190 OUTLINES OF CHORDATE DEVELOPMENT 



of the veins. Outside the endothelium is a syncytial layer 

 or network of striated muscle fibers, which commence rhythmic 

 contraction about the time the mouth opens. 



Shortly after hatching (7.5-8 mm.) two lymphatic vessels 

 may be seen passing anteriorly and posteriorly from each 

 heart, along the lateral nerve, in the connective tissue beneath 

 the integument. The anterior vessel extends forward into the 

 head region, while the posterior vessel extends along the sides of 

 the trunk for a considerable distance. The openings of these 



FIG. 69. Dorsal, lateral and ventral views of the lymphatics in a 26 mm. 

 tadpole of R. temporaria. From Hoyer. For description see text. 



vessels into the lymph hearts, and of the hearts into the veins, 

 are guarded by long valves. These vessels are formed as 

 blind tubular outgrowths from the endothelium of the lymph 

 heart; they grow rapidly and give off a rich network of fine 

 lymphatic capillaries and vessels which spread generally among 

 the other tissues and especially just beneath the skin. 



In the older tadpole of about 26 mm. (Hoyer) the lymphatic 

 system is quite extensively developed. At this time the an- 

 terior vessel runs forward and downward, connecting with a 

 large lymph sinus around the mouth and heart and branchial 

 region (Fig. 69), while the posterior trunk passes to the base 



