THE MOSAIC STAGE OF DIFFERENTIATION 209 



fortunately, it has not yet been found possible to test the directional 

 eifects of gravity on the developing ear- vesicle by forcing the em- 

 bryo to adopt abnormal positions, for the embryo invariably rights 

 itself also, and explantation methods have not been applied to this 

 interesting problem. ^ 



§3 

 The principle of self-differentiation is further illustrated by ex- 

 periments of tissue- culture, from which it emerges clearly that the 

 cells of any particular tissue are permanently determined (except 

 in so far as metaplasia may occur: see below). Mesenchyme, 

 smooth muscle, heart-muscle, striped muscle, epithelium, endo- 

 thelium, kidney-epithelium, and blood-corpuscles of adult birds 

 and Mammals have been shown to preserve their specific character 

 in a wide range of media, and experiments have now been conducted 

 long enough to show that they can preserve them indefinitely. 

 Fibroblasts of the fowl have been cultured in vitro for over 20 years 

 (a much longer period than the maximum length of life of the fowl) 

 and show unchanged characters and an unchanged rate of growth. 

 In many cases, particular characteristics assumed by a cell are 

 a function of the environment or medium in which it finds itself. 

 Epidermis which, like that of the chorio-allantoic membrane of the 

 avian embryo, does not normally show keratinisation, may do so 

 as a reaction to grafts of tissue placed upon it.^ Under certain con- 

 ditions of the medium, an apparent loss of specific characters, or 

 dedifferentiation, may occur, and the tissue reverts to an undiffer- 

 entiated type. Such dedifferentiation is, however, a reversible 

 phenomenon. Cartilage-cells^ or kidney-epithelium* may undergo 

 dedifferentiation and grow as sheets of embryonic cells, but on 

 restoration of the original conditions, the cells readopt the differ- 

 entiated character typical of the tissue to which they belong. This 

 may take place in vitro, or after interplantation subcutaneously 

 under the wing of a young chick. Cartilage-cells, epithelial cells, 



^ A further interesting fact is that in those cases in which the ear-vesicle has 

 been inverted and has failed to rectify its position completely, the resulting tad- 

 poles have an altered sense of balance, which they show by swimming in abnormal 

 attitudes and upside down (Spemann, 1906 a). 



^ Huxley and Murray, 1924. 



3 Strangeways, 1924. ^ A.H.Drew, 1923. 



HEE 14 



