CELLS AND TISSUES 19 



group tends to act as a unit. Thus without protoplasmic continuity 

 the cells may still have a physiological continuity; it is the possession 

 of both that conditions the coordinated action of most tissues. In 

 any case the field of force which pervades the whole organism and which 

 shapes its development is not interrupted by cell partitions (Thompson, 

 1917). 



The differentiation of specialized tracts with a correlating function 

 is exemplified not only in the nervous system of higher animals but also 

 in the neuromotor apparatus of certain Protozoa. It has been found 

 that after the delicate strands leading to the several groups of locomotor 

 organelles in Euplotes are cut, the action of the groups is no longer 

 properly coordinated (C. V. Taylor, 1920, 1929). It has been contended 

 that the vascular bundle regions of higher plants serve to conduct 

 correlating stimuli, but the evidence for this is as yet inconclusive. 



In the vertebrates correlation is maintained in part through the agency 

 of hormones. These are chemical substances which are elaborated in the 

 endocrine glands and carried by the blood or lymph to the various parts of 

 the body, where they exercise profound effects upon growth and other 

 activities of the organs and tissues. The discovery of these endocrine 

 secretions and the application of knowledge concerning them to the treat- 

 ment of certain human disorders constitute one of the most significant 

 developments in modern medical biology. ^^ The question of the possible 

 role of hormones in plants is still a disputed one, although Went (1931) 

 and others have adduced evidence indicating the presence of one, called 

 "auxin," in growing points, while Haberlandt (1921 et seq.) attributes 

 the healing of wounds to a hormone. 



One of the most important contributions to our understanding of 

 correlation is the conception of physiological gradients developed by 

 Child (1911 et seq.) and his associates. It has been shown in a number 

 of animals and plants that along each of the axes of symmetry there 

 exists a physiological gradient (also called "metabolic gradient" and 

 "axial gradient"), the rate of the physiological processes being highest 

 at one end of the axis and diminishing progressively toward the other 

 end. The anterior end of a planarian, for example, exceeds the posterior 

 end in its rate of oxygen consumption and carbon dioxide output and 

 in its susceptibility to poisons. Furthermore, the portions having a 

 higher rate exercise a "dominating" influence over the development of 

 those portions having a lower rate, with the result that the individual 

 maintains a definite physiological correlation of anterior and posterior 

 parts. Similarly in individuals with more than one axis of symmetry 

 there may be a corresponding dorsal-ventral, as well as an axial-marginal, 

 correlation. The gradient arises in the first place, according to Child, 

 as a response to differential factors in the environment; and, although 



^^ For a convenient account of hormones and their effects, see Guyer (1931). 



