124 Animal Life and Intelligence. 



suppose, as we seem to have every right to suppose, that 

 it is the nucleus that controls the formative processes in 

 the cell, there is not much difficulty in understanding how, 

 when the nucleus divides into two similar portions, each 

 directs, so to speak, the similar refashioning of its own 

 separated protoplasmic territory. 



Frpm the protozoa we may pass to such a comparatively 

 simple metazoon as the hydra. Here the organism is com- 

 posed, not of a single cell, but of a number of cells. These 

 cells are, moreover, not all alike, but have undergone 

 differentiation with physiological division of labour. There 

 is an inner layer of large nutritive cells, and an outer layer 

 of protective cells, some of which are conical with fine pro- 

 cesses proceeding from the point of the cone ; others are 

 smaller, and fill in the interstices between the apices of the 

 cones, while others have developed into thread-cells, each 

 with a fine stinging filament. Between the two layers 

 there is a thin supporting lamella. The essential point we 

 have here to notice is that there are two distinct layers 

 with cells of different form and function. 



Now, it has again and again been experimentally proved 

 that if a hydra be divided into a number of fragments, each 

 will grow up into a complete and perfect hydra. All that 

 is essential is that, in the separated fragment, there shall 

 be samples of the cells of both layers. Under these con- 

 ditions, the separated cells of the outer layer regenerate a 

 complete external wall, and the separated cells of the inner 

 layer similarly regenerate a complete internal lining. From 

 these facts, it would appear that such a small adequately 

 sampled fragment has the power, when isolated, of 

 assimilating nutriment and growing by the multiplication 

 of the constituent cells, and that the growth takes such 

 lines that the original form of the hydra is reproduced. 



Here we may note, by way of analogy, what takes place 

 in the case of inorganic crystals. If a fragment of an 

 alum crystal be suspended in a strong solution of alum, 

 the crystal will be recompleted by the growth of new parts 

 along the broken edges. We say that this is effected under 



