410 Royal Institution. 



persists for a longer or shorter time, and may divide and subdivide, 

 but never becomes metamorphosed into any kind of tissue. 



The periplastic substance, on the other hand, midergoes meta- 

 morphoses quite independently of the endoplast (a point which has 

 been greatly overlooked, and which the lecturer illustrated by the 

 mode in which the peculiar cells of the Sphagnum leaf acquire their 

 thickening fibre offer the total disappearance of the primordial 

 utricle); these changes are, — 1. chemical; 2. morphological. The 

 chemical changes may be either the conversion of the cellulose into 

 xylogeu, &c., or the deposit of salts, silica, &c. Again, the periplastic 

 substance around each endoplast may remain of one chemical com- 

 position, or it may be different in the outer part (intercellular sub- 

 stance, woody matter) from what it is in the inner (cellulose wall). 



Then, as to the morphological changes in the periplastic substance, 

 they may consist in the development of cavities — vaciiolation (develop- 

 ment of intercellular passages), or in fibrillation (spiral fibres, &c.). 



It is precisely the same in the Animal. 



The endoplast may here develope a nucleus {e.g. cartilage cor- 

 puscle in some cases), or, as is more usual, it does not ; it persists 

 for a longer or shorter time ; it divides and subdiiides, but it never 

 becomes metamorphosed into any tissue. 



The periplastic substance, on the other hand, undergoes quite 

 independent modifications. By chemical change or deposit it acquires 

 horn, collagen, choudrin, syntonin, fats, calcareous salts, according 

 as it becomes epithelium, connective tissue, cartilage, muscle, nerve 

 or bone ; and in some cases the chemical change in the immediate 

 neighbourhood of the endoplast is different from that exteriorly, 

 whence the assumption of distinct walls to the cartilage and bone 

 corpuscles; of " cell-contents " and of " intercellular substance," as 

 distinct histological elements. 



The morphological changes in the periplastic substance in 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 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, 

 Fibro-cartilage and Muscle. 



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

 the Endoplast, which does nothing but vegetatively repeat itself : the 

 other element, the Periplastic substance, is the subject of all the 

 chemical and morphological metamorphoses in consequence of which 

 specific tissues arise. The differences between them are mainly, — 

 1. That in the Plant the endoplast grows and, as the primordial 

 utricle, attains a large comparative size ; while in the Animal the 

 endoplast remains small, the principal bulk being fonned by the 

 periplastic substance ; and 2. in the nature of the chemical changes 

 in the periplastic substance in each case. This does not, however, 

 always hold good, the Ascidians furnishing examples of animals whose 

 periplastic substance contains cellulose. 



In conclusion, the lecturer endeavoured to point out that the value 



