6o 



A. T. MASTER MAN 



[.JANUARY 



presence of yolk may, in certain cases, enable the embryo to, as it were, 

 take a short cut to axo-symmetry (unequal segmentation), otherwise 

 the blastula in due course becomes the gastrula, a typical axo- 

 symmetric organism, with locomotion in one direction and rotation 

 about the axis so defined. Whilst the lower forms (Coelentera) remain at 

 this stage, the higher pass on to the type of piano-symmetry, and thus 

 the phyletic history is repeated (Fig. 7). 



Eeference has been made above to the secondary centres of symmetry 

 and the predominant number of these in the three types of symmetry 

 was stated to be six, four, and two respectively. Any organs in an 

 organism must be centric or peripheral. If they are in the former 



ASYMMETRY. 



CENTRO-SYMMETRY. 



AXO-SYMMETRY. 



PLANO-SYMMETRY. 



STEREO- SYM 



PROTOZOA 



COELENTERATA 



COELOMATA 



Fig. 7. — Diagram to indicate the relative predominance of each main type of symmetry in 

 the chief divisions of the animal kingdom, and the gradual advance of type botli in 

 phylogeny and ontogeny. 



category and conform to the symmetry of the organism, they must be 

 single, and must be conformable to the centre of symmetry, whether a 

 point, an axis, or a plane. If they are peripheral they must, in order 

 to conform to the symmetry of the whole organism, be repeated a 

 certain number of times round secondary centres of symmetry, and the 

 primary numbers corresponding in each case to twice the number of 

 symmetrical dimensions, work out as six, four, and two. 



This subject is intimately connected with the theory of "metameric 

 segmentation " and its attendant phenomenon and will be more fully 

 dealt with later. 



4. Stereo- Symmetry. — We have discussed in turn the three forms of 

 symmetry, and the question remains — Is there not yet another ? Our de- 

 finition of the symmetry of an organism reads as follows : — The system 

 of arrangement of its constituent parts in relation to each other and to a 



