176 OUTLINES OF CHORDATE DEVELOPMENT 



between the splanchnic mesoderm and the enteron, cells that 

 have been formed from the endoderm in the same way that 

 much of the lateral plate mesoderm has been, i.e., through a 

 splitting off of cell groups from the surface of the enteric wall 

 (Fig. 61). These scattered mesoderm cells are often regarded 

 as belonging primarily with the ventral ends of the hyoid 

 visceral arches. They become distinct by the time two 

 mesodermal somites are formed. 



Fig. 61 shows how the layers of the lateral plate extend 

 beneath the pharynx, remaining fused in the mid-line as the 



ra 



FIG. 62. Diagrams of frontal projections of the hearts of early frog embryos- 

 After Weber. A. Heart of an embryo of 2.7 mm. showing the median bulbus 

 arteriosus and the separate auricular and ventricular cavities. B, Heart of a 

 3.2 mm. embryo showing the fusion of the auricular and ventricular cavities. 

 The broken line marks the incomplete separation between the endothelial auri- 

 cular and ventricular regions. C. Heart of a 3.5 mm. embryo. At this stage the 

 ventricle is strongly looped ventrally. a, Auricle; ba, bulbus arteriosus; ra, roots 

 of aortic arches; s, incomplete septum between endothelial tubes of auricle and 

 ventricle; v, ventricle; vl, root of left vitelline vein; vr, root of right vitelline vein. 



ventral mesocardium. The inner or splanchnic wall of the 

 pericardial cavity now folds together dorsally, enclosing the 

 endothelial cells, which have become arranged in the form of a 

 short tube. Finally the splanchnic folds meet and fuse dor- 

 sally, forming a tube outside of the endothelial tube and con- 

 nected with the dorsal wall of the pericardial cavity; this tube 

 forms the muscular wall of the heart and the connection is the 

 dorsal mesocardium. 



The endothelial tube, which is to be regarded as the primary 

 rudiment of the heart, really consists of a pair of short tubes 



