PLATE XIV 



Two LATER STAGES OF CLEAVAGE OF THE EGG OF THE SEA-URCHIN, x 1000 (Wilson) 



FIG. i. "The embryo is here shown in vertical section at the moment preceding the divi- 

 sion from 8 to 1 6 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." 



FIG. 2. "The blastula. Sixteen-celled stage. 



" This section shows the blastomeres arranged in a hollow sphere surrounding a central 

 blastocoel or cleavage-cavity. The nucleus is visible in each cell, and some of them show also 

 the attraction-spheres (asters)." 



The figures in Plates XIII and XIV are reproduced from the original negatives made for 

 Wilson's Atlas. The eggs were fixed with sublimate-acetic, afterward preserved in alcohol, 

 embedded in paraffin, sectioned and stained on the slide with Heidenhain's iron-hematoxylin 

 for twenty-four hours, and " differentiated in a one per cent solution of iron-alum to a bright 

 but delicate blue." The figures were .printed in the Atlas in black. They are printed here 

 in blue, but without an attempt to reproduce the exact shade of the objects. 



