2 GENERAL CONCEPTIONS. 



The Segmented Animals. Metamerism. 



All vertebrates and certain invertebrates haye_-their bodies divided into a 

 series of parts, which begins in the region of the head and extends to the caudal 

 end of the body. These parts are called segments or metameres. In man each 

 vertebra corresponds to one segment. In the earthworm and other annelids 

 the segments are plainly marked on the outer surface of the body. Since all 

 segmented animals are bilaterally symmetrical, each segment is bilaterally sym- 

 metrical and may therefore be described as a paired structure. Embryology 

 has demonstrated that each segment arises as a pair of masses (somites] situated 

 between the digestive canal and the outer surface of the body. Each pair of 

 masses is formed from the middle germ-layer (mesoderm) exclusively (compare pages 

 84-85) and is known as "a primitive segment." The mesodermic somites are 

 to be considered the essential primary morphological segments, and in the course 

 of the development of the individual they produce adult metameric structures, 

 among which may be muscles, skeletal elements, excretory organs, etc., the whole 

 history of which depends upon their segmental origin. The nervous system and 

 to a considerable extent the blood-vessels exhibit a segmental arrangement, which 

 is usually regarded as a secondary correlation of these structures with the primary 

 mesodermic metamerism. The spinal nerves exemplify the correlation. In brief, 

 where the segmental organization exists, it dominates the anatomy alike of the embryo 

 and the adult; therefore the student of embryology should pay special attention 

 to the segmentation of the body in all its chief stages. 



In regard to the bodily segmentation two general observations may be made : 

 First, the development^ of segments begins at the- cephalic end and progresses 

 tailward; hence so long as the development of segments continues various stages 

 of their differentiation may be found in a single embryo, the more advanced 

 stages being always cephalad from the less advanced. Second, there is a funda- 

 mental difference between the metamerism of vertebrates and that of annelids 

 and many other invertebrates, which consists in the unlike extent of the segments; 

 for the primitive segments of vertebrates are confined to the dorsal region of the 

 body, while in the other forms the segmentation extends from the start through the 

 dorsal and ventral regions both. It is probable that the segments of vertebrates 

 are homologous only with the dorsal part of the segments of annelids. 



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 statement, 

 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 



