356 GEORGE A. BATES 



that structure reaches the ectoderm which were referred to when 

 the growth of the pronephros was described. This substance 

 seems to fasten itself to the ectoderm and thus unites that layer 

 with the duct. It will be remembered that Van Wijhe claimed 

 that the relation was one of fusion, not a mere contact. The 

 presence of this substance acting as a medium of union between 

 the two layers would, in a way, seem to bear out his statement, 

 although not in the sense in which he intended it ; for it is entirely 

 independent of the ectoderm, its relation to that layer being wholly 

 secondary. It will be readily seen, however, that in places where 

 the entire surface of contact is covered by this substance it might 

 very easily be interpreted as a fusion, were the method of prepara- 

 tion such as to render the limiting membrane invisible. In some 

 places it seems to exert tension upon the ectoderm. This is es- 

 pecially apparent in figure 35, where it forms a broad band and 

 has apparently pulled the ectoderm toward itself, as if it had con- 

 tracted and drawn the ectoderm out of line. The peculiar bend 

 in the ectoderm brings the ectodermal cell, dorsal to the duct, into 

 such relation with the latter that, but for the fact that the cell 

 outlines and limiting membrane are so clearly manifest, the struc- 

 tures might be interpreted as being continuous, and the presence 

 of a mitotic figure in the ectoderm at the point where this layer 

 bends towards the duct would seem to present corroborative evi- 

 dence of ectodermic contribution to the latter. But the independ- 

 ent character of the two layers is so clearly demonstrated in the 

 section under discussion that no such inference is possible. 



Figures 33, 34 and 35 present much the same conditions; except 

 that in figure' 35 the mitosis is in metaphase and the cell is cut 

 transversely so that the axis of the spindle is in the wrong direction 

 for the daughter cell to be contributed to the duct. This was the 

 case in 'the example drawn by Riickert in his figure 36, as cited 

 above and pointed out by Rabl ('96). In figures 33 and 34 the 

 mitotic figures are so placed that in division they would be parallel 

 to the surface of the ectoderm and thus, but for the clearly dem- 

 onstrated limiting membrane, the possibility of ectodermic con- 

 tribution might be suggested. Another feature to be noted in 

 this, as well as in the preceding series, is the irregular manner of 



