I2 4 THE SHAPES AND SIZES OF ANIMALS 



main or longitudinal axis of the animal, the rounded 

 end of the bell going first, the open mouth of the 

 bell last, yet that it is a matter of indifference 

 which part of the rim of the bell is uppermost. 

 The animal, in fact, may be said to have anterior 

 and posterior ends, but no distinction between 

 dorsal and ventral surfaces, or between right and 

 left sides ; and this is the characteristic arrange- 

 ment in a radially symmetrical animal. 



If the spherical form is primitive, then the 

 radially symmetrical form must be derived from it, 

 and of this we have direct evidence in the fact that 

 every radially symmetrical animal is developed 

 directly or indirectly from a spherical egg. Con- 

 cerning the actual historical mode of derivation of 

 the radial from the spherical form however there 

 has been much discussion, and the question cannot 

 yet be regarded as settled. The chief difficulty arises 

 from the fact that in actual development there are 

 at least two quite different ways in which the 

 spherical ovum may give rise to a radial larva, and 

 it has not yet been determined which of these 

 modes is the more primitive, and in what way one 

 of them could have been derived from the other. 



The more usual mode of development is as 

 follows : The ovum, after fertilisation, divides into 

 two cells ; each of these again divides, giving four 

 in all ; the process is repeated until a solid heap of 

 cells, the morula, is produced ; then this becomes 

 converted into a blastula by the cells moving away 

 from the centre and becoming arranged so as to 

 form a spherical ball, consisting of a single layer of 



