc- 

 ture of the duct, and compares its growth to that of the nerve of 

 the lateral line organ which, according to his interpretation, is 

 developed in part at least from cells contributed by the ectoderm. 

 He admits the possibility of the duct growing by the division of 

 its own cells, but considers it improbable, his general conclusion 

 being that the ectoderm is in considerable measure responsible 

 for its growth. He says: 



Was nun seine Abstammung betrifft, so betheiligt sich das Ektoderm 

 sicher an seiner Bildung, indem er in ahnlicher Weise wie die Nerven 

 der Seitenorgane weiterwachst. Ebensowenig wie bei diesen Nerven 

 mochte ich eine ausschliessliche Abstammung von der Haut behaupten, 



