PRONEPHRIC DUCT IN ELASMOBRANCHS 353 



the duct and the ectoderm, at the point of contact, shows beyond 

 question the true relation between the two layers. 



Figure 9 gives a comprehensive view of the entire embryo at 

 the time when the anlage of the duct has a length of about three 

 somites on one side and a little more than two on the other, show- 

 ing the asymmetrical growth of the two sides and also the entire 

 independence of the anlage at its first appearance. It will im- 

 mediately come into contact with the ectoderm as shown in figure 

 3. It also shows the relation of the pronephros to the ectoderm. 

 The appearance of the growing tip and its relation to the ecto- 

 derm present some peculiar and characteristic features at the 

 point of contact. The ectoderm becomes thinner, apparently 

 by the drawing out or extension of its cells. This modification of 

 the ectoderm is very pronounced in many places, but particularly 

 at the point where the growing tip of the duct impinges upon it. 

 In many instances, where the two layers are closely attached, the 

 ectoderm becomes very thin, while the extreme point of the anlage 

 is reduced to a few cells, usually to only a single cell in diameter, 

 but there is always to be distinguished a limiting membrane be- 

 tween the two. If this were not so the structure would present 

 the appearance of thickening, as described by Riickert, and also, 

 but for the limiting membrane, one would conclude that the thin- 

 ning of the ectoderm was caused by the splitting off of cells to be 

 contributed to the growing tip of the duct. 



The pecuhar modification of the ectoderm seems to be the result 

 of some influence, probably pressure, exerted by the growing tip 

 of the duct. It often happens that in the process of preparation 

 the two structures are separated, and then an indentation is left 

 on the ectoderm where the anlage has pressed against it. Many 

 instances of this kind, as well as cases where the same condition 

 exists with the structures still intact, are clearly displayed in the 

 series of cross sections. It will be observed, however, that no 

 matter how close the contact may be, there is always to be demon- 

 strated a limiting membrane separating the duct from the ecto- 

 derm. It will readily be appreciated that any method of prepara- 

 tion which obliterated cytoplasmic outlines and made limiting 

 membranes invisible would show the two layers as a single mass, or 



