2O6 



SENESCENCE AND REJUVENESCENCE 



the axes of symmetry differ in different groups, and it is impossible 

 to consider the various details here. 



In the vertebrates the developmental gradients of the longi- 

 tudinal and transverse axes like those of most bilaterally symmet- 

 rical invertebrates, show a decrease in rate from the anterior region 

 posteriorly and from the median region laterally, but the gradient 

 along the dorso-ventral axis is the reverse of that in the inverte- 

 brates, the dorsal region preceding instead of the ventral. Fig. 71 



ns 



nc 



ac 



12 



FIGS. 71, 72. Axial developmental gradients in the fish embryo: in Fig. 71 the 

 embryo consists chiefly of the median dorsal region, in which the nervous system, ns, is 

 developing; in Fig. 72 development has proceeded laterally and ventrally, the somites 

 s, the notochord nc, and the alimentary canal ac being present. From H. V. Wil- 

 son, '89. 



represents a transverse section of a fish embryo at an early stage of 

 development. At this stage the embryo consists chiefly of the 

 embryonic nervous system (ns), the other parts being represented 

 by only a few cells. Ventral to the embryo is a very large mass of 

 yolk, not shown in the figure. Here the median dorsal region pre- 

 cedes lateral and ventral regions in morphogenesis. Fig. 72 shows 

 a later stage in which morphogenesis has advanced both laterally 

 and ventrally from the median dorsal region. The development 



