PRONEPHRIC DUCT IN ELASMOBRANCHS 357 



the growth of the duct. This irregularity ig much more marked 

 in earher than in later stages, and, in cases where the dividing line 

 between it and the ectoderm is indistinguishable, would present 

 conditions where the relative position of parts of the two layers 

 would seem to warrant the conclusions reached by sonae observers. 

 This is particularly obvious in the case of Riickert's figure 35, 

 where the three sections present these conditions very markedly. 



Figures 36 to 39 present a series of sections from an embryo of 

 10 mm. In this series the anlage is not so irregular in outline 

 and the terminal cells are larger and not so closely attached to the 

 ectoderm. It will also be observed that the anlage becomes 

 freed from ectoderm much more abruptly, its attachment cover- 

 ing only four sections of 10 microns each. A change is also ob- 

 servable in the ectoderm; the cells being much thicker and more 

 shortened, while the nuclei, in most cases, lie with their long diam- 

 eter at right angles with that of the layer itself. It still, however, 

 shows modification at the point of contact with the duct; at figure 

 36 profoundly so. Figure 36 is an example of the close contact 

 between the ectoderm and anlage of the duct, and demonstrates 

 how readily the conditions might be misinterpreted if the limiting 

 membrane were invisible and the structures thus blended into a 

 syncytium. The striking modification of the ectoderm and the 

 relative position of the ectodermic cell dorsal to the duct might 

 easily lead to the suggestion of ectodermic contribution. With 

 the limiting membrane so clearly demonstrated such inference 

 would seem to be unwarranted. 



Figure 39a is a very interesting example of the close contact 

 between the two layers. It shows a terminal cell of the duct and 

 is very closely attached to the ectoderm. At either end of the 

 cell the clear cytoplasm of attachment is very conspicuous. The 

 whole cell seems, indeed, to be composed of it, the center around 

 the nucleus being translucent. If the limiting membrane be- 

 tween this terminal cell and the ectoderm were invisible the dis- 

 tinction between the two layers, duct and ectoderm, would be 

 impossible. 



Figures 40 and 41 are drawn from sections of a 9 mm. embryo. 

 The ectoderm has become very much thickened and the cells 



