THE ASCENT OF THE BODY 8i 



number of chambers is so great that count is lost, 

 and the activity becomes so vigorous in every 

 direction that one ceases to notice individual cells 

 at all. The tenement in fact consists now of 

 innumerable groups of cells congregated together, 

 suites of apartments as it were, which have quickly 

 arranged themselves in symmetrical, definite, and 

 withal different forms. Were these forms not 

 different as well as definite we should hardly call 

 it an evolution, nor should we characterize the 

 resulting aggregation as a higher organism. A 

 hundred cottages placed in a row would never 

 form a castle. What makes the castle superior 

 to the hundred cottages is not the number of 

 its rooms, for they are possibly fewer ; nor their 

 difference in shape, for that is immaterial. It lies 

 in the number and nature and variety of useful 



trula stage. That some of the lower Metazoa, indeed, never 

 develop much beyond it, a glance at the structure of the 

 humbler Coelenterates will show — the simplest of all illus- 

 trations of the fact that embryonic forms of higher animals 

 are often permanently represented by the adult forms of lower. 

 The chief thing however to mark here is the doubling-in of 

 the ovum to gain a double instead of a single wall of cells. 

 For these two different layers, the ectoderm and the endoderm, 

 or the animal layer and the vegetal layer, play a unique part 

 in the after-history. All the organs of movement and sensation 

 spring from the one, all the organs of nutrition and reproduc- 

 tion develop from the other. 



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