198 MORGAN. [Vol. IV. 



a number of small nuclei lying in close contact with the follicle 

 cells. These seem to belong to the stroma of the ovary, and 

 to be added secondarily to the growing ovum. 



In later stages the test-cells do not seem materially to change 

 either in number, or size, or structure, but the follicular cells 

 continue to increase in size and become much vacuolated. 



By the methods of preparation ordinarily employed, the boun- 

 daries of the cells of the follicle do not appear in sections of young- 

 ova. In order to determine the shape and arrangement of the 

 follicular cells, I experimented with other methods and found one 

 which made clear the structure of the cells, and indirectly con- 

 firmed the conclusions reached by previous methods of prepara- 

 tions. The fresh ovaries were teased apart in very dilute osmic 

 acid, washed in distilled water, and placed in a one per cent 

 solution of silver nitrate, where they remained for half an hour ; 

 then put into a two per cent solution of acetic acid for the 

 same length of time, and placed in the sunlight. They were 

 then examined under the microscope, and the cell boundaries 

 were distinctly seen. The eggs were then carried through the 

 usual processes, imbedded in paraffin, and cut into thin sections. 



In young ova, the follicular cells were found from surface views 

 to have irregular outlines, and in general appearance to resemble 

 peritoneal epithelial cells. A bit of the periphery of the ^gg is 

 shown in Fig. 7, which corresponds approximately to an egg 

 at an age represented in Fig. 3. 



As the Q.^^ increases in size, the outer surface area of each 

 follicular cell diminishes ; but it will be remembered they are 

 increasing in thickness during this period. At the time when 

 the test-cells are forming, corresponding to Fig. 5, the follic- 

 ular cells have reached a size shown in surface view by Fig. 8. 

 In Figs. 9 and 10 are drawn sections of the periphery of eggs 

 at this stage, showing the origin of the test-cells. Here it 

 is seen that one of the follicular cells pushes itself inside of 

 the follicular zone and becomes a test-cell. Comparing Fig. 8 

 with 9 and 10, the size of the test-cells is seen to exactly corre- 

 spond to that of the follicular cells ; so that we find at the time 

 when the test-cells are formed that the follicular cells have 

 reached exactly the same size as the test-cells, and this by itself 

 would lend probability to the view that the inner cells are 

 derived from the outer. Taken in connection with what we learn 



