SURVEY OF CELL-LIFE. 171
surface and partly around these medullary canals. Now, the structure of bone becomes most simple, if we assume that, in consequence of the firmness of the osseous substance, this process goes on in layers, which do not completely coalesce together. It must consist of a double system of layers, one being concentric to each of the medullary canals, and the other to the external surface of the bone. When the bone is hollow, the layers must also be concentric to the cavity; and when small medullary cavities exist in the place of canals, as in the spongy bones, the layers must also be concentric to them. The difference in the growth of animals and plants also rests upon the same law. In plants, the nutritive fluid is not so equably distributed throughout the entire tissues, as it is in the organized tissues of animals, but is conveyed in isolated fasciculi of vessels, widely separated from one another, more after the manner of bone. These fasciculi of vessels are also observed to be surrounded with small (most likely younger) cells, so that, in all probability, the formation of their new cells also takes place around these vessels, as it does in bones around the medullary canaliculi. In the stem of dicotyledonous plants the sap is conducted between the bark and the wood, and on that account the new cells are generated in strata concentric to the layers of the previous year. The variety in the mode of growth, as to whether the new cells are developed merely in separate situations in the tissue, or equally throughout its whole thickness, does not, therefore, constitute any primary distinction, but is the consequence of a difference in the mode in which their nutritive fluid is conveyed.
The generation of cells of a different character, such as fat- cells, in the interior of a non-vascular tissue (in cartilage which does not as yet contain vessels, for example), appears at first sight to form an exception to the law just laid down. But such is not really the case; the circumstance is capable of two explanations, either the cytoblastema for this kind of cells is furnished by the true cells of the tissue only when they have attained a certain stage of their development, or, the cytoblastema which penetrates into the depth of the tissue contains the nutritive material for the true cells of the tissue in a less concentrated state, whilst it is still sufficiently
