CYTOPLASMIC INEQUALITY AND NUCLEAR EQUALITY 333 



distinct ooplasmic substances which possess different developmental poten- 

 cies. This segregation of different substances into separate blastomeric chan- 

 nels is one of the functions of cleavage. 



2. Nuclear Equality of the Early Blastomeres 



Another question next arises: Is there a similar sorting out of nuclear sub- 

 stances during the cleavage period and do the nuclei in certain cells become 

 different from those of other cells? Or, do all of the nuclei retain an equality 

 during cleavage and development? Experimental evidence indicates a negative 

 answer to the former question and a positive one to the latter. 



A precise and illuminating experiment demonstrating nuclear equality of 

 the early blastomeres may be performed by the hair-loop constriction method 

 (Spemann, '28; Fankhauser, '48). For example, the fertilized egg of the 

 newt, Triturus viridescens, may be constricted partially by a hair loop so that 

 the zygotic nucleus is confined to one side (fig. 165A, B). The side possessing 

 the nucleus divides, but the other side does not divide (fig. 165B, C). By 

 releasing the ligature between the two sides at various stages of development 

 of the cleaving side, i.e., 2-, 4-, 8-, 16-, and 32-cell stages, a nucleus is per- 

 mitted to "escape" into the cytoplasm of the uncleaved side (fig. 165C, E; in 

 D the escaped nucleus is seen in the blastomere to the left). By tightening 

 the loop again after the escaping nucleus has entered the uncleaved cytoplasm, 

 further nuclear "invasion" of the uncleaved part is blocked. If the original 

 constriction was made so that the plane of constriction coincides with the 

 plane of bilateral symmetry, i.e., if it constricts the gray crescent into two 

 halves, the result is two normal embryos. This occurs after the 2-, 4-, 8- and 

 16-cell stages of the cleaving half of the egg. Nuclei permitted to escape 

 when the cleaving side has reached the 32-cell stage do not produce normal 

 embryos in the uncleaved side, probably because of the changes which have 

 occurred in the meantime in the cytoplasm of the uncleaved side and not to 

 the qualitative differences in the nuclei at this stage. 



Another type of experiment upon the early cleaving blastomeres which 

 demonstrates nuclear equality may be performed. It has been shown by 

 Pfluger, Roux, and Driesch (Wilson, E. B., '25, p. 1059) that a cleaving 

 egg pressed between two glass surfaces will divide parallel to the pressure 

 surfaces. That is, the mitotic spindle is moved into a position parallel to the 

 pressure surfaces. Under these circumstances, the spindle obeys the second 

 law of Hertwig, namely, that the mitotic spindle tends to coincide with the 

 long axis of the protoplasmic mass. Cleavage under pressure so applied, there- 

 fore, will result in a series of vertical cleavage planes. In the sea urchin 

 (fig. 166) if pressure is applied in the four-cell stage, the mitotic spindles 

 will form in a horizontal position, as shown in figure 166E, instead of in the 

 vertical position, as indicated in figure 166B, C, where no pressure is applied. 

 In other words, all of the nuclei shown in white in the upper blastomeres of 



