222 EMBRYOLOGICAL TYPES 



branching of the ureter. Each capsule is vascularised by a 

 glomerulus, and forms a Malpighian corpuscle. The meta- 

 nephric tubules never have ccelomic funnels. 



So long as the mesonephros functions as an excretory organ, 

 there is a renal portal system formed by the hinder region of 

 the posterior cardinal veins. These veins filter through the 

 mesonephros, and are collected into the so-called subcardinal 

 veins, which contribute to the formation of the inferior vena 

 cava. The renal portal system disappears with the excretory 

 function of the mesonephros ; meanwhile the metanephros 

 has developed and renal veins connect its glomeruli to the 

 inferior vena cava. The posterior cardinals then run into these 

 renal veins. 



The Mullerian ducts develop in both sexes as grooves in 

 the coelomic epithelium which become closed over to form 

 tubes, and these tubes grow back to the cloaca. In the male, 

 both these ducts disappear ; in the female the right duct is 

 lost and that on the left side persists as the definitive oviduct. 



Limbs. — The limbs appear as buds at a stage relatively 

 earlier than that at which they arise in the frog. Their interest 

 lies in the fact that the cartilaginous skeleton of the limbs in 

 larvas reflects the primitive condition, before the modifications 

 of the wings and the mesotarsal joints arose. In the wrist, 

 the distal carpals and the metacarpals are at first separate ; 

 there is as yet no carpo-metacarpus. There are also vestiges 

 of the i st and 5th digits, so that the wing at this stage resembles 

 a more normal pentadactyl fore limb. Similarly in the hind 

 limb, the proximal tarsal cartilages are not yet fused on to the 

 tibia to form a tibio-tarsus, neither are the distal tarsals yet 

 joined on to the metatarsals to give rise to a tarso-metatarsus. 



The early stages of the pelvic girdle are of great interest, 

 for the pubis when it first arises in cartilage points forwards 

 and downwards ; it is only later that it extends back beneath 

 the ischium. 



Hatching. — Meanwhile the mouth and the anus have broken 

 through into the gut as the result of the sinking in of the 

 stomodaeum and proctodeum. As the chick grows during its 

 development, its position changes to accommodate it to the 



