PRONEPHRIC DUCT IN ELASMOBRANCHS 349 



da die Mcglichkeit nicht ausgeschlossen ist, diss Zellen des Pro- 

 nephros iinter fortgesetzten Theilungen den Gang in seiner ganzen Lange 

 mitbilden helfen. Doch kommt mir Letzteres nicht wahrscheinlich vor. 



He quotes Balfour ('87, p. 127). Here Balfour describes the 

 pronephros as 



. . . . arising as a sohd knob from the somatic layer of the meso- 

 blast and growing outward toward the epiblast. The knob consists of 

 from 20 to 30 cells agreeing in character with the neighboring cells of the 

 intermediate cell-mass and are, at this period, rounded. It is mainly, 

 if not entirely, derived from the somatic layer of the mesoblast. From 

 this knob there grows backward a solid rod of cells which keeps in very 

 close contact with the epiblast, and rapidly diminishes in size toward 

 its posterior extremity. Its hindermost part consists in section of, at 

 most, one or two cells. It keeps so close to the epiblast that it might 

 be supposed to be derived from that layer were it not for the sections 

 showing its origin from the knob above mentioned. We have in this 

 rod the commencement of what I have called the segmental duct. 



Van Wijhe, commenting on Balfour's statement, thinks that 

 Balfour was working with the technical methods of his time, which 

 were defective. The solid knob was the pronephros and did not 

 arise from the middle, but from the lateral plate, and he con- 

 jectures that, because Balfour interpreted the knob as consisting 

 of from 20 to 30 cells, he must have drawn his conclusions from a 

 single section instead of from a series, as he himself had done. He 

 states that his series cut the knob into 23 sections. He further 

 states that the close contact of the duct with the epiblast, spoken 

 of by Balfour, is not a mere touching or contact, but an actual 

 fusion and quotes Beard, Rabl and Riickert as substantiating his 

 statement. He quotes Riickert as having seen an invagination 

 of the ectoderm on the duct and claims to have observed, in excep- 

 tional cases, the same thing. He states that the contribution of 

 cells by the ectoderm to the structure of the duct is made certain 

 through the occurrence of a mitotic figure in one of his sections, 

 in which one daughter cell was situated in the skin and the other 

 in the duct. He says: "Die Betheiligung des Ectoderms an der 

 Bildung des Ganges wird sicher gestallt durch Kerntheilungs- 

 figuren, bei welchen der eine Tochterkern in der Haut, der andere 

 in der Anlage des Ganges liegt. Dies zeigt Fig. 5b, welche einem 

 Schnitt durch einem Embryo von Scyllium catulus mit 37 Somiten 

 entnommen ist." 



