RELATIVE SENSITIVENESS OF FISH EMBRYOS 319 



of the Fundulus embryo to rapid and great variations in the 

 concentration of the sea-water must rest (at least partly 

 [1903]) upon properties of the germ-plasm. 



I will not try to say, however, what condition determines 

 that the sensitiveness to loss of water is much greater during 

 the process of cleavage than during the formation of the 

 embryo. 



III. CONCLUDING REMARKS 



The experiments which have just been detailed were made 

 with a view to obtaining further data for a theory of embry- 

 onal organization, or as this is ordinarily termed for a 

 theory of heredity. It seems to me that some of these 

 theories for example, Weissmann's theory of determinants 

 assume more for the germ-plasm than it contains. According 

 to this theory, things are already definitely determined in 

 the germ-plasm which, to my mind, are functions of circum- 

 stances that first make their appearance in much later stages 

 of development. To give an example: According to the 

 theory of determinants, one would have to imagine that the 

 marking of the yolk-sac of Fundulus is already prearranged 

 by the spatial grouping of the determinants in the germ- 

 plasm which are responsible for the marking, while I found 

 that the marking is produced by the protoplasm of the chro- 

 matophores being compelled through its "chemotropism" to 

 spread over the surface of the blood-vessels. The formation 

 of blood-vessels, as well as the formation of pigment-cells, 

 may perhaps be traced back to the original germ, but the 

 spatial arrangement of the pigment-cell is, as we have seen, 

 the effect of a stimulus which the fully developed vessels, or 

 rather the blood which they contain, exercises upon the fully 

 developed chromatophores. These facts led me to the idea 

 that the chemical circumstances determining organization 

 do not all exist ready-formed in the germ-plasm, but arise 

 gradually in the different stages of development. The 



