XI.] SPIROGYRA. 401 



After a time the contents of the cells which first under- 

 went contraction begin to move into the connecting tube. 

 The entire contents pass over and fuse completely with 

 those of the cell which receives them. Recent researches 

 have shown that the corresponding constituents of the 

 contents of each cell unite one with another, primordial 

 utricle fusing with primordial utricle, chromatophore with 

 chromatophore, and, which is the most important point, 

 nucleus with nucleus. 



The cell formed by conjugation is termed the zygospore. 

 It is at first spherical in shape and destitute of any cell-wall 

 of its own. It soon increases in size, by taking up water, 

 assumes an elliptical outline, and becomes surrounded by a 

 cell-wall. The latter consists, in its mature condition, of 

 three layers, the middle one of which is of a brown colour, 

 while those on either side of it are colourless. The wall is 

 cuticularized, that is to say, it has undergone a chemical 

 change which renders it almost wholly impermeable to 

 water. Meanwhile changes go on in the contents of the 

 zygospore, the most important being the appearance of oil 

 in place of starch. In this condition the zygospore passes 

 through a long stage of rest, during which it can withstand 

 great extremes of temperature, and is not injured by periods 

 of drought. On germination each zygospore bursts its cuti- 

 cularized wall and grows out directly into a new plant. 



We have seen that in the normal case of conjugation just 

 described a distinct differentiation of the filaments into 

 male and female individuals is shown, as all the cells of 

 each of the conjugating filaments behave in the same way, 

 either giving up their own contents or receiving those of the 

 other thread. This sexual difference however has not yet 

 become completely fixed, for in certain cases adjoining cells 

 of one and the same filament conjugate with each othei 

 M, 26 



