Oi\ THE IDENTITY OF STRUCTURE OF PLANTS AND ANIMALS 219 



The Chemical changes may consist in the conversion of the 

 cellulose into xylogen, &c., &c., or in the deposit of salts, silica, &c., 

 in the periplastic substance. Again, the periplastic substance around 

 each endoplast may remain of one chemical composition, or it may be 

 different in the outer part (so-called intercellular substance) from what 

 it is in the inner (so-called cell-wall). 



As to Morphological changes in the periplastic substance, they 

 consist either in the development of cavities in its substance — 

 vaaiolation (development of so-called intercellular passages) or in 

 fibrillation (spiral fibres, &c.). 



It is precisely the same in the Animal. 



The Endoplast may here become differentiated into a nucleus and 

 a primordial utricle (as sometimes in Cartilage) or more usually it 

 does not, — one or two small solid particles merely arising or existing 

 from the first, as the so-called " nudeoli ; " — it persists for a longer or 

 shorter time ; it divides and subdivides, but it never (except perhaps 

 in the case of the spermatozoa and the thread-cells of Medusse, &c.) 

 becomes metamorphosed into any tissue. 



The periplastic substance, on the other hand, undergoes quite 

 independent modifications. B)- chemical change or deposit it acquires 

 Horn, Collagen, Chondrin, Syntonin, Fats, Calcareous Salts, accord- 

 ing as it becomes Epithelium, Connective Tissue, Cartilage, Muscle, 

 Nerve or Bone, and in some cases the chemical change in the imme- 

 diate neighbourhood of the endoplast is different from that which has 

 taken place exteriorly, — so that the one portion becomes separable 

 from the other by chemical or mechanical means ; — whence, for 

 instance, has arisen the assumption of distinct walls for the bone- 

 lacuns and cartilage cavities ; of cell-contents and of intercellular 

 substance as distinct histological elements. 



The Morphological changes in the periplastic substance of the 

 animal, again, are of the same nature as in the plant : — Vacuolation 

 and Fibrillation (by which latter term is understood, not only the 

 actual breaking up of a tissue in definite lines, but the tendency to do 

 so) — Vacuolation of the periplastic substance is seen to its greatest 

 extent in the " Areolar " connective tissue ; — Fibrillation in tendons, 

 nbro-cartilages and muscles. 



In both Plants and Animals, then, there is one histological 

 element, the Endoplast, which does nothing but grow and vegetatively 

 repeat itself ; the other element, the periplastic substance, being the 

 subject of all the chemical and morphological metamorphoses, in 

 consequence of which specific Tissues arise. The differences between 

 the two kingdoms are, mainly : i. That in the Plant the Endoplast 



