EARLY DEVELOPMENT OF THE PIGEON'S EGG. 239 



ter of this blastoderm is shown in Fig. 5 . The accessory cleav- 

 age is confined to the region immediately outside of the large 

 marginal cells of the blastoderm, but the sperm nuclei have mi- 

 grated peripherally into the unsegmented part. These nuclei 

 were more abundant than this drawing suggests ; for on 

 the right hand side of the section there were four more 

 nuclei in superficial positions in the unsegmented part beyond 

 the limits of the figure. The sperm nuclei were just as abundant 

 in every other section of this egg. But these nuclei have mi- 

 grated not only peripherally ; they are also under the large mar- 

 ginal blastomeres. The latter, however, are definitely separated 

 from the sperm nuclei by cleavage planes whose significance will 

 be better appreciated in contrast with a stage after the disappear- 

 ance of the sperm nuclei as shown in Figs. 6 and 9. 



Fig. 6. Longitudinal section of pigeon's egg at the time of disappearance of the 

 sperm nuclei ; on the left (anterior), the marginal cell has become open, i. e., contin- 

 uous with the marginal periblast. On the right the marginal cell is still slightly sep- 

 arated from the periblast at the surface. Surface view of the egg showed traces of 

 accessory cleavage ; note continuity of the central cells with central periblast, t . Mar- 

 ginal cells. 2. Cone of protoplasm. 3. Marginal periblast. 4. Neck of latebra 

 (white yolk). 5. Yellow yolk. Egg taken 7 A. M., about eleven hours from ferti- 

 lization (estimated). 



