PLATE 7 



EXPLANATION OF FIGURES 



17 Section of a 4-celled egg No. 81 (2), showing characteristic appearance of 

 yolk in this batch of eggs. In some of these eggs the blastomeres are without 

 clear-cut cell membranes (X 390). 



18 Section through three blastomeres of the 4-celled egg No. 83 (7). The 

 blastomeres of the 4-celled stages are in all cases so oriented that no section con- 

 tains more than three cells (X 390). 



19 Section through a 7-celled egg, No. 85 (7). The tendency of the cells to 

 flatten out against the zona is already manifested at this stage. The peripheral 

 arrangement of 3'olk remaining within the blastomeres is well shown (X 390). 



20 Section through 9-celled egg, No. 85 (3). The blastomeres are in contact 

 with the albumen layer. The cells are large and little yolk is seen in the cleav- 

 age cavity. The vacuolization in the inner margins of cells is due to breaking 

 down of deutoplasmic content of cells in this region. The yolk has not all as 

 yet been crowded into the cleavage cavity (X 390). 



21 Section through the four blastomeres of the 12-celled egg No. 85 (18) b. 

 The blastomeres are numbered to correspond to the figures employed in text 

 figure 3 B. Cells 5 and 6 are in mitosis. Unlike the two preceding specimens, 

 the cells of this egg are not 3'et in intimate contact with the zona (X 390). 



22 Section through the 16-celled egg No. 85 (2). The centrifugal migration 

 of the cells and their flattening out against the zona are well shown. The yolk 

 and coaguluni occupy the cleavage cavity (X 390). 



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