540 CELL DIVISION IN EGGS OF CREPIDULA. 



stopped in early stages, and if eggs remain in such solutions for a considerable 

 time (9 hrs. or more) all cleavage is permanently stopped, though the nuclei 

 may sometimes resume division when the eggs are put back into normal sea water. 

 On the other hand if these solutions are allowed to act on eggs for a shorter time 

 the cleavage of the protoplasm may be resumed when the eggs are returned to 

 normal sea water (figs. 204, 207, 212, 213) but the cleavage of the yolk is perma- 

 nently suppressed. 



I do not find, as Loeb did, that the protoplasm in such cases begins to segment 

 into as many cells as there are nuclei preformed; on the other hand there are 

 usually several nuclei (or karyomeres) in each of the cells so formed. Further- 

 more the cleavage of the protoplasm never occurs during the resting stages of 

 nuclei and centrosomes, but only during periods of mitosis, and there is an evident 

 tendency for the protoplasm to segment around each of the superficially placed 

 centrosomes (fig. 204). Only in cases where these centrosomes are very numerous 

 or where they lie some distance from the surface are there no constrictions in the 

 protoplasm around each astral system as a center (figs. 203, 219). 



Loeb believes that the failure of plasma to divide, in hypertonic solutions, 

 is due to the withdrawal of water from the egg. In my experiments hypotonic 

 solutions as well as hypertonic ones cause a suppression of cell division. In the 

 former the egg takes up water, in the latter it loses water, and it does not seem 

 possible that diametrically opposite conditions should produce identical results. 

 Many other conditions also lead to the suppression of cleavage, especially in the 

 yolk, — indeed this is the one abnormality of development which is most readily 

 produced by any change in the environment, — and it shows that in such eggs as 

 those of Crepidula, holoblastic cleavage may easily be transformed into mero- 

 blastic. A study of the catalogue of these experiments given on pp. 559-576 will 

 reveal the fact that suppression of cleavage, especially in the yolk, may be caused by 

 pressure, electric current, cold, ether, reduced oxygen tension, increased carbonic acid, 

 diluted sea water and concentrated sea water. If cleavage is the result of some simple 

 physical process, such results are difficult to understand, but if it be the result of more 

 complex physiological processes, such as contractility or movements on the part of the 

 plasma, then it is possible to understand how anything which weakens or disturbs this 

 function of the plasma may cause stoppage of cell division. When eggs are returned 

 to normal sea water after having been subjected to hypertonic solutions the 

 various cell constituents, if they recover their power of division, return to it in the 

 inverse order of their suppression in the solutions, activity reappearing first in the 

 centrosomes, then in the nuclei, then in the protoplasm and finally in the yolk. 



3. Suppression of all Forms of Division without Stopping Nuclear Growth. 



(Figs. 171-173, 176.) 



In solutions of 1 per cent. KC1, 2 per cent. NaCl, or 4 per cent. MgCl 2 in 

 sea water, which are allowed to act for approximately 9 hrs., or in stronger 



