ORIGINS OF AGAMIC PATTERNS 603 



pole, a rhizoid develops from it, and the chlorophyll-bearing portion be- 

 comes the thallus. Since the colorless flagellate pole is apical or anterior, 

 as far as locomotion is concerned, this course of development suggests that 

 the thallus filament represents a new polarity arising from the primarily 

 basal pole, as the hydranth-stem axis of calyptoblast hydroids develops 

 from the originally basal end of the planula (p. 96). These zoospores 

 and gametes are usually asymmetrical as regards position of the stigma 

 and apparently, at least in some forms, as regards nuclear position. 

 Among the brown algae zoospores and gametes occur with two flagella 

 arising from one side — the longer extending anteriorly, the shorter pos- 

 teriorly, in relation to direction of locomotion— and stigma closely asso- 

 ciated with point of flagellar origin (Fig. 186, C). Whether these asym- 

 metries show any definite relation to particular factors in the environ- 

 ment of the developing spores is apparently not known. 



In certain other algae — for example, Oedogonium and Vaucheria — a 

 zoospore develops without division from an elongated cell of the thallus. 

 The zoospore of Oedogonium possesses a circle of cilia near one end and a 

 polar cytoplasmic differentiation into colorless and chlorophyll-bearing 

 zone at the level of the cilia (Fig. 186, D). It appears, however, that this 

 pattern develops at right angles to the thallus axis (Fig. 186, E), but the 

 factor determining it is not evident. This spore also attaches by its color- 

 less "apical" pole, from which rhizoids develop; and the thallus forms from 

 the chlorophyll-bearing part. 



The large multinucleate zoospore of Vaucheria, developing at the tip 

 of a branch, becomes separated by a cell wall from the rest of the thallus, 

 in which cell walls are absent, and becomes cihated over the entire sur- 

 face, two cilia developing above each of the numerous superficial nuclei. 

 In germination, local budlike outgrowths, becoming new filaments, de- 

 velop from one or more regions. Apparently at least some of these out- 

 growths represent new axiate patterns, resulting from local activations, 

 determined by some internal or external differential. 



In various algae the zygotes themselves or products of their division 

 become oospores, quiescent stages showing no indication of polarity and 

 giving rise on germination to a bud axis in the same way as the agamic 

 spores. Here, also, new axiate pattern apparently originates from a local 

 activation. The fertilized egg of Chara becomes an oospore; germination 

 consists in the origin of a bud from some point of its surface; the bud be- 

 comes a filament; and the axis of the plant develops as a branch of this 

 filament, that is, from another bud. It is sufficiently evident from these 



