The Freshwater Algae of Africa 567 



the membranes takes place. These are of a very complicated nature. 

 In the first place a dense cylinder of thickening gradually develops all 

 round the inner face of the wall of the conjugation-tube (Fig. 34, b, d, 

 /'.;/. i . Simultaneously, appreciable thickening arises on the inner 

 side of the concave wall of each conjugating cell at a point opposite 

 the conjugation-tube (Fig. 34, &and <l ; in the latter this thickening is 

 left unshaded). It would appear that this thickening gradually 

 increases until it extends right up to the fused protoplasts of the con- 

 jugating cells, within the conjugation-tube (cf. Fig, 34, b, especially 

 the second zygospore, and Fig. 34, d). Both of the zones of 

 thickening mentioned show more or less marked stratification. 

 Further, as the protoplasts pass towards the conjugation-tube, the 

 empty ends of the conjugating cells become filled with an, usually 

 irregularly stratified, mass of an obviously different texture (Fig. 34, 

 ". /', </. <n to that of the thickenings previously described. This is 

 borne out by staining with methyl blue, when the extensive thickening 

 in the ends of the cells rapidly takes on an almost homogeneous deep 

 blue colour, whilst the remaining thickening is much more faintly 

 stained (Fig. 34, d). In all probability the thickening in the ends of 

 the cells is semi-mucilaginous in character, a view which is in accord- 

 ance with its appearance. The figures show that the extent of 

 development of these thickenings in the ends of the conjugating cells 

 is rather variable, a larger or smaller part of the lumen being left 

 unoccupied. Even this, however, appears to contain some mucilage, 

 as methyl blue stains the whole empty cell-cavity almost uniformly 

 (Fig. 34, d). 



Owing to the spiral twisting of the conjugating threads, it is easy to 

 obtain a side-view of the zygospore. In such a view (Fig. 34./, g, i) the 

 zygospore appears provided with a uniform, thick, stratified membrane, 

 formed by the thickening of the wall of the conjugation-tube. In the 

 ordinary aspect of the zygospore (e. g. Fig. 34. d) this would form a 

 vertical girdle, whilst the thickenings on the walls of the conjugating 

 cells, opposite the conjugation-tube, would form an incomplete equa- 

 torial girdle. There does not, however, usually appear to be a com- 

 plete membrane encircling the zygospore in front-view (Fig. 34, l>, d), 

 since at the points where the latter faces towards the four ends of the 

 conjugating cells there does not seem to be any covering, apart from 

 the mucilage above mentioned (cf. Fig. 34, d). The integument of 

 the zygospore would thus consist of two broad encircling bands of 

 dense membrane-thickening at right angles to one another, whilst the 

 gaps are filled out by (he paired halves of the conjugating cells, with 

 their mucilaginous thickening. It seems probable that, after conjuga- 



