286 GEORGE W. BARTELMEZ 



food yolk. Thus one of the main features of the bilateral struc- 

 ture of the mature ovum is determined. The position of the 

 germinal vesicle nearer to one end of the long axis determines the 

 other, as will be shown below. 



C. The period of differentiation: {O.4 to 5 mm.) 



As the oocyte grows from three to four-tenths of a millimeter 

 and the yolk spherules are laid down only at the periphery, the 

 germinal vesicle remains relatively stationary in position, so that 

 it comes to lie nearer and nearer to the center of the cell (com- 

 pare figures 14 to 17, 19 and 20). During the ensuing stages this 

 process is continued as figures 21 to 23 show. Does it ever become 

 quite central in position? This is a matter of theoretical im- 

 portance and, since the evidence from paraffin sections cannot be 

 relied upon, it has been decided by a study of creasote prepara- 

 tions of entire oocytes and by the use of thick free hand sections. 

 Figure 6 is from an oocyte drawn in creasote and in this, as in all 

 oocytes studied, the germinal vesicle was nearer one pole, the ani- 

 mal pole. This is the only stage at which there could be any ques- 

 tion as to the eccentricity of the germinal vesicle and the fact that 

 it is always nearer one pole, defining the polar axis perpendic- 

 ular to the long axis, completes the evidence that we are dealing 

 throughout development with the same polar axis. If the ger- 

 minal vesicle migrated about in response to external forces it 

 should be possible to find cases in which it is quite central. Such 

 cases are not found and this agrees with the statements of all 

 recent workers on the bird and reptile ovary. 



The appearance of the oocyte during the earlier stages of the 

 period of differentiation is shown in figures 21 to 23. The first, 

 from an oocyte of 0.51 mm. stained with Sudan III, illustrates 

 the position of the sperule zone (sz) near the periphery. The 

 next period of growth does not involve the deposition of yolk 

 spherules and so the peripheral protoplasm becomes wider. Some- 

 what later, in oocytes of about one millimeter, another zone of 

 spherules is laid down and this is the first evidence of the periodic- 

 ity in yolk formation which characterizes the final growth period 

 (Riddle, '11). In oocytes from 0.5 to 0.8 mm. (figs. 22 and 23), 



