CYTOKINESIS. 101 



tion in these cells is dexiotropic. the second qnartette is formed by Inotropic cleav- 

 age and the rotation in these cells is Igeotropic, etc. 



(3). The extent of the rotation diifers somewhat in different cell divisions and 

 for different cell constituents, but in all cases there is an evident tendencj^ to carry 

 the poles of the spindle axis to that portion of each daughter cell which lies nearest 

 the animal pole, tliough this movement is limited by the fact that the spheres do 

 not move away from a free surface and under other cells. Normally the nuclei lie 

 close to the centrosomes and although they may move into that portion of the 

 cell which is overlapped b}- other cells, they do not separate from the centrosomes ; 

 hence it may be concluded that their movements are indirectly limited b}- this 

 tendency of the sphei-es to keep in contact with a free surfiice of the cell. 



(4). As a result of the fact that the spheres do not move under overlapping 

 cells, but lie close to a free surface, the centrosomes, nuclei and cytoplasmic areas 

 of the macromeres move down over the periphery of these cells as the cap of ecto- 

 blast cells extends until finally they are carried clear around to the vegetal pole. In 

 this way the polarity of these cells is apparently reversed, the nuclei, centrosomes 

 and cj'toplasmic areas being car)'ied from the animal to the vegetal pole, in front 

 of the maro'in of overorowino- ectoblast cells. Another result of the fact that the 

 centrosomes and spheres lie in contact with a free surface of the cell is that the 

 cells are formed in a one laj-ered epithelium and not in a many la3^ered one or in 

 a solid mass. Cells are not budded off from the macromeres under the cap of ecto- 

 derm cells but at its edge, and in the subdivision of the ectoderm cells the same 

 principle is operative ; thus although the ectoderm cells may overlap one another to 

 a certain extent they are never completely covered hy other cells but always preserve 

 a free surface. Heidenhain ('94) has shown that in one layered intestinal epithelium 

 the centrosomes during the rest lie between the nucleus and the free surface of the 

 cell; in division the centrosomes lie 90° from this position, the spindle being para- 

 tangential with the free surface of the cell ; he has pointed out the fact that if 

 the angle of rotation of the centrosome were different an entirely different form 

 of cell complex might result. In Crepidula it is not the angle of rotation which 

 determines that the ectoderm shall form a one-layered epithelium, since this angle 

 varies with every cleavage, but the fact that in the rest the centrosomes and spheres 

 lie next a free surface of the cell. 



In the formation of the mesentoblast (4d), however, there is an important ex- 

 ception to this general rule. In the late anaphase of this cleavage the centrosome 

 and sphere are still in contact with a free surface of the cell 4d, (text fig. XVI) but 

 in the telophase the nuclei, centrosomes, spheres and cytoplasmic areas are carried 

 under the overlying ectoderm cells, only that portion of the cell which contains 

 3'olk remaining at the surface. It is difficult to observe the centrosomes and spheres 

 in the cells derived from 4d, but during the first two or three cleavages they lie on 

 the apical, i. e., animal pole, side of the nuclei during the resting period, fig. 100. In 

 all these derivatives of 4d the spheres stain less densely and are larger and less definite 

 in outline than in those cells in which thev are in contact with a fi'ee surface. I 



