86 CLEAVAGE AND DIFFERENTIATION 



to pass across from one side to the other. This may be done at the 

 2-cell, 4-cell, 8-cell, i6-cell, or 32-cell stage of the cleaved side. If 

 the constriction lay in the plane of bilateral symmetry of the original 

 fertilised tgg, and if after the passage of one nucleus from the 

 cleaved side to the other the ligature is then drawn tight again so 

 as to separate the two halves completely, each half will develop into 

 a normal little newt. One of these little embryos will contain only 

 the nuclear material of one blastomere of the normal 2-, 4-, 8-, or 

 i6-cell stage, or as we may for brevity write it, a 1/2, 1/4, 1/8 or 

 1/16 nucleus, depending on the time when the two halves were 

 separated ; the other embryo will contain all the rest of the nuclear 

 material. This means that in normal development, the nuclei of the 

 blastomeres of the i6-cell stage contain material which is equivalent 

 to that of the nucleus of the fertilised egg. Nothing has been lost 

 by the nuclei in the process of cleavage, at least up to and including 

 the i6-cell stage. Further, all the 1/16 nuclei have retained this 

 equivalence, for in the numerous experiments performed it would 

 not have been possible for the nucleus which passed across from 

 one side to the other to be the same^ (fig. 37). 



When cleavage in one-half of the dumb-bell has reached the 32- 

 cell stage, the passage of a nucleus into the other half is insufiicient 

 to enable the latter to undergo normal development. This is, how- 

 ever, probably not to be attributed to a qualitative insufficiency of 

 a 1/32 nucleus. It is more likely that the failure to develop is due 

 to some alteration of the cytoplasm of the uncleaved half, in turn 

 due to the length of time during which it has been deprived of a 

 nucleus, and therefore prevented from prosecuting its normal 

 physiological activities. This explanation follows from the fact that 

 a 1/16 nucleus is incapable of ensuring development beyond the 

 late gastrula, or, rarely, early neurula stage, if the constriction had 

 been placed in such a way as to separate dorsal and ventral halves 

 of the future embryo, and the zygote nucleus had been restricted 

 to the ventral half. A 1/16 nucleus is therefore unable to do in a 

 previously enucleate dorsal half what it can do perfectly well in a 

 previously enucleate lateral half. It would appear that the failure 

 in this case lies with the cytoplasm. The susceptibility of the cyto- 

 plasm of a dorsal half is greater than that of a lateral half (see 



^ Spemann, 1928. 



