organisers: inducers of differentiation 137 



organiser before gastrulation,i by preventing the organiser from in- 

 vaginating, which can be effected either by removing it entirely 

 by killing part of it and so preventing invagination on one side;'- 

 or by reducing its activities by means of exposure of the organiser 

 region to cold or deprivation of oxygen.^ In spite of the absence of 

 an organiser or of any invagination, distinctive but somewhat 



.^C^\\ 



-J 



Fig. 63 

 a. Embryo of Pleiirodeles in which gastrulation has been prevented by reducing 

 oxygen-access to the region of the dorsal Hp; neural folds are nevertheless 

 formed, b, Transverse section through the same embryo, showing neural tube, 

 but absence of notochord ; the lining of mesoderm and endoderm has been de- 

 rived from the floor of the blastocoel, which, here, is the large central cavity. 

 (From Vogt, Verh. deiitsch. Zool. Ges. xxxii, 1928.) 



imperfect neural folds and tubes are developed. It is of interest 

 to note that in the absence of an underlying organism, the brain 

 achieves a more perfect differentiation than the spinal cord.^ 



In experiments of a different nature, in w^hich developing Uro- 

 dele eggs are subjected to a lateral temperature-gradient (seep. 342), 

 it is found that on the warmed side, structures appear in the ecto- 

 derm resembling neural material in cell structure, but may differ 

 considerably from neural folds in form.* These structures arise in 



^ Lehmann, 1926, 1928 a. 

 3 Vogt, 1928A. 



2 Goerttler, 1925, 1926. 

 * Gilchrist, 1929. 



