EMBRYOLOGICAL CRITERION OF HOMOLOGY. W] 



mation of the arms may be only incidental to the production 

 of the spicules, and we need only assume in the ectoderm a 

 general 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. 



B 



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 



