158 DYNAMICS OF LIVING MATTER 



Positive stereotropism is apparently that form of irritability which 

 next to chemotropism is most instrumental in bringing about the union 

 of the two sexes. The holding of the female by the male during copu- 

 lation is evidently in many forms purely a form of stereotropism. In 

 frogs such contact irritability develops during the spawning season, 

 on the' ventral side of the chest. At that time the contact of the ventral 

 side of the chest with any sohd body causes a reflex closing of the arms 

 of the male frog around the solid body. The embrace becomes lasting, 

 however, only in case the embraced object is a female frog. In this 

 case, obviously, other stimuU contribute toward making the embrace last- 

 ing. What the nature of these stimuli is, is not yet known.* Holmes 

 has called attention to the fact that the embrace of the female Gammarus 

 by the male is a similar case of stereotropism.f These reactions can 

 even be demonstrated in the decapitated frog. 



6. Concluding Remarks concerning Tropismlike Reactions 



It is obvious that the tropisms furnish the understanding for many 

 purposeful instinctive reactions, and that what is generally called an 

 instinct.is often nothing more than a compulsory turning and moving of 

 an organism in a given direction. I have carried out such an analysis 

 of animal instincts in another book, and therefore do not wish to enter 

 upon this subject here. I believe, indeed, that the tropisms and trop- 

 ismlike reactions will one day form the main contents of a scientific 

 psychology of lower forms. The tropisms, however, and tropismlike 

 compulsory reactions also play a r6le in the mutual arrangements of 

 organs and tissues. The first case of this kind mentioned was the 

 observation that the tigerUke coloration of the yolk sac of the Fundulus 

 embryo is due to a creeping of the chromatophores upon the blood 

 vessels. At first the chromatophores and blood vessels are formed with- 

 out any definite relation to each other, but by and by every chromato- 

 phore creeps on the capillary, enveloping it completely. J I am not able 

 to state whether this is a case of chemotropism caused by the oxygen in 

 the capillary tubes, or a case of stereotropism. Driesch § suggested 

 later that the migration of the mesenchyme cells to those spaces in the 

 gastrula of the sea urchin where the skeleton is to be formed, might 

 be due to a tropism. Herbst || has pointed out the possibility of a wide 

 application of the tropisms in ontogenetic processes. 



* Goltz, Beitr'dge zur Lehre von den Nervenceniren des Frosches, Berlin, 1869. 



t S. J, Holmes, Biological Bulletin, Vol. 5, p. 288, 1903. 



X Loeb, PflUger's Arc!iiv,\o\. 54, p. 525, 1893; andyi3«n of Morphology, NiA. 8, p. 161, 

 1893. § Driesch, Archiv fur Entwickelungsmechanik, Vol. 3, 1896. 



II Herbst, Ueher die Bedeutung der Reiiphysiologie fur die causale Auffassung der 

 Ontogenese, Biologisches Centralblatt, Bd. 14 and Bd. 15, 1894 and 1895. 



