organisers: inducers of differentiation 159 



formed from each of the two invaginations. ^ The former two 

 branches represent the posterior portions of the primitive gut-roof: 

 the latter two branches represent the anterior portions (figs. 73-76). 

 Overlying the cross-shaped gut-roof, neural folds arise, and a 

 monstrous double embryo is formed, the hinder portions of which 

 have each been induced by a single organiser, while the anterior 

 portions have been induced by tissue derived from two organisers. 

 Furthermore, these anterior portions are formed from induced 

 tissues w^hich had very different normal presumptive fates. The 

 relative lengths of the arms of the cross, or of the composite an- 

 terior and of the simple posterior portions of the double embryo, 

 can be controlled by varying the distance which separates the two 

 blastopore lips at the start of gastrulation. If they are far apart, the 

 primitive gut-roofs will travel a long way forwards before they 

 meet and cross, and the anterior composite portions will be short : 

 if they are close together, the gut-roofs will meet and cross very 

 soon, and continue their invagination as parts of the composite 

 anterior ends. Crossed doubling can also be obtained by grafting 

 an organiser into a normal embryo in such a way that the anterior 

 ends of the primary and secondary embryos meet and obstruct one 

 another.'"^ 



§5 



Experiments on the blastoderm of the chick and duck have pro- 

 duced results of the greatest interest. They have shown that the 

 primitive streak has organising powers similar to those of the am- 

 phibian dorsal lip of the blastopore (with which it is morpho- 

 logically homologous), and they have confirmed and extended the 

 results obtained from experiments with amphibian material.^ 



In these experiments, the method of tissue culture has been used. 

 The embryonic rudiment of the bird at a very early stage consists 

 of an upper layer (ecto-mesoderm), and a lower layer (endoderm). 

 These layers can be separated from one another, and cultured 

 separately in vitro. The upper layer will differentiate into neural 

 folds, notochord, and mesodermal somites, but the lower layer will 

 not differentiate at all. This is due to the fact that the lower layer 



^ Wessel, 1926. 2 Bautzmann, 1926. 



^ R, Wetzel, 1929; Hunt, 1929; Waddington, 1930, 1932, 1933 a, b, c. 



