THE VERTEBRATE TYPE OF STRUCTURE. 3 



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

 tinguish 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, 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 may have put on his understanding of the matter. 



A. The six most important characteristics are: 



/ i. The pharynx and pharyngeal structures (gill-clefts, nerves, aortic arches, 



heart). 

 ./ 2. The notochord or structural axis. 



3. Tubular central nervous system. 



4. Limbs. 



5. Position of mouth. 



6. Division of the ccelom into: 



(a) dorsal segmented part or cavities of the somites. 



(b) ventral unsegmented part (splanchnocele), which is subdivided by 

 the septum transversum into a thoracic and an abdominal portion. 



B. Other fundamental but less distinctive characteristics are: 



7. Stomach, intestine, and mesentery. 



8. Position of liver, and its relation to veins. 



9. Wolffian tubules and ovotestis ( = urogenital ridge). 



10. Urogenital ducts (Wolffian and Miillerian). 



11. Special sense-organs (nose, eye, and ear). 



12. Hypophysis. 



The pig embryo illustrates all these characteristics, and we shall study the 

 ways in which the typical mammalian modifications of the type are gradually 

 evolved. 



Let us now pass in review these twelve characteristics. 



i. The pharynx is the cephalic portion of the digestive canal, and it acquires 

 in all vertebrates a somewhat complicated structure. This complication de- 

 pends primarily upon a series of lateral outgrowths from the pharynx which are 

 known by the name of gill-pouches. They are symmetrically arranged and there- 



