THE BLOOD- VASCULAR SYSTEM AND THE BODY-CAVITY. 



89 



(Compare with the development of the heart in the Annelida and 

 in the Mollusca.) 



The isolated cells which had become grouped together into a strand become 

 blood-corpuscles. Their crowded condition and their extremely close connec- 

 tion with the walls of the primitive segments suggested the idea that the 

 heart was derived from a solid mesodermal strand extending along the dorsal 

 middle line (Balfour), but this view cannot be verified ; the formation of the 

 heart may be directly compared with the similar process in the Annelida. The 

 cavity of the heart corresponds to a part of the primary body-cavity, enclosed on 

 each side by the primitive segments. 



Fig. 47.— Transverse sections through the abdomen of embryos of Theridium macula 

 showing the formation of the heart (after Mokin). U, blood-corpuscles ; c, coelomic cavity ; 

 d, yolk ; dz, yolk-cells ; ec, ectoderm ; h, heart; so, somatic, sp, splanchnic mesobl 



The heart lies in a depression of the yolk (Fig. 47 B). The 

 latter is covered only by the splanchnic layer of the mesoderm, as 

 the entodermic epithelium is still wanting. From this part of the 

 splanchnopleure, a mesodermal lamella is said to separate and grow 

 round the heart to form the pericardium (Schimkewitsch). The 

 alary muscles of the heart are then formed from the somatic 

 mesoblast. The pulmonary veins arise as outgrowths of the peri- 



