266 



PRINCIPLES OF EMBRYOLOGY 



This is a tube which leads, in the early stages, from the pronephros to the 

 posterior where it opens into the cloaca. It is at that time known as the 

 pronephric duct. Later the meson ephric tubules become connected with it; 

 and in the higher vertebrates the pronephros degenerates and disappears; 

 the duct is then entitled to be called the mesonephric duct. (It is also re- 

 ferred to as the Wolffian duct.) Finally, a diverticulum is pushed out 

 from it, starting from the region near the cloaca; this makes contact with 



Neural crest 



Figuiie"i2.7 



Diagrammatic section through the anterior trunk of a vertebrate, showing 

 the relations of the mesonephric tubules. 



the metanephros, and is known as the metanephric duct (Fig. 12.8). 

 Closely associated with it is another duct, the Mullerian duct; this leads 

 from the exterior to the gonads, which originate near the kidneys, though 

 they may later shift into another region of the abdomen. 



The comparative study of kidney formation in the different classes o. 

 vertebrates, besides being of great interest from the anatomical point of 

 view, illustrates one or two points of importance for general embryology. 



In the first place, we may note that the pronephros, which is an actively 

 functional excretory organ in amphibian tadpoles, is formed opposite a 

 well-defined group of somites; numbers 2, 3 and 4 in Anura, and num- 

 bers 3 and 4 in urodeles (cf Cambar 1949)- hi the chick, in which the 

 pronephric tubules are rudimentary, and never function as excretory 

 organs, they are developed transitorily opposite somites 5 to 16. This is a 

 good example of an organ being moved along the length of the body 



