The Early Development of the Pigeon's Egg. 31 



accessory cleavage on the surface, and in the sections no nuclei 

 outside of the blastomeres (the egg of Chart V) was monospermic. 

 I cannot prove the contrary in the case of this particular egg. My 

 argument for the disappearance of the supernumerary sperm nuclei, 

 the establishment of a zone of junction and the organization of the 

 periblast with nuclei from the marginal blastomeres does not rest 

 upon this alone. The presence of the well marked line of cleavage 

 separating the blastodisc from the periblast where the sperm nuclei 

 exist and the absence of those cleavage planes from other parts of 

 the same egg where there are no sperm nuclei, and the constant con- 

 tinuity between blastodisc and periblast after about twelve or fourteen 

 hours after fertilization, are facts which argue strongly for the 

 difference in character of the free nuclei in early and late stages. 



My argument for the disappearance of the supernumerary sperm 

 nuclei and the subsequent opening of the marginal cells to become 

 continuous with the periblast, is further supported by observations 

 of a living egg through about eight hours of development. I saw 

 the egg first at 2.15 a. m. and made a free hand sketch of the surface 

 view (Fig. 16 A). Fig 16 B is a sketch of the same egg at 2.50 a.m. 

 The accessory cleavage has increased slightly. The cleavage planes 

 have become more distinct. A little later, I saw white spots appear- 

 ing in the periblast. They probably marked the position of the 

 supernumerary sperm nuclei. I watched one particular spot, and 

 saw it elongate and divide into two. As the supernumerary nuclei 

 multiplied, the peripheral outlines of the marginal cells became more 

 distinct, but where there were no nuclei in the periblast, the marginal 

 cells were continuous with it. At 5.15 a. m. the lines of accessory 

 cleavage had become indistinct, and finally, at 10.00 a. m. (when 

 the egg was killed) there were only faint traces of accessory cleavage, 

 although some white spots remained, indicating the presence of 

 supernumerary nuclei. They disappeared more slowly in this egg, 

 perhaps, than in normal conditions, at any rate there was little change 

 in the egg during the last three hours that it was under observation. 

 (The egg has not yet been sectioned). 



I have placed no emphasis on the histological characters of the 

 nuclei, because there is great danger of misinterpretation here. 



