468 The Bird 



immediately begins to divide into numerous cells, as in 

 the case of the frog's egg. This goes on until the egg is 

 laid, and when we break the shell, we see at the uppermost 

 part of the sphere of yellow yolk a well-defined portion, 

 in appearance a tiny ring of cloudy, opaque matter enclos- 

 ing a transparent circle. So now we see the use of begin- 

 ning our investigation with the frog's egg, that of the fowl 

 having reached quite an advanced stage before it is laid. 



The ring and circle of the embryonic spot on the yolk 

 consists of a laj-er of small, even cells, like cobblestones. 

 These are spread over the top of the yolk, while just be- 

 neath is a jumbled mass of many larger cells. The opaque 

 ring is caused by a thicker, denser concentric layer of these 

 lower cells. When heat is applied, this outer layer begins 

 to segment rapidly, the new cells spreading down over 

 the surface of the great ball of yolk; a curving depression 

 dimples the surface of the little transparent circle, pushing 

 in deeper and deeper; and behold! we have the very same 

 condition — the gastrula stage — which we saw in the frog's 

 egg. To make this stage in the egg of the hen more real, 

 squeeze the rubber ball into a hemisphere and clap it 

 upon an orange so that the two layers of rubber fit, cap- 

 like, upon the fruit. 



This is all very wonderful, but what special significance 

 has it? What particular point upon which we may sus- 

 pend it in our memory, so that it will always return to us 

 with a thrill of interest and wonder whenever we see an 

 egg ? Just this. ^\Tien we first examined the frog's egg, 

 and when the egg of the chick was still attached to the 

 ovary, they were comparable to the one-celled creatures 



