HETEROMORPHOSIS. Not all individuals of a given species show all characters in normal form and number. Mon strosities occur which have more members or heads than usual, or fail in the development of some part (see MONSTER). A special class of abnormalities is characterized by the substitution of one kind of organ or region for another. This has been termed "Heteromorphosis." If the head-end of a living being is substi tuted by a tail-end, or vice-versa, we speak of "Polar Heteromor phosis"; substitution of a member by one of the kind normally found on another segment of the body is "Homoeosis." We know of three causes producing Heteromorphosis : First, when very plas tic forms such as hydroids are acted on by external forces their head- and tail-ends may be transposed. These animals are attached by rootlike stolons to the ground. A point of the body getting into close contact with the substratum may grow such a stolon. On the other hand, when detached the posterior end may produce hydroid heads. The operating factors may be light, oxygen or gravity, the latter when the stem is turned upside down. The sensitiveness towards these factors varies with different species. The second cause of heteromorphic structures is the process of regeneration under the condition that the lost part cannot be restored as such, whereas the faculty to grow its like has not been lost in the torso. The tail of a tadpole will not be able to regenerate the body, but has been known to grow forward into a second tail. If the head of a flatworm is removed it will grow a heteromorphic head from its cut surface. In this case the inability to produce body and tail is only temporary ; tail-ends will appear between the heads, each of these moving with all body regions restored. Thirdly, certain cases of heteromorphic appendages may be explained by a portion of cells getting detached in the embryo and again coalescing, but in a wrong place.