different stages of development, to obtain some 

 light on another problem which arose. This per- 

 tained to their peculiar identity with the sea. No 

 other group of animals is so restricted. Was there 

 in the echinoderm constitution some fundamental 

 feature that required it forever to remain in a 

 habitat strictly marine, that prohibited it perpetu- 

 ally even from the fresh-waters of the land? It 

 would seem that there was. Something I had seen 

 in the larvae swimming in the dark-room tank ap- 

 peared partly to supply an answer. 



Of the hundred strictly embryological observa- 

 tions that the course of my laboratory work has 

 required, not one has ever palled on me. Never yet 

 have I become wearied with the routine of watch- 

 ing those apparently simple changes taking place 

 in the beginning organism; and doubtless I never 

 shall. I know of nothing in the whole range of my 

 activities that to me is more impressive, more pro- 

 found than the dividing cells of a fertilized egg. 

 For here one has to do with the very fundamentals 

 of life itself; here one passes all outposts of labo- 

 ratory research and stands at the actual frontier of 

 the Unknowable; here, in short, arise those unan- 



[96] 



