REJUVENESCENCE IN NATURE. 19 i 



nest, the middle the thickest ; both the inner layers are 

 colourless, the outermost is of a pale yellow tint. When 

 treated with dilute sulphuric and tincture of iodine, they 

 exhibit different behaviour. The external layer, which 

 I shall call the cuticle, neither swells up evidently, nor 

 becomes coloured; the inner two, on the contrary, acquire 

 a violet colour, more or less intense according to the 

 circumstances, and swell up, so that the membrane as a 

 whole appears twice or thrice as thick as at first. The 

 innermost layer here becomes thicker than the middle 

 one, which was previously the thickest, at the same time 

 becoming folded in waves on its surface, and allowing us 

 to make out clearly a composition out of two subordinate 

 layers. If strong sulphuric acid is applied, the circum- 

 ference of the entire cell is perceptibly contracted ; the 

 cuticle, however, does not share in this diminution of 

 size, but gradually separates, forming numerous wavy or 

 vesicular elevations, from the inner layers of the cell- 

 membrane, under which circumstances it can be readily 

 ascertained that it extends over the surfaces of articulation 

 of the cells, and therefore completely encloses the indi- 

 vidual cells, like the inner layers. A, long-continued 

 action of strong sulphuric acid dissolves the cell- wall, the 

 cuticle however appearing to withstand this longer. A 

 swelling-up of the cell-membrane, exactly similar to that 

 produced artificially by acids, takes place in the natural 

 course of life of the Water-net, at the period of 

 propagation. As soon as the germ-cells are formed in 

 the interior, the cell-membrane appears thickened, and 

 its internal surface somewhat wavy. While the spores 

 are commencing their originally very slight movements, 

 the cuticle tears. From this time those cells in which 

 large gonidia, destined to form new nets, have been 

 produced, are distinguished from those which contain 

 small gonidia destined to swarm out.* In the former, 

 namely, the entire cell-membrane .expands very perceptibly 

 and uniformly in the course of a few minutes, the cuticle, 



* Vide supra, p. 137. 



