110 CYTOKINESIS. 



cleavage is that of the maternal and not of the paternal species ; in this case the 

 I'hythm depends upon the egg cell and not upon the sperm and therefore, in all 

 probability, upon the cytoplasm and not upon the nucleus or centrosome. 



While the divisions of nucleus, centrosome and cell body may occasionally go 

 on more or less independently, it is certain that they are normall}- intimately con- 

 nected, and I believe that the normal rhythm of division is largely determined by 

 the interrelation of these structures. In Crepidula it is always possible to deter- 

 mine whether or not a cell will soon divide by the relative size of the nucleus as 

 compared with the cell bodj'. Both nucleus and cytoplasm increase in volume after 

 each division, but the nucleus increases much more rapidly than the cytoplasm.^ 

 When the nucleus has reached a certain maximum size relative to that of the cell 

 body it enters upon the prophase of the next division. The centrosome sometimes 

 divides and gives rise to the initial spindle before the nuclear prophase and in such 

 cases it seems to wait for the nucleus before going through the further stages of its 

 separation. The cytoplasm also sometimes elongates in the direction of the coming 

 division, but seems to wait for the nuclear prophases before undergoing constriction. 



Strasburger ('93) determined the relative size of the nucleus to the cell body in 

 some forty species of plants. He found that while this ratio differed in different 

 species and in different organs of the same species, j-et in a given organ of a given 

 species it was quite constant. The average ratio of nuclear to cell diameter in em- 

 bryonic cells he found to be about as 2:.3. When the diameter of the cell, as com- 

 pared with that of the nucleus, exceeds this ratio, cell division occurs and the ratio 

 is thus restored. 



In Crepidula it is difficult to establish such a ratio owing to the differences in 

 in the shapes and dimensions of cells in the early cleavage ; I have, however, meas- 

 ured a number of cells and nuclei in the prophase of the first maturation and of the 

 first, second and third cleavages, and the ratio of the nuclear to the cell diameter 

 in these yolk laden cells is about 2:7. Such measurements show that at the 

 moment of division there is a fairly definite ratio between the diameter of the 

 nucleus and that of the cell, but whether the growth of the nucleus beyond this 

 ratio is a stimulus to division or is merely an accompaniment of it, is not indicated. 



[ti] Direction of Division. — Upon the direction of division depends the rela- 

 tive positions of the daughter cells and consequently the type of cleavage, viz.: 

 radial, spiral, bilateral or teloblastic. In determinate cleavage this is an important 

 factor of difterentiation since it leads to the localization of cells and different cell 

 substances. 



The direction of cell division has been attributed by various authors to a variety 

 of factors ; thus it is said to be due to the fact that the mitotic figure lies in the di- 

 rection of least resistance (Pfliiger), or in the longest axis of the protoplasmic mass 

 (Hertwig) ; the shape and position of cells and consequently to a certain extent the 

 direction of division, are said to be due to the rectangular intersection of cleavage 



' Since the egg as a whole does not increase in size until the gastrula stage the increase in the 

 quantity of cytoplasm must be at the expense of the yolk. 



