96 THE AMERICAN ARBACIA 



The sperm aster becomes quite large, easily visible in the living 

 egg, indicating the position of the male pronucleus as it approaches 

 the female pronucleus. The small sperm nucleus flattens over and unites 

 with the large egg nucleus and the astral rays spread through the egg ; 

 this is the monaster stage. The rays disappear and the centrosome 

 (probably) divides, forming a curved disk over the nucleus; this is the 

 "streak" stage (Plate III, Photograph 6; Plate V, Photograph 4). The 

 cell wall at this stage is not smooth but somewhat crenate. The 

 hyaline layer which starts to form soon after fertilization has now 

 become 2 to 3 (jl thick. 



The nucleus enlarges from 1 1.5 [jl to 16 u. in diameter, nearly three 

 times in volume; it becomes elliptical, and then the nuclear membrane 

 breaks (Plate III, Photograph 7). The chromatin material, meanwhile, 

 forms into chromosomes, and a spindle with an aster at each pole is 

 formed on which the chromosomes lie, at first irregularly scattered 

 (prophase), then lined up at the equator (metaphase) (Plate III, Photo- 

 graph 8; Plate V, Photograph 6). Then the chromosomes divide, half 

 of each chromosome going to each pole (anaphase). After reaching 

 the pole, each chromosome becomes vesicular and the vesicles fuse 

 (telophase). The centrosphere is largest and the astral rays most 

 extended during the late anaphase and telophase (in Arbacia). 



b. Cleavage. Plates III and V 



Now the egg elongates and the cleavage furrow comes in, usually 

 asymmetrically, on one side first, the side nearest the excentrically 

 placed spindle. There is a heaping up of the hyaline layer in the furrow 

 and a corresponding thinning at the poles. The red pigment granules 

 also tend to accumulate in the furrow, as noted by many observers, 

 e.g., McClendon, 1910b. See under Chromatophores, Part IV. 



The two cells are at first well separated but later become closely 

 pressed together owing probably to the formation of the next mitotic 

 figure (Plate III, Photographs 10, 11). The first cleavage plane passes 

 through the polar axis, i.e., in the region where the polar bodies 

 formed, the animal pole. The second cleavage plane is also meri- 

 dional and at right angles to the first. The third cleavage plane is 

 equatorial, and we have eight equal blastomeres. The fourth cleavage 

 plane is differential, cutting off four small unpigmented cells, the mi- 

 cromeres, at the vegetal pole (Plate III, Photographs 14, 15). More 

 cleavages follow, and a blastula of many cells is formed. It will be 

 noticed in the photographs that the spindles in the later cleavages 

 have no asters, a characteristic of plant cells (Plate V, Photograph 12). 



