AMPHIBIA 209 



in this mesenchyme that the skeleton of the vertebral column 

 is developed. 



Vascular System. The mesoderm also yields a general 

 mesenchyme in which the blood and blood-vessels and the 

 lymph and lymph-vessels form. It is said by some investi- 

 gators that the heart is formed from yolk cells that is to say, 

 directly from endoderm ; and Schwink explained the appear- 

 ance as being due to a delamination of the yolk cells into 

 endoderm and mesoderm. From a consideration of the 

 happenings in bony fishes, it is probable that the mesenchyme 

 of the heart, which is enclosed by the primitive pericardium, 

 is due to wandering cells of the mesenchyme. 



In the adult frog the blood consists of a clear plasma 

 which bears oval flat nucleated erythrocytes containing the 

 haemoglobin, together with leucocytes and thrombocytes. 

 The latter are active in producing thrombin, and so fibrin, in 

 the plasma in coagulation. 



The blood is brought to the heart by the three caval veins. 

 These combine to form the triangular-shaped sinus venosus 

 on the dorsal side of the heart. From the sinus venosus it is 

 propelled into the thin-walled right auricle through the sinu- 

 auricular opening, which is guarded by two valves. A smaller 

 left auricle receives the blood from the lungs at the same 

 time by the pulmonary veins, which join to form a small 

 aperture in the dorsal wall of the left auricle. Both the 

 auricles contract next, and the blood is sent into the single 

 ventricle. Two valves guard the opening common to the 

 two auricles, and the septum which separates the auricles is 

 continued on to their anterior surfaces. The valves are 

 attached to the walls of the ventricle by deep folds which may 

 be regarded as incipient chordae tendineae. The walls of the 

 ventricle are muscular, and the muscles form folds and pits 

 in the inner aspect of the wall. 



The blood is discharged from the ventricle by the conus 

 arteriosus. The conus is provided with a row of semilunar 

 valves at each end, and it is partially divided into two longi- 

 tudinal halves by a stout fold depending from the dorsal 

 wall. The longitudinal valve is disposed in a slightly spiral 

 direction from the right anterior aspect to near the left of 



