PRONEPHRIC DUCT IN ELASMOBRANCHS 361 



have existed in figures 33 and 34, might not the conclusion of 

 such contribution have been reached? Indeed this is what did 

 happen in the case figured by Van Wijhe. In this case, too, it 

 seems to the writer, Rabl has given the right answer, where he 

 claims that both parts of the dividing cell are in the anlage of the 

 duct. 



Figure 57 shows a mitosis which seems closely comparable to 

 that figured by Van Wijhe. The section was stained with Dela- 

 field's hematoxylin and, as it originally appeared, figure 57 was 

 very strikingly like Van Wijhe's figure. By accident it was 

 broken apart (in the act of focusing a one-twelfth oil immersion 

 objective upon it), and when the duct parted from the ectoderm, 

 it left the latter intact. The drawing, figure 58, was made with 

 the duct moved back to the ectoderm, where it so lies that the 

 latter overlaps it, but it very clearly illustrates the fact that the 

 two layers are entirely separate and that they were never fused, 

 in the sense in which the term is repeatedly used; and also that 

 contribution from the dividing cell in the ectoderm to the duct 

 was impossible. 



Figure 60 presents a similar instance. Here the terminal cell 

 of the duct is very closely united to the ectoderm and particularly 

 so with an ectodermic cell in mitosis. It is obvious that if the 

 limiting membrane were not present and the structure were a 

 syncytium, the interpretation would be very difficult and contri- 

 bution of a daughter cell to the duct from the mitosis would seem 

 more than probable. These are examples out of many instances 

 and might be indefinitely multipHed. At figure 56 is shown a 

 mitotic figure in the terminal cell of the duct and, at 55, in the 

 anlage just above the terminal cell. 



Other mitotic figures appear very frequently in the duct and in 

 many instances in such positions and relation to the ectoderm as 

 to lead to misapprehension as to their significance. One such is 

 shown in figure 59. Here a mitotic figure occm's in the duct in 

 such relation to the ectoderm that, but for the clearly distinguish- 

 able cell outline and the presence of a Umiting membrane, it would 

 be difficult to determine to which layer the cell belongs. Under 

 conditions as above presented the question could not be raised. 



JOURNAL OF MORPHOLOGY, VOL. 25, NO. 2 



