224 The Mechanistic Conception of Life 



relation between blood-vessels and chromatophores. As soon 

 as a ramification of a chromatophore comes in contact with 

 a blood-vessel the whole mass of the chromatophore creeps 

 gradually on the blood-vessel (Fig. 37) and forms a complete 

 sheath aromid the vessel, until finally all the chromatophores 

 form a sheath around the vessels and no more pigment cells 

 are found in the meshes between the vessels (Fig. 38). Nobody 

 who has not actually watched the process of the creeping of 

 the chromatophores upon the blood-vessels would anticipate 

 that the tiger-like coloration of the yolk sac in the later 

 stages of development was brought about in this way. Similar 

 facts can be observed in regard to the first marking of 

 the embryo itself. The writer is inclined to believe that we 

 are here dealing with a case of chemotropism, and that the 

 oxygen of the blood may be the cause of the spreading of the 

 chromatophores around the blood-vessels. Certain observa- 

 tions seem to indicate the possibility that in the adult the 

 chromatophores have, in some forms at least, a more rigid 

 structure and are prevented from acting in the way indicated. 

 It seems to the writer that such observations as those made on 

 Fundulus might simplify the problem of the hereditary trans- 

 mission of certain markings. 



Driesch has found that a tropism underlies the arrangement 

 of the skeleton in the pluteus larvae of the sea-urchin. The 

 position of this skeleton is predetermined by the arrangement 

 of the mesenchyme cells, and Driesch has showTi that these 

 cells migrate actively to the place of their destination, possibly 

 led there under the influence of certain chemical substances. 

 When Driesch scattered these cells mechanically before their 

 migration, they nevertheless reached their destination. 



In the developing eggs of insects the nuclei, together with 

 some cytoplasm, migrate to the peripherj^ of the egg. Herbst 

 pointed out that this might be a case of chemotropism, caused 

 by the oxygen surrounding the egg. The writer has expressed 



