PLATE 4 
EXPLANATION OF FIGURES 
45 Sixty-three-cell stage from the upper pole. 
46 Same stage as the preceding, from the ventral pole; very few of the ecto- 
meres are visible. 
47 Seventy-cell-stage, ventral view. Increase in number due to division of 
small cells. Consult table of cleavage. First nearly equal division of the pro- 
teloblasts, X, X. This division separates the neural and nephridial elements. 
48 Seventy-two-cell stage, ventral view. 
49 Little later than the preceding stage, to show x® and x’. 
50 Embryo, ventral view, to show the first division of the nephroblasts; 
transverse axis greater than the longitudinal. 
51 Embryo, ventral view, turned anteriorly to show the blastopore. 
52 Embryo to show the lengthening of the anterios-posterior axis. The 
small cells, x®, x®, are good points to mark the orientation of the different figures. 
All figures on plate are similarly orientiated with reference to the right and left 
sides of the embryo. 
53 Stage a little later than the preceding. 
54 Embryo from upper pole, to show derivatives of x® between the nephro- 
blasts. 
55 Embryo with upper surface turned posteriorly, to showthe rapid division 
of the ectoblast cells. : 
56 and 57 Show that either nephroblast X(2) or X() may divide, to form the 
three nephroblasts on either side. 
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