ARTIFICIAL PARTHENOGENESIS OF ASTERIAS. 159 



served in which the first polar spindle may apparent lv become 

 directly the first cleavage amphiaster (Fig. 16). In this case the 

 first polar spindle is formed deep within the cytoplasm near the 

 center of the cell. It is very much larger than the normal, and 

 the ensuing division results in two almost equal hlastomeres. A 

 careful search through the egg gives no indication of polar bodies 

 having been formed. Apparently the amphiaster of this figure is 

 the first polar spindle which has become very much enlarged. It 

 seems to form in the usual manner, but instead of migrating to the 

 periphery of the egg, it recedes deeper into the cytoplasm. 



It is common also for the first polar amphiaster to form deep 

 within the cytoplasm and divide to form two nuclei similar to 

 those formed by the submerged second polar spindle. The two 

 nuclei have been observed to fuse and give rise to the cleavage 

 nucleus. A monaster arises at the point of contact and by division 

 forms the amphiaster of the first cleavage spindle. 



Some eggs have been found where the first polar division is 

 carried through submerged, and gives rise to two nuclei (Fig. 17). 

 In Fig. 1 8 one of the nuclei has developed asters apparently 

 simultaneously, and is migrating to a central position of the egg. 

 Here it will become directly the first cleavage amphiaster which 

 may divide the egg into two equal blastomeres. I have been un- 

 able to trace the fate of the other submerged nucleus, except to 

 observe that it comes to rest in one of the blastomeres. It seems 

 probable, however, that it might develop an aster which would 

 divide to form the cleavage amphiaster of the blastomere. 



The two nuclei that are formed by the first submerged polar 

 spindle may develop asters which form amphiasters and divide 

 the two nuclei, forming four nuclei in the egg. These four nuclei 

 have been observed to fuse and form the zygote of the egg. 



Monocentric Mitosis. When a one-aster egg is studied soon 

 after activation, it cannot be determined at that time whether the 

 aster will later become bipolar and give rise to an amphiaster, or 

 will only pass through rhythmic fluctuations that never lead to 

 division. These eccentric monasters pass through growth and re- 

 cession stages which may or may not be repeated. These cycles 

 correspond in time, roughly, with the formation of the first polar 

 amphiaster, the second polar monaster, and amphiaster of the- 



