504 



EMBRYOLOGY OF THE CHICK 





The Wolffian duct has already been mentioned. The embryonic kidney 

 in the chick is called the Wolffian body or mesonephros. This embryonic 

 kidney ceases to function very soon after hatching, and is then replaced 

 by the metanephros. 



One of the lowest forms of a chordate (an animal which possesses a 



notochord), is the small fish- 

 like amphioxus or lanceola- 

 tus. In this animal a primi- 

 tive form of excretory system 



fg, B,^ develops and persists 



$, ^ throughout the adult life of 



the animal. It is called a 

 pronephros, or head kidney. 

 This structure develops in 

 the frog and other amphibia 

 during the embryonic pe- 

 riod, but it is followed by 

 the mesonephros or Wolffian 

 body, which becomes the 

 permanent kidney of the 

 amphibian, while in the 

 chick, as in all amniotes, the 

 mesonephros serves as the 

 embryonal kidney, which is 

 then followed by the devel- 

 opment of a metanephros or 

 permanent amniote kidney. 



Schematic arrangement to show relationship of v^ 1 ^' JvAj.) 



metanephros and mesonephros. I, in Gymnophiona (trop- Nntwith:tpnrlinrr + V, & 



ical amphibians without tails or legs). II, in ad- INOtWlt 



vanced chick embryo. Ill, one type of its appearance in 

 man. IV, in rabbit. The Wolffian duct and ureters are 

 black. The canaliculae of the mesonephros are hatched. 

 The canaliculae of the metanephros are dotted. (After 

 Felix.) 



higher animal forms develop what the immediately lower animal form 

 possesses, plus the next succeeding type of pronephros, mesonephros, or 

 metanephros. 



Amphioxus therefore has the pronephros as its permanent kidney, 

 amphibians have the pronephros as a sort of embryonic kidney with the 

 mesonephros in the adult form, while all higher types of animals have 

 a pronephros (which just appears and degenerates during the early em- 

 bryonic period) with a mesonephros acting as an embryonic organ of 

 excretion, and then, later, from the caudal region of the mesonephric 

 duct the adult permanent kidney or metanephros develops. 



To obtain a clear and accurate view of the functional and structural 

 relations of the three kidney-forms, it is important to summarize the 

 nephridial theory here. 



9 



Fig. 300. 



type of these three kidneys 

 which an animal may pos- 

 sess in adult life, all of the 



