DEVELOPMENT OF EPITHELIUM. 105 



round elements ; next to this we find pyriform cells of 

 somewhat larger size, whose rounded heads are directed out- 

 wards, while their pointed ends dip down into the interstices 

 between the round cells of the deepest layer. That these cells 

 belonged originally to the deepest layer, that they were raised 

 into the second layer by the propulsive force of cells of later 

 growth, while remaining attached to their place of origin for a 

 time by their inferior ends, this is an assumption which flows 

 most naturally, as I think, from their pyriform shape. The cells 

 of the third layer present, at first sight, the strangest peculiarities 

 of form. They are flattened, and provided on their under surface 

 with angular projections and shallow depressions which correspond 

 to the heads of the cells of the second layer in much the same 

 way as the juga cei^ehralia and impressiones digitated of the tabula 

 vitrea correspond to the convolutions and sulci of the brain. 

 These forms can only be explained by supposing that a cell, be- 

 longing to the second layer, is set free from its attachment to the 

 connective tissue, that it protrudes towards the free surface, and 

 that it is flattened and squeezed into the seams and irregularities 

 of the second layer, by the centrifugal pressure of the urine 

 which accumulates periodically in the bladder. 



We cannot account so satisfactorily for the morphological 

 variety exhibited by the constituent cells of every epithelium. 

 Authors are unanimous, however, on one point, sc. that the 

 epithelial cells appear everywhere to originate immediately upon 

 the connective tissue, to be subsequently extruded from their 

 original seat by the pressure of new elements. This unanimity 

 however refers only to the place ^ and in nowise to the mode of 

 origin of the cells. To explain the latter — omitting for the pre- 

 sent to take generatio cequivoca into account at all* — two hypotheses 

 lie open to us. We may either suppose the new epithelial cells 

 to lie produced by fission of the older ones, or to be extruded 

 from the underlying connective tissue. 



There seems to be no a priori reason why the cells should not 

 originate in both ways. It must however be admitted that the 

 epithelial cells have seldom been actually seen to divide. The 



* J, Arnold {Virchow's Archiv, Bd. 46) has published some views on the 

 regeneration of epithelial structures, which amount to a sort of generatio 

 cequivoca; they are worked out with an amount of genuine care and research 

 which render them worthy of the utmost attention. 



