ORGANS OF LARVA 643 



layer appears. It is known as the glomus ^ and becomes filled 

 with blood from the systemic arch. The outer part of the thicken- 

 ing for the kidney becomes a longitudinal tube, the segmental 

 or archinephric duct, into which the pronephric tubules open 

 at their outer ends. This duct grows backwards and at the time 

 of hatching opens into the cloaca. Later the adult frog's kidney 

 (mesonephros, see p. 617) arises as a series of paired masses 

 of cells in the kidney rudiment along the inner side of the seg- 

 mental duct, behind the pronephros. The part of the segmental 

 duct in this region is now called the Wolffian duct. Each of the 

 cell masses develops into one of the kidney tubules, having at 

 one end an opening to the Wolffian duct, and at the other a 

 Malpighian capsule (secondary nephrocoele) with a glomerulus, 

 and beyond the capsule a ' peritoneal funnel ' which opens to 

 the main coelom. In later stages this funnel loses its connection 

 with the tubule and acquires instead a communication with a 

 vein which it retains in the adult. ^ Outgrowths from certain 

 Malpighian capsules to the testis form the vasa efferentia. 



Just before metamorphosis the pronephros and the front part 

 of the segmental duct degenerate. The oviduct arises as a structure 

 called the Miillerian duct, which is present in the late tadpole 

 in both sexes, but degenerates in the male, leaving only a minute 

 vestige. It is formed as a longitudinal tract of the peritoneal 

 epithelium outside the kidneys, which becomes converted into 

 a canal, the front part by being grooved and then closing in, 

 the hinder part by hollowing out. Part of the groove does not 

 close, but remains as the internal opening of the oviduct. The 

 gonads are formed as thickenings of the coelomic epithelium, one 

 on either side of the mesentery, on the dorsal wall of the peritoneal 

 cavity. No distinction between the sexes can be seen until the 

 metamorphosis takes place. 



KINDS OF SEGMENTATION 



The difference between the mode of cleavage of the ovum of 

 the lancelet and that of the ovum of the frog is due to the presence 



1 This is exceptional. Usually each tubule has its own glomerulus, enclosed in a 

 nephrocoele so as to form a Malpighian capsule. 



2 In this condition of the peritoneal funnels, frogs are unique. It is probably, 

 like the lymph-hearts, an expedient for facilitating the return to the blood vessels 

 of lymph, which in the frog is copious and less confined in vessels than that of 

 higher vertebrates. 



