THE VERTEBRATE TYPE OF STRUCTURE. 19 



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). 



1 1. 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 : 



1 . 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 

 therefore form pairs. They are designated by numbers, the pouch which lies 

 nearest to the mouth being called the first, the next the second, and so on. In 

 many of the lower vertebrates the number of these gill pouches varies from five 

 to perhaps nine. In mammals there are always four pairs on each side. In 

 aquatic vertebrates the pouches acquire each an opening to the exterior at the 

 side of the neck, and are then designated as gill clefts or branchial clefts. We 

 find that the position of the clefts determines the distribution of a series of the 

 most important of the cephalic nerves and the primitive distribution of the 

 branches of the aorta and of certain important muscles, hence the morphological 

 features of the pharynx have a profound influence upon the entire anatomy of 

 the body in that region. No similar pouches are formed from any other part of 

 the digestive canal. 



2 . The notochord is a rod of cells which extends nearly the entire length of 

 the embryo. It lies in the median plane, a little below the ventral edge of the 

 central nervous system. Its cephalic termination is always in the neighborhood 

 of the pituitary body. It may be considered the primitive structural axis of the 

 vertebrates. There are vertebrates in which it is the only structural axis ever pro- 

 duced, but in the great majority of vertebrates there is developed around the 

 notochord a series of skeletal elements which we know as vertebrae, and which 

 make a new structural axis in these forms. The notochord in these animals is 

 found to run through the bodies of the vertebrae. The notochord diminishes in 

 size as we ascend the vertebrate series. It is of very considerable diameter in 

 the lowest fishes, smaller in amphibia and reptiles, and smallest of all in mam- 

 mals. Its duration through the life-history of the individual also diminishes as 



