THE WAY LIFE BEGINS 43 



the duct. They are said to gain more yolk as they pass along. 

 Turning about on their long axes, they gather albumin or 

 'white' of the egg from the glands lining the oviduct. Near 

 the lower end of the duct the egg receives its final covering, 

 the lime shell, which on passing out of the tube, is soft and 

 pliable until hardened in the air. For a land animal whose 

 young are formed outside the body of the mother the shell 

 with its rigid yet porous wall is a genuine discovery. How 

 great an improvement it is, too, over the gelatinous covering 

 of the fish or the frog! 



The Egg is Started on Its Course of Development 

 by a Free Swimming Cell from the Male 



From time to time the male bird deposits at the entrance 

 of the oviduct hundreds of thousands of male sperm cells. 

 These are very much smaller than the ova — so small, indeed, 

 that it is necessary to use the highest powers of the microscope 

 before they are clearly visible. Each sperm cell is equipped 

 with a long swimming tail, with which to make its way up 

 the oviduct. In the upper part of this duct, above the region 

 of the albumin and shell glands, they await the coming of 

 the ova. Although so many thousand sperms are ready to 

 fertilize the ovum, but one really does so. A few may enter 

 the great yolk sac and move by some subtle attraction 

 toward the mother-germ; all but one, however, are halted 

 before reaching the nucleus of the egg. The fortunate sperm 

 that passes within the prohibited zone unites with the egg 

 nucleus, leaving its tail in the yolk. 



The sperm cell undoubtedly starts the egg on its course 

 of development, but it also does something more. It con- 

 tributes its share of inheritance from the father, and, through 

 him, from all his ancestors. 



Modeling the Chick Out of the Yolk 



As soon as the two germ cells have united, the work of 

 building the new body begins. The united cells divide and 

 subdivide. At first there are two, then four, then eight, and 

 so on, the divided cells remaining in contact to form layers, 



