DEVELOPMENT OF KIDNEYS AND FAT-BODIES IN THE FROG. 139 



duct, become clearly unsuitable terms in tbese later stages, and 

 either segmental duct or archinephric duct becomes preferable. 



3. The Changes undergone by the Head Kidney and its Duct. — The 

 head kidney is the excretory organ of the tadpole during the earlier 

 part of its existence. At the time of hatching, the head kidney is 

 already convoluted, and has acquired its three nephrostomial openings 

 into the body cavity, while its duct opens behind into the cloaca, and 

 through the cloacal aperture to the exterior. 



The mouth is not formed till a much later period, and inasmuch 

 as until the mouth perforation is completed, no food can be taken 

 into the alimentary canal, it seems not improbable that the early 

 formation of the cloacal opening is for the sake of providing an exit 

 for the excretory matters separated by the head kidneys. The large 

 size of the head kidneys and their further rapid growth certainly 

 point to their being in physiological activity from the time of 

 hatching of the tadpole, and the histological characters of the 

 epithelial cells of the tubules afford corroborative evidence. 



After hatching, the head kidneys for a time grow rapidly, and 

 become extremely complicated, as described in the last section. They 

 receive an abundant vascular supply from the posterior cardinal 

 veins, which surround them, and send in branches between the 

 individual tubules. The head kidneys attain their greatest size in 

 tadpoles of about 12 mm. length, in which the opercular folds have 

 grown back over the gills, and the hind limbs are just visible as little 

 buds at the sides of the anal spout. At this stage the tubules of the 

 Wolffian body are beginning to develope : as these increase in number 

 and in size they gradually replace the head kidneys functionally, 

 while these latter undergo degenerative changes and ultimately dis- 

 appear completely. 



In tadpoles of from 12 to 18 or 20 mm. in length, the head kidneys 

 remain of about the same size, and show but slight structural changes ; 

 but in tadpoles of from 20 mm. to 24 mm. in length, by which time 

 the "Wolffian bodies have attained considerable size and complexity, 

 the head kidneys, though as large as before, or nearly so, have under- 

 gone important degenerative changes. 



Transverse sections at the latter period (Fig. 6) show that the tubules 

 of the head kidney are enormously dilated at places (Fig. 6, N C), 

 forming irregular cavities as large in transverse section as the lungs ; 



