VERTEBRATE ZOOLOGY 



structural differentiation that are largely independent of the pnma.ry 

 axis and have Uttle reference to the polarity of the body as a whole. 

 In the embryo, however, the axis is less distorted than in the adult. 

 The Dorso-Ventral Axis (Secondary Axis).— If we take a cross 

 section at any level of the primary axis, we at once perceive that a 



line drawn from the 



mid-dorsal to the 

 mid-ventral line rep- 

 resents another main 

 architectural axis. In 

 the diagram (Fig. 2) 

 it will be noted that 

 the central nervous 

 system occupies the 

 high point or apical 

 end of the axis and 

 that, at this level, 

 one loop of the intes- 

 tine occupies the ven- 

 tral or basal point of 

 the axis. The grad- 

 ient of functions in 

 between the two poles 

 has been so decidedly 

 disturbed by lateral 

 foldings and second- 

 ary displacements, 

 that any attempt to 

 construct a list of 

 graded functions 

 Fig. 2. — Diagrammatic transverse section of a verte- would be futile. We 

 brate to illustrate the dorso-ventral (secondary) and tjjow in eeneral how- 

 bilateral (tertiary) axes of organization, al, alimentary j.u i u j 



tract; ao, aorta; d. TO, dorsal mesentery; mj/, myotome; 6Ver, that the dorsal 

 nc, notochord; n, nephrotome; o, omentum; sc, spinal side is the dominant 

 chord; vm, ventral mesentery. (Modified after ^^ ^f ^^e axis and 

 Kmgsley.) f-u a j • i, 



that, durmg embry- 

 onic development, the various functions differentiate almost exactly 

 in the order of their relative dynamic activity. 



The Bilateral Axis (Tertiary Axis). — The dorso-ventral axis divides 



