io8 THE MAIN CURRENTS OF ZOOLOGY 



organs, as the digestive system, the circulatory sys- 

 tem, the nervous system, etc. It aims to establish 

 homologies between organs by tracing the origin and 

 modifications of structures occurring in series of 

 animals. 



Histology. By the microscopic study of the 

 tissues composing the organs we come to histology. 

 This reaches a deeper level of morphological analysis 

 than comparative anatomy. In its full scope, it dis- 

 tinguishes the cells as well as the tissues, and cytology 

 is an offshoot of histology. The study of cells, how- 

 ever, has assumed such importance and opened so 

 many new questions that cytology stands out as 

 an independent division. The arrangement, the form 

 and staining qualities of cells often do much to show 

 the use of tissues, and, therefore, cytology has its 

 physiological side. 



Embryology. This likewise has become a broad 

 subject including observation of all stages of devel- 

 opment from the egg to hatching or birth. Among 

 its especial topics come the analysis of the germinal 

 elements, the nature of fertilization, the formation of 

 the germ-layers, the physical basis of heredity, the 

 histological differentiation (histogenesis) of tissues, 

 etc. 



