Tbo Dcvelopincut of the TTcart in Shad. 



247 



pression and is, in its turn, crowded back so as to approach the hyo- 

 mandibular anlage. The backward movement of the above mentioned 

 structures produces an effect of advance in the tip of the notochord, 

 this is relative only and not actual. 



Having found that the heart-tube represents conns, ventricle and 

 atrium, it remains to be seen how tlie different parts of the wall 

 of the pericardial coelom attain their definitive positions ; this can be 

 most conveniently studied from the left side. 



Ectoderm cover 

 ing yolk 



Position of 

 of notochord 



Ventricl 

 Mandibular pouch { 

 Con 



Hyo- 

 branchial 

 pouch 

 anlage 



Pharynx 



Fig. 13. — Ventral surface of a reconstriictiou of the pericardial region 

 of shad, stage of 6.2 mm. x 100 diams. 



Tart of the ventral wall of the pericardial coelom has been removed to 

 uncover the conns, ventricle, and atrium. 



To make a satisfactory drawing of the heart at this, and the 

 following, stage, from the left side would be very difficult; a com- 

 plete view of the organ is obstructed partly by the dorsal and partly 

 by the ventral wall of the pericardial coelom. The parts considered 

 essential to the elucidation of the changes about to occur are, there- 

 fore, shown diagrammatically and the less important parts elimi- 

 nated altogether. 



