NO. 1788. 



NORTH AMERICAN ERGASILID^— WILSON. 



317 



Their rate of growth is well shown in figure 29, which gives three stages 

 of development. At the beginning one egg evidently ripened alone; 

 in the second group there were three eggs on one side and four on the 

 other, whUe in the third group there were again three eggs on one side 

 and four on the other. 



So long as the eggs remain in the oviduct they are granular and 

 homogeneous and surrounded by only a single delicate membrane. 

 The formation of the second outer shell takes place in the external 

 sacks. As development proceeds these two shells separate more and 

 more from each other, and the outer one thickens and hardens a little. 



Coming thus separately, with intervals between the successive 

 groups, each egg has time to assume a spherical shape in the proximal 



Fig. 29.— Three successive stages in the growth of the egg cases of Ergasilus centrarchida- 



RUM, showing that THE EGGS RIPEN IN GROUPS AND NOT SINGLY. 



end of the sack before the next lot issues. And the pressure being 

 diffused around an angle, is distributed in different directions; while 

 some of it may continue to be parallel to the axis of the egg-tube by 

 far the greater part is tangential. In consequence the eggs do not 

 assume a biscuit shape but remain more or less spherical, being flat- 

 tened only where they press agamst one another. 



Again there are no partitions here between the individual eggs as 

 in the CaligidaB, but the eggs lie in actual contact one with another. 

 This allows the pressure to be evenly distributed throughout the 

 entire tube, thereby reducing it to the lowest possible terms. The 

 eggs are also capable of moving about within the tube and can read- 

 just themselves whenever necessary. Consequently there is usually 

 very little flattening even at the points of contact. 



