ARTIFICIAL PARTHENOGENESIS OF ASTERIAS. 



157 



outside the spot where the nuclear wall first disappeared. The aster 

 has apparently arisen " de novo " as described by Tennent and 

 Hogue ('06), Scott ('06), and Tharaldsen ('26). The nucleolus 

 fragments and apparently gives rise to chromosomes which can be 

 seen resting free in the nuclear material (Jordan, '08 ; Wilson, '01 ) . 

 The aster enlarges for a time sending its rays well into the nuclear 

 mass where they become attached to the chromosomes (Fig. 2). 

 The aster together with its centrosome then divides to form the 

 amphiaster of the first polar division (Fig. 3). The first polar 

 spindle is formed in a position tangential to the surface of the 

 egg. When the mitotic figure has reached its maximum size it 

 rotates at right angles to this position, and migrates to the surface 

 of the cell (Fig. 4). Here the chromosomes divide and move to 

 the foci of the two centers (Fig. 5). With the extrusion and 

 pinching off of the first polar body the inner half of the amphiaster 

 with its central body persists for a short time in the outer region 

 of the cytoplasm after the astral rays are lost. The central body 

 soon fades away, leaving a deeply stained area which gradually 

 disappears. The chromosomes migrate a short distance deeper 

 into the cytoplasm of the egg. 



On the median side of the scattered chromosomes a small 

 centrosome and central body appears. An aster is formed (Fig. 

 6) which divides, and the two centers move apart, drawing the 

 chromosomes into an equatorial position. When the asters reach 

 an antipodal position the spindle rotates approximately 90 degrees 

 and migrates to the surface of the egg. Division takes place and 

 the extrusion of the second polar body follows (Fig 7.). 



After the extrusion of the second polar body the inner half 

 of the amphiaster together with its centrosome and chromosomes 

 persists for a short time, but soon degenerates as did the first, 

 leaving the chromosomes in a deeply stained area. These give 

 rise to chromosomal vesicles which are drawn together to form 

 the egg nucleus by a monaster (Fig. 8). This condition has been 

 described by Tennent and Hogue ('06) and Tharaldsen ('26). 

 After the vesicular monaster has performed its function it disap- 

 pears, leaving its center as a darkly stained body at the periphery 

 of the newly formed nuclear membrane. The deeply stained area 

 soon vanishes, followed shortly after by the disappearance of the 



