STRUCTURE AND DEVELOPMENT 61 



that of the frog, it starts upon its course of development 

 as a single unitary biological element — a cell. During 

 the earliest subsequent hours the first cell divides again 

 and again to form a small disk upon the surface of the 

 yolk. Soon the cells along the middle line of this small 

 sheet become rearranged to make an obvious streak 

 or band, and about this line a simple tube is constructed 

 which is destined to become the future brain and 

 spinal cord. The whole disk continues to enlarge by 

 further division of its constituent elements so that it 

 encloses more and more of the yolk mass, but the little 

 chick itself is made out of the cells along the central 

 line of the original plate, from which it folds at the sides 

 and in front and behind so as to lie somewhat above 

 and apart from the flatter enclosing cell layers which 

 partly surround the yolk. 



At the sides of the primitive nerve-tube small blocks 

 of cells arise to develop into primitive muscles and other 

 structures. As nourishment is brought to the embryo 

 from the surrounding layers enclosing the nutrient 

 yolk, one system after another takes its shape and 

 builds its several parts into organs which can be rec- 

 ognized as elementary structures of a chick. Among 

 the more interesting ones are small clefts or slits formed 

 in the side walls of the rudimentary throat or pharynx. 

 Blood-vessels go forward from the simple heart to run 

 up through the intervening bars exactly as in the tadpole 

 and the fish. In brief, the young chick possesses a 

 series of gill-slits, for these structures are the same in 

 essential plan and relations as the clefts of tadpoles and 

 fishes. Does this mean that even birds have descended 

 from gill-breathing ancestors? Science answers in 



