CYTOLOGICAL CHANGES IX UNFERTILIZED 

 TUBAL EGGS OF THE RAT. 



MARGARET C. MAXX, 

 LABORATORIES OF ZOOLOGY, UNIVERSITY OF CALIFORNIA. 



INTRODUCTION. 



Recent work on artificial parthenogenesis in vertebrates has 

 reopened the question as to the possibility of some degree of 

 parthenogenesis in mammals. Many authors have described 

 division comparable to cleavage in eggs of atretic follicles, but no 

 thorough-going study of the cytological changes in the tubal eggs 

 of mammals has been made until recently. This lack of obser- 

 vation was natural as long as ovulation was thought of as occur- 

 ring only at copulation, since under these circumstances the possi- 

 bility that fertilization might have occurred could not have been 

 excluded. Other work indicating that ovulation occurs at 

 regular intervals in several mammals including the rat quite 

 independently of copulation, lead to the question as to how far 

 the cytological changes of the unfertilized tubal egg of the rat 

 might parallel normal development. It \\as of interest to com- 

 pare the changes in the unfertilized eggs in atretic follicles, as 

 well as with eggs which have been artifically stimulated to par- 

 thenogenetic development. The conditions which have been 

 described in these three cases are so much alike that one is 

 inclined to believe that similar physiological changes are going on 

 in each. Whether these processes are wholly disintegrative in 

 the tubal rat egg; or represent an abortive beginning of partheno- 

 genetic development appears to be a matter of interpretation. 

 Several authors have stated, and this work corroborates the 

 observation, that mammalian eggs disintegrate in the second 

 polar spindle stage if fertilization does not occur soon after 

 ovulation. Many other authors have noted the occurrence of 

 divided eggs, simulating, at least, two, four, or more cell stages, 

 in the lower portion of the oviduct and uterus of mammals. It 

 is the purpose of this paper to trace the cytological ch.m^rs which 

 occur between ovulation and such apparent cleavage stages. 



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