324 HARRY H. CHARLTON. 



has traced an apparent normal cleavage as far as the six-cell 

 stage. He also described multipolar spindles and occasionally 

 two spindles in the same egg. The first polar body is occa- 

 sionally seen to divide. 



Kirkham ('07) gives an excellent review of the literature on 

 the early development of mammalian eggs previous to 1907. 

 The common occurrence of spindles in the polar body led him 

 to conclude that the polar body would divide mitotically. Ab- 

 normal eggs containing tripolar spindles and in one case an egg 

 containing two spindles were observed. Kirkham agrees with 

 Sobotta ('95) and Rubaschkin ('06) that an egg never develops 

 after the formation of the first polar body and the second polar 

 spindle unless fertilization takes place, the egg degenerating 

 within the ovary or in the Fallopian tube. 



Athias ('09) also takes the view that the process is entirely 

 one of degenerative fragmentation. 



MATERIAL AND METHODS. 



The mouse is an animal well adapted to such a study, for we 

 know from the investigations of Sobotta, Kirkham, and later 

 of Mark and Long, that ovulation takes place without the 

 necessity of copulation in from thirteen and three-quarter to 

 twenty-eight and one-half hours (Mark and Long) after parturi- 

 tion. The sexes w r ere kept together in suitable cages and the 

 females examined frequently for signs of pregnancy. Whenever 

 a female was found to be pregnant she was placed in a cage and 

 examined each morning. As soon as a litter was found it was 

 removed, and a record of the time made. It is from this time 

 record that all the ages for the various eggs are figured. 



Animals were killed at intervals varying from one and one 

 half days to four days and nine hours after the finding of the 

 litter. The body was immediately cut open and the ovaries 

 with tube and uterus removed and placed in the fixing solution. 

 For this purpose Zenker with acetic, and Carnoy's 6-3-1 solu- 

 tion, were found to be about equally good. For tube eggs 

 strong Flemming proved excellent. The prepared sections 

 were stained in Heidenhain's iron haematoxylin, Ehrlich's 

 hsematoxylin, or Flemming's Triple stain, usually with suitable 

 counter stain. 



