The Devclcpiiicnf of the Heart in Shad. 230 



movement takes place around the aorta as a center, and becomes 

 progressively greater as the base of the triangle referred to is reached. 

 In the region of Fig. lOB (anterior to the mandibular pouch) the 

 columnar area of the splanchnic mesoderm, being about two-thirds 

 to the left of the mid-line, has accomplished its migration. The 

 movement of the columnar area of the splanchnic mesoderm is not 

 quite simple; it is accompanied, and partly brought about, by a 

 slighter movement to the left of the entire pericardial plate (see 

 Fig. lOB). The slighter sinistral movement of the entire pericardial 

 plate began as early as the stage of 32 somites (see Fig. YB). 



In Fig. lOD the initiation of heart-tube formation can be recog- 

 nized. Union is about to take place between the splanchnic meso- 

 derm in the region of the original medial margins of the right and 

 left lateral plates. This union takes place around the lateral and 

 posterior circumference of the aortic root. The splanchnic mesoderm 

 to the right and left of the aortic root is seen in Fig. 10 D to be some- 

 what prominent ventrally, so that, by uniting in the mid-line (which 

 it is just about to do), it will enclose a small chamber, the conus 

 arteriosus ; the latter contains a small amount of endocardium directly 

 continuous with the endothelium of the aorta. 



At a stage of 36 somites (see Fig. 11, one hour older than the pre- 

 ceding stage) the formation of the heart-tube has made considerable 

 ]irogress. The arterial extremity of the heart-tube has been formed 

 by the blending of the splanchnic mesoderm al)out the circumference 

 of the aortic root symmetrically in the mid-line. The remainder 

 of the heart-tube is also undergoing formation by the blending of 

 splanchnic mesoderm on either side. The axis about which the 

 further blending of splanchnic mesoderm occurs corresponds to a 

 straight line connecting the middle of the base of the triangle 

 spoken of with its apex. The apex of the triangle corresponds in 

 position with the aortic root ; the middle of the base is now placed 

 (owing to rotation of the triangle to the left around its apex) quite 

 near to the left side of the anterior margin of the pericardial plate. 

 The axis, then, along which union of the splanchnic mesoderm is pro- 

 ceeding, extends from the (medially placed) aortic region forward; 

 as it passes forward it diverges to the left so as to form an acute 

 angle with the mid-sagittal plane of the embryo. 



