238 



H. D. Sen 



becomes apparent. In mapping ont the heart anlage (see page 231) 

 it has been shown that the splanchnic mesoderm contained within 

 a triangle drawn with its apex at the aortic region and its base near 

 the anterior end of the pericardial plate would correspond to the 

 future conus ventricle and atrium. By referring to Figs. 7B, 7C 

 and 7D it will be seen that the splanchnic mesothelial cells included 

 in this triangle are columnar in shape, while those lateral to the 

 triangle are much flatter. Passing forward from the region 7B, 

 the cells within the triangnilar area diminish in height until they 

 become cubical, and finally, near the anterior limit of the pericardial 

 plate, quite flat. 



Endocardium 



Sex cell' 



lOB 



Columnar cells of 

 (future) hean tube 



Carotid cells Ectodermal and 



fudermal mandibular 



pouches (blended) 



-l.-'!»^'^^Ventral aortic cells 

 \Ca\ii> of (closing) 

 . ^ coiius ancnosus 



lOD 



Fio. 10. — Two transverse sections of shad, stage of 34-35 somites x 1^0 

 clianis. lOB passes through the mid-sagittal plane of the embyro anterior to 

 the mandibular pouch, lOD mid\yay between the mandibular and hyoid pouches. 

 The left side of the figures represents a region anterior to that seen on the 

 right. Fixation by Pereny's method. 



The splanchnic mesoderm included within the triangular area is 

 soon to form a tube, which may provisionally be called the heart-tube ; 

 the columnar mesothelial cells will form the myo-epicardium. 



Before formation of the heart-tube begins, the triangular area 

 spoken of undergoes a migration toward the left, carrying with it 

 the underlying endocardium ; this is the movement which begins at 

 33 somites. The sinistral movement of the "columnar area" of the 

 splanchnic mesoderm and of the endocardium, which begins at 33 

 somites, is very pronounced an hour later at the stage of 34-35 

 somites from which Fig, lOB is taken. Reference to Fig. lOD shows 

 that in the aortic region there has been no sinistral movement; this 



