460 WINTERTON C. CURTIS, 



poses them to be produced by a bipolar cell, like the one above de- 

 scribed, but growing a both ends. I have figured on PI. 32, Fig. 12, 

 fibres from the main longitudinal system of Planaria simplissima^ 

 showing merely the outlines of the fibres and the dark areas, with 

 no attempt to represent the fibrilke or any of the finer structure. 

 The fibres are very commonly branched after the manner shown and 

 the dark areas almost as distinct as the black and white of the figure. 

 These dark areas are perhaps in line with the differentiation observed 

 by Lang, Ijima and Woodworth, which Chichkoff discredits thinking 

 they may have mistaken the supporting substance between the fibres 

 for a "Rindenschicht". The fibres shown in the figure were so far 

 from each other that it was impossible to mistake any supporting 

 substance for a part of the fibre. Furthermore the dark areas 

 are evidently within the fibre-substance, from the fact that the 

 edge of the fibre in the region of such an area is much more 

 definite with no increase in size. They occur at frequent inter- 

 vals along the fibres when stained in iron haematoxylin after fixation 

 in corrosive sublimate, and are almost as well set ofi as the black 

 and white of the figure. The fibres were not cut longitudinally for 

 any distance being of less diameter than the thickness of the section 

 and hence could be studied entire. The dark areas appear on the 

 upper surface of a fibre as a somewhat lighter spot (f) or are found 

 as a much denser substance on either side (/'") as though the thin 

 sheet first seen from a surface view were turned on edge. Again 

 there occur other cases where the dark area has the appearance of 

 only reaching up around one side to about the middle of the upper 

 surface and showing darker on only one edge of the fibre (/"')• In 

 transverse section the muscle fibres are oval and often found with an 

 inner transparent and an outer zone of a denser substance corre- 

 sponding to what was seen in longitudinal view. The demonstration, 

 however, is not so satisfactory as in the other case. 



It does not seem that the appearances above described are due 

 to a pathological condition or any extreme distortion caused by the 

 reagents used, for the fibrillae in the preparation from which they are 

 described were beautifully preserved. In view of the carefull de- 

 scription of the muscles cited above from the work of Chichkoff one 

 would not expect to find the nuclear material in such a condition. 

 It can be said, however, that the dark areas are definite stainable 

 portions of the fibre substance. 



