CHAPTER XIII. 

 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



I. Primary Causes of Differential Growth. 



Before we attempt to explain the meaning of the various embryonic stages in 

 arthropods and vertebrates, it is desirable to reach some conclusion, if possible, 

 as to the general nature of the causes, or conditions that are likely to control the 

 initial stages of growth. Even if it is quite impossible to detect the real causes, 

 it is well to make perfectly clear, merely as an aid to exposition, the mental attitude 

 in which the writer approaches his problem. 



It is apparently assumed by some authors that differential growth takes place 

 in developing eggs because in some predetermined manner certain agents dis- 

 tribute to their proper places preformed materials, which then develop into 

 definite organs because they are made of the same material from which those 

 organs arose. 



Such artificial explanations are now found only in the biological sciences and 

 are to be regarded as the decadent offspring of the doctrine of special creation. 

 They are pernicious, because the clever juggling of words and images fixes the 

 attention solely on the imaginary structure of imaginary things, thus leading one 

 to overlook the sequence of form and of physical and chemical conditions that 

 constitute the only measurable or accessible properties of living things. 



An erratic boulder does not grow into a mountain like the one from which 

 it came, even if it is made of precisely the same kind of materials. And even if 

 some metaphysical geologist should succeed in picturing a planatasmal geo- 

 phore that was a mountain in miniature, or that stood for one, or represented one, 

 or was capable of becoming one, it makes little difference what expression one 

 uses, the real problem, and the only one of interest to a matter of fact geologist, 

 would be: What was the sequence of events in the evolution of that mountain? 

 What were the conditions immediately preceding each step in the process ? 



The biologist should approach the problems of growth and morphology in 

 the same spirit. Let him study the changes of form and action as they occur, 

 with the hope of discovering a sufficient cause for each one. He should not use 

 omnipotence, either of a creator, or of heredity, or of chromosomes, to short 

 circuit the process of evolution. 



I shall therefore throw aside, for the time being, all preconceived ideas as to 

 the ultimate composition of the ovum, or of the growing embryo, and shall consider, 

 as they appear at successive periods, some of the simpler physical and chemical 

 conditions likely to be determining factors in differential growth. It is assumed 



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