220 Cc. M. CHILD 
reproduction in the same individual, the asexual reproduction 
occurs earlier in the life cycle than the sexual, and in organisms 
which produce naturally both parthenogenetic and non-partheno- 
genetic eggs, the parthenogenetic eggs appear earlier than the 
non-parthenogenetic. In both these cases the earlier product is 
usually capable of a greater degree of regulation when it is iso- 
lated from the pareat body, than the later; in other words, both 
the non-parthenogenetic egg and the sperm appear from their 
behavior to be more highly specified or differentiated than the 
asexual or parthenogenetic reproductive elements. There is then 
considerable evidence in support of the view that the history of 
the germ cells, like that of other organs, is in part the result of 
physiological correlation. 
As regards the ‘stimulus to development,’ I have shown by 
experiment (Child, ’11b) that in Planaria the procass of reconstitu- 
tution after physical isolation as well as extreme starvation fol- 
lowed by feeding, accomplish rejuvenation and that it is highly 
probable that various other factors bring about similar changes. 
And finally, I have considered the facts which indicate that the 
process of fertilization and the conditions inducing artificial par- 
thenogenesis produce changes in the egg similar in character to the 
rejuvenation occurring in the other cases. 
From this point of view, the stimulus to development of the 
egg is essentially a process or the beginning of a process of recon- 
stitution and so is similar in its physiological effect to the factors 
initiating the various processes of asexual reproduction. Experi- 
mental reconstitution following section is then merely a special 
case of reproduction occurring under certain conditions, or we 
may say just as correctly that each form of reproduction in nature 
or experiment is a special case of reconstitution occurring under 
certain special conditions. 
If this view be correct, then the fundamental problems of devel- 
opment and heredity are before us in every case where a physically 
or physiologically isolated part of an organism produces a new 
organism, just as truly as they are in sexual reproduction. In fact 
sexual reproduction constitutes the most complex case of all, but 
I am convinced that a recognition of its essential similarity tothe 
