THE URINARY ORGANS. 187 



well-defined ventral or outer lip, and a less distinct dorsal 

 boundary. 



In the hinder part of the body the lips of the groove meet, 

 so as to form a tube, lying between the mesoblast and the epi- 

 blast ; and this tube is the segmental duct. At the anterior end, 

 the lips remain separate for a time, so that the duct opens in 

 front into the body cavity by a longitudinal slit-like mouth. 



FIG. 82. Transverse section through the body of a Tadpole at the time of 

 hatching ; the section passing through the second pair of nephrostomes, 

 and the third pair of myotomes. x 50. 



A, aorta. C, ccelom, or body cavity. CH, notochord. CJ, subiiotochortlal rod. 

 GM, glornerulus. KP, segmental duct. KS, second nephrostoine of left side. 

 ME, sornatopleuric layer of mesoblast. MH, splanclmopleuric layer of mesoblast. 

 ML, myotome. NL, lateral line branch of ] meuiaogastric nerve. TfS, spinal cord. 

 T, intestinal region of mesenteron. VC, posterior cardinal vein. VH, hepatic vein. 

 "W, liver diverticulum. 



By meeting of the lips at two places this slit becomes divided 

 into three openings or nephrostomes, lying one behind another, 

 and immediately below the ventral edges of the second, third, 

 and fourth myotomes respectively (cf. Figs. 74 and 82). 



The anterior end of the duct, and especially the part between 

 the second and third nephrostomes, now increases rapidly in 

 length, becoming twisted on itself; and at the same time 

 the nephrostomes become drawn out into short tubes. At its 



