472 The Bird 
upper part of the shell may be carefully picked away with 
a pin and the little embryo exposed to view. 
When thirty-six hours old it measures almost one 
quarter of an inch in length and shows many interesting 
things. The embryo is set off from the rest of the yolk, 
much as one’s hand is if placed under a piece of cloth, the 
latter then being tucked in beneath the palm in all direc- 
tions, until the gathered portion is closely constricted. 
We are able with a good lens to make out which is the 
head and which the tail end of the future chick, the former 
being broader and showing the beginning of the two tiny 
swellings—the future eyes. Behind these, four faintly 
outlined enlargements along the central line show the 
anlagen of the various parts of the brain. These take up 
about one third of the entire length of the embryo, showing 
the importance of the organs of the head. Still farther 
back are two rows of little segments strung along the 
centre line—the false back-bone, hinting of the worm-like 
series of muscles, of which we have already spoken (page 
69). 
A heart is even now hinted at, but is seen better in a 
later stage. An interesting thing about it, however, is 
that, at this stage, it is really in the head region, vividly 
recalling the condition existing in fishes, where it is very 
far forward in the body, in fact only just behind the gills. 
At this period in the chick embryo the heart, instead of 
being a complicated organ, divided into four complete 
cavities, is very similar to that organ in our old friend 
Amphioxus, that lowliest of all fishes, where it is nothing 
but a slightly enlarged, contractile blood-vessel. In this 
