238 OUTLINES OF CHORDATE DEVELOPMENT 



or follicle, forming there the germinal disc or spot. When the 

 ovum reaches a diameter of something more than 0.5 mm. 

 the nucleus migrates into the germinal disc, where it remains 



FIG. 87. Growth stages in the oogenesis of the hen's egg. After Sonnen- 

 brodt. A. Oocyte measuring 0.012 X0.016 mm., the nucleus of which is 0.006 

 mm. in diameter. B. Oocyte measuring 0.018 X0.028 mm., the nucleus of which 

 is 0.0105 X0.014 mm. Enclosed in follicle. C. Oocyte measuring 0.040 X0.045 

 mm., the nucleus of which is 0.020X0.022 mm. D. The nucleus only, of an 

 oocyte measuring 5.84 X 6. 16 mm., the nucleus itself measuring 0.214 X 0.238 mm. 

 Total view showing the small chromosomes in the midst of a collection of chro- 

 matin nucleoli. E. Vertical section through the nucleus only, of an oocyte, the 

 follicle of which measured 37 mm. in diameter. The nucleus itself is 0.455 mm. 

 in diameter and 0.072 mm. in greatest thickness, c, Chromosomes; cr, extra- 

 nuclear chromsome-like bodies; /, follicle; ra, nuclear membrane; mf, folds in 

 nuclear membrane; n, nucleus; nu, chromatin nucleolus; ps, pseud ochromosomes; 

 s, centrosphere; v, yolk nucleus or vitellogenous body. 



throughout the remainder of its ovarian history (Fig. 88). It 

 should be noted that the eccentricity of the nucleus, which 



