LEPTO MIT ALES 863 



rudiment, but eventually a central vacuole forms, around which, at 

 least in certain species, they become regularly disposed in one or two 

 layers. Lines of cleavage then extend from the vacuole outward (Behrens, 

 1931) and divide the contents into uninucleated segments of approxi- 

 mately equal size. These segments, the zoospore initials, become flagellate 

 before their discharge from the sporangium. The precise method of 

 formation of the delicate whip-like flagella is, however, understandably 

 obscure. 



In the motile primary pip-shaped zoospores of Apodachlya the nucleus 

 is in the anterior part of the body. The two apical flagella are connected 

 to separate "basal granules" of nuclear origin (Cotner, 1930b), and 

 these, in turn, by delicate fibrillar structures to the tip of the beaked 

 nucleus (Cotner, loc. cit.\ Kevorkian, 1935) (Fig. 67 E). The secondary 

 swarmers in all genera of the order appear to be nearly uniform in 

 internal structure and organization of the nucleus, which is more cen- 

 trally disposed than in the primary zoospore. In all secondary types of 

 zoospores the nucleus is somewhat beaked. Occasionally two opposite 

 beaks are formed (Cotner, loc. cit.). In A. brachynema there emerge from 

 the tip of the beak, which is usually darker-staining, either two delicate 

 fibrillar strands or a single one. The latter is supposedly composed of 

 two fused fibrils (Cotner, loc. cit.) connecting with basal granules from 

 which the oppositely directed flagella have their origin (Fig. 67 F). In 

 the secondary zoospore of Rhipidium interrupt um (Cotner, loc. cit.) the 

 basal granules are united with a more massive chromatic body which is 

 adjacent to the nucleus. Cotner considers these basal granules of nuclear 

 origin, and intimately connected with the formation and control of the 

 flagella. He points out that they are present wherever flagella are found 

 and that if more than the usual number of granules are formed a corres- 

 ponding number of flagella appear. He describes the "basal apparatus" 

 as consisting of several granules, two of which usually remain within 

 the nucleus to form its dark-staining beak, whereas the third is on the 

 periphery of the cytoplasm at the base of the flagellum. Cotner also 

 asserts that even though the granules are of nuclear origin, once formed 

 they appear to function even after separation from the nucleus. He thus 

 indicates that the flagella may arise from and be controlled by differen- 

 tiated parts of the nuclear material rather than by the whole nucleus. 



