The Early Development of the Pigeon's Egg. 35 



Unfortunately, this cannot be demonstrated experimentally. It 

 seems to me that the entrance of more than one spermatozoon is 

 a casual event. The same condition in the egg that secures fertiliza- 

 tion (the attractive influence) also takes care of the supernumerary- 

 spermatozoa, be they many or few. I do not consider it a protective 

 device ( Schutzvorrichtung) . 



IV. Area of Primary Cleavage. 



. Maturation Stage. — The surface view of the pigeon's egg before 

 the ai^pearance of the first cleavage plane has been previously 

 described (p. 4, Chart I A, and Eig. 45). The periblastic zone is 

 not conspicuous in all eggs in the maturation stage. What causes 

 the differentiation between periblast and blastodisc, I cannot explain 

 at present. Possibly the two areas are not distinguished from each 

 other until the migrating sperm nuclei come to rest at the inner 

 margin of the periblast. 



Direction of the First Cleavage Plane. — The first cleavage plane 

 may appear about 1 a. m., five hours after fertilization. Its position, 

 and direction in relation to the axis of the future embryo seem not 

 to be constant. There are, of course, occasional variations in the 

 orientation of the embryo. I have found pigeon eggs in which the 

 primitive streak was nearly parallel to the chalazal axis, and others 

 in which the angle with the chalazal axis was about 60°, but the 

 anterior end was toward the left instead of the right (see Fig. 1). 

 These abnormalities in orientation are comparatively rare. But it 

 may be that some of the eggs which I have studied in the two-celled 

 stage which present variations in direction of the first cleavage plane, 

 would have shown an abnormal orientation of the embryo. One 

 egg obtained about 1.15 a. m., 51/4 hours after fertilization, had the 

 first cleavage plane makijig an angle of about 7° with the chalazal 

 axis. Another egg (Fig. 18) apparently has the first cleavage 

 plane parallel to the longitudinal axis of the embryo, although this 

 may be the second cleavage, and the first may have nearly disap- 

 peared. 



I watched the development in another egg (Egg 404) (Fig. 17). 

 When I first saw this egg at 3 a. m., there was one cleavage plane 



