PROMORPHGLOG Y. 



163 



then reach the most usual form, bilateral symmetry. The 

 dissimilar ends of the sagittal axes are called "dorsal" 

 and "ventral," and further the terms "right" and "left" 

 are given to the ends of the transverse axis ; a bilaterally 

 symmetrical animal can be divided symmetrically into a 

 right and a left half by one plane, the median, passing in 

 the direction of the longitudinal sagittal axis; a frontal 

 section (a section through the longitudinal and transverse 

 axes) always gives dissimilar parts, dorsal and ventral sides. 



FIG. 86. Cross-section through a fish passing through the five limbs. Dl', sagittal axis ; KL, 

 transverse axis ; , aorta descendens; c, body-cavity ; d, gut ; ck l chorda ; g, shoulder-gir- 

 dle ; A, heart ; >, muscles ; n anterior end of the kidneys ; /, pericardium ; t>6, neural 

 arch ; n&, haemal arch ; r, spinal cord. 



Antimeres and Metameres. The symmetrical parts 

 of an animal are called antimercs ; each antimere has organs 

 which occur likewise in its adjacent antimere. The right 

 arm of man corresponds to the left, the right eye to the 

 left, etc. The same organs are repeated in the direction 

 of the transverse axis. Frequently however the repetition 

 of organs occurs not only in the direction of the transverse 

 axis, but also in the direction of the long axis. Thus the 



