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 
im bones around the medullary canaliculi. In the stem of 
dicotyledonous plants the sap is conducted hetween 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 
