REGENERATION AND DEATH 191 



ing parts it ceases. In all the cases known to me the 

 cells in the foci of growth are small and embryonic in 

 character, while the surrounding cells, which do not 

 multiply, are larger and more or less differentiated. 

 Fig. 64 illustrates the matter. The drawing on the 

 left, A, represents the beginning (Anlage) of the lens 

 of a chicken embryo, as seen in section. The mitotic 

 figures, produced by the dividing cells, are scattered 

 and may occur anywhere. The drawing on the right, 

 B, represents a section of a differentiated but still grow- 

 ing lens. The inner layer, B, has completely changed 

 owing to the differentiation of its cells, which have 

 been transformed into lens fibres and no longer mul- 

 tiply. The outer layer, A, on the contrary is still un- 

 differentiated and that its cells are still capable of 

 muliplication is evidenced by the mitotic figures scat- 

 tered through it. The mitotic figures, owing to their 

 deep staining, are conspicuous. The main volume of 

 the structure is formed by lens fibres, each of which is 

 a differentiated cell and has never been found in pro- 

 cess of cell division. Only at its edge can the inner 

 layer of the lens acquire new elements; the cells there 

 produced are added to it and give rise to additional 

 lens fibres. In A, we observe the lens growing inter- 

 stitially; in B, by apposition. Appositional growth 

 plays many important roles in the ontogeny of verte- 

 brates. 1 It occurs in the retina, in the enamel organ 



1 Schaperund Cohen, "Beitrage zur Analyse des thierischen Wachsthume," II. 

 Theil, Arch, fur Entwickelungs-mechanik, xix., 3.48-445 (1905). This article 

 gives a clear exposition of the distinction between diffuse, or interstitial, and focal. 



