SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 225 



through a stage comparable in this respect to adult fish. There are 

 many examples of such comparisons among animal groups. It is 

 not likely that any animal in its development repeats every stage 

 in its racial development or phylogeny, because many are fused and 

 new ones are introduced. In general, however, the statement, ''On- 

 togeny recapitulates phylogeny/' is recognized as having some appli- 

 cation. Ontogeny refers to the embryonic development of the in- 

 dividual. 



Homology. — The principle of homology is another concept which 

 is employed in studies of development as well as in taxonomy and 

 anatomy. Structures are said to be homologous when they have simi- 

 lar morphological nature and similar embryonic origin. For example, 

 a pectoral fin of a fish, the wing of a bird, and the arm of the human 

 are homologous. The eye of the fish and that of man are homologous, 

 but the eye of a crayfish is analogous rather than homologous to these. 

 Its function is similar but its structure and embryonic origin are 

 different. 



Theory of Metabolic Gradients (Axial Gradients). — In connection 

 with the orientation and polarity of developing embryos, there is 

 definite and normal order in the physiological phenomena involved. 

 Dr. C. M. Child has related this to regions along the principal axis 

 in forms which possess one. He holds that there is a relative meta- 

 bolic dominance of certain regions of such a body over succeeding 

 regions with a center or centers of high rate of metabolism and a 

 gradation to lower rates as progressing away from the center. The 

 animal pole of the embryo is such a center; this area sets the pace 

 in the development. In vertebrates the head structures develop 

 most rapidly and tend to dominate the developmental activity pos- 

 teriorly along the axis of the body. The posterior portions are 

 slower to develop. According to this theory, the organization and 

 orientation of developing parts are determined by the interaction of 

 chemical substances of cellular origin, with the rate of metabolism 

 as the most important factor in infiuencing the sequence of appear- 

 ance of parts. 



References 



Arey, Leslie B.: Developmental Anatomy, Philadelphia, 1934, W. B. Saunders 



Company. 

 Richards, A.: Comparative Embryology, New York, 1931, John Wiley & Sons, 



Inc. 

 Wieman, H. L. : An Introduction to Vertebrate Embryology, New York, 1930, 



McGraw-Hill Book Company. 



