Influence of Environment on Animals 217 



of the protoplasm to flow back to its normal position was 

 partially successful and led to a partial or complete separation 

 of the living from the dead half; whereby the former was 

 enabled to form a whole embryo, which, of course, possessed 

 only half the size of an embryo originating from a whole egg. 



b) Experiments on hydroids. — A striking influence of gravita- 

 tion can be observed in a hydroid, Antennularia antennina, 

 from the Bay of Naples. This hydroid consists of a long, 

 straight, main stem which grows vertically upward and 

 which has at regular intervals very fine and short bristle- 

 like lateral branches, on the upper side of which the polyps 

 grow. The main stem is negatively geotropic, i.e., its apex 

 continues to grow vertically upward when we put it obliquely 

 into the aquarium, while the roots grow vertically downward. 

 The writer observed that when the stem is put horizontally 

 into the water the short lateral branches on the lower side 

 give rise to an altogether different kind of organ, namely, to 

 roots, and these roots grow indefinitely in length and attach 

 themselves to solid bodies; while if the stem had remained in its 

 normal position no further growth would have occurred in the 

 lateral branches. From the upper side of the horizontal stem 

 new stems grow out, mostly directly from the original stem, 

 occasionally also from the short lateral branches. It is thus 

 possible to force upon this hydroid an arrangement of organs 

 which is altogether different from the hereditary arrangement. 

 The writer had called the change in the hereditary arrange- 

 ment of organs or the transformation of organs by external 

 forces heteromorphosis. We cannot now go any farther into 

 this subject, which should, however, prove of interest in rela- 

 tion to the problem of heredity. 



If it is correct to apply inferences drawn from the observa- 

 tion on the frog's egg to the behavior of Antennularia, one might 

 conclude that the cells of Antennularia also contain non-miscible 

 substances of different specific gravity, and that wherever 



