BILATERALTTY OF THE PIGEON's EGG 287 



two ooplasmic zones may be more or less clearly distinguished, 

 the central and peripheral protoplasm of authors. At this time 

 when the germinal vesicle is still near the center of the oocyte, 

 the first definitive yolk granules (white yolk) appear, as the photo- 

 graphs show. 



Now as the oocyte continues to grow, the germinal vesicle 

 begins to migrate peripherally along the polar axis to the animal 

 pole (figs. 24 and 25). This is no* wandering of the germinal 

 vesicle to the best nourished region of the cell as Sonnebrodt 

 seems inclined to think; the path to the periphery is predeter- 

 mined, for the polar axis is never changed. The migration begins 

 in oocytes of about 0.9 mm. and the germinal vesicle is usually 

 quite peripheral when they have reached a diameter of 1.5 mm. 

 Shortly after the beginning of the migration ver}^ fine yolk sphe- 

 rules appear in the central protoplasm so that they are again 

 present in all parts of the ooplasm except in the peripheral pro- 

 toplasm. 



During this period characteristic changes occur within the 

 nucleus which have been best described by Sonnenbrodt ('08) 

 for the hen. So far as my examination goes, I can confirm his 

 descriptions for the pigeon in most particulars. It may not be 

 out of place to say that in every oocyte I have studied it was 

 possible to demonstrate the chromosomes or their morphological 

 equivalents. 



At any given time one finds in an ovary about twelve, rarely 

 more, oocytes from 2 to 5.5 mm. in diameter. The growth at 

 this stage is due chiefly to an increase of the fluid content and a 

 periodic deposition of yolk granules in the central protoplasm, 

 while the peripheral protoplasm grows thinner and thinner (com- 

 pare figures 27 and 8 from oocytes of 2.5 and 3.2 mm. respectively). 

 In figures 26 and 27 the path of the germinal vesicle to the periphery 

 is marked by a trail of fine reticulum and in the latter the region 

 that surrounded the germinal vesicle during its stay near the 

 center of the oocyte is clearly defined by the smaller yolk granules. 

 This whole flask-shaped area (lat) is the anlage of the latebra. 

 Its position was determined by that of the germinal vesicle and 

 accordingly it is near the animal pole and nearer one end of the 



