PRINCIPLES OF VERTEBRATE MORPHOLOGY 5 



any level of the antero-posterior axis into two mirror-image halves 

 (Fig. 2), so that each side is the reversed counterpart of the other. It 

 would appear then that the bilateral axis is a necessary consequence of 

 the dorso-ventral axis. The axis has a single median or apical point 

 and two lateral or basal points. Any vertical level in the dorso-ven- 

 tral axis may constitute the apical point of a double bilateral axis. 

 A good example of this type of axial organization is seen in connection 

 with the differentiation of the segmented mesoblast of the chick 

 (Fig. 3). The median dorsal part of the somite forms the myotome 

 (segmental voluntary muscles), the most highly dynamic of all meso- 

 dermal structures; next comes the derma tome which forms the deeper 

 skin and many complex sensory and glandular structures, etc.; next 

 comes the nephrotome or primordium of the excretory system; next, 



Coel. 



FIG. 3. Transverse section across the primary axis of a 10 somite chick em- 

 bryo, to illustrate the bilateral (tertiary) axis of vertebrate organization. Ao, 

 aorta; Coel, ccelom; ect, ectoderm; ent, entoderm; nch, notochord; N.F., neural 

 furrow; N. ph, nephrotome; s, somite; sow. Mes, somato-pleure; spl. Mes, splan- 

 chopleure. (From Lillie's " Development of the Chick " [Henry Holt and Co].) 



but only at posterior levels, the gonatome or primordium of the re- 

 productive system; next, the embryonic ccelom, from which are de- 

 rived mainly such passive structures as the peritoneal lining of the 

 body cavities and the mesenteries; and finally, the extra-embryonic 

 body cavity, which has to do primarily with the formation of embry- 

 onic membranes that serve as protective and respiratory organs dur- 

 ing the life of the embryo, but play no role in the organization of the 

 adult. 



It should be understood that the appendages are, as the name indi- 

 cates, truly added structures and are not to be considered as part of 

 the original bilateral gradient. Each appendage has its own three 

 gradients, with the free end representing the apical end and the fixed 

 end, the basal. It need hardly be said that the main specializations 

 of the appendages take place at the apical or free end, while the basal 

 parts remain comparatively conservative and undifferentiated. 



