258 



COMPARATIVE ANATOMY 



contractile fibre. A large nucleus, with a distinct nucleolus 

 (sometimes there are two nucleoli) and a network of very fine 

 chromatin particles, occupies the middle of the cell. The ex- 

 ternal moieties of adjacent cells are fused together by intimate 

 union of their protoplasmic walls, hence it is very difficult to 

 isolate a single cell, and if one is isolated its edges look torn 

 and jagged, as is shown in the outermost cells in fig. 55, A. 



Fig- 55- 



Isolated cells from Hydra, fusca. A, a group of ectoderm cells from the body 

 wall showing the muscular fibres. B, an ectoderm cell from the basal disc 

 showing the secretory granules. C, two interstitial cells from the ectoderm. 

 Z>, an interstitial cell dividing, very highly magnified. E, an endoderm 

 cell, not very highly vacuo'lated, with muscular process, nucleus, and 

 plastids. F and G, two forms of gland cell from the endoderm of the body 

 wall. 



The cells are limited externally by a cuticle which, under the 

 highest powers of the microscope, is seen to contain a number 

 of minute refringent particles. When the animal is fully 

 extended the cuticle forms a continuous even layer over the 

 surface. When it is contracted, and its walls are thrown into 

 furrows and wrinkles, the cuticle dips down into the valleys 

 between the wrinkles. To sum up, the ectoderm of Hydra is 

 mainly composed of columnar epithelial cells, differing from an 

 ordinary epithelium chiefly in the fact that each cell has formed 



