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 j 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 



