CELL DIVISION IN EGGS OF CREPIDULA. 531 



Cases of the suppression of yolk cleavage, with interesting results, are shown 

 in figs. 123, 125, 126, 127, 128, 129. In figs. 123, 128, and 129, the macromere 

 AB did not divide at the second cleavage, though its nucleus did. In fig. 123, 

 the first micromere (lab) formed from this undivided macromere is abnormal, 

 all the other micromeres being normal. In fig. 125, the half of the second cleavage 

 furrow between C and D was suppressed, while the half between A and B was 

 abnormal in position giving rise to a large macromere A and a small macromere B, 

 lying above C and D. Each of these macromeres has given rise to a micromere 

 which is normal in size and appearance, though lc is abnormal in position. In 

 fig. 128, the second cleavage furrow was suppressed in both of the first two 

 blastomeres, and at the third cleavage the two mitotic figures in each blastomere 

 came so near each other at one pole that triasters were formed, with the result 

 that each double macromere gave off a single micromere of the first set (lab, led), 

 each with two or more nuclei and spheres. At the fourth cleavage, the two 

 spindles in each macromere were distinct, one of them being directed dexio- 

 tropically, the other laeotropically, and as the result of this cleavage each double 

 macromere gave off two separate micromeres (2a, 2b, 2c, 2d), the two lying on 

 opposite sides of the first micromere. In fig. 129, all the micromeres of the 

 24-cell stage are present and all are approximately normal. The fact that two 

 nuclei may exist within the same cell and may divide in normal fashion giving 

 rise to typical micromeres is thus plainly demonstrated. Where abnormal 

 micromeres arise from such binucleate macromeres, it is usually due to the fact 

 that the two mitotic figures lie so near each other that triasters or tetrasters 

 are formed. One interesting fact brought out in this, and in other experiments, 

 is that when the nucleus of a cell divides and the cell body does not, the two 

 nuclei usually take up positions at opposite sides of the cell, and in subsequent 

 divisions of these nuclei the direction of division is frequently dexiotropic in 

 one and Isotropic in the other, whereas in normal cleavage the position of the 

 resting nuclei and the direction of division is dexiotropic or Isotropic in both. 

 In fig. 123, the nuclei 2A and 2B lie at opposite sides of the macromere, though 

 the cleavages by which 2a and 26 were formed were not exactly at right angles 

 to each other. In fig. 128, the direction of division in forming 2o and 2c is 

 Isotropic, in forming 2b and 2d it is dexiotropic. In fig. 129, the direction of divi- 

 sion in forming 3a was Isotropic, in forming 36, dexiotropic. Fig. 131 represents 

 an egg in which the direction of division in one of the first two blastomeres 

 was at right angles to that in the other with the result that a T-shaped cleavage 

 mass was formed. Each of the four macromeres has given rise in typical manner 

 to three sets of micromeres and the first and second sets have subdivided, as 

 they do in normal eggs, but the relation of these micromeres to one another 

 is quite atypical, the micromeres being arranged in sets of three over the three 

 contiguous macromeres, while the micromeres from the displaced macromere D 

 are applied to one side of this three-cornered micromere plate. 



The other figures shown on Plate LII are more abnormal than the ones just 



