The Development of the Heart in Shad. 



251 



splanchnic mesoderm between the points X and Y, which has hitherto 

 formed the ventral wall of the pericardial coelom, has imdergone 

 considerable contraction and is stouter than before ; it will form the 

 anterior wall of the sinus venosus (compare Figs. 16 and 17). 



Between "the stage of 8.75 mm. and that of 114 hours, noticed in 

 the introduction (see Fig. 17), the heart is brought into the adult 

 position; the arterial end is fixed (by the ventral aorta) to the floor 

 of the pharynx, while the venous end follows the retreating yolk. 



Branchial cleft 1 



Common trunk of Left 

 branchial arteries 3 and 4 



Br. cleft 2 Somatic mesoderm (pericardial) 



Mouth 



Somatic mesoderm (pericardial) 



Ectoderm [cut] 

 Yolk [surface] 



Fig. 16. — The left side of a reconstruction from the gill region of shad, 

 stage of 8.75 mm., x 100 diams. 



The pericardial coelom and adjacent parts have been laid open to the left 

 of the mid-sagittal plane. 



The main axis of the ventricle (approximately transverse at 8.75 

 mm. ) becomes longitudinal ; the venous orifice of the atrium moves 

 from left to right until it reaches the mid-sagittal plane of the 

 embryo. 



On comparing the parts of the pericardium and heart, as described 

 in the introduction (Fig. 17), with those of the original heart anlage 

 (Fig. 7), it will be seen that the entire parietal pericardium has 

 been derived from somatic, and the myo-epicardium from splanchnic 



