332 ORGANIC GROWTH sec. 



lower multicellular animals now living, all multicellular 

 animals have been derived from forms whose only organs 

 were two or three germ layers, then the external germ layer, 

 the ectoderm, must naturally have originally been the 

 medium of those relations to the external world. And in 

 these multicellular forms which still consist only of these 

 " j)i'iniitive organs " the ectoderm must be still the medium 

 of relation. This most external of the three layers of cells 

 of the larva or of the body came, and comes, first into con- 

 tact with the outer world. All external stimuli acted first 

 upon it. By the repeated incidence of the stimuli this layer 

 must have become more and more fitted for their reception, 

 and subsequently for dealing with them, must have had 

 these capacities exercised and strengthened. Only thus can 

 it be explained that the nervous system, that brain and 

 ganglia wherever they can be distinguished, arise from the 

 epiblast. 



Again, the most important parts of the nervous system, 

 the brain and higher sense organs, the organs of sight and 

 hearing, and also of taste and smell, are always situated in 

 those parts of the body wdiich come most into contact with 

 the outer world, which are so placed as to be oftenest affected 

 by stimuli — in worms, molluscs, arthropods, and vertebrates 

 at the anterior end, in Medusae on the edge of the bell and 

 so on. 



The sense organs are not only derived from the epiblast, 

 but usually remain situated in the ectoderm. The sensory 

 cells moreover are developed from ordinary ectoderm or 

 epiblast cells. 



At first only simple epiblast or epidermis-cells were present 

 everywhere ; from these touch-cells were developed. All the 

 higher sense- cells have been developed from touch- cells — 

 this is irrefutably proved by comparative anatomy. 



The origin of the various sense-organs from the epidermis- 



