INTRODUCTION 



23 



others. Many of the processes which occur in the developing em- 

 bryo are also included here. Much of the present study referred 

 to as cytology is physiological. Physiology, like morphology, is an 

 old branch of zoology; physiology, however, remained in a crude 

 state long after morphology was fairly well developed. Physiology 

 depends upon an understanding of physics and chemistry on one 

 hand, and anatomv on the other. This field of study could not 

 develop until the sciences of physics and chemistry came forward 

 during the nineteenth century. 



4. Pathology is the study of the abnormal structures and abnor- 

 mal functioning of life processes. It is really the science of disease 

 in all of its manifestations. There is a fundamental similarity in 

 diseases in the different groups of animals, and a study of pathol- 

 ogy is likely to involve certain forms from all groups of animals. 

 This field has advanced rapidly during the last seventy years. 



5. Embryology is a study of the origin and development of the 

 mdividual. It usually involves the changes occurring in the or- 

 ganism from the time of fertilization by the union of two cells, one 

 derived from each parent, through the numerous cell divisions, 

 growth, organization, and differentiation leading to the adult con- 

 dition. This process includes both morphological and physiological 

 changes. The beginning of this study dates back to the work of 

 K. E. von Baer before the middle of the nineteenth century. In 

 recent years the field of experimental embryology has developed 

 rapidly. The development of the individual may be referred to as 

 ontogeny. 



6. Genetics is the division which deals with the study of varia- 

 tions, resemblances, and their inheritance from one generation to 

 the next or from parent to offspring. The characteristic features 

 of an animal or plant may be transmitted to the offspring somewhat 

 independently of one another, bringing about a variety of combina- 

 tions in the progeny. Fairly definite laws governing this inherit- 

 ance of qualities have been established by the geneticists. Some of 

 the factors which control this distribution of characteristics are 

 morphological in their nature, others are physiological. 



7. Phylogeny is a study of the origin and relationships of the 

 different groups and races of organisms. It is based on the results 

 of studies of morphology, embryology, genetics, zoogeography, and 

 paleontology. 



