18 GENERAL CONCEPTIONS. 



tration of some general principle. To facilitate his reaching this result the 

 following pages are arranged so as to offer a digest of some of the more important 

 generalizations and fundamental laws of embryology. 



The Vertebrate Type of Structure. 



When one traces the course of development of any vertebrate, one finds, 

 speaking in general terms, that the fundamental characteristics, which are more 

 or less common to all vertebrates, are those which first appear. Later, there 

 come in the secondary characteristics, which distinguish one class from another, 

 and still later the subordinate characteristics by which the smaller subdivisions 

 of the vertebrate type become differentiated one from another. This state- 

 ment, however, is correct only if we add to it certain indispensable limitations. 

 Every embryo at every stage of its development is an individual of the particular 

 genus and species to which it belongs. It has at every stage peculiarities which 

 distinguish it from every other species. The embryos of allied forms resemble 

 one another more closely than do the embryos of forms which are only distantly 

 related to one another. The specific qualities of an embryo are, however, far 

 more difficult to recognize than those of the adult, and the student will be far 

 more impressed by the resemblances between embryos than by their differences. 

 It is owing to this very fact that the distinctive peculiarities of the species are not 

 accentuated in the embryo. We are able to derive from the embryos themselves 

 a series of conceptions which render it comparatively easy to perceive the domi- 

 nant morphological features of the vertebrate type. 



It will be convenient to put down six fundamental characteristics of the ver- 

 tebrate type as the most important, and to add to these six others which are 

 also fundamental, but perhaps less distinctive. This enumeration is necessarily 

 arbitrary, and can serve only to facilitate the work of the student. When his 

 knowledge deepens, he will be able to free himself from the limitations which 

 such a numerical classification has put on his understanding of the matter. 



A. The six most important characteristics are : 



1. The pharynx and pharyngeal structures (gill clefts, nerves, aortic 



arches, heart). 



2. The notochord or structural axis. 



3. Hollow nervous system. 



4. Limbs. 



5. Position of mouth. 



6. Division of the coelom into : 



(a) segmented part (mesomeres) ; 



(6) unsegmented part (splanchnocele), which is subdivided by 

 the septum transversum. 



