organisers: inducers of differentiation 179 



be induced by balancer grafts even in species which normally have 

 no balancer.^ 



In Anura, the nervous portion of the pituitary (infundibulum 

 and pars nervosa) is dependent for its full differentiation and growth 

 upon contact with the epithelial or hypophysial invagination, 

 which originates from the epidermis of the front of the head, and 

 later gives rise to the pars anterior, intermedia, and tuberalis.'- If 

 the hypophysial rudiment be extirpated or destroyed, the infundi- 

 bulum and pars nervosa fail to develop normally, both as regards 

 size and qualitative differentiation. 



In chick embryos, it appears probable that contact of the heart 

 rudiment with the endodermal gut-floor is necessary for the latter 

 to undergo differentiation into a liver.^ (In Amphibia, however, the 

 liver appears to possess marked powers of self-differentiation : see 

 Chap. VII, p. 203.) 



In tissue cultures, it has been conclusively shown that the 

 differentiation of kidney-epithelium into characteristic tubules is 

 dependent on the presence of connective tissue. When cultivated 

 alone, the kidney-tissue merely forms an undifferentiated sheet.* 

 Similarly, tissue-cultures of mammary gland carcinoma may be 

 induced to redifferentiate into structures resembling the acini 

 of m^ammary gland by addition of connective tissue. Again,^ 

 epithelial tissues grown in culture tend to dedifferentiate unless 

 connective tissue is present also,^ and cultures of chick-epithelium 

 can be induced to differentiate into structures resembling salivary 

 glands by the addition of fibroblasts." 



The perforation of the mouth in Urodele embryos is preceded by 

 a reduction of the ectoderm from a two-layered to a one-layered 

 condition, and by the sinking in of the stomodaeal depression. It 

 has been found that these processes, and the consequent perfora- 

 tion of the mouth aperture, is dependent on the establishment of 

 contact between the ectoderm and the underlying endoderm of the 

 fore-gut. The latter is capable of inducing these changes even when 

 ectoderm from other regions is grafted in place of the normal 



' Harrison, 1925 b. - Smith, 1920. 



^ Willier and Rawles, 193 1 a. * A.H.Drew, 1923 ; see also Rienhoff, 1922. 



^ A. H. Drew, 1923. '^ Champy, 1914. 



^ Ebeling and Fischer, 1922. 



