REMAK. 63 



the Science of the Evolution of Tissues, as fii^st elaborated 

 by Remak and by KoUiker in this comprehensive sense. 

 By thus proving more definitely the part which the germ- 

 layers take in the formation of the various tissues and 

 systems of organs, and applying the Theory of Epigenesis 

 to the cells and the tissues formed from them, Remak raised 

 the Germ-layer Theory, at least as regards Vertebrates, to 

 that degree of perfection in which we shall find it hereafter 

 when we examine it in detail. According to him, the two 

 germ-layers, of which the so-called germinal disc, the first 

 simple leaf-shaped formation of the body of a Vertebrate, 

 is composed, are soon increased by another layer, produced 

 by the lower layer separating into two. These three have 

 entirely distinct relations to the various tissues. First, 

 from the upper layer proceed those cells which compose the 

 outer skin (epidermis) of the body, together with the parts 

 belonging and necessary to it (hair, nails, and the like) — 

 that is, the external covering which envelops the whole 

 body; and, remarkable as it is, it produces also the cells 

 which constitute the central nervous system, — the brain and 

 spinal marrow. Secondly, from the lower germ-layer spring 

 the cells which form the intestinal epithelium, — that is the 

 whole inner coating of the intestinal canal and its append- 

 ages (liver, lungs, salivary glands, and the like) ; in other 

 words, the tissues which take up the food of the animal body 

 and attend to its digestion. Finally, from the middle layer, 

 lying between these two, arise all .the other tissues of the 

 body of the Vertebrate; flesh and blood, bones and liga- 

 ments, and the like. Remak also proved that the middle 

 layer, which he calls the " motor-germinative " layer, again 

 separates, secondarily, into two layers. In this way we get 



