No. 642] SEROLOGICAL REACTIONS 



In organisms with radial symmetry, special centers of 

 growth occur, and only at a certain distance from a given 

 center can another arise. 



After differences in protoplasmic constitution have 

 arisen at different levels of the gradient, a system of 

 chemical or transportatiye correlation probably begins 

 to operate, and out of it all finally comes the various sup- 

 portive and mechanical tissues, vascular tissues, tissues 

 of excretion and secretion, nervous tissues, etc., which 

 constitute the underlying mechanisms of correlation and 

 integration in the finished organism. 



The hypothesis says little of chromosomes, or of genes, 

 for its objective differs somewhat from that of the genet- 

 icist, but it in no wise denies the existence of such 

 entities. Child argues, however, that since each cell of the 

 body is a descendant of the original zygote and therefore 

 presumably possesses the full complement of chromo- 

 somes of that zygote, something other than the nuclear 

 pattern per se must be responsible for the fact that cells 

 become different in different parts of the body. And 

 this something, he would say, is the metabolic gradient 

 initiated by differential excitation, since the very estab- 

 lisliincnt of such a gradient means the concomitant es- 

 tablishment of local differences along its path. As he 

 says, if all the cells are originally alike they cannot of 

 themselves become different." Tiie specific character of 

 the differentiation, the kind of organ or organism pro- 

 duced, he reiterates, is determined "by the specific in- 

 herited constitution of the protoplasm." 



So much for Child's hypothesis of axial or metabolic 

 gradients, in its bearings on embryogeny and differentia- 

 tion. T have reviewed it at some length because it shows 

 more clearlv than any other theory of development with 

 which T am acquainted that there is no necessity for 

 believinu- that ceIN hvcomo .po( -.ali/ed tlioy lose part of 



occurred— the cell may 1>e iiu-apMhle of de.i, th-n-nt intion 

 -but nothing constitutional, call it gene or what you will, 



