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; 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. 
