86 



THE CELL AND PROTOPLASM 



areas determined by this means are, how- 

 ever, not organ-forming areas in the strict 

 sense, for practically any piece of the egg 

 of sufficient size may, if isolated or placed 

 in new position, give rise to more than it 

 would have — or at least to something dif- 

 ferent — had it been left in place. This is 

 shown by many types of isolation and re- 

 combination experiments (Bautzmann 1929 ; 

 Holtfreter 1929, 1931, 1938). 



It has been shown beyond doubt that the 

 egg after fertilization has a bilaterally 

 symmetrical structure, which may, in fact, 

 even be antecedent to fertilization. When 

 the first cleavage furrow divides the egg 

 approximately in the plane of symmetry, 

 which is the presumptive median plane of 

 the embryo, separation of the blastomeres 

 is followed by the development of each into 

 a complete embryo (Ruud 1925). The co- 

 blastomere of the two-cell stage therefore 

 constitutes an internal environment which, 

 if present, restricts its mate to forming half 

 an embryo. Here, however, the regulative 

 process may be little more than a flow of 

 materials within the single remaining cell, 

 for a dead cell, if not removed, may have 

 the same effect as the living cell (Roux 

 1888). The amphibian egg contents are 

 very fluid and in them there is obviously 

 not the same hindrance to flow as in 

 ascidian eggs. When the first cleavage is 

 at right angles to the plane of symmetry, 

 it divides the germinal material into pre- 

 sumptive dorsal and ventral halves. The 

 former, when separated from the rest of 

 the egg, has sufficient power to readjust its 

 internal arrangement to form a normal 

 whole embryo. The latter, which contains 

 no gray crescent material, lacks this power 

 and forms a very defective '^Bauchstiick" 

 or ventral piece, which is without notochord 

 and nervous system. Some readjustment 

 of material takes place in this piece too, 

 but it is insufficient to produce a normal 

 embryo. An essential constituent is want- 

 ing. Two eggs fused together in the two- 

 cell stage may form either a single normal 

 embryo of large size or double or multiple 

 monsters, according to the relative position 

 of the gray crescents in the combination. 



This, likewise, shows the power of adjust- 

 ment of the egg substance and the limita- 

 tions imposed upon it by cytoplasmic dif- 

 ferentiation (Mangold and Seidel 1927). 



Even through the blastula stage the two 

 lateral halves of the newt embryo retain the 

 ability to readjust and form a complete 

 embryo after separation (Spemann and 

 Falkenburg 1919; Ruud and Spemann 

 1922), except that the side originally 

 turned toward the other cell may be de- 

 fective. The right-hand partners in pairs 

 of twins produced in this way may have 

 their asymmetry (situs viscerum) reversed. 

 Regional areas along other axes of the 

 blastula become more and more specialized 

 as development proceeds, but at the same 

 time show much dependence upon sur- 

 rounding parts (internal environment) for 

 their proper differentiation. 



The most striking case of dependent dif- 

 ferentiation is that of the central nervous 

 system, which is formed in the outer layer 

 of the embryo by reaction with the material 

 underlying it, which is turned in during 

 gastrulation. This material, because of its 

 marked general effect upon the develop- 

 ment of the embyro, has been termed the 

 organizer by Spemann (1924, 1927), its 

 discoverer. The material which is invagi- 

 nated and which acts upon the overlying 

 tissues forms the pharyngeal roof, head 

 mesenchyme, axial musculature and noto- 

 chord. 



Several of the earlier experiments lead- 

 ing to the conception of the organizer are 

 the following: (1) When two newt's eggs 

 are divided down the midline in the blastula 

 stage and the two right halves or the two 

 left halves are grafted together, so that the 

 two half dorsal lips are not exactly opposite 

 one another and point in opposite direc- 

 tions, each one forms or organizes a whole 

 embryonic axis with the incorporation of 

 adjacent material from the other half, and 

 double embrj'-os result. (2) When half the 

 dorsal lip and adjacent material is grafted 

 into another less specialized region of the 

 embryo, it invaginates and reacts upon its 

 surroundings so as to produce a whole new 

 embryonic axis in the host organism. (3) 



