The Mother Decides the Sex 



Its pebble, the other on its bough. Their 

 spherical configuration is therefore very prac- 

 tical. 



The nest of the Mason-bee of the Walls 

 consists of a cluster of upright cells backing 

 against one another. For the whole to take 

 a spherical form, the height of the chambers 

 must diminish from the centre of the dome to 

 the circumference. Their elevation is the 

 sine of the meridian arc starting from the 

 plane of the pebble. Therefore, if they are 

 to have any solidity, there must be large cells 

 in the middle and small cells at the edges. 

 And, as the work begins with the central 

 chambers and ends with those on the circum- 

 ference, the laying of the females, destined 

 for the large cells, must precede that of the 

 males, destined for the small cells. So the 

 females come first and the males at the finish. 



This is all very well when the mother her- 

 self founds the dwelling, when she lays the 

 first rows of bricks. But, when she is in 

 the presence of an old nest, of which she 

 is quite unable to alter the general arrange- 

 ment, how is she to make use of the few va- 

 cant rooms, the large and the small alike, 

 if the sex of the egg be already irrevocably 



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