The Early Development of the Pigeon's Egg. 9 



is determined by the depth of the vertical cleavage plane. The 

 latter measures 0.12 mm. in this egg, and the first horizontal cleavage 

 (Fig. 5) which marks the position of the future segmentation cavity 

 occurs later at the same depth. Compare Figs. 2, 3, 4, 5, 6, and 11, 



"^ +■■■-..... 



which are drawn to the same scale from central transverse sections 

 of different eggs. 



Five primary cleavage nuclei were found in this egg, Chart II B, 

 the nucleus of the cell at the left having divided without a cleavage 

 plane being yet formed. 



