EMBRYOLOGY 



503 



body cavity. Opposite the funnels a sacculated outgrowth 

 of the splanchnic layer appears. It is known as the 

 glomerulus x and becomes filled with blood from the systemic 

 arch. The hinder part of the thickening which forms the 

 pronephros becomes a longitudinal tube, the segmental 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. Some time later the mid-kidney or 

 mesonephros arises as a series 

 of paired masses of cells along 

 the inner sides of the segmental 

 duct, behind the pronephros. 

 This part of the duct becomes 

 the Wolffian duct. Each of 

 the masses in question de- 

 velops into one of the tubules 

 of the kidney, acquiring at 

 one end an opening to the 

 Wolffian duct and at the other A„ 3 

 a glomerulus and a nephros- 

 tome. Just before metamor- 

 phosis the pronephros and the 

 front part of the segmental 

 duct degenerate. The oviduct 

 arises as a structure called 

 the Mullerian duct, which is 

 present in the late tadpole 

 in both sexes, but degener- 

 ates 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 ccelomic 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. 



1 Better as the glomus, a glomerulus being a small glomus for a single 

 tubule. 



A diagrammatic sec- 

 tion of the egg of a bird. — 

 From Thomson. 



a.c, Air chamber; ck., twisted cords 

 in the white known as " chalazia " ; 

 g.v., small patch of protoplasm 

 comparatively free from yolk, in 

 which lies the "germinal vesicle" 

 or nucleus ; y., yolk, in alternate 

 layers of yellow and white sub- 

 stance. The yolk is surrounded 

 by the " white of egg." Note the 

 two membranes underlying the 

 shell and separating to enclose 

 the air chamber. 



