32 ATLAS OF THE FERTILIZATION AND KARYOKINESIS OF THE OVUM. 



Plate X. Phototype 39. 



The Fourth Cleavage. 



The embryo is here shown in vertical section at the moment preceding" the division from S to 16 cells. The 

 two lower cells are dividing equally and vertically, one of the amphiasters being seen endwise, the other en face. 

 The two upper cells are about to divide unequally to form two smaller cells or micromeres (p. 2, Fig. I.). 

 As the phototype shows, the chromosomes are equal on the two sides, but the asters are conspicuously unequal, and 

 in this fact perhaps lies the immediate cause of the inequality of division. Why the aster should be unequal is not 

 known. It is perhaps due to the position of the karyokinetic figure, which, as the figure shows, lies excentrically 

 in the cell. 



Plate X. Phototype 40. 

 The Blast II la. Sixteen-celled Stage. 



This section shows the blastomeres arranged in a hollow sphere surrounding a central blastocozl or cleavage- 

 cavity. The nucleus is visible in each cell, and some of them show also the attraction-spheres (asters). 



