THE EMBRYOLOGY OF THE OPOSSUM 29 



(fig. 8). The ovum has become very elongate and equatorially 

 constricted. The chromosomes are grouped about two cen- 

 trosomes at opposite poles, with the exception in each group 

 of one chromosome which lags nearer the equator. This 

 lagging chromosome is much nearer the equator in one case 

 than in the other. Indeed, a closer inspection reveals that all 

 the chromosomes on one side of the equator are at a slightly 

 different stage of development from those on the other, those 

 approaching one pole (the lower pole in fig. 8, B) being nearer 

 the telophase condition than those approaching the other. 





A B 



Fig. 8 Late anaphase of first cleavage from one of Hartman's specimens 

 (16265). A, the entire ovum showing elongation and equatorial constriction of 

 the cytosome, much deutoplasmic material in the cytoplasm, the beginning of 

 yolk extrusion in the lower hemisphere, and the situation of the spindle a little 

 above center. B, enlarged view of the spindle to show the difference in stage 

 of vacuolatiou of the chromatin at the two poles. 



Furthermore, the yolk is not equally divided between the two 

 poles. A count of the fat droplets in camera lucida tracings 

 of serial sections of this specimen reveals that about 60% 

 of them are in that half of the egg which has the more ad- 

 vanced chromosomes. This is not a difference in concentra- 

 tion of yolk, but merely in volume of cytoplasm plus its in- 

 clusions at the two poles. The location of the spindle and 

 the plane of division are off center so that the two daughter 

 cells will be unequal in size. These early indications of a dif- 

 ference between the two poles of the egg are of interest as 

 they foreshadow the fact that the cells at one pole of the 

 prospective blastocyst will divide more rapidly than those at 



