442 BENJAMIN H. PRATT AND J. A. LONG 



The rat was selected because of its interest in connection 

 with other studies and because of the possibility of obtaining a 

 chronological series of embryos of known ages, and, consequently, 

 of securing a natural series of stages. 



MATERIAL AND METHOD 



Since the period of synapsis in the egg of the rat is well along 

 toward completion at birth, it was found necessary to obtain 

 embryos of known age in order to complete the necessary series 

 of slides. These were obtained in the following way. It having 

 been clearly demonstrated that the female albino rat normally 

 ovulates within twenty-four hom*s after having given birth to a 

 litter (Long, '12), mother rats were taken from their young im- 

 mediately after parturition and placed with sexually active 

 males. Twenty-four hours later these females were taken from 

 the males and left alone or with females only. Upon subsequent 

 examination about 50 per cent of the rats so treated were found 

 to be pregnant, and the age of their embryos was consequently 

 known to within a fraction of a day. For the most part the sex 

 organs were dissected out of these embryos before fixing, but in 

 a few cases the entire embryo was fixed and sectioned. For those 

 stages occurring after birth the ovaries of young of appropriate 

 ages were used. The fixing solutions employed were Zenker's, 

 Flemming's strong solution, Bouin's formol-picro-acetic, and 

 sublimate acetic. The first two gave the best results and were 

 used almost exclusively. The material was sectioned from 3 

 to 10 micra thick and for the most part stained in Heidenhain's 

 iron haematoxylin. Usually a counterstain was employed, 

 orange G or acid fuchsin, or a mixture of the two being most 

 satisfactory. Erythrosin was used to some extent. A few slides 

 were stained in safranin and gentian violet. 



GENERAL CONSIDERATIONS 



The process of synapsis in the rat lasts for approximately 

 ten days, starting about seven days previous to birth and being 

 practically completed by three days thereafter (table, p. 444). 

 Following von Winiwarter ('00) it has been found possible to 



