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 layer 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 1s 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 
ege. 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 
ege? Just this. When 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 
