238 Thomas H. Huxley, Esq., on the 



case of the spermatozoa and the thread-cells of Medusa?, &c.) 

 hecomes metamorphosed into any tissue. 



The periplastic substance, on the other hand, undergoes 

 quite independent modifications. By chemical change or de- 

 posit it acquires horn, collagen, chondrin, syntonin, fats, 

 calcareous salts, according as it becomes epithelium, con- 

 nective tissue, cartilage, muscle, nerve, or bone ; and in 

 some cases the chemical change in the immediate neighbour- 

 hood 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-lacunae 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 under- 

 stood, 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 

 11 areolar" connective tissue ; Fibrillation in tendons, fibro- 

 cartilages, and muscles. 



In both plants and animals, then, there is one histological 

 element, the endoplast, which does nothing but grow and vege- 

 tatively repeat itself ; the other element, the periplastic sub- 

 stance, being the subject of all the chemical and morphologi- 

 cal metamorphoses, in consequence of which specific tissues 

 arise. The differences between the two kingdoms are main- 

 ly, 1. That in the plant the endoplast grows, and as the pri- 

 mordial utricle, attains a large comparative size — while in 

 the animal the endoplast remains small, the principal bulk 

 of its tissues being formed by the periplastic substance ; and 

 2 ; in the nature of the chemical changes which take place 

 in the periplastic substance in each case. This distinction, 

 however, does not always hold good, the Ascidians furnishing 

 examples of animals whose periplastic substance contains 

 cellulose. 



"The plant, then, is an animal confined in a wooden case ; 

 and Nature, like Sycorax, holds thousands of ' delicate Ariels' 



