EARLY DEVELOPMENT OF THE PIGEON'S EGG. 



237 



3. The 16-cell Stage. — Fig. 4 is a photograph of a pigeon egg 

 of sixteen cells. Although of later development, it was obtained 

 an hour earlier than the egg shown in Fig. 2, — i. e., it was taken 

 from the oviduct at 3:45 A. M., seven hours and forty-five min- 

 utes after the time from which fertilization is reckoned. The 

 arrow indicates the direction of the axis of the embryo, and the 

 anterior side of the blastoderm is in the direction of the point of 

 the arrow. 



Three principal regions in the blastoderm of the bird's egg 

 are to be recognized in surface view at this stage : (1) the cen- 



FlG. 4. Photograph of a pigeon's egg 7|^ hours after fertilization. 3.45 A. 

 M. The anterior side of the blastoderm is toward the point of the arrow. 



tral area, (2) the marginal cells, (3) the periblast. In this egg 

 (Fig. 4), the central area is occupied by six cells [the Furch- 

 ungskugeln of Kolliker (6)] and there are ten marginal cells 

 (Kolliker's Segmenten). The periblast is the zone outside the 

 marginal cells. At the inner margin of this zone is the 

 accessory cleavage caused by the supernumerary sperm nuclei. 

 This is on all sides, but at a few places where there are no 

 sperm nuclei, the large marginal cells are open peripherally. 



