160 



THE AMEBIC AN NATURALIST [Vol. XLIX 



low protozoon colony; it does so simply because at every 

 stage it must do what its ancestors did under like condi- 

 tions. We can also see that progressive evolution must 

 follow from the gradual accumulation of additions at 

 the end of each ontogeny, these additions being rendered 

 possible by the better start which each individual gets 

 at the commencement of its career. 



Let us now glance for a moment at the next stage in 

 phylogeny, the conversion of the hollow spherical proto- 

 zoon colony into the ccelenterate type of organization, 

 represented in ontogeny by the process of gastrulation. 

 Here again it is probable that this process is explicable 

 to a large extent upon mechanical principles. Accord- 

 ing to Ehumbler, 4 the migration of endoderm cells into 

 the interior of the blastula is partly due to chemotaxis 

 and partly to changes of surface tension, which decreases 

 on the inner side of the vegetative cells owing to chem- 

 ical changes set up in the blastocoel fluid. 



We may, at this point, profitably ask the question, Is 

 the endoderm thus formed an inherited feature of the 

 organism? The material of which it is composed is, of 

 course, derived from the egg-cell continuously by re- 

 peated cell-division, but the way in which that material 

 is used by the organism depends upon the environment, 

 and we know from experiment that modifications of the 

 environment actually do produce corresponding modifi- 

 cations in the arrangement of the material. We know, 

 for example, that the addition of salts of lithium to the 

 water in which certain embryos are developing causes 

 the endoderm to be protruded instead of invaginated, 

 so that we get a kind of inside-out gastrula, the well- 

 known lithium larva. 



It appears, then, than an organism really inherits from 

 its parents two things: (1) a certain amount of proto- 

 plasm loaded with potential energy, with which to begin 

 operations, and (2) an appropriate environment. Ob- 



* Quoted by Przibram, " Experimental Zoology, ' ' English Trans., Part 



