PLATE 1 
EXPLANATION OF FIGURES 
1 Surface view of an unsegmented ovum, to show the polar bodies and 
the cleavage nucleus. 
2 Two-cell stage from the upper pole. 
3 Two-cell stage; cell CD dividing. 
4 Three-cell stage from the upper pole; division of CD is complete. 
5 Same stage as preceding; cell AB dividing. 
6 Four-cell stage from the upper pole; the cleavage spindle for the first 
ectomere forming. 
7 Four-cell stage, ventral view. 
8 Four-cell stage viewed from the left side. 
9 Same as the preceding, viewed from the right side. 
10 Four-cell stage from the upper pole, showing the formation of the first 
ectomere. 
11 Five-cell stage from the upper pole, d! formed. 
12 Stage showing the cleavage spindle of the second ectomere. 
13 Six-cell stage from the upper pole; cleavage spindles for third and fourth 
ectomere forming. 
14 Hight-cell stage from the upper pole; the macromeres are considerably 
flattened. 
15 Same as the preceding, from the ventral pole; similar to the same view of 
the four-cell stage. 
16 Eight-cell stage from the left side. 
17 Same as the last showing the behavior of the macromere D in the forma- 
tion of the first somatoblast. 
18 Stage a little later than the preceding. 
19 Nine-cell stage from the left side after the formation of the first somato- 
blast. 
20 Nine-cell stage turned a little to the left, so that all the cells are visible. 
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