The Development of the Heart in Shad. 



243 



Before passing from the stage of 42 somites to that of 6.2 mm. 

 (the next one to be examined) it will be necessary to make a digres- 



Somatic mesod 

 Entodermal pharynx 

 Mandibular pouch 



Ventral aorta 



Pericardial coelom 



Fig. 12.— Ventral surface of a reconstruction of the pericardial region of 

 shad, stage of 42 somites x WO diams. 



Over an area mainly to the right of the mid-line the ventral wall of the 

 pericardial ccelom has been removed to show heart-tube and somatopleure ; 

 also, laterally (where somatopleure has been removed), the pharynx and 

 ventral aorta. The continuous black line encloses the area covered by 

 endocardium. 



si on; firstly, to correlate the stages as estimated by the number of 

 somites with those designated by the length in millimeters ; secondly, 

 to mention some points in connection with the general circulation. 



