654 INVERTEBKATA CHAP. 



maturing genital cells. Sooner or later we may fairly assume that some 

 of these hormones may become incorporated in the nuclear matter of 

 the genital cells ; and then, when these cells develop into embryos, the 

 hormones are set free at the corresponding period of development to 

 that at which they were originally formed. They reinforce the action 

 of the environment and cause it to produce greater effects ; they 

 may become free even before the stimulus of the environment reaches 

 them, and produce the appropriate structural change at an earlier 

 period of development. In this way we explain the tendency not only to 

 recapitulate, but to reflect back ancestral structures into progressively 

 earlier periods of development. 



We do not assume for a moment that this is a full and satisfactory 

 explanation of recapitulation, we regard it merely as a sketch of the 

 direction in which the explanation may be found, and as a call for 

 further investigation. 



But recapitulation of ancestral structure is by no means the only 

 factor in development ; and we must now inquire whether our studies 

 have led us to form an idea of what the other factors are. One has 

 been already alluded to, viz. the tendency for changes in structure to 

 make their appearance in successively earlier periods of growth, and 

 consequently for the larva to be reduced in size. That this reduction 

 in size entails changes in the larval organs has already been pointed 

 out. We have seen that serially repeated organs, like the legs of the 

 Nauplius, may be reduced to two or three pairs of those which are 

 functionally the most important ; that organs which should occur in 

 pairs, arranged in a bilaterally symmetrical manner, may be repre- 

 sented by one-sided structures, such as the eye of the Ascidian tadpole 

 and the " primary " gill-slits of Ampliioxus. 



Even here the researches of experimental embryologists may aid 

 us. Driesch (1900, 1910) has succeeded in inducing two or three 

 early larvae of Echinus to fuse into a single compound organism. 

 This compound larva naturally possessed three guts, but one only 

 grew large and became functional, the others dwindled, no doubt 

 owing to some inhibiting influence on their growth emitted by the 

 larger one. So we may imagine that larval organs which are 

 functionally necessary, and which increase, owing to their use, out of 

 all proportion to the proportion they should sustain to the organism 

 in which they find themselves, must inhibit the growth of their less 

 fortunate sisters. 



Again, we must remember that whilst the reaction to the final 

 environment has evoked and sustains the adult structure, the larva 

 has also its environment. Whilst in many, nay most cases, we have 

 reason to believe that the larval environment has the same general 

 characters as the environment of the ancestral stage which the larva 

 represents, yet in no case is it probable that the two environments 

 are exactly alike, and in some cases the larval environment has 

 become markedly different from what the ancestral one must have 

 been. 



