46 .KARYOKINESIS. 



versally. in the early divisions of ova, though they are not usually found in other 

 mitoses. What is the cause of this difference ? It occurs to me that it may be due 

 to diiferences in the size and in the rapidit}- of division of blastomeres as comjDared 

 with tissue cells.-^ The following observations favor this view : — The chromosomal 

 vesicles are proportional in size to the size of the cell (quantity of cj'toplasm) in 

 which they lie. The daughter chromosomes which go to the two poles of the spindle 

 are always equal in size however unequal the cell division may be, until the time 

 when the daughter cells are separated bj- the new cell wall. Immediatel}' after this 

 separation a difference ajDpears in the size of the vesicles in the two cells, if the 

 division was unequal, the larger cell containing large chromosomal vesicles while 

 in the smaller cell they remain small or do not show the vesicular structure at all. 



The chromosomes which go into the polar bodies do not appear vesicular at anj- 

 stage, though after the division of the first polar body they fuse into a single nucleus 

 in each cell which contains very little achromatic material. The smallest cells in 

 the early stages of cleavage ar-e the '' trochoblasts" (fig. 97, la--ld-); these cells 

 do not again divide for a very long period, and in them the chromosomal vesicles 

 are at first very small. Chromosomal vesicles appear in the anaphase of all the 

 other cleavages, but as the cleavage advances and the blastomeres grow smaller 

 these vesicles become less and less apparent. 



From these observations I conclude that in large cells where divisions succeed 

 one another at short intervals the chromosomes begin the growth characteristic of 

 the daughter nuclei, i.e.^ the absorption of substances from the cell body, before 

 the}' have fused together, whereas in small cells or cells Avhich divide only at long 

 intervals the chromosomes fuse before the absorption of achromatic material begins. 



After the fusion of the chromosomal vesicles to form the daughter nuclei, the 

 latter continue to absorb achromatic material, growing larger and larger, until the 

 prophase of the next division. A part at least of the achromatic material absorbed 

 is derived from the sphere which in turn contains interfilar substance of the 

 spindle and aster. This recalls the conclusions of 0. Hertwig ('75), in which he 

 points out that in the formation of the daughter nucleus the chromosomes absorb 

 ^'' Kernsaff and become vesicular, the process being the reverse of what occurs in 

 the beginning of division, Avhen ^'Kernsaft'' is set free into the cell body. A similar 

 view was held by Butschli ('76). 



In the growth of the nucleus the nuclear membrane has the properties of a 

 semi-permiable membrane, i. e., substances pass readily through the membrane in 

 one direction, but not in the other. Eeinke (1900) has suggested that the nuclear 

 ground substance is a diosmotic material, which, b}' taking up substances from the 

 cell, produces a substance of higher osmotic pressure. When, on the other hand, 

 the nuclear membrane dissolves and the ground substance of the nucleus mingles 

 with the fluid substance of the cell, the peripheral laj-er of the latter assumes the 



' Flemming ('92) formerly held that all chromosomal vesicles were artifacts. Now that they have 

 been found, however, in so large a number of ova, pi'epared hy the best modern methods, such an idea 

 cannot be maintained. 



