ORIENTATION 



upon a series of changes mediated by cellular groups known as organizers 

 which appear at the correct time and locus in development. 



It soon became apparent, however, that the terminology employed in ex- 

 perimental embryology was vague because it substituted indefinite terms such 

 as "inductors" or "organizers" as an explanation of developmental events. 

 The use of the word organizer means little unless one is able to describe the 

 manner of operation of the physical and chemical substances which effect 

 the results produced by the organizer. Consequently, embryologists with physi- 

 ological and biochemical training are concerned now with the effort of deter- 

 mining the specific chemical factors concerned with the various processes and 

 steps involved in development. This type of embryological study is called 

 biochemical or chemical embryology. Chemical embryology is divisible into 

 two main lines of attack, namely, an investigation of the chemistry of cells 

 and cellular parts or cytochemistry, and a study of the chemistry of groups 

 of cells or histochemistry. 



11. Free-living Versus Sheltered Embryological Forms; Periods 

 of Development 



The independence of a free-Uving existence on the part of developing young 

 is assumed at different stages of development depending upon the species 

 involved. For example, in the case of the frog, the developing embryo becomes 

 free-living at an early stage and it experiences a free-living larval existence 

 for an extended period before its metamorphosis into the adult or definitive 

 form of the frog. In the chick, the young undertakes a kind of free-living 

 existence at the time of hatching or about a week after it has assumed the 

 definitive body form. The human young, on the other hand, experiences an 

 extensive period of fetal development for about five months in utero after 

 it has achieved definitive body form. Moreover, it is most helpless and de- 

 pendent even after birth. 



Regardless of the time during its development when an animal species 

 assumes a free-living, independent existence, it is apparent that the develop- 

 ment of the individual as a whole may be divided into two general periods, 

 viz., embryonic and post-embryonic periods. The embryonic period of devel- 

 opment begins at fertilization of the egg and continues for a time after definitive 

 body form is achieved. The end of the embryonic period may be regarded as 

 the time of birth in viviparous forms, hatching in oviparous species, and the 

 end of metamorphosis in free-living larval species. This is an arbitrary and, 

 for some forms, quhte comprehensive definition. Nevertheless, for comparative 

 purposes this definition is suitable. The post-embryonic period begins at the 

 termination of the embryonic phase of development and continues through 

 sexual maturity into later life. 



