58 J. F. McCLEXDON. 



spheres increase in size by thickening of the hyaloplasm layer 

 around the centrosome and become surrounded by protoplasm 

 drawn in along the rays. One sphere grows larger than 

 the other and moves to the surface of the egg. Some yolk 

 granules are caught between the astral rays and form a clear 

 space between the sphere and the egg membrane and push the 

 astral rays outward. We thus have a central space almost free 

 from rays, surrounded by an annular area in which the rays are 

 especially aggregated. The center is bulged out and the annular 

 area sunken in by the stress (Fig. 20). Some astral rays connect 

 with those of the other pole, forming "spindle fibers" outside 

 the mantle fibers. The sphere at the surface of the egg soon 

 pulls all the hyaloplasm from between the yolk granules in that 

 half of the egg and the astral rays that pass through the yolk 

 break and are drawn into the sphere. The mantle fibers con- 

 nected with the outer pole shorten more than those of the oppo- 

 site pole, and the equatorial plate moves to the plane of the 

 ensuing cell division. The cell division is very unequal, sepa- 

 rating a cell containing very little yolk (ah' 1 } from one containing 

 practically all the yolk (cd z , Fig. 21). 



Going back a little, by the dissolution of the nuclear mem- 

 branes a good deal of nuclear sap is liberated. This fluid is 

 hard to follow, but I have some slides that seem to show that 

 most of it goes toward the sphere that reaches the surface, and 

 is therefore included in the cell ab'-. The first cleavage plane is 

 parallel to the chief axis, but is very eccentric because of the 

 great inequality of the division. (See the section on orientation 

 of the egg.) 



B. Second Cleavage. After fusion of the chromosomal ves- 

 icles in the two cell stages (Fig. 21), the nuclei thus formed 

 remain connected for a short time by interzonal fibers. I have 

 no stages in the division of the centrosome, but soon after this 

 division the two centrosomes are at the ends of a spindle shaped 

 sac or centrodesmus (Fig. 21, small figure to right below). 

 Mantle fibers become attached to the nuclear membrane and 

 the chromosomes gather in that side of the nucleus nearest these 

 points of attachment. From this stage on, the histories of the 

 protoplasmic cell and the yolk cell are different and will be 

 treated separately. 



