THE HYPOMERE (LATERAL PLATE MESODERM) 233 



It will be remembered that the mesoderm of the ventral lip of the 

 blastopore is at first continuous with the floor endoderm of the gut 

 by way of the yolk. It may be suggested that the separation of the 

 heart endothelial cells may be a later (hence more anterior) phase 

 of the same separation of endoderm and mesoderm. It is purely an 

 academic consideration whether the endocardium is endodermal or 

 mesodermal in origin, for it arises from original endoderm in a manner 

 similar to most mesoderm. The converging sheets of mesoderm then 

 fuse above and below, enclosing these endothelial cells. The wall of 

 the inner tube thus formed becomes greatly thickened during this 

 convergence and is known as the myocardium. It will begin to give 

 rise to the striated, syncytial, and involuntary muscles of the heart 

 by the 5 mm. body length stage of development. The enclosed endo- 

 thelial cells will form the lining of the heart known as the endocar- 

 dium. The cavity surrounding the myocardium is the pericardial 

 cavity which itself is enclosed by an outer (splanchnic) pericardium. 

 The pericardial portion of the embryonic coelom is separated from 

 the peritoneal cavity by growth of the common cardinal veins, the 

 liver, and peritoneal folds, all of which comprise the septum trans- 

 versum. This process is completed during metamorphosis. 



As in the gut region there will develop a double-layered suspensory 

 membrane of the heart known as the dorsal mesocardium. This re- 

 mains attached to the heart at the anterior and posterior extremities 

 of that organ, but ruptures at the level of the elongating and expand- 

 ing heart itself. There is also a temporary ventral mesocardium, 

 which arises before the dorsal mesocardium. It soon ruptures through 

 to provide a single continuous enclosing pericardial cavity completely 

 surrounding the heart except for the previously described dorsal 

 mesentery. 



The mesocardium then disappears both dorsally and ventrally at 

 the level of the heart, and this is accomplished in part by the rapid 

 growth and expansion of the heart itself. The heart arises by the 

 bilateral downgrowth of mesenchyme which becomes a single short 

 tube. This tube finally lengthens and becomes divided to form the 

 coiled, folded, and three-chambered heart of the adult. 



The early differentiated heart consists of a simple tube which be- 

 comes divided into three chambers. The fused pair of vitelline veins 

 becomes the meatus venosus which empties into the first division of 

 the heart, the sinus venosus. Then follows (more anteriorly) the thin- 



