T7O C. M. CHILD. 



these physiological axial gradients. Thus far every living physio- 

 logical axis examined has given evidence of the existence, at least 

 in the earlier developmental stages of such a gradient, and in 

 many cases the experimental methods show the presence of a 

 gradient where structural, or other directly visible indications of 

 its presence are absent. In whatever terms we may finally inter- 

 pret these gradients, there can be no doubt concerning their ex- 

 istence. They are physiological facts, and their significance for 

 localization, differentiation and functional relation is already 

 demonstrated. 



THE ORIGIN OF THE AXIATE PATTERN IN NATURE AND 



EXPERIMENT. 



It is a familiar fact that physiological axes persist through 

 certain agamic reproductive processes and are therefore inherited 

 by the individuals resulting from such reproduction, but in at 

 least many eggs the axiate pattern apparently arises de novo dur- 

 ing the growth of the egg. Granting that axiate pattern in its 

 simplest form is a gradient pattern as already pointed out, we 

 have at present no grounds for believing that such pattern is 

 inherent in protoplasm, or that it can arise de novo in protoplasm 

 apart from the action of environmental factors. Apparently a 

 differential exposure of the protoplasm to some environmental 

 factor or factors which affect its rate of activity, i.e., which are 

 primarily quantitative rather than specific or qualitative in their 

 action, is necessary for the origination and establishment of an 

 axial gradient in protoplasm. In other words, the gradients must 

 arise through the differential action of environmental factors 

 which affect primarily the rate of general protoplasmic activity. 



Origin of Axiate Pattern in Plants and Simpler Animals. As 

 regards the plants, Winkler ('oofr) and Kniep ('07) showed that 

 in certain algae, e.g., various species of Fucaceae, differential 

 illumination of the two sides of the egg determines the polarity 

 of the plant developing from the egg, though in the absence of 

 light polarity appears and germination occurs, but more slowly. 

 Stahl ('85) showed that in the spore of Equisetum polarity is de- 

 termined in the same way. Winkler ('ooa) also showed that 

 polarity in Bryopsis could be determined by light. In various 



