376 HOLMES. [Vol. XVI. 



The two cells are equal in size, as is the rule in gastero- 

 pod eggs in which there is not a very large amount of yolk. 

 The nuclei are large and vesicular, containing, in proportion 

 to their size, only a small amount of chromatin. The yolk 

 spheres again encroach upon the protoplasmic area around the 

 nuclei, from which they had been extruded during mitosis, 

 and the cells gradually flatten against each other until they 

 resemble a single undivided sphere. The behavior of the yolk 

 in relation to the processes of cell division seems to indicate 

 that the regions around the poles of the spindle are the seat of 

 a tension which squeezes out the deutoplasm spheres as the 

 contraction of a sponge would squeeze out the water contained 

 in its meshes. With the disappearance of the astral radiations 

 and the decrease of tension in those regions, the yolk spheres 

 become free to distribute themselves more uniformly through 

 the ^ZZ- -^t seems probable that the rounding off of the cells 

 after division is due to the persistence for a time of the same 

 central tension which excluded the deutoplasm spheres from 

 the region of nuclear division, and that the subsequent flat- 

 tening of the cells after the resting period has begun is due to 

 a relaxation of the tension which at the same time permits the 

 more even distribution of the yolk, and allows those agencies 

 tending to draw the cells together to become dominant. This 

 flattening of the two blastomeres upon each other occurs very 

 soon after their complete separation, and it continues until 

 each blastomere becomes almost a hemisphere. A lenticular 

 cleavage cavity, if we may call it such, makes its appearance 

 at this stage, reaching its maximum development just before 

 the next cleavage. 



TJie Second Cleavage. 



The two cells usually begin to divide at the same time. 

 The division of one cell sometimes occurs a short time before 

 that of its fellow, but the cleavage is never completed before 

 the process of division in the other cell is well under way. 

 Both spindles are at first parallel to the plane where the cells 

 join. When the elongation of the cells occurs, the spindles 



