56 AQUATIC PHYCOMYCETES 



nucleate "spore initials." The contents of certain sporangia, instead 

 of being uniformly distributed, are disposed around a large vacuole. 

 Such a disposition has been noted in several other chytrids, notably in 

 Rhizosiphon erassum (Scherffel, 1926a). In Cladocliytrium replication 

 cleavage furrows appear to originate on the periphery of the vacuole 

 and progress outward as broad wedge-shaped radial arms. Very rarely 

 both methods of cleavage occur in the same sporangium. After delim- 

 itation the spore initials become somewhat swollen, possibly because 

 of the intake of water. At this time the contours of the pentagonal, 

 hexagonal, or somewhat cuneiform segments become nearly but not 

 completely obliterated, and the sporangium appears to have just under- 

 gone simultaneous division into polyhedral segments. Because of this 

 Karling suggests that the accounts of simultaneous division recorded 

 in the literature are due to the observers' failure to note the early phases, 

 which are actually progressive. The fact that in living material it is 

 usually difficult to follow the process of segmentation has no doubt 

 led to the conflicting accounts of cytokinesis in the chytrids. Cyto- 

 logical observations of -this process on other representatives of the 

 order confirm it as being progressive (Antikajian, 1949; Ajello, 1948a; 

 J. M. Roberts, 1948; etc.). 



Obviously the number of zoospores formed in a sporangium is de- 

 pendent upon the size of the individual spore and of the sporangium. 

 The motile zoospores of a species appear to be relatively constant in 

 size, although most of them are too minute to enable one to obtain 

 any significant measurements. For example, though plants of Chytrid- 

 ium lagenaria when growing on different types of algae ranged from 

 8 to 29 (j. in diameter, the zoospores constantly remained 5 \i (Sparrow, 

 1936a). On the other hand, the size of the sporangium, as just indicated, 

 may vary considerably in a single species. The factors which influence 

 this are not known with certainty. Nature of the substratum, available 

 space, competition, size of the cell of the substratum (in monophagous 

 types), and available nutriment have all been suggested, however, as 

 possible determinants. In a preliminary study of Chytridium lagenaria 

 (Sparrow, op. eit.) striking variations in the size of sporangia were 

 noted on three different algae, Rhizoclonium, Spirogyra, and a slender 

 species of Oec/ogoniion. Such variations were not, however, coordinated 



