MODIFICATION OF REPRODUCTIVE PROCESS 73 



and the zygote so produced slowly travels along the 

 tubular passage (oviduct) towards the outside. As it 

 does so the lining of the tube deposits on the surface of 

 the zygote layers of covering material first the white 

 or albumen (Fig. 43, alb), with thicker strands (ch) 

 towards each end to moor the zygote in position, then a 



FIG. 42. 

 Pipa amcricana, female with young escaping from the cells upon her back. 



tough white membrane (s.m), and lastly, as the egg 

 approaches the exterior, a layer of limy material which 

 solidifies to form the hard porous shell. The membrane 

 is split into two layers towards the broad end of the 

 shell the end which is directed towards the external 

 opening of the oviduct as the egg travels along and 

 after the egg is laid, as the white gradually shrinks in 

 volume, air accumulates in the space between these two 

 layers (Fig. 43, a, s). Hence it is that when an egg not 

 quite fresh is placed in water the broad end instantly 



