REPOET ON THE DEEP-SEA MEDUSAE. xxv 



II. GENERAL HISTOLOGY OF THE MEDUSAE. 



§ 39. Primitive germinal layer (" blastoderma "). In all Medusae as in all other 

 Metazoa, the aggregate cells of the developed body are descendants of the homogeneous, 

 indifferent " segmentation cells," which are formed by repeated division of the fecundated 

 egg cells, and which compose, first of all, the solid multicellular "mulberry germ" 

 ("morula"). As fluid gathers inside this solid spheroidal accumulation of cells, and its 

 homogeneous cells appear on the upper surface, this important hollow sphere, the 

 "germinal vesicle" or "vesicular germ" ("blastosphera" or "blastula"), is originated, 

 whose wall is composed of a single, simple layer of cells. This simple cellular membrane 

 itself is the germinal membrane (" blastoderma") or the " primitive germinal layer." As 

 the hollow sphere then forms a depression at one point of its upper surface, and this 

 depression always deepens, the germinal membrane becomes invaginated, and thus 

 differentiates into the two primary germinal layers composing the " gastrula." As the 

 formation of the gastrula by invagination of the blastula in the Medusas has been observed 

 in very different groups, we may assume that it happens universally in this class, and 

 supposed exceptions (e.g., Geryonia) are founded on erroneous observations. 



§ 40. Primary germinal layers ("ectoderma" and " endoderma"). The two primary 

 germinal layers, which first of all arise from the primitive germinal layer, have the same 

 fundamental morphological significance for the Medusas as for all other Metazoa (gastrasa 

 theory). As these two layers regularly recur now in the gastrula of all Metazoa, we may 

 assume that they have been transmitted by inheritance to all the groups from their 

 common ancestral form, the Gastrcea. According to the fundamental biogenetic law, they 

 therefore appear to be constant in the gastrula of all Medusas, which first develop by 

 invagination of the blastula. The inner or vegetative germinal layer, the intestinal layer 

 ("endoderma," or " endoblastus"), limits the cavity of the primitive intestine, as a 

 simple nutritive cell layer, whilst the outer or animal germinal layer, the dermal layer 

 (" ectoderma," or " ectoblastus "), covers and protects the former layer from the outside, 

 as simple sensitive cell layer. In Medusas generally, the cells of the two primary 

 germinal layers (both the inner and the outer) are flagellate, high, cylindrical cells, each 

 of which bears a single, long vibrating flagellum. Whilst the vibrating flagella are 

 constant on the epithelial surface in the majority of the endoderm cells, they are lost in 

 the majority of the ectoderm cells. The two great organic systems of the Medusas stand 

 in definite relation to the two primary germinal layers, just as they are distributed over 

 the two principal sections of the body, the central umbrella disc, and the peripheric 

 umbrella corona. The more numerous and most important parts of the neurodermal 

 system arise from the ectoderm, those of the gastrovascular system, on the contrary, 

 more usually from the endoderm. During the development of the Medusas from the 

 gastrula a histological differentiation of the two primary germinal layers appears every- 



(ZOOL. CHALL. EXP. PART XII. 1881.) M d 



