A CENTURY OF ZOOLOGY IN AMERICA 431 



ever, there has been made a concerted attempt to analyze 

 the factors which determine the amount and rate of 

 regeneration. Much progress has been made toward the 

 postulation of definite laws applicable to the regenerative 

 processes of the parts of each organism. The critical 

 analyses of Morgan, Loeb and Child have been particu- 

 larly stimulating. 



Tissue Culture. Another line of experimental work 

 which has been developed within the past few years by 

 Harrison, Carrell, and others is the culture of body 

 tissues in artificial media. These experiments have 

 included the cultivation in tubes or on glass slides of the 

 various tissues of numerous species of animals. They 

 have yielded much information regarding the structure, 

 growth and multiplication of cells, the formation of tis- 

 sues, and the healing of wounds. 



Transplantation and Grafting. Closely associated 

 experiments consist in the transplantation of organs or 

 other portions of the body to abnormal positions, to the 

 bodies of other animals of the same species or of other 

 species. In this way much has been learned about the 

 potentiality of organs for self-differentiation, for regula- 

 tion, for regeneration and for compensatory adaptations. 

 The experiments have shown, further, the independence 

 of soma and germplasm and have revealed the nature of 

 certain organs whose functions were previously obscure. 



Tropisms and Instincts. Another field of experimen- 

 tal biology concerns the analysis of behavior of organ- 

 isms in response to various forms of stimuli. These 

 studies are being prosecuted on all groups of organisms, 

 including the larval stages of many animals, and are 

 yielding most remarkable results. The success in this 

 field of research is largely due to stimulating influence of 

 Jacques Loeb, Parker, Jennings, and their co-workers. 



Biological Chemistry. Still another experimental field 

 which has developed into one of the most important of 

 the biological sciences relates to the fundamental^ chem- 

 ical and physical changes which underlie all organic phe- 

 nomena. A knowledge of both physiological and physi- 

 cal chemistry is to-day essential for all advanced 

 biological work. The peculiar nature of life itself, of 

 growth, disease, old-age, degeneration, death and dissolu- 



