EARLY DEVELOPMENT OF THE CHICK. 



67 



When the ovum escapes from the ovary it immediately enters the oviduct. 

 The spermatozoa having traversed the oviduct, at once from four to twenty- 

 five of them enter the cytoplasm. Polyspermy is apparently a normal inci- 

 dent, although only one sperm nucleus unites with the egg nucleus. The 

 entrance of the sperms seems to stimulate the formation of the polar bodies 

 (maturation), and as soon as this is accomplished one of the sperm nuclei 

 unites with the mature egg nucleus to form the nucleus of the fertilized ovum. 



c d 



FIG. 38. Cleavage in hen's egg. Coste. Germinal disk and part of yolk, seen from above. 



^ Cleavage. As soon as the two pronuclei unite in the cytoplasmic disk 

 a spindle appears in preparation for the first division. The spindle is parallel 

 to the surface and the first cleavage is therefore vertical. The cleavage 

 plane is indicated on the surface by a slight furrow near the center of the disk, 

 the margin of the disk being undivided. The first two blastomeres are there- 

 fore not completely separated from each other. The new spindle in each 

 primary blastomere forms parallel to the long axis of the cell, and the second 

 division occurs at right angles to the first and is also vertical. The plane of 



