ARTIFICIAL PARTHENOGENESIS OF ASTERIAS. l6l 



condition is usually found after a longer exposure to the activating 

 agent. 



Fig. 23 is an equatorial plate stage of a tripolar second matura- 

 tion spindle. The first maturation division has been carried 

 through normally and the astral rays of the distal pole are fading 

 away; while the second polar spindle has united with a cytaster 

 which will divide the chromosomes somewhat unequally among 

 the three centers, thus forming three nuclei with unequal chro- 

 matin content. 



Fig. 24 shows a telophase of the above tripolar condition. The 

 chromosomes have all been collected at the centers, and are broken 

 down. The asters are beginning to recede and the result is three 

 separate nuclei. These nuclei may or may not fuse. In most 

 cases they have been observed to fuse (Figs. 25-26). In such 

 cases where fusion takes place, all trace of the astral radiations, 

 centrosomes and centrioles, of the three centers are completely 

 lost to view. The chromatin material that has resulted from a 

 fusion of the three nuclei may develop an individual monaster 

 which may never divide ; or in some instances they may form an 

 amphiaster which becomes the first cleavage spindle. If the three 

 nuclei do not fuse but remain separate, each nucleus may develop 

 a monaster which forms an amphiaster ; this divides the chro- 

 matin material again, so that six small nuclei results within the 

 egg (Fig. 29). In another case the two nuclei resulting from the 

 first or second polar mitosis (Figs. 11-17) nave formed asters and 

 divided again (Fig. 27) to form four small nuclei, as seen in Fig. 

 28. In Fig. 28 the four nuclei are in pairs. Each of the two 

 pairs has developed an aster at its point of contact and is fusing 

 to form two large nuclei as in the original case. 



Fig. 29 shows six nuclei with asters which are the result of 

 a division of a triaster which forms three nuclei, on each of which 

 an aster has developed. These asters divide, thus bringing about 

 a submerged mitosis, forming three sets of small nuclei without 

 division of the cytosome. These six nuclei may fuse to form 

 three nuclei again, which may in turn fuse to form the cleavage 

 nucleus. Here again the chromosomal count could not be made, 

 due to the many divisions of the chromosomes. 



In many eggs several asters and cy tasters have been observed. 



