ORIGIN OF AXIATE PATTERN 65 



to the order or pattern which we call axiate or axiate- 

 symmetrical. 1 



The egg in most cases requires fertilization or the 

 action of some other factor external to it to initiate 

 development, but the specific hereditary constitution 

 of the egg protoplasm with its potentialities of develop- 

 ment is present, whether fertilization or initiation of 

 development by other means occurs or not. Physio- 

 logically speaking, the spermatozoon, or some other 

 factor, merely sets the mechanism in motion or gives it 

 the necessary speed, and development proceeds. Simi- 

 larly, the gradient in the egg, whether it persists from 

 earlier cell generations, or arises anew in the egg through 

 differential exposure, is merely a physiological condition 

 which determines that the hereditary mechanism shall 

 give rise to a particular order or pattern. Alteration of 

 the gradient relations alters the pattern, though the 

 hereditary mechanism remains the same. The gradient 

 is then nothing more than one of the physiological con- 

 ditions under which the development of axiate organ- 

 isms occurs and the surf ace -interior relation, whether a 

 gradient or not, is merely another even more general 

 physiological condition of organismic development. 



The idea of a quantitative gradient in physiological 

 condition as the condition initiating axiate development 



1 In his recently published Vertebrate Zoology (1920), Newman has 

 employed the gradient conception in connection with various problems 

 of vertebrate evolution. I have found it desirable thus far to consider 

 the gradient primarily as a physiological factor in individual develop- 

 ment rather than as itself a factor in evolution, but even if the direct 

 transfer of the conception to the evolutionary field is justifiable, some 

 of the applications which this author has made seem to me to be at 

 least open to question. In the present state of our knowledge, how- 

 ever, differences of opinion concerning tin- scope and limitations of 

 this, as of most other biological com eptions, arc to In- expected. 



