582 Animal Biology 



ficially separated, they would have collaborated to form the two sides 

 of a complete individual. In other scientifically performed experiments 

 it appears that stimulating chemical substances are produced which in- 

 fluence the development of structures in certain places, when and where 

 such specific substances are formed. These specific chemical substances 

 have been moved experimentally from one part of an animal to another 

 part of the same animal with the subsequent development of a rather 

 typical structure in the newly stimulated region. The stimulating influ- 

 ence may even be transplanted successfully to another animal. This 

 suggests that not only genes are at work but the latter are influenced by 

 environmental, chemical substances. Possibly the production of the 

 specific chemical substances is influenced by the action of specific genes. 

 It is to be expected that the type and amount of the development of a 

 particular structure or ability will be influenced by such factors as ( 1 ) 

 the specific genes involved, (2) the quality and quantity of the specific 

 chemical substances available at that particular region, (3) the suscepti- 

 bility of the particular tissue or organ involved, (4) the age of the or- 

 ganism itself, etc. Undoubtedly, enzymes, hormones, vitamins, and 

 similar substances influence the specific type of development in an area 

 where such substances are available in the proper quantity. It is known 

 that these substances are usually specific; that is, they are capable of 

 stimulating specific actions in certain places. In other words, the spe- 

 cific chemical stimulator must be present in the proper area in the proper 

 amount, and the tissues, or organ, must be susceptible to its influence 

 before a reaction can take place. Abnormalities of either tissue or 

 stimulator may bring about abnormal reactions, or possibly none at all. 

 A point to be borne in mind is that various forces are necessary to initi- 

 ate and maintain a particular development, but there is also an equally 

 important phenomenon of discontinuing development at the proper time. 

 For example, the necessary factors must be present in order to develop 

 a finger embryologically, but there must also be a cessation of develop- 

 ment when the finger has reached normality. Undoubtedly, modifica- 

 tions of such factors are responsible for certain abnormal anomalies 

 (a-nom'ali) (Gr. anomalos, uneven). With the incidence of old age 

 and its attendant atrophy of certain structures (and functions), we 

 encounter other problems, probably the opposite of the morphogenic 

 development type. Other closely related problems are those associated 

 with the death of tissues and organisms as a whole. Causes and eflPects 

 of death are still unknown, and much scientific work must still be done 

 in this field. 



