INDIVIDUATION AND REPRODUCTION 207 



of the chick is essentially similar. Fig. 73 is from a transverse 

 section of a very early stage in which cells from what will later 

 become the median dorsal region are separating from the outer 

 ectodermal layer to form the mesoderm. Somewhat later the 

 central nervous system arises by an infolding of the ectoderm, 

 beginning at the anterior end and proceeding posteriorly in this 

 same region. In Fig. 74, a more advanced stage, the embryonic 

 nervous system is already present in the form of the neural tube, 

 and it is evident that morphogenesis is proceeding both laterally 

 and ventrally from the median dorsal region. The developmental 

 gradient along the longitudinal axis is also indicated by Figs. 73 

 and 74, for both are from the same embryo, the latter from a more 

 anterior, the former from a more posterior, level of the body. The 

 more posterior level has only attained the stage of Fig. 73, while 

 the more anterior level has passed far beyond this stage. 



Particular parts and organs of the individual very often possess 

 an axis or axes of their own and without any uniform relation to the 

 axis of the body as a whole. Although but little attention has been 

 paid to this point, there are many facts which indicate that meta- 

 bolic gradients exist along these axes, at least in the earlier stages 

 of development. 



In many animals the chief axial gradient along the longitudinal 

 axis and often also the symmetry gradients persist throughout life 

 or disappear only in advanced stages of development. In fact, 

 as will appear below, the continued existence of the individual in 

 the lower organisms is dependent upon the persistence of the 

 gradients. In higher forms where agamic reproduction from pieces 

 of the body does not occur it is possible that in the adult the gradi- 

 ents may be altered or eliminated without altering the individuation 

 to any marked degree. 



The axial gradients arise in various ways which cannot be con- 

 sidered in detail here, but the different lines of evidence indicate that 

 in the final analysis they result from the differential action of factors 

 external to the protoplasm, cell, or cell mass concerned. We see 

 gradients arising in nature in this way, and it is possible to produce 

 them experimentally by these means. In many cases of the recon- 

 stitution of pieces into new individuals the stimulation of the 



