512 CELL DIVISION IN EGGS OF CREPIDULA. 



or lobes may be formed on the surface of the cells, usually opposite the poles of 

 the spindle. Figs. 46, 47, 49 show eggs in which the first cleavage was made 

 unequal; figs. 47, 48, eggs in which lobes were formed near the vegetal pole; fig. 50 

 is an egg which was pressed in the direction of the second spindle axis and in 

 which cleavage was halted in one of the macromeres (CD) while the other con- 

 tinued to divide; fig. 57 is an egg, pressed in the same direction as fig. 50, in which 

 the four macromeres are arranged in linear series; fig. 51, an egg which has 

 divided into three cells, each with several nuclei, probably the result of a triaster. 

 In no instance, however, have I found evidence that triasters or tetrasters may be 

 formed as the immediate result of pressure. On the other hand they are fre- 

 quently due to the continuance of nuclear and centrosomal division after the 

 suppression of cell division, the result being that two or more nuclei and cen- 

 trosomes are left in a single cell body and in subsequent mitoses triasters, tetras- 

 ters or polyasters may be formed. Figs. 69-72, 74 show eggs in which the second 

 cleavage spindles are out of their usual positions, and in which the division of 

 the cell body is partially or entirely suppressed. These eggs were subjected to 

 the action of the electric current, while being pressed between graphite-plate elec- 

 trodes, but it is probable that the abnormalities shown are due to pressure rather 

 than to the electric current. In fig. 73 an egg is shown in which one of the first 

 two blastomeres contains two nuclei, while the other one contains a well formed 

 spindle without any chromatin. This is probably the result of pressure during 

 the first cleavage, whereby both daughter nuclei were forced into one blastomere, 

 leaving only a centrosome in the other blastomere, as in fig. 13. Figs. 69, 72, 75 

 represent successive stages in the development of three different eggs in which 

 the first cleavage furrow was suppressed, probably by pressure between the 



graphite-plate electrodes. 



Pressure during the third and fourth cleavages in the direction of the chief 

 axis of the egg leads to some very interesting modifications of the cleavage type. 

 The micromeres which normally lie on top of the macromeres and which are about 

 1/20 the volume of the latter, may thus be caused to lie in nearly the same plane 

 as the macromeres and to equal them in size. Of course this is brought about 

 by the turning of the spindles from a position nearly parallel with the egg axis 

 to one at right angles to that axis. It has been a matter of surprise to me to see 

 how difficult it is to bring about this change in the position of the spindles and 

 how much the eggs must be flattened in order to accomplish it. Even under the 

 greatest flattening which the eggs will stand without bursting, the spindles still 

 preserve a little of their original slant, the end of the spindle nearer the polar 

 bodies being at a little higher level than the opposite end. Consequently m 

 most of my experiments the cells which are formed at the apical (central) ends 

 the spindles are at a little higher level than those at the outer ends, and 

 former are usually smaller than the latter, though containing yolk and bei^g 

 much larger than typical micromeres. Fig. 52 represents an 8-ceU stage ^ 

 which the third cleavage in quadrants C and D was nearly normal ; in A an 



