136 



MORPHOLOGICAL DEVELOPMENT. 



diatom passes, will be alike; and so, too, on the average, 

 will be the relations of the two edges. In desmids 



of the type exemplified by the second individual in Fig. 2, 

 a kindred equalization of dimensions is otherwise insured. 

 There is nothing to keep one of the two surfaces uppermost 

 rather than the other; and hence, in the long succession of 

 individuals, the two surfaces are sure to be similarly exposed 

 to light and agencies in general. When to this is added the 

 fact that spontaneous fission occurs transversely in a constant 

 way, it becomes manifest that the two ends, while they are 

 maintained in conditions like one another, are maintained in 

 conditions unlike those of the two edges. Here then, as 

 before, triple bilateral symmetry in form, coexists with a triple 

 bilateral symmetry in the average distribution of actions. 



Still confining our attention to aggregates of the fir 

 order, let us next note what results when the two ends are 

 permanently subject to different conditions. The fixed 

 unicellular plants, of which examples are given in Figs. 4, 

 5, and 6, severally illustrate the contrast in shape arising 



between the part that is applied to the supporting surfac 

 and the part that extends into the surrounding medium. 

 These two parts which are the most unlike in their relations 

 to incident forces, are the most unlike in the forms. Observe, 

 next, that the part which lifts itself into the water or air, is 

 more or less decidedly radial. Each outward growing tubule 

 of Codium adhwrens, Fig. 4, has its parts disposed with some 

 regularity around its axis; the upper stem and spore-vessel 



