VIU PRESIDKNTIAL ADDRKHS 



ments, which extended the theory to the minute structure of 

 animals, and enunciated the comprehensive generalisation that 

 animals resemble plants in being composed of cells. If we 

 examine under a high power a little human blood, we find 

 examples of two forms of cells floating in a colourless liquid — 

 the red corpuscles, which impart to the blood its red colour, and 

 the white ones. The examination of a section of spinal chord, 

 or of cartilage or bone, shows that animals, not less than plants, 

 are aggregates of cells and the products or derivatives of cells. 

 In animals, however, the amount of formed material, as it is 

 called, derived from the cells bears a much larger proportion to 

 the whole, and the cellular structure is consequently frequently 

 obscured. Again, as we descend in the scale of animal life, we 

 meet with organisms which are little more than a double layer 

 of cells, as in the hydra, a little creature frequently met with in 

 fresh water ponds, the outer layer acting as a sensory system 

 and the inner as a digestive apparatus. And in such lowly 

 forms as the sunanimalcule, a minute-rayed animalcule often 

 found in standing water, we reach at length the unit or single 

 cell existing as a separate individual. The correspondence 

 therefore between animals and plants, in either consisting of 

 one of these elementary units, or being built up of a number of 

 them, and their derived material and secretions, is evident. 

 Schleiden and Schwann, however, believed that the cell was in 

 all cases, as the name implies, a cellula or enclosed vesicle ; and 

 although they did not overlook the cell contents, they thought 

 that the cell wall was the essentially vital portion of the celU 

 and that the interior parts were merely subsidiary. Schwann 

 seems to have been led to this belief by observing that the cell 

 wall was the most persistent part, and that there were many 

 cells in which the contents had dwindled away, while the cell 

 wall remained. But further research has demonstrated that 

 these conceptions are erroneous. In the first place, it was 

 discovered that many of these elementary units, both in the 

 constitution of complex organisms and in those lowly forms 

 which each consist of but one unit, have no cell wall whatever.. 

 In the sunanimalcule, for example, which is but a single cell, 

 no cell wall exists : the most that can be detected is a slightly 

 greater density at the surface and a slightly increased fluidity 

 within. The same is the case with the amieba — a microscopic 

 jelly-like speck, found frequently in stagnant water. The outer 

 portion of the cell is, perhaps, slightly denser than the inner ; 

 but the difference can scarcely be detected, and the animalcule 



