HISTOLOGY OF THE ECTODERM. 



113 



Fig. 49. 



Tlie fibrillatcd tissue is largely developed in tlie umbrella of the medusa, where the fibres 

 are arranged circularly, and where, instead of lying between the ectoderm and endoderm, they 

 are situated on the concave surface of the umbrella, as well as in the membranous velum, of 

 which they constitute nearly the entire substance. We shall examine this tissue in connection 

 with the other structures which enter into the umbrella of the medusa. 



Umbrella of medusa. — A remarkable and important modification of the ectoderm is seen in 

 the umbrella of the medusa, where it forms nearly its entire mass, the ectoderm becoming 

 almost whollv changed into a perfectly transparent, elastic, gelatinous-looking substance, traversed 

 by the gastro-vascular canals, which are lined by endoderm. 



The following observations on the structure of the umbrella have been chiefly made on the 

 medusa oi Syncoryne pulckella shortly after its liberation from the trophosome. In this medusa 

 (PI. VI, fig. 3, and woodcut, fig. 49) the convex surface of the umbrella is covered by a distinct 

 epithelial layer (woodcut, fig. 49 a, b) formed of cells (b), containing a nucleolated nucleus, 

 and separated from one another by narrow 

 tracts of homogeneous intercellular substance. 

 These cells appear to consist of simple masses 

 of protoplasm, without distinctly differentiated 

 walls. Under the action of acetic acid they 

 undergo changes of form, becoming elongated 

 oval or fusiform or conical, or even extended into 

 irregular processes, while the protoplasm assumes 

 a moi'e distinctly granular ajipearance, and the 

 nucleus, with its nucleolus, frequently becomes 

 more evident. Immediately beneath the epithelial 

 layer the proper gelatinous substance (a, a) of the 

 umbrella begins. It is transparent, colourless, 

 and elastic, and ap|)arently of the consistence of 

 soft jelly. I have in vain sought for anything 

 like definite structure in it, though IlaeckeP has 

 shown that in the GeryoniclfB the homogeneous 

 gelatinous substance of the umbrella is traversed 

 by numerous fine branching fibres in a direction 

 chiefly perpendicular to the surfaces, while the 

 researches of jMax Schultze" have proved that in 

 the BiicojjJwra or stcganophthalmic medusae the 

 corresponding part consists of widely separated 

 cells, which send out prolongations from their 

 walls to meet similar prolongations from the neigh- 

 boiu'ing cells, and wMth a voluminous intercellular 

 substance, which is composed of a semifluid matter in the meshes of a loose, elastic, fibrous 



Structure of medusii of Syneorytie puhhella Bhortly after 

 liberation from the troijhoaome. 



A. Projected view of medusa from tlie summit of the um- 

 brella ; a, gelatinous substance of the umbrella; 6, external epi- 

 thelium-layer, and c, internal epithelium-layer of the um- 

 brella ; d, subumbrellar muscular sac united to the umbrella 

 along the lines of tlie four radiating canals and along four in- 

 termediate lines corresponding to the ibur meridional furrows 

 e, e, e, e, observable on the outer surface of the umbrella; f, 

 summit of mauubrium. 



n. Cells composing the external epithelium of the umbrella. 



C. Fibrillated tissue of the subumbrellar muscular sac. 



D. Fibrillie of the muscular sac treated with acetic acid, 

 ami viewed under a high power. 



' Op. cit., p. IGO. 



' Schultze, "Ueber den Bau dor Gallertscheibe der IMcdnsen," IMiillcr's 'Arch.," 185G. 



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