238 CHARLES R. STOCKARD AND G. N. PAPANICOLAOU 



approaching. The orifice of the vagina is sometimes open dur- 

 ing this stage and thus explains why this sign, which was observed 

 before, does not make it possible to detect the actual time of 

 the regular oestrous activity. Rubaschkin has observed the 

 opening of the vagina ten to twelve days after parturition, but 

 this period of time is certainly too short ta indicate the return 

 of heat. We agree with Rubaschkin in stating that during the 

 ovulation the vagina is open, but we do not admit that the oppo- 

 site is also true, that the opening of the vagina indicates unmis- 

 takably the return of the ovulation process. 



4. MICROSCOPIC STRUCTURE AND CHANGES OF THE VAGINAL 



FLUID 



A microscopical examination of the smears prepared from the 

 vaginal fluid taken at the several stages separated above shows 

 decidedly typical differences. The cellular character of a smear 

 made at a given stage differs from the cellular make-up of all 

 other stages. The relative numbers of various cell types in the 

 fluid at different stages are so definite that one with a little ex- 

 perience may diagnose the exact sexual stage of the animal 

 concerned solely by an examination of the smear. 



A photomicrograph from a smear of the vaginal content dur- 

 ing the first stage of mucous secretion is shown by figure 1. 

 This mucous fluid is seen to contain an abundant mass of cells 

 which, as shown in the figure, are of a squamous type with very 

 small pycnotic nuclei sometimes broken into pieces. The cell 

 protoplasm is also greatly degenerated having only a weak 

 affinity for the plasma stains and exhibits a reticular structure. 

 These cells derived from the wall of the vagina (fig. 17) char- 

 acterize by their presence and great superiority in numbers this 

 first stage. There are, however, to be seen particularly toward 

 the end of the first stage a certain number of elongate, cornified 

 viells without nuclei, which are desquamated from the more 

 external portions of the vagina. These cells contrast in appear- 

 ance with the fu'st type cells since in smears stained with haema- 

 toxylin and eosin they present a decidedly red color, while the 

 abundant first type cells are almost grey. The red cells rather 



