122 THE MAIN CURRENTS OF ZOOLOGY 



the experimental study of heredity, including Men- 

 delism, neurology, protozoology and parasitology, 

 ecology, animal psychology and behavior, paleo- 

 zoology and economic entomology. Morphological 

 investigations on broad lines have been largely taken 

 over by the anatomists, while the zoologists are more 

 engaged with cytology, ecology, genetics, etc. 



In the line of cellular studies (cytology) there has 

 been great refinement of technique and of observa- 

 tion. The minute constitution of the germinal ele- 

 ments and of the cellular phenomena of develop- 

 ment have been worked out, the physical basis of 

 heredity has been located in the chromosomes and, 

 more recently, partly within the cytoplasm of the 

 egg. These studies are closely allied with those of 

 experimental embryology in which so many advances 

 are being made the nature of fertilization as a 

 physiological process, etc. 



The theory of sex-determination and the biology of 

 twins have aspects of especial interest and, recently, 

 have been investigated by zoologists with important 

 results. 



In morphology, studies in neurology have taken 

 the center of the field. Although there are many 

 individual pieces of morphological investigation car- 



