DEVELOPMENT OF THE CHICK 135 



from the intermediate cell-mass lying between 

 the paraxial and lateral mesoblast. It consists 

 of tubules apparently of no functional value, 

 and disappears after the fourth day. As- 

 sociated with the pronephros is the Wolffian 

 duct, which, from its mode of formation, may 

 be divided into two parts. The anterior part 

 is produced by the union of pronephric tubules, 

 while the posterior portion consists of an 

 extension of the anterior part backwards to 

 the cloaca. 



2. The mesonephros or Wolffian body is of 

 importance since it acts as a urinary organ 

 during a certain part of embryonic life. Some 

 of its tubules, moreover, contribute to the 

 formation of the male reproductive organs. 

 The origin of the mesonephros is from epithelial 

 vesicles, which make their appearance in the 

 intermediate cell-mass. The vesicles elongate 

 into tubules, one end of which joins the Wolffian 

 duct. The other end of each tubule becomes 

 invaginated to form a capsule enclosing a 

 glomerulus of blood-vessels : that is to say, 

 Malpighian corpuscles are formed comparable 

 to those present in the adult kidney. Ad- 

 ditional mesonephric tubules form until the 

 fifth day, when new formation ceases. De- 



