CELLULAR DIFP^RENTIATION AND INTERNAL ENVIRONMENT 



87 



Pieces of presumptive ectoderm and pre- 

 sumptive neural plate, when interchanged 

 in the blastula or early gastrula, develop 

 in harmony with their new surroundings 

 (Spemann 1918). (4) Pieces of presump- 

 tive ectoderm, when grafted into the dorsal 

 lip of the early gastrula, acquire the prop- 

 erties of the latter and are able to act as 

 organizer material (Spemann and Geinitz 

 1927). 



What it is that gives the organizer cells 

 these properties, which are so effective as 

 an internal environment to the overlying 

 cells, and what the nature of the reaction 

 in the surrounding tissues is has not been 

 ascertained. Spemann (1931a) himself ex- 

 perimented with organizer material the 

 structure of which had been destroyed. 

 Later Holtfreter (1933, 1933a, 1934) dis- 

 covered that a great variety of animal tis- 

 sues both adult and embryonic and from 

 almost any phylum could, when placed in 

 the blastocoel so that it ultimately came to 

 lie under the ectoderm, stimulate the forma- 

 tion of a thickened area (neural plate) in 

 the overlying tissue, which might fold over 

 and close in as the neural tube does. 

 Even pure chemical substances held in an 

 agar matrix may have a similar effect. 

 There is a difference, however, between the 

 response to living normal organizing ma- 

 terial and that to dead material, the former 

 stimulating the formation of a complete 

 neural plate and tube with normal re- 

 gional differences, while the latter produces 

 simply a thickened plate which may close 

 to a tube without, however, showing such 

 regional differentiation. 



Much effort has been spent to find out 

 the nature of the chemical substance that 

 produces these effects, and several differ- 

 ent conclusions have been tentatively 

 reached. The fact that certain embryonic 

 tissues which have no organizing effect 

 when living do so act when dead has led 

 to the view that the substance responsible 

 for the effect is free only in the active 

 regions but exists in a bound condition in 

 other parts of the embryo. Needham, 

 Waddington and co-workers (1935, 1938) 

 have isolated sterol derivatives from the or- 



ganizer and have found that synthetic 

 sterols when applied in an agar matrix do 

 induce the formation of neural-plate-like 

 structures. Another fact that may be of 

 significance is the occurrence of an intense 

 carbohydrate metabolism in the organizer 

 (Woerdeman 1933; Heatley and Lindahl 

 1937; Boell et al. 1939; Brachet 1939). 



Differences between the action of the 

 natural organizer in its normal position and 

 that of dead material and chemical sub- 

 stances may be due to the circumstance that 

 the former is distributed in a gradient hav- 

 ing different local concentrations. Another 

 factor that may have some influence is the 

 possible variation of the intensity of action 

 with respect to time. A normal induction 

 might be achieved only through the action 

 of a living system, because this might re- 

 quire the action to start with a low in- 

 tensity, increase to a maximum, and then 

 taper off; or it might require several 

 cycles of variation in intensity, which dead 

 material could not afford. Regional differ- 

 ences in the reacting material must also be 

 taken into consideration, as shown by the 

 different respective reactions of presump- 

 tive head and trunk regions to similar im- 

 planted organizer material (Spemann 

 1931). 



"What happens in the ectodermal cells 

 that become transformed into neural plate 

 is of much importance in any theory of 

 organizer action but it is almost totally un- 

 known. The neural plate contains rela- 

 tively more water than the embryo as a 

 whole (Glaser 1914), and it seems to be the 

 inner portions of the neural plate cells that 

 are the most swollen. We might assume, 

 then, that the organizer produces a chemi- 

 cal change, which leads first to an orien- 

 tation of particles that makes the cells 

 columnar, followed by a greater hydration 

 of the protein lattice localized in the basal 

 halves of the cells. This would lead to 

 the folding of the plate to form a groove 

 and its ultimate closing into a tube. The 

 changes underlying the process would thus 

 be of a chemical and paracrystalline nature. 



It must also be borne in mind that the 

 agencies effecting the differential changes 



