42 K. T. Günther 



Histology. 



The Ectoderm of the Exumbrella. 



The ectoderm for tbe most part consists of flattened polygonal 

 cells with small round nuclei. It is generally easy to demonstrate 

 that tbe cells are linked together by slender Strands of protoplasm 

 wbich bridge tbe relatively large intercellular Spaces (iigs. 3, lü). 

 In some individuals wbich were examined in tbe living state, tbe 

 cell outlines altbougb retaining indications of a polygonal sbape, 

 were rounded and tbe slender protoplasmic connections were not to 

 be Seen (fig. 5): it is supposed tbat in tbese individuals tbe proto- 

 plasmic processes bad been witbdrawn eitber on account of old age 

 or on account of bad bealtb. 



In tbe vicinity of tbe cnidacts or streaks of cnidoblasts upon 

 tbe exumbrella, tbe ectoderma! cells are of a peculiar form (fig. 10). 

 Wbile still retaining tbe polygonal cbaracter, tbey are more elongate 

 tban cells remote from a cnidact. Tbe cells are connected botb 

 with their neighbours and witb tbe cells of tbe streak by attenuated 

 processes of granular protoplasm whicb may be as mucb as tbree 

 or four times as long as tbe cells to wbicb tbey belong. These 

 processes niay be easily demonstrated in osmic preparations, in 

 wbich too a few granules witbin tbe cytoplasm stain a conspicuous 

 brown. 



The Cnidoblast Streaks. 



All round tbe margin of tbe umbrella and along tbe cnidacts, 

 tbe ectoderm is of a strikingly different cbaracter. Eising in sligbt 

 relief from tbe exumbral surface, tbe cells of tbe streak are more 

 closely packed togetber and bave smaller nuclei. Two sorts of 

 cells are distinguisbable — interstitial cells and cnidoblasts (tig. 7). 



Tbe interstitial cells are in some parts delimited witb well 

 defined contours, but in otbers tbeir nuclei lie in an undivided matrix 

 of cytoplasm. 



Tbe cnid()l)lasts are distributed somewhat irregularly along tbe 

 streaks, whicb vary considerably in brcadtb. In tbe broader parts 

 six or eigbt ranks of nematocysts fili tbe width; a single row of 



