CELL DIVISION IN EGGS OF CREPIDULA. 557 



present in subsequent mitoses, since the four centrosomes which are typically 

 present in such an egg are sure to interfere; subsequent cleavages lead to th< 

 formation of ectomeres which may be formed in sets of one, though the number 

 of sets and the directions of division are difficult to identify; there seems no 



doubt however of the tendency on the part of the cytoplasm to cut off charac- 

 teristic ectomeres at the animal pole, even though the nuclei and their divisions 

 may be very atypical. (Figs. 6-8, 120, 121, 134, 135, 145, 148, 197-208; p. 390.) 

 (4) When either the first or second cleavage furrow is suppressed but the 

 nuclear division goes on, subsequent cleavages may form more or less normally 

 but the suppressed cleavage is never repeated; if the egg is prevented from com- 

 pleting one of these divisions in its proper sequence it never returns to that 

 division but goes on to the next in order. On the other hand if the third cleavage 

 is turned out of its typical position, so that it becomes meridional instead of 

 latitudinal, the next cleavage in order is morphogenet ically the third clea I 

 and is typical barring the number of macromeres and micromcres, which 1- 

 double the typical number; the imposed meridional cleavage is a new one, inter- 

 calated between the typical second and third cleavages. 



c 



C. Mitosis and Amitosis. 



16. In eggs subjected to an electric current there is no evidence tl t tl 

 centrosomes and chromosomes carry electric charges which differ in sign; nor 

 that the mitotic spindle and the astral rays are chains of granules along lines of 

 force in an electric field; nor that the movements of chromosomes into or out of 

 the equatorial plate are due to the attractions or repulsions of electrically charged 

 bodies. On the contrary, when mitotic figures are displaced by convection 

 currents they move as a whole; the spindle fibers are actual threads of more 

 consistent plasma than the surrounding parts, and may undergo bending and 

 stretching without interrupting their continuity; the typical movement! of 

 chromosomes into and out of the equatorial plate cannot be explained con- 

 sistently on the hypothesis that these movements are due to electrical at t Tactions 

 or repulsions between centrosomes and chromosomes. (Pp. 520-524.) 



17. As the result of former observations (1902) and present experiment 

 I conclude that the mitotic figure is, in the main, the expression of complicated 

 diffusion phenomena between nucleus, centrosome and cell body ; at the beginning 

 of mitosis the " chromatic nuclear sap" (achromatin, archiplasm) escapes from 

 the nuclear vesicle, when the membrane dissolves; it fills the areas around the 

 centrosomes and radiates from these areas and from the entire amphiastei r into 

 the cell body; the spindle fibers, although consistent threads, are not preformed 

 structures, but they grow, when first in contact with the <* romo ^™' m 

 manner suggestive of the formation of fibrin threads m clotting blood it » 

 theoretically possible to explain the division of chromosomes and then mn*+ 

 ment into and out of the equatorial plate by such formation and ^* <^£ 

 fibers and of inter chromosomal (interzonal) fibers; in later stages of mitosis 



I 



