The Early Development of the Pigeon's Egg. 13 



No attempt was made to count the supernumerary sperm nuclei 

 in this egg. They were very abundant, so that in almost any section 

 of the blastoderm several were found in both the marginal and central 

 periblast. They had evidently multiplied by repeated divisions. In 

 earlier stages, the cells of the primary area were separated from the 

 sperm nuclei merely by the cleavage planes enclosing the small acces- 

 sory cells (see Fig. 5). But in this stage, when the sperm nuclei 

 have become so numerous, the cytoplasm of the blastodisc is definitely 

 separated from the periblast, i. e., there are diagonal or submarginal 

 cleavage planes ventral to the marginal cells of the primary area. 

 See Chart IV C and Figs. 6, 7, and 8. The subgerminal super- 

 numerary nuclei are found as far centrally as the margins of the 

 nucleus of Pander. 



Chaet V. 



Chart V, Fig. A, presents an incomplete drawing of the surface 

 of an egg taken from the oviduct at 7 a.m. — about eleven hours 

 after fertilization. There ds no accessory cleavage. The marginal 

 cells are all open peripherally, and only a few of the central cells 

 are drawai to show their relative size. This chart presents a striking- 

 contrast to all those that have preceded ; for in this Qgg there was not 

 a single supernumerary sj)erm nucleus. The nuclei that were found 

 in the sections were all in the cells of primary cleavage with no nuclei 

 in either the marginal or central periblast. There is not usually 

 such an abrupt change as that indicated between the Charts IV 

 and V. Sections of other eggs of about this stage show a diminishing 

 number of sperm nuclei. They do not usually disappear simultane- 

 ously on all sides of the blastoderm as seems to have happened in 

 this egg (Chart V). And I have not been able to discover that there 

 is any particular side where they regularly persist longest. Of 

 course, their original position in the egg is variable. But wherever 

 the sperm nuclei do persist, the marginal cells are limited peripherally 

 and ventrally in such a manner as is shown in Chart IV A and C, 

 and Figs. 6, 7, 8, and 10 D to H. And where the sperm nuclei 

 have disappeared, the marginal cells are open peripherally and ven- 

 trally, as in Chart V A and C, and Fig. 10 A and B. 



