FATE OF UNFERTILIZED EGG IN WHITE MOUSE. 325 



OBSERVATIONS. 



Thirty-six hours after finding the litter, eggs are almost certain 

 to be found in the upper part of the Fallopian tube. They 

 always show the second maturation spindle and may or may 

 not have the polar body attached. If present, the polar body 

 has the chromosomes arranged in the form of a spindle (Fig. I.) 

 During the next twelve hours there is a change in the polar 

 bodies. The chromatin of the spindle breaks down and arranges 

 itself into one, two or many resting nuclei. The formation of 

 two nuclei is well illustrated in Fig. 2. Here the vesicles have 

 been formed but a number of chromosomes or chromatin bodies 

 have not yet entirely lost their individuality. To one side of 

 the polar body a small body, the first indication of a mitochon- 

 drial substance, may be seen. Fig. 3 shows a later stage in 

 which the chromatin of the polar body has formed eight vesicle- 

 like nuclei. Since eggs with three or more polar bodies are not 

 infrequent (see Fig. 4), one is led to believe that the cytoplasm 

 segments, forming itself around the various vesicles. Long and 

 Mark ('n) find that the first polar cell often divides amitotically. 

 They consider that this aids the degeneration and absorbtion 

 of the polar bodies due to the increase in the exposed surfaces. 

 Kirkham also, in work which has never been published, has 

 observed that the first polar cell occasionally divides forming a 

 number of fragments. 



The achromatic fibers of the second maturation spindle dis- 

 appear early, since in only a few cases have they been noted in 

 eggs as old as forty-eight hours. The spindle stands out clear 

 and distinct with the chromosomes in their natural relations 

 but no signs of fibers can be observed. 



Between forty-eight and seventy-two hours a renewal of 

 activity begins in the unfertilized egg. Its effect is first apparent 

 in the second maturation spindle, which may break down and 

 form a single resting nucleus. Three eggs from a seventy-six 

 hour mouse all have a single nucleus both in the eggs and in 

 their polar bodies. More often however numerous vesicles 

 instead of one result from the breaking down of the spindle. 

 An examination of Fig. 5 will make it clear how this takes place. 

 This egg is from a mouse in which one of the uterine horns was 



