238 NATURAL SCIENCE [October 



members of a family were always sterile and different in character 

 and endowments from the intermediate, fertile children, but where 

 every fertile couple produced among its progeny some resembling 

 the parents, others with the endowments and characters of the 

 sterile uncles and aunts ; x we must, however, bear in mind that any 

 comparison of a strict cellular pedigree with the genealogical table 

 of the members of a Metazoan race is only an analogy. 



While the main features of reproduction in the Higher Animals 

 run on the same general lines as the Sponges, certain of them may 

 present differences ; and especially, as above noted, the relation of 

 the middle and the reproductive cells to those of the two original 

 germ layers respectively, varies in different groups. 



Propagation by budding! in the higher animals, and regener- 

 ation, or the repair of injuries, are essentially two different aspects 

 of the same phenomenon. In both cases the cells of one or more 

 tissues multiply rapidly, and revert more or less closely to the state 

 they possessed in the developing embryo. In some cases these 

 ' embryonic cells ' can only give rise to tissues like those they 

 respectively sprung from, or, at least, to tissues belonging to the 

 same layer ; but in the lowest Worms the middle- cells are capable 

 of thus forming other layers. In the Vertebrata the regenerative 

 functions are strictly limited ; thus, if the surface of the skin is 

 completely removed over an ulcer or burn, the new epidermis only 

 grows over by gradual extension of the living epidermis at the 

 edges, not by its direct growth upon the raw. This is the rationale 

 of the modern practice of ' skin grafts,' which implanted at intervals 

 over the surface of a healing wound give so many centres for the 

 new overgrowth of epidermis to start from, thus accelerating the 

 process of ' skinning over.' 



Most tissues of the Higher Animals retain sufficient ' vitality ' 

 to be able to enter at once on processes of regeneration of their own 

 individual kind in cases of wounds ; and in the Newts, for instance, 

 even a complete structure like a limb or an eye can be renewed 

 after amputation. The epiderm of Vertebrates retains in its 

 deepest layer an almost indefinite power of growth and reproduction, 

 the cells next the true skin forming a continuous stratum, each cell 

 of which is constantly growing and dividing, the upper cell at each 

 division becoming horny, to be ultimately cast off as other horny 

 cells are formed beneath it, while the lower retains the original 

 power of growth and division. This layer is absolutely comparable 

 to the layer of cells that forms cork in most green plants. The 

 periosteum or layer of cells overlying the bone has similar but less 

 active powers. 



1 The case we have suggested for comparison is actually found in social Insects with 

 their ' sterile castes ' in each generation. 



