CLASSIFICATION OF THE PRIMORDIA 



79 



bicellular specimen of Euastropsis is more or less comparable 

 with a uniaxial system with two segments. In Pediastrum tetras 

 the cells are ordinarily four in number, united into a rect- 

 angular cross (radial system? rectangular biaxial system?). 

 In other species of Pediastrum certain specimens consist of 

 eight cells, one of which is central and the seven others 

 marginal. This disposition in (more or less distinct) concentric 



Fig. 



3. — I. Euastropsis. 2. Pediastrum tetras. 3. Pediastrum 

 consisting of sixteen cells. (Schematic, after WEST) 



rings is also observed when the cells are more numerous. 

 According to NAEGELI,^ the following dispositions are the 

 commonest : — 



Specimen of 8 cells 1+7 



„ 16 „ i + 5 + io(Fig. 3. 3) 

 „ 32 » I + 5 + 10 + 16 



Although these regular arrangements are not always observed, 

 it is incontestable that a remarkable plan of structure exists in 

 these algae. This calls for reflection. 



§65.— SPECIAL SYSTEMS OF SEGMENTATION. III. 

 PEDIASTRUM AND EUASTROPSIS (continued). FRUIT 

 OF MYXOMYCETES AND GASTEROMYCETES. POLYP- 

 IFORM ANIMALS AND OTHER EXAMPLES. MORPHO- 

 LOGICAL HOMOLOGY AND MECHANICAL CONCORD- 

 ANCE. — The classical morphology is based upon embryological 

 investigation. We try to explain the structure of a given adult 

 organism by the knowledge of its development. Starting from 

 the initial cell and proceeding step by step, we look upon each 

 state of development as being produced by a transformation of 



» Quoted after G. S. WEST, loc. cit., p. 209. 



