53o 



THE QUARTERLY REVIEW OF BIOLOGY 





a two layered cup (a gastrula) with the 

 axis passing through the center of the 

 opening and of the opposite pole, and the 

 walls the same in all the radii. 



RADIAL SYMMETRY 



If such an animal form should continue 

 its development to the adult stage, follow- 

 ing to its logical conclusion the preceding 

 line of geometrical development, the 

 result would be an animal radially sym- 

 metrical and composed of two layers of 

 cells. Such an animal is represented by 

 the hydra and allied creatures, the so- 

 called ccelenterates. 



We know that all animals begin life 

 as a single cell which divides into two, 

 and these derivatives continue to divide 

 in the same way. Undoubtedly the 

 original animal forms had a similar life 

 history. But there is no logic in the 

 assumption that the earliest animals were 

 single celled creatures of the protozoan 

 type. It is far more likely that from the 

 very first the dividing cells would take 

 all three of the courses outlined, complete 

 separation, formation of an irregular 

 mass, or formation of a geometrical body 

 developing into a two-layered creature 

 with radial symmetry. 



While it is most reasonable to suppose 

 that all three alternatives were realized 

 from the start, if any of the three were to 

 precede the others it would presumably 

 be the development of a more or less form- 

 less sponge-like mass from which on the 

 one hand single celled creatures were 

 derived, and on the other the geometrical 

 multicellular types. 



As typical of the single celled animals 

 let us take the amoeba as the best known. 

 As typical of the animals developing into 

 a formless mass we may take the sponges. 

 And as typical of the result of geometrical 

 development we may take the hydra. 



The amoeba, the sponges and the hydra 



arc all radially symmetrical. The amoeba 

 is composed of a single cell; when it has 

 reached the limit of its growth from the 

 economic viewpoint it divides into two. 

 The sponges, with their systems of canals 

 which penetrate the mass, are capable of 

 almost indefinite growth without trans- 

 gressing economic boundaries. 



But with the hydra it is different. The 

 hydra, or any creature similarly formed, 

 must remain of a size suitable for the 

 capture of its prey. There are four ways 

 in which this may be done without inter- 

 rupting a continuous increase in bulk. 

 The animal may do as the single celled 

 amoeba does, divide into two, or even 

 into more, by the formation of buds which 

 separate off and grow into complete and 

 independent animals. This process occurs 

 in hydra, in the sea-anemones, and in the 

 solitary corals. Or the animal may do as 

 the sponges do and grow into a large, more 

 or less radially symmetrical mass with the 

 food collecting mechanism distributed 

 over the surface, as in the case of the brain 

 corals. Or the animal may form a bud 

 which eventually develops into another 

 individual remaining attached to the first, 

 the second individual may in the same way 

 produce a third, and the third a fourth, 

 and so on, until a more or less geometri- 

 cally arranged plant-like structure results 

 bearing many individuals or polyps, as 

 in the case of the stag-horn corals, red 

 corals, sea-pens, sea-fans, and similar 

 things. 



Thus it is evident that the individual 

 radially symmetrical animals repeat the 

 same processes that are seen in the develop- 

 ment of the single cell. That is, when 

 the limit of size is reached they divide into 

 two individuals which separate from each 

 other; they divide into two individuals 

 which remain attached in such a way that 

 further division results in the formation of 

 a more or less symmetrical mass; or they 



