INTRODUCTION 11 



in both axis and tip. When the flagellum is dismembered it disinte- 

 grates into eleven longitudinal strands, including a central pair; both 

 sheath and whiplash have vanished. The presence of coarse dark bands 

 crossing the fibrils in the basal part of the flagellum suggests the reten- 

 tion of some material which binds the fibrils together in the living spore. 

 In Olpidium brassicae fibrillar disintegration also reveals the presence 

 of eleven strands, including a central pair. In this instance, however, 

 a smokelike prolongation extending beyond the apparent end of each 

 strand is visible. This has been conjectured to be the remains of the 

 whiplash. If so, the whiplash is composed of the same eleven strands. 

 In the Olpidium zoospore there was no conspicuous sheath. 1 



Zoospores of the anteriorly uniflagellate type (Fig. 2 H) characteris- 

 tic of the Hyphochytriales and exemplified by Rhizidiomyces apophy- 

 satus (p. 754), have a very different flagellar structure. In these, accord- 

 ing to Couch (1941), the flagellum bears along its length a series of 

 fine, short, lateral threads arranged either in two opposite rows or 

 spirally or irregularly. This "tinsel" or "Flimmer" type is also typical 

 of the anterior flagellum of a biflagellate zoospore (Fig. 2 M). 



The zoospores of the laterally biflagellate type (Fig. 2 J-K, M) formed 

 by the "Saprolegniineen — Peronosporineen" series (including the 

 Lagenidiales) are essentially alike in flagellar structure. With respect 

 to the primary zoospores of Saprolegnia, commonly referred to as 

 "anteriorly biflagellate,' , Couch (1941) noted that, although the almost 

 equal flagella are indeed attached near the anterior end, one is directed 

 backwards and is of the whiplash type, whereas the other is directed 

 forward and is of the tinsel type (Fig. 2 I). In the second swimming 

 stage ("secondary zoospore") the anterior flagellum is distinctly shorter 

 than it was on the primary spore and its tinsel hairs are more conspic- 

 uous. The longer posterior flagellum is of the usual whiplash type 

 (Fig. 2 M). 



1 See also Koch (1956) for further details of the structure of the flagellar apparatus 

 of the chytrid zoospore as revealed by the electron microscope. Among other things, 

 he finds the fibrils of the flagellum to be composed of parallel subfibrils, the whole 

 surrounded by a membrane of spiral substructure. The whiplash is a prolongation 

 of the central thinner pair of fibrils, which extend beyond the decreasingly numerous 

 peripheral ones. Koch also gives evidence for the presence in the body of the chytrid 

 zoospore of a second, vestigial blepharoplast. This suggests (to him) a possible 

 biflagellate ancestry for the group. See also Koch (1958) for details of internal 

 structure of chytrid zoospore with light microscopy. 



