THE STRUCTURE OF THE FERTILISED EGG 35 



dimensional structure, consisting of material particles, the 

 determinants. In the course of the nuclear divisions during 

 cleavage the id would be distributed over the various cleavage 

 nuclei. The nucleus of each blastomere would receive only part 

 of the determinants, and, eventually, there would be only one 

 determinant in each cell. This would then exert its influence 

 upon the cell, thereby unequivocally determining its different- 

 iation. In other words, the spatial multiplicity of the embryo 

 would be preformed in the architecture of the id; the rules 

 governing the distribution of the determinants during cleavage 

 would keep the process of development on its normal course. 



Two deductions from Weismann's theory are open to ex- 

 perimental verification: (1) there must be qualitative differ- 

 ences among the nuclei formed during cleavage, and (2) 

 disturbances of the normal course of cleavage must result in 

 an abnormal distribution of determinants over the cells, and 

 therefore in abnormal development. If it can be shown ex- 

 perimentally that these conclusions do not correspond to reality, 

 it follows that the theory is not correct. 



Now it can already be seen from the experiments on the 

 isolation of cleavage cells (see above p. 28) that the nuclei of 

 the first few blastomeres are not qualitatively different. We have 

 seen that a single blastomere from the two-cell stage of a newt 

 or the 4-cell stage of a sea urchin can still produce a complete, 

 normally built embryo. This proves that the nucleus of these 

 cells must still be equivalent to the original nucleus of the 

 fertilised egg. In other words, no division has occurred, up 

 to this stage of cleavage, that led to non-equivalent results. 

 This is demonstrated even more clearly by the following 

 experiments. Driesch (1893) kept developing sea urchin eggs 

 between two sheets of glass, so that they were slightly flatten- 

 ed. This changed the direction of the plane of the divisions, and 

 resulted in abnormal positions of the cleavage cells relative 

 to one another. Consequently, the cleavage nuclei became located 

 in the "wrong" cells (Fig. 12). In spite of this, the eggs devel- 

 oped into normal embryos. It follows that disturbance of the 

 course of cleavage does not influence further development. 



Spemann (1928) constricted fertilised, but uncleaved newt 



