THE ORIGIN OF LIFE 85 



has only one muscle wherewith to close its shell, and 

 many other bivalves have two, the young oyster begins 

 with one, then as it grows a little larger develops two, 

 and later drops one and returns to a single one. The 

 rationale of this is that in the young animal the only 

 muscle needed is the anterior or front one. A little 

 later, as the shell becomes wider, a second, the 

 posterior, is needed. Later still the form and hinge 

 are such that one is sufficient, and the anterior the 

 one first developed becomes abortive, while the 

 posterior remains. Other changes might be noticed, 

 but let us think of the significance of these. The egg 

 of the oyster is absolutely undistinguishable from 

 that of any other invertebrate animal. Still it must 

 have within it structures or predetermined powers 

 which denote the animal that is to result from it. 

 The next stage, that of the early embryo, presents a 

 form which we could perhaps decide from its structures 

 to be molluscan ; but we could not tell previous to 

 experience whether, for instance, it would be a uni 

 valve or a bivalve. The next stages determine it to 

 be a bivalve, but rather one of those with rounded 

 and smooth shells and two muscles than those like 

 the oyster. Here it is to be observed that this dis 

 tinction of one or two muscles is used to divide 

 the whole of the bivalves into two great groups, so 

 that in this early stage our oyster might be either a 

 monomyarian or a dimyarian. In this stage it becomes 

 fixed, and begins to spread out its valves into the 



