INTRODUCTION 3 



similar pattern to the development of all forms. The earlier in develop- 

 ment that the comparison is made, the greater the similarity among 

 even widespread species. This may mean that all other possible types 

 of development have failed to survive in a harshly selective environ- 

 ment. Or it may suggest a common ancestry. In any case, all embryos 

 do begin with the fertilized egg. (With the rare exceptions of natural 

 parthenogenesis.) The activated egg immediately manifests certain 

 metabolic changes which may be correlated with the kinetic move- 

 ments leading toward the completion of maturation or toward the first 

 cleavage. Division results in a shifting of the nucleo-cytoplasmic vol- 

 ume relations from the unbalanced condition of the ovum to the more 

 stable ratio found in the somatic cells. Each division of the egg means 

 more nuclear material, less cytoplasmic material, and more rigid cells. 

 This rigidity, the adhesiveness of cells, and their activity together 

 cause the development of sheets of cells which soon cover (chick) 

 or surround (frog) a cavity, known as the blastocoel. As this sheet 

 of cells expands it becomes necessary for it to fold under (chick), to 

 push inward (frog), or to split into layers (mammal), and thereby 

 form the 2-layered gastrula. In most forms almost immediately the 

 third layer (mesoderm) develops between the two preceding layers, 

 epiblast and endoderm. After the derivation of the mesoderm from 

 the outer layer, the latter is then known as the ectoderm. All of this 

 occurs before the appearance of any discernible organs. There is 

 reason to believe that these sheets of cells have topographical rather 

 than functional significance and that parts of any one of them could 

 be exchanged experimentally with any other at this time without 

 seriously disrupting the normal development of the embryo. 



At this point, however, the process of cell division becomes second- 

 ary to the process known as differentiation or specialization of cell 

 areas. This is the very beginning of organ formation. Many embryos 

 begin to develop transient (larval) organs which are replaced as the 

 more permanent organs appear and begin to function. Eventually 

 there is produced a highly complex but completely integrated organ- 

 ism which is able to ingest, digest, and assimilate food, and subsist 

 for itself independently of its parent organism. It is then no longer 

 considered an embryo. 



This period of ontogeny is the subject matter of embryology. We 

 attempt not to answer the major biological question "why" but 

 rather to confine ourselves to a detailed description as to "how" an 



