18J> HOFMEISTEE, ON 



septa cutting those which diverge from the longitudinal axis 

 of the organ at an angle of about 45° (PI. XXIV, fig. 8). 

 By this means the foundation is laid for the two-lobed form 

 of the prothalliura. The cells of the edge of the wings of 

 the fore end of the prothallium then divide very frequently 

 by septa parallel to the chord of the arc of their circum- 

 ference. After a series of such divisions there are produced 

 in the marginal cells, longitudinal septa at right angles to 

 the latest formed transverse septa. This cell-multiplication, 

 by which both lobes of the prothallium are rapidly and 

 remarkably enlarged, is least active on the outside of the 

 lobes, where it soon ceases. The cessation progresses from 

 the hinder part of the prot allium to the apex of each of the 

 side lobes. On the other hand those cells of the two lobes 

 which are directed towards the deep indentation of the 

 fore edge, continue to multiply for a longer period. The 

 multiplication, however, is most active in the two cells 

 which occupy the base of the notch between the two wings 

 of the prothallium, which notch is constantly increasing in 

 depth. These cells divide continually and repeatedly by 

 means of transverse septa at right angles to the longitudinal 

 axis of the prothallium, alternating with divisions by means 

 of longitudinal septa which converge slightly to the longi- 

 tudinal axis. Those of the new cells thus formed which are 

 farthest from the middle point of the indentation begin 

 immediately to diverge in their growth from the longitu- 

 dinal axis of the prothallium to the extent of about 05°. 

 By this means the new cells thus produced drive the cells 

 adjoining them on the outside upwards and outwards. 

 During their change of position the growth of these cells 

 diverges more and more from the longitudinal axis of the 

 prothallium, ultimately to the extent of 90°. Those 

 daughter-cells which are separated from the cells in the 

 lowest portion of the indentation push themselves upwards 

 laterally by the side of their mother-cells, by means of an 

 innate power of growth. Whilst they push the adjoining 

 older cells outwards, they take part in the formation of the 

 two side wings of the prothallium, which consequently in- 

 crease continually in size and in the number of their cells, not- 

 withstanding the progressive cessation of the multiplication 



