Emhryologic Development of Animals 443 



phibia, reptiles, and birds. The fishes have typical two-chambered hearts, 

 the amphibia have three-chambered hearts, the reptiles have three- or 

 four-chambered hearts (depending on the type), the birds and mammals 

 have four-chambered hearts. 



One of the most important phenomena in the embryologic develop- 

 ment of individual organisms is the actual origin and development of the 

 definite structures and forms of specific tissues, organs, and systems. 

 How do so many different kinds of structures originate within a single 

 embryo? A study of the origin and development of the form and struc- 

 ture in organisms is known as morphogenesis (mor fo -jen' e sis) (Gr. 

 morphe, form; genesis, origin) . Naturally, the specific inheritance of the 

 particular embryo, together with the influences of environmental factors, 

 both external and internal, are influential in determining morphogenesis. 

 Many of these are not well understood, but the action and interactions 

 of specific genes, as well as the action of the cytoplasm,, assist in laying the 

 groundwork or blueprint for the detailed construction of the individual. 

 These are affected by physical and chemical factors (environmental) so 

 that specific traits are developed. The earliest stages in the development 

 of an animal seem to be influenced by the cytoplasm of the ^^g, which is 

 maternal. Within each species of animal there is a certain pattern of 

 development which is typical and normal. When this pattern is disturbed, 

 either by genetic or environmental factors, there follow abnormal proc- 

 esses which cause abnormalities in growth and developments. The 

 specific abnormalities depend upon the particular genetic materials in- 

 volved or the quantity and quality of the environmental factors at work 

 or both. Just after fertilization, various currents appear in the cytoplasm 

 of the cells which initiate the developmental pattern unique for that 

 species. Environmental influences may modify the normal cytoplasmic 

 currents and thus alter normal development. Later developments appear 

 to be influenced by chemical factors of the genes and cytoplasm and may 

 also be affected by environmental factors. However, unless these condi- 

 tions are extreme, development proceeds according to the normal pat- 

 tern for that species. 



It is thought that genes possess a high degree of autonomy (o -ton'- 

 omi) (Gr. autos, self; nemo, distribute) and that they consist of highly 

 specific nucleoproteins which may be constructed from the simple mole- ^ 

 cules of the nutrients of the cells. Many processes which occur in cells 

 are the result of the actions of various enzymes which in turn are thought 

 to be produced by gene action. It is believed that these enzymes are also 

 protein in composition. It is thought that genes reproduce themselves. 



