130 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



crine glands are unknown, it is probable that there are, nevertheless, 

 regulating centers constituting various links in the developmental 

 chain which starts with the fertilized egg and its chromosomes 

 and ends with the fully formed parts and tissues. These en- 

 docrine glands and other developing controlling tissues are what 

 the chromosomes make them. 



This hypothesis receives support from the observations that 

 have been made upon the physiology of the later stages of de- 

 velopment, with their processes of folding of membranes, or 

 concrescence, of disruption of parts by mechanical and histo- 

 lytic processes, and the development of special tissues with their 

 special kinds of form and substance such as are seen in the various 

 connective tissues of the higher animals. These tissues are in- 

 deed responsive in turn, not only to the hormones which seem 

 so largely to control their development but also to external agents, 

 such as pressure, gravity and radiant energy. 



The hypothesis advanced by Davenport assumes new 

 significance when viewed in the light of S3Tiibionticism and 

 the known responses of microorganisms to a continually 

 changing environment. It is not only the metaboUc prod- 

 ucts elaborated by living cells, however, that play the 

 role of stimulators in cellular differentiation. The signifi- 

 cance of iodine in relation to normal development and 

 growth illustrates the dependence of some organisms upon 

 chemicals derived from the environment. 



