GENERAL CONCEPTIONS. 



fore 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. Among 

 the lower vertebrates the number of these gill-pouches varies from five to 

 perhaps nine pairs. In mammals there are always four distinct pairs. 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 di- 

 gestive canal. The pharynx also gives rise to the thyroid gland, the anlage of 

 which starts as an outgrowth from the median ventral side of the pharynx. The 

 entoderm of the third pair of gill-pouches produces the anlages of thymus glands, 

 and that of the fourth pair the anlages of the parathyroids. 



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. In the lower forms it persists throughout life as a continuous rod. In 

 the higher forms it tends to become attenuated in the vertebral, expanded in the 

 intervertebral, regions, and in adult mammals persists only as a /series of dis- 

 connected thickenings (nuclei pulposi} between the vertebras. 



3. The tubular central nervous system. This is found in vertebrates only, or 

 in animals which are closely related to vertebrates, so closely that b.y many 

 naturalists they are included in the same subkingdom. The hollow nervous system 

 is enlarged in the region of the head, the enlargement constituting the brain. 

 The rest of it is of smaller size and constitutes the spinal cord. That the brain 

 and spinal cord form the wall of a tube is one . of the fundamental conceptions 

 of anatomy. 



4- The limbs. There are two pairs, which are lateral extensions of the sur- 

 face of the body and acquire in their interior a skeleton by which they are 

 supported and muscles by which they are moved. No structures in any invertebrate 

 animal are known to be homologous with vertebrate limbs. 



5. The position of the mouth. The typical invertebrate mouth is surrounded 



