576 



CONTINUI 



in the form of a theory, called the axial 

 gradient theory, which has been helpful in 

 explaining some abnormalities appearing 

 in various kinds of embryos, including man. 

 Supposing some toxic or foreign substance 

 were introduced in the human egg, at the 



TY OF LIFE 



scribed above, and the end result would be 

 the same. Other abnormalities in embryo- 

 logical development — of which there are 

 many — may similarly be explained. 



Study of the regeneration of parts in 

 adult animals is a natural outgrowth of 



S 



LL 



<W^^W> i»»<MM»<M»^^^* 



C 



i 



^W^MWMM»ii»^*iii I*! Ai*^ 



\owO2 ^ higloOg 



Fig 23-14. The first character that appears in embryonic development is polar.ty and this can be demon- 

 strated by performing some experiments on hydroids. Segments cut out of the stem w.M regenerate 

 tentacles and bases oriented in the same manner as in the parent hydroid. If a constriction 's made 

 in the middle of a segment, tentacles form on both ends. If one end is placed in a region of high 

 oxygen level, tentacles will form on that end regardless of its position in the parent hydroid. 



time of fertilization or shortly thereafter, 

 which was able to stimulate the formation 

 of two regions of high activity, that is to 

 say, two axes of polarity. The resulting em- 

 bryo would then be doubled in some sort 

 of twinning, partial or complete. This would 

 have the same effect as the ligaturing de- 



work on embryos. Many interesting ques- 

 tions raise themselves in the field of regen- 

 eration. For instance, why do limbs of sala- 

 manders regenerate completely, whereas 

 those of the closely related frogs do not? 

 Further, why have the tissues of higher ani- 

 mals lost most of the power of regeneration, 



