98 ANIMAL PHYSIOLOGY. 
The excretory system is also foreshadowed at an early pe- 
riod by the Wolffian duct (Fig. 114), a mass of mesoblast cells 
near which the cleavage of the mesoblast takes place. 
, 
o ls 
Fia. Sem ghee ae of the heart and principal arteries of the chick (Quain). A represents an 
earlier, and B and C later stages. 1, 1, omphalo-mesenteric veins ; 2, auricle; 3, ventricle; 
4, aortic bulb ; 5, 5, primitive aorte ; 6, 6, omphalo-mesenteric arteries ; 4, united aorte. 
During the latter part of the second day the vascular system, 
including the heart, makes great progress. The latter, in con- 
re) 
Fie. 111.—Diagrammatic outlines of the early arterial system of the mammal vertebrate em- P 
bryo (after Allen Thomson). A. At a period corresponding to the thirty-sixth or thirty- f 
eighth hour of incubation. B. Later stage, with two pairs of aortic arches. h, bulbus ! 
arteriosus of heart ; v, vitelline arteries ; 1—5, the aortic arches, The dotted lines indicate : 
the position of the future arches. 
sequence of excessive growth and the alteration of the relative 
position of other parts, becomes bent up on itself, so that it 
