4 INTRODUCTION. 



a segmented character, especially in the axial portion of the body, and this 

 segmentation affects more or less, not merely the skeletal parts of its structure, 

 but also, to some extent, its other component organs. 



Fig. 2. Fig. 2. — Semidiagrammatic 



View of a Longitudikal 

 Section of the Embryo 

 repeesented in figure 1 ; 

 showing the relations 

 OF THE Principal Systems 

 AND Organs to each 

 other in the horizontal 

 Position of the Verte- 

 bral Axis. (A. T.) 



1, 2, 3, 4, 5, primary 

 divisions of the Lrain in the 

 cranial part of the nenral 

 canal ; n, n, sjnnal cord in 

 the vertebral part of the 

 same ; s, one of the spinous 

 processes of the vertebro3 

 (4th dorsal) ; cJi, chorda dorsalis running through the axis of the vertebral centra or 

 bodies ; ch', the same extending into the base of the cranium ; a, dorsal aorta ; j^, 

 phaiyngeal cavity ; /, i, alimentary canal ; /(, ventricular part of the heart, from wluch 

 the arterial bullj is seen joining the aorta by arches ; b, brancliial plates ; I, liver ; v\ 

 Wolffian boily ; r, urinary vesicle or aUantois, joining the intestine in the cloaca ; u, «', 

 umbilicus. 



Fig. 3. 



Fig. 4. 



-TEANS^'ERSE Section (diagrammatic) of the Trunk, of the Embryo 



THROUGH the ANTERIOR LiMBS. (A. T. ) 



in, spinal cord ; n, neural or dorsal arch, including bone, muscle, skin, roots of tlie 

 nerves, &c. ; cit, chorda dorsalis, surrounded by the vertebral bo(ly or centrum ; r, 

 ventral or visceral arch, or wall of the body ; p, p, pleuro-peritoneal cavity ; i, alimen- 

 tary canal ; h, heart ; I, I, the rudimentary lindjs. 



Fig. 4. — First Dorsal Vertebra ■with the First Rib and upper part of thi: 

 Sternum, seen from above. I 



C, body or centrum ; .Y, neural arch or vertebral ring ; V, cavity of the chest, visceral 



cavity. 



It is true that a segmented plan of construction is not restricted to vertebrate 

 animals, but exists in several other classes of the animal kingdom, as is most 

 conspicuously seen in the class Articulata. such as insects and crustacea. In 

 these animals^ however, there are many important deviations from the verte- 



