602 



THE EMBRYOLOGY OF THE FROG 

 THE LYMPHATIC SYSTEM 



By the time the tadpole is 6.5 millimeters in length, one may see 

 a single pair of "lymph hearts" (Fig. 11). They are sac-like, and grow 

 out of the intersegmental veins, usually from the fourth pair. That is, 

 they are outgrowths from the veins which run between the fourth and 

 fifth myotomes. These "lymph hearts" empty into the posterior cardinal 

 veins at the more caudal end of the pronephros. 



The "hearts" themselves lie between the peritoneum and the outer 

 covering, and below the level of the myotomes. The endothelial lining 

 of the "hearts" and the blood vessels is continuous. 



The "beating" of the lymph hearts is due to a syncytial layer or net- 

 work of striated muscle fibers immediately outside of the endothelium. 

 The "beating" begins about the time the mouth opens. 



A short time after hatching, that is, when the tadpole is about 7.5 

 to 8.0 millimeters in length, two lymphatic vessels develop from each 

 heart. They are known as anterior and posterior lymph vessels. They 

 follow the lateral nerve in direction, the anterior vessel extending into 

 the head, and the posterior along the sides of the trunk. Valves guard 

 the openings of the lymph vessels into the "hearts" as well as into the 

 veins where they empty. 



Fig. 347. 



Frog. A, showing anterior lymph-hearts, from the dor- 

 sal side. B, showing posterior pair of lymph-hearts seen 

 from the ventral side, gl, gluteus muscle ; ic, iliococcygeal 

 muscle ; L, lymph-heart ; Is, levator scapvlae muscle ; N, 

 spinal nerve; p, piriformis muscle: r, vastus muscles; ta, 

 transverse scapularis major muscle ; ve, vastus externus 

 muscle; 1-5, vertebrae. (After Wiedersheim.) 



Immediately after these lymph vessels begin growing, they develop 

 a rich network of capillaries which spread out in all directions, being 

 greatest in number close to the skin. Later, as the tadpole becomes 

 quite large (about twenty-six millimeters), the lymphatic system be- 

 comes well developed. 



The anterior lymph vessel, running downward and forward, connects 

 with a large lymph sinus, around the mouth, heart, and branchial 

 region. The posterior vessel passes caudad into the tail, and there 

 divides into dorsal and ventral branches. These dorsal and ventral 



