GENERAL CONSIDERATIONS ON ANIMAL FORM 3 



indication that the vertebrates arose from homonomously segmented forms. In the embryonic 

 development of vertebrates the change from a somewhat homonomous condition to an extreme 

 heteronomy can be directly followed. 



E. CEPHALIZATION 



In the evolution of animals there is a pronounced tendency for the anterior end of the 

 body to become more and more distinctly separated and differentiated from the rest of the 

 body as a head. This differentiation of the head consists chiefly of the localization within 

 the head of the main part of the nervous system i.e., the brain and of the most important 

 sense organs. Since the brain and the sense organs control, to a very large degree, the activi- 

 ties and responses of the rest of the body, the head thus becomes the dominant part of the 

 organism. This centralization or localization of nervous structures and functions in the head 

 with accompanying dominance of the head is called cephalization. Cephalization is more 

 and more marked the higher one ascends in the animal kingdom, and is particularly prominent 

 as a structural and functional feature of the vertebrates. 



In segmented animals the advance in cephalization is correlated with the progression 

 of the heteronomous condition. Heteronomy, in fact, appears first hi the head region and 

 gradually progresses posteriorly. The anterior end thus retains the least and the posterior 

 end the most resemblance to the original homonomous condition. This results in an illu- 

 sion of a retreat of certain systems toward the posterior regions of the body, whereas the 

 situation in reality arises from the fact that these systems have disappeared from the anterior 

 segments and are retained in the posterior segments. In the case of certain vertebrate organs, 

 as the heart, a real posterior descent occurs during the evolution of the vertebrates. In 

 the vertebrates, as in other heteronomously segmented animals, the head is produced through 

 the fusion of a certain number of the most anterior segments with a loss of some segments 

 or of parts of segments and the disappearance from these head segments of nearly all systems 

 except the nervous system. As cephalization progresses the head appropriates more and 

 more of the adjacent segments, incorporating them into its structure, so that in general it 

 may be said that the higher the degree of cephalization, the greater is the number of segments 

 composing the head. In advanced cephalization, such as is possessed by vertebrates, it is 

 very difficult, indeed, almost impossible, to decipher the number and boundaries of the segments 

 which originally went into the composition of the head; in fact, the problem of the segmenta- 

 tion of the vertebrate head has not been completely solved, although it has received the atten- 

 tion of the foremost vertebrate anatomists. 



The vertebrates are, then, animals characterized by the possession of bilateral symmetry, 

 internal and markedly heteronomous segmentation, and a high degree of cephalization. The 

 details of their structure are understandable only in the light of these three broad anatomical 

 conditions. 



F. HOMOLOGY AND ANALOGY 



Homologous structures are those which, however unlike in function or superficial appear- 

 ance, have the same origin, as demonstrated by the study of their embryonic origin and develop- 

 ment and their paleontological history. Thus the wing of birds, the flipper of the seals, and the 

 fore leg of the cat are homologous structures because they are all modifications of the original 

 fore limb and develop in the same way up to a certain point. The whole aim of comparative 

 anatomy is to discover what structures are homologous and to trace the modifications of such 

 homologous structures in the course of evolution. 



Analogous structures are those which resemble each other either as to superficial appear- 

 ance or in function but which have had different origins. Thus, both fish and snakes are 



