214 PRINCIPLES OF EMBRYOLOGY 



mesoderm and the ectoderm, no induction takes place and neither does 

 the colour become transferred from one group of cells to the other. This 

 seems to indicate that the transfer of dye is brought about by the passage 

 of rather large particles which are not able to pass through the membrane, 

 and thus supports the suggestion that particles of the size of microsomes 

 are able to migrate from cell to cell. None of these observations are, how- 

 ever, as clear cut as one would desire to establish such an important con- 

 clusion as the intercellular migration of large microsome-like particles. 



Niu and Twitty (1953) have recently made the important observation 

 that if pieces of axial mesoderm are cultured in saline solution for some 

 days, and small fragments of gastrula ectoderm then added to the culture, 

 these may become induced to differentiate into neural tissue, even 

 when they do not estabhsh any direct cell-to-cell contact with the original 

 mesodermal explant. The induction must be due to substances given off 

 by the mesoderm into the culture medium. It is possible that these sub- 

 stances are relatively unspecific and act through a relay mechanism (as 

 would, for instance, acids) but it seems more probable that we are really 

 confronted here with a diffusion of the normal evocator itself. Prehmin- 

 ary spectroscopic study shows that the medium, after conditioning by 

 the mesoderm implant, contains substances which may be nucleic acids; 

 but the biochemical analysis is still in a very early stage. It has been 

 mentioned earlier (p. 196) that Bautzmann (1929) had already shown 

 that the body fluids of older larva exert an organiser-like influence on 

 fragments of ectoderm isolated in it. 



Some attempts have been made to get further information by the use 

 of radioactive labelling. Waddington (1950I)) showed that, if yeast is 

 labelled by being cultivated for some time in solutions containing phos- 

 phorus-32, then dried and used as a graft in the gastrula, the radioactive 

 material passes from the implant into the neurahsing ectoderm. It is not 

 clear however whether the phosphorus is carried by large complexes 

 containing nucleic acids, or whether it is in the form of small groups such 

 as the phosphate ion when it makes the passage from one tissue to the 

 other. Ficq (1954) has also used organiser grafts labelled by cultivation in 

 radioactive amino-acid solutions. She found that after a few days the 

 radioactivity was no longer solely confined to the graft. It occurred in 

 the evocated neural tissue, but it was also found in the neural system of the 

 host embryo. It is therefore probable that we are dealing not with a 

 straightforward diffusion of the radioactive material out of the graft into 

 the surroundings, but rather with a selective accumulation by the most 

 rapidly metabohsing tissues of substances released from the graft itself, 

 which become available throughout the whole embryo. Again it is un- 



