xxviii THE VOYAGE OF H.M.S. CHALLENGER. 



and teleoses are developed from the simplest beginnings, step by step before our 

 eyes. Whilst at definite parts of the body (that is in many higher and larger Medusas) all 

 the four principal forms of the animal tissues are already secreted as independent layers, 

 they appear in other parts of the body (that is, in many smaller and lower Medusa?) still 

 in a dependent form, as mere appendages of a single fundamental tissue, the epithelium. 

 The most different degrees of formation of tissue are represented beside each other in 

 genetic connection, within this long series of perfection and differentiation, so that the 

 most important forms of the higher tissues are to be found here " in statu nascenti." In 

 this respect the Medusas furnish an excellent argument in favour of the tenet, recently 

 brought forward in the gastrasa theory, that there is only one primary tissue, the 

 epithelial tissue, and that all other forms of tissue have arisen secondarily from it. 

 The simplest and phylogenetically oldest form of this primary tissue is the blastoderm 

 of the "blastula," this simple single-layered epithelium, which alone forms the wall of 

 this hollow sphere in the germ of all Medusas, in the same way as it does in the germ 

 of all other groups of Metazoa. When the two-layered gastrula is formed by in- 

 vagination of the blastula, the blastoderm (or the simple " primitive germinal layer ") 

 of the former is divided into the " primary germinal layers " of the latter, which 

 are likewise simple epithelia. All other formations of tissue (connective, muscular, 

 and nervous tissues) have arisen, both ontogenetically and phylogenetically, from 

 these two epithelia. 



§ 45. Epithelia] tissue (" tela epithelialis "). The protective tissue or epithelium, which 

 in the gastrula of the Medusas, as of all Metazoa, is formed first of all by the simple 

 tissue of the multicellular germ, in the mature and developed Medusa, covers firstly, as 

 outer covering (" ectoderm "), the whole upper surface of the body; and secondly, as inner 

 covering (" endoderm "), the whole inner surface of the gastrovascular system. These 

 coverings are everywhere separated from one another by secondary formations of tissues, 

 secreted between them, and only pass uninterruptedly into one another at the oral mar- 

 gin. This oral margin (am) is identical with the " primitive oral margin " of the gastrula 

 or the '" invagination opening" of the invaginated blastula. The inner covering ("epi- 

 thelium endodermale ") shows far simpler and more uniform conditions of formation in 

 both coverings. However, most of the differentiations recur in it, which appear more 

 expressed and varied in the outer covering (" epithelium ectodermale "), corresponding to 

 its manifold adaptations and relations to the outer world. 



§ 46. Outer covering (" epithelium ectodermale " or " chrotale ") . The outer cover- 

 ing or chrotal epithelium (which may also be termed " ectoderma " in the more restricted 

 sense) in all Medusas covers the entire outer upper surface of the umbrella as a connected 

 dermal covering, and only passes into the endodermal epithelium at the umbrella margin 

 (in some Medusas at the excretory papillae of the umbrella). Corresponding to the form 

 of the concave-convex umbrella, we distinguish two different principal parts of its 



