XVI ORIENTATION 



tive embryonic conditions of the various systems are present, a common, 

 basic, primitive embryonic body form also is present. Hence, all vertebrate 

 embryos tend to pass through a stage of development in which the shape 

 and form of the developing body resembles that of all other vertebrate species 

 at this stage of development. This stage of body formation is known as the 

 primitive, embryonic body- form stage. 



2. The larval period or period of transition 



During this phase of development, the basic conditions of the organ-systems, 

 which are present at the end of primitive body formation, are transformed 

 into the structural conditions present in definitive body form. At the end of 

 this period of development the general form of the organ-systems, and of the 

 embryo as a whole, resembles the adult morphology of the species. Hence the 

 term: "definitive body form." 



3. The late embryonic period 



This part of development intervenes between the time when definitive body 

 form is established and the episode of hatching or birth. In free-living larval 

 species it comprises a brief period at the end of metamorphosis. 



C. Post-embryonic Development 



Post-embryonic development may be divided into the following periods: 



1 . Prepuberal period 



During this time the organ-systems grow and enlarge, and the reproductive 

 mechanisms mature. 



2. Puberal period and the adult 



The organism now is capable of reproduction, and in size, activity, and 

 appearance is recognized as an adult. 



3. Period of senescence and decline 



The sexual activities lessen and the organ-systems of the body may very 

 slowly undergo regressive changes. 



IV. A Classification of the Vertebrates and Related Species 



A. Characteristics of the Phylum Chordata 



The vertebrates belong to the phylum Chordata. This phylum is character- 

 ized by three main features which appear in the early embryo, viz., ( 1 ) a 

 dor sally situated nerve cord which in most instances is hollow or tube-like; 

 (2) a dorsally placed notochordal or median skeletal axis located always 

 immediately ventral to the nerve cord, and (3) a complicated anterior portion 



