INTRODUCTION 517 



E. Summary of later derivatives of presumptive, major, organ-forming areas of the late 

 blastula and gastrula 



1. Neural plate area (ectoderm) 



2. Epidermal area (ectoderm) 



3. Entodermal area 



4. Notochordal area 



5. Mesodermal areas 



6. Germ-cell area 



F. Metamerism 



1. Fundamental metameric character of the trunk and tail regions of the vertebrate 

 body 



2. Metamerism and the basic morphology of the vertebrate head 



G. Basic homology of the vertebrate organ systems 



1. Definition 



2. Basic homology of vertebrate blastulae. gastrulae. and tubulated embryos 



A. Introduction 



1. Purpose of This Chapter 



In this chapter, the basic morphogenetic features which give origin to the 

 later organ systems are emphasized. These features arise from the stream of 

 morphogenetic phenomena which come down from the fertihzed egg through 

 the periods of cleavage, biastulation, gastrulation, and tubulation. This chapter 

 thus serves to connect the developmental processes, outlined in Chapters 6 

 to 10, with those which follow in Chapters 12 to 21. As such, it emphasizes 

 certain definitions and basic structural features involved in the later morpho- 

 genetic activities which mold the adult body form. 



2. Definitions 



a. Morphogenesis and Related Terms 



The word morphogenesis means the development of form or shape. It in- 

 volves the elaboration of structural relationships. The morphogenesis of a 

 particular shape and structure of a cell is called cytomorphosis or cytogenesis 

 and is synonymous with the term cellular differentiation, considered from the 

 structural aspect. In the Metazoa, the body is composed of groups of cells, 

 each cellular group possessing cells of similar form and function. That is, each 

 cell group is similarly differentiated and specialized. A cellular group, com- 

 posed of cells similar in form (structure) and function, is called a tissue. 

 Histology is the study of tissues, and the word histogenesis relates to that 

 phase of developmental morphology which deals with the genesis or develop- 

 ment of tissues. An organ is an anatomical structure, produced by an asso- 

 ciation of different tissues which fulfills one or several specialized functions. 

 For example, the esophagus, stomach, liver, etc., are organs of the body. 

 During development, each of the major organ-forming areas, delineated in 



