54 THE EVOLUTION OF MAN 



segments are developed all the muscles of the body, the axial 

 part of the skeleton, and the membranes surrounding the 

 brain and spinal cord. Although the muscles are at first 



FIG. 20. THE HUMAN HEART 

 Diagram showing its development and changes of form and external appearance 



at different stages. 

 Modified from His's models. 



Figs. IIIB and IVB are side views; the others are front views, a, primitive 

 ventral aorta; b, bulbus cordis; c, ventricle; c', left ventricle; d, auricle; d', left 

 auricle; e, sinus venosus; f, atrio-ventricular canal ; g, position of orifice of atrio- 

 ventricular canal; h, vitelline vein. 



Redrawn from Cunningham, Anatomy. 



