556 CELL DIVISION IN EGGS OF CREPIDULA. 



due to several factors: (1) The separation of daughter centrosomes at right 

 angles to the cell axis (the axis passing through centrosome, nucleus and mid- 

 body), and in the plane which separates the gonomeres. (2) Telokinetic move- 

 ments at the close of each mitosis by which the cell axis undergoes regular 

 changes with respect to the axis of the egg as a whole, the poles of the cell axis 

 always moving toward the animal pole and toward the point where the previous 

 cell constriction began. (3) Local reductions of tension of the cell membrane 

 in certain axes, or increase of tension in other axes, by which an axis of least 

 resistance is established in the cell. (4) Mitotic movements of the cell contents, 

 which begin with the dissolution of the nuclear membrane in the prophase, and 

 which cause the spindle and the surrounding plasma to move into certain definite 

 axes and positions in the cell. Of all of these factors the last named is, during 

 the early cleavages at least, the most important and the most difficult of ex- 



plana t ion. 



14. Cleavage furrows may be suppressed, without stopping subsequent 



nuclear and centrosomal divisions, by shaking, pressure, increased temperature, 



carbonic acid, diluted sea water and concentrated sea water. The first and 



second cleavages may be suppressed without stopping subsequent cleavages, and 



in this way a holoblastic type of cleavage may be transformed into a meroblastic 



type. 



(1) When the second cleavage furrow is suppressed subsequent cleavages 



may go forward in typical manner, provided the two mitotic figures in each 

 blastomere (AB, CD) remain so far apart that they do not interfere; in this way 



three quartets of typical ectomeres (la-Id, 2a-2d, 3a-3d) may be formed and 



j 



at the fourth cleavage of the blastomere CD, two mesentoblasts (4c, 4d) may 

 form simultaneously, at the 24-cell stage, although in typical development 4c is 

 not a mesomere but an entomere, and does not form until the 52-cell stage. 

 Within the same cell two mitoses are always simultaneous, and the resulting 

 daughter cells are similar. The importance of division walls and of the isolation 

 of morphogenetically and chemically different substances in differentiation is 

 thus clearly indicated. (Figs. 123, 129, 157, 158; pp. 531, 532, 534, 538-540.) 



(2) If the two mitotic figures in each of the macromeres (AB, CD) interfere 

 thus forming triasters or tetrasters, subsequent cleavages are not typical, but 

 each macromere usually forms a single ectomere at each cleavage (lab-led, 

 2ab-2cd, Sab-Scd) , the ectomeres thus being formed in sets of two instead of m 

 sets of four, as in typical cleavage ; of course the nuclei and centrosomes in sucn 

 ectomeres are usually abnormal; however the fact that, in such cases, three sets 

 of ectomeres may be formed in the typical directions, although the mitotic 

 figures are very abnormal, indicates that typical, differential cleavage is a function 

 of the cytoplasm rather than of the nucleus, and that the positions of the s P m( ^ 

 and the locations of the cleavage planes are principally determined by 

 cytoplasm. (Figs. 123, 128, 137, 138, 211, 212, 213; pp. 531, 535.) 



(3) If both first and second cleavages are suppressed multipolar spmaies 



