HOW THE MAMMALS DEVELOPED 203 



These cells swim nearly to the top of the tubes in 

 which they are placed, and there fertilize the descend- 

 ing eggs. Farther down the canal the shell is se- 

 creted about the now swollen mass of yolk and white, 

 completing the egg just before it leaves the parent. 



If the evolutionist understands properly the line of 

 descent, the birds and mammals are both the descend- 

 ants of the reptiles. While there is less exterior re- 

 semblance between a chicken and a turtle than be- 

 tween a cat and a turtle, the real relationship in the 

 first case is much closer than in the second. This is 

 perhaps most easily seen in the scaley legs of both 

 bird and reptile. Another remarkable resemblance 

 lies in the fact that in both cases the eggs are large, 

 well stored with nourishment, and protected by a re- 

 sistant shell. 



So few people know the turtle's egg that it will be 

 better to describe that of the hen, which it largely 

 resembles. Underneath the hard shell is a tough but 

 flexible membrane which lies against the limey coat- 

 ing, except at the blunt end, where a separation be- 

 tween the two gives room for a bubble of air. In- 

 side of this shell and its membrane lies the white of 

 the egg, which is nourishment for the chick, though 

 not nearly so rich as the yolk. This, besides the 

 albumen which it contains, is stored with large quan- 



