36 



TEXT-BOOK OF EMBRYOLOGY. 



the second polar body. The sperm nucleus and centrosome then traverse 

 the yolk area to meet the mature egg nucleus which in the meantime has 

 migrated toward, but not quite to, the center of the egg. The division of the 

 ^perrn centrosome to form a disaster and the arrangement of the chromosomes 

 of the two pronuclei in the equatorial plane comprise the preparatory step 

 for the first cleavage. These phenomena are identical with the prophase of 

 mitosis (Fig. 19). 



The position that the spindle assumes is determined by three factors: 

 the point where the first polar body is extruded, the point where the sperm 



enters, and the location of the yolk-free 

 area. A plane bisecting this area and pass- 

 ing through the other two points will divide 

 the egg into symmetrical halves. The spindle 

 takes its position at right angles to this 

 plane. The first cleavage therefore will pro- 

 duce two equal and symmetrical daughter 

 cells, or blastomeres, the first cleavage plane 

 coinciding with the plane of symmetry of the 

 ovum. These two blastomeres will become 

 the right and left halves of the embryo, the 

 plane of symmetry of the ovum representing 



FIG. 19. Prophase of first cleavage the sagittal plane of the embryo. With the 



anterior portion already indicated by the 

 point of extrusion of the first polar body, the 

 orientation of the first two blastomeres rela- 

 tive to the future embryo is now complete. 



The second cleavage plane falls at a right angle to the first, cutting both 

 the animal and the vegetative pole. The division is slightly unequal, how- 

 ever, the result being two slightly smaller blastomeres and two slightly 

 larger blastomeres (Fig. 20, A ) . These are arranged symmetrically on the two 

 sides of the median plane. The third cleavage plane lies at right angles to 

 the other two, and division of the cells is again slightly unequal (a condition 

 often called subequal), the result being four pairs of cells of four different 

 sizes (Fig. 20, B) . The smallest cells are those derived from the portion of the 

 ovum which contained less yolk, the largest are those derived from the por- 

 tion which contained more yolk. All the cells have divided completely, a 

 circumstance which gives rise to the term total cleavage; and this condition 

 obtains throughout the later stages. All the cells at a given cleavage thus far 

 have divided at the same time, a fact which is expressed in the term regular 

 cleavage. If cleavage were to continue regularly the result at succeeding 

 divisions would be 16, 32, 64, 128 cells, and so on. Regularity is lost, how- 





figure in ovum of Amphioxus. 

 The chromosomes of the male 

 and female pronuclei are ming- 

 led in the equatorial plane. 

 Sobotta, from Kellicott. 



