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 



