THE VEINS AND LYMPHATICS. 185 



i. The p ulmonary vein s (Fig. 76, vp) develop early, but 

 not quite so soon as the pulmonary arteries. They appear, 

 about the time of establishment of the mouth opening, as a 

 series of irregular lacunar spaces along the inner or median 

 walls of the lungs, and at first do not reach the heart. Very 

 shortly afterwards, they are completed along their whole length, 

 about the time of formation of the inter-auricular septum in the 

 heart. 



6 . The Lymphati c System. 



The most striking point about the lymphatic system in the 

 tadpole stages is the great development of the subcutaneous 

 lymph spaces. In tadpoles of about 20 mm. length, and up- 

 wards, these form sacs of literally enormous size, filled with a 

 coagulable fluid, and almost completely surrounding the sides 

 and ventral surface of the body and head (cf. Figs. 71, 75, and 88, 

 Ly) ; in the living tadpole they give rise to the appearance of a 

 semi-transparent border surrounding the head and trunk. 



The eai'ly development of the lymphatics appears to take 

 place in the same manner as that of the blood-vessels; the 

 lymphatics appearing first as intercellular lacunar spaces, to 

 which proper walls are formed later by the surrounding tissues. 



The lymph-cells appear rather late, after the internal gills 

 are well established ; according to Maurer, they are derived from 

 the hypoblast cells of the wall of the mesenteron, and wander 

 thence into the connective tissue. 



The spleen arises as a spherical bud on the mesenteric 

 artery ; it consists of cells similar to those of the lymphatic 

 tissues, and, like these, is said to be derived originally from the 

 hypoblast cells of the mesenteron. 



DEVELOPMENT OF THE URINARY AND 

 REPRODUCTIVE ORGANS. 



1. Preliminary Account. 



The e xcretory organs of t he tadpo le are the head kidneys, a 

 pai r of globular bodi es (Fig. 83, kp), i mbedded ia the dors al 

 bo dy- wall, imm ediately behind the pericardial cavity . Each 

 h ead kidn ey, or p ronephros, consists of a convoluted and comp lex 

 mass of tubules, with glandular walls, opening into the body 



