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. Diagram showing the development of the human heart and the 

 changes of form and external appearance at different stages. Modified from 

 His's models. Figs. Ill B and IV B 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 Cunning- 

 ham, Anatomy. 



