EMBRYOLOGICAL CRITERION OF HOMOLOGY. 117 



mation 

 of the 

 general 



of the arms may be only incidental to the production 



spicules, and we need only assume in the ectoderm a 



power of growth which is exerted at particular points 



under stimulus acting at those 

 points. In this case the neces- 

 sary condition of development 

 is a certain internal stimulus 

 (formation of spicules). This 

 stimulus, itself, however, is 

 directly dependent on exter- 

 nal conditions (the chemical 

 environment), and hence the 

 formation of the arms is de- 

 termined both by internal and 

 external conditions. 



Accurately determined cases 

 like these are at present far 

 from common, though some 

 others might be mentioned. 



DIAGRAM III. 



A. Normal Pluteus of Strongylocentrotus lividus, from the side. 



B. Potassium-larva " of Spharechinus granuluris at a stage corresponding 

 with the last (after Herbst). 



But even a single such case opens the way to a rational concep- 

 tion of epigenesis ; for it enables us in a measure to compre- 

 hend how a single property of the germ-plasm may involve a 

 whole train or cluster of events in development how, in the 

 words of Herbert Spencer, development involves a multiplication 



