508 THE FORMS OF TISSUES [ch. 



reason of perfect uniformity of force and perfect equality of the 

 individual cells, are not so numerous. The hexagonal cells of the 

 pigmented epithelium of the retina are a good example. Here we 

 have a single layer of uniform cells, reposing on the one hand upon 

 a basement membrane, supported behind by the sohd. wall of the 

 sclerotic, and exposed on the other hand to the uniform fluid 

 pressure of the vitreous humour. The conditions all point, and 

 lead, to a symmetrical result: the cells, uniform in size, are flattened 

 out to a uniform thickness by uniform pressure, and their reaction 

 one upon another converts each flattened disc into a regular 

 hexagon. An equally symmetrical case, one of the first-known 

 examples of an "epithehum," is to be found on the inner wall of 

 the amnion, where, as Theodor Schwann remarked, "die sechs- 

 eckige Plattchen sind sehr schon und gross*." 



Fig. 187. Epidermis of Girardia. After GoebeJ. 



In an ordinary columnar epithelium, such as that of the intestine, 

 again the columnar cells are compressed into hexagonal prisms; 

 but here the cells are less uniform in size, small cells are apt to 

 be intercalated among the larger, and the perfect symmetry is 

 lost accordingly. But obviously, wherever we have, in- addition 

 to the forces which tend to produce the regular hexagonal sym- 

 metry, some other component arising asymmetrically from growth 

 or traction, then our regular hexagons will be distorted in various 

 simple ways. Thus in the delicate epidermis of a leaf or young 

 shoot we begin with hexagonal cells of exquisite regularity: on 

 which, however, subsequent longitudinal growth may impose an 

 equally simple and symmetrical deformation or polarity (Fig. 187). 



In the growth of an ordinary dicotyledonous leaf, we see reflected 

 in the form of its cells the tractions, irregular but on the whole 

 longitudinal, which growth has superposed on the tensions of the 



* Untersuchungen, p. 84; cf. Sydenham Society's translation, p. 75. 



