DEVELOPMENT OF KIDNEYS AND FAT-BODIES IN THE FROG. 137 



We find that in tadpoles of 3| mm. to 4 mm. length the head 

 kidney is just commencing to form, and is present as a longitudinal 

 fold of the somatopleure at the anterior end of the body, and imme- 

 diately below the ventral borders of the myotomes. The fold 

 extends back some distance on either side as the rudiment of the 

 duct, but does not as yet reach the cloaca. 



As regards the formation of the nephrostomes, the relations between 

 the nephrostomes and the convoluted part of the duct, the outgrowths 

 of blind diverticula in the head kidney, and the growth backwards 

 of the duct towards the cloaca, our observations agree exactly with 

 Fiirbringer's. 



We find that the duct developes in situ along its whole length, 

 and not by growth backwards from its anterior portion. We also 

 find that though it lies very close to the surface epiblast, yet that 

 the epiblast takes no share whatever in its formation. 



At the time of first appearance of the head kidney, the body 

 cavity or coelom is a potential rather than an actual space between 

 the parietal and splanchnic layers of the mesoblast. By the time the 

 three nephrostomes are established, the dorsal part of the body cavity, 

 opposite to these and immediately below the myotomes, becomes 

 dilated, and into it projects on either side the glomerulus of the head 

 kidney, a thick Availed sacculated diverticulum from the dorsal 

 aorta. This dorsal portion of the body cavity, with the glomerulus 

 projecting into it from the inner or median side, and the nephro- 

 stomes leading outwards from it into the tubules of the head kidney, 

 has been compared to the Bowman's capsule of a Malpighian body, 

 a comparison based at present on very insufficient evidence. Some 

 stress has been laid on the fact that at this early period, about the 

 time of hatching of the tadpole, this dorsal portion of the body 

 cavity does not communicate freely with the ventral part of the 

 body cavity ; but this in the frog is simply due to the mass of the 

 food-yolk, which distends the splanchnopleure and keeps it in close 

 contact with the somatopleure, preventing the formation of a con- 

 spicuous body cavity in this region until a considerable portion of 

 the yolk has been absorbed. 



The segmental arrangement of the three nephrostomes of the frog's 

 head kidney has been a subject of discussion. We find that a true 

 segmental arrangement is present in the early stages. The first or 



