12 VERTEBRATE ZOOLOGY 



In the neck region there is very little differentiation of subordinate 

 elements belonging to the ventral ranges of the dorso-ventral axis; 

 muscles, glands, and skeletal elements are present, but no excretory or 

 reproductive organs differentiate in that region. In the thoracic 

 region, where the dominance of the anterior end of the dorsal struc- 

 tures is less intense, the organs of respiration, circulation, and locomo- 

 tion differentiate fully; but there are no digestive, excretory, or re- 

 productive specializations. Still further back in the upper abdominal 

 region the digestive functions reach their maximum importance and 

 the uro-genital functions begin to appear. And, finally, in the lower 

 abdominal region, the excretory and genital functions develop fully; 

 but the genital tissues do not fully differentiate till comparatively late 

 in the life cycle. 



The tail of the vertebrate appears to be a developmental after- 

 thought. It is the last part to develop and in many specialized ver- 

 tebrates scarcely develops at all, as though the developmental momen- 

 tum slowed down to such an extent that there was not enough force 

 left to push out the tail. When the tail does form, however, it is 

 usually, though not always, the most primitive part of the body. A 

 tailless condition is very common in highly specialized races with 

 prolonged developmental period. 



GENERALIZED, SPECIALIZED, SENESCENT, AND RETARDED 

 (P^DOGENETIC) TYPES OF VERTEBRATES 



A generalized vertebrate of any class is one in which the axial 

 relations are well in balance; the primary axis distinctly dominant 

 over the secondary and tertiary axes. Such animals as a dog-shark, 

 a salamander, a lizard, a shrew, are typical generalized vertebrates. 

 They have in common certain characteristics of which the following 

 are the most important: the body is rather elongated and cylindrical 

 in shape; with head, trunk, and tail in normal balance; the fore and 

 hind limbs of approximately equal value; and no pronounced special- 

 izations either externally or internally. Types such as these are 

 usually looked upon as prototypic of the groups to which they be- 

 long and therefore as affording a close approximation to the ancestral 

 stock from which the group in question has been derived. 



The history of vertebrate evolution has been closely associated 

 with a series of radical geographic and climatic changes, that have 

 had the effect of periodically eliminating large numbers of specialized 



