260 TJniversitu of California ruhlications. [Zoology 



conditions obtainino' during embryonic life. The adult stem 

 changes its polarity readily when subjected to certain experi- 

 mental conditions. Since the embryo and larva are morpho- 

 logically far less specialized, they are presumably more plastic 

 and capable of more facile transformations.^ 



The oral end of the polyp is determined by both internal and 

 external factors. As has been indicated in a previous account 

 of the locomotion of the larva (:04, p. 419), the latter early 

 exhibits a marked negative geotropism combined with a negative 

 thigmotropism. The simple expedient of rearing eggs on a plate 

 inclined at an angle with' the horizontal, was employed 

 without positive results in the early stages. Yet it ma.y be 

 said that geotropism is exhibited by the time the ten- 

 tacles begin to arise. As to thigmotropism, there is no doubt 

 that the region of the egg which tirst emerges from the 

 egg capsule is not in contact with the substratum, and never 

 becomes attached to it in the future. In this respect, it behaves 

 like the buds which develop on the free upper surfaces of the 

 stolons in campanularian hydroids. Gravity does not appear to 

 determine the point of emergence, because this point is only 

 occasionally uppermost when the egg is resting on a horizontal 

 surface (cf. Fig. 7). The processes which develop in contact 

 with the substratum never develop into hj^dranths. It would 

 appear, then, that the fate of any extension of the subspherical 

 intracapsular embryo is fixed in part at least by its relation to 

 the substratum. The emergence of any part is conditioned by 

 its ability to dissolve the wall of the capsule. That in turn is a 

 function of the ectoderm in all apical growing regions on stems 

 or stolons. External factors do not seem to account for an 

 accelerated growth of the embryo at any given point. That must 

 then be referred to internal conditions of life which, from all 

 obtainable evidence, are not constant in all individuals and do 

 not suggest fixed polar relations in the egg or embryo previous 



^ In this connection, attention may be called to Perkins' (:02) de- 

 scription of the formation of buds on the larva of Gonionema murbachi. 

 The distal end of the bud fastens to the substratum, instead of develop- 

 ing free, as in Hydra, Cwnina, and many hybrids, and becomes the aboral 

 instead of the oral end of the polyp. There appears to be more than 

 an accidental correlation here between contact and differentiation. 



