296 W. B. KIRKHAM AND H. S. BURR 



centh^ occurred. Cases such as this show that ovulation usually 

 occurs about twenty-four hours after parturition. The individ- 

 ual variation is so great that any complicated apparatus for 

 determining the exact date of parturition is valueless. We have 

 obtained the best results by killing the females at half hour inter- 

 vals, beginning in the later afternoon and continuing through the 

 early evening. By doing this practically all the stages of matu- 

 ration can be obtained. 



In a number of instances the senior author dissected out the 

 Fallopian tubes, and after placing them in warm salt solution, 

 by slitting the tubes he was enabled to obtain two eggs fertilized 

 but unsegmented, three eggs in the two cell stage and three 

 eggs so obscured by follicle cells as to prevent any exact infor- 

 mation as to their condition. The technique of this operation 

 is so simple, requiring only a binocular microscope, two needles, 

 some warm physiological salt solution, and a female rat that 

 has given birth to a litter at least twenty-four hours before and 

 not more than five days previously, that we recommend the rat 

 as highly as the mouse for obtaining live mammalian eggs for 

 class demonstration. 



In all other instances the ovaries (and also the tubes, wher- 

 ever ovulation was thought to have occurred) were fixed in either 

 Zenker's 'fluid or in a strong solution of Flemming, imbedded 

 in paraffin, cut serially into sections 0.010 mm, thick and stained 

 in Delafield's haematoxylin. Such sections as were found on 

 subsequent examination to be worthy of detailed study were 

 later decolorized with acid alcohol and restained with Heiden- 

 hain's iron-haematoxylin. 



A study of the ovaries of the above rats showed that there 

 is a progressive development of the egg until it is ready to leave 

 the ovary at ovulation. The developing eggs of any adult ovary 

 can be readily divided into six groups. The first of these (fig. 1) 

 includes all those eggs that are in the resting condition. These 

 vary considerably in size, as do also their follicles. The earlier 

 stages show a small egg with a follicle consisting of from one to 

 three layers of radially arranged follicle cells with scattered cells 

 lying between the layers, the later stages lie in larger follicles 



