148 EARLY EMBRYOLOGY OF THE CHICK 



completely patent, and never acquire the vascular connections 

 characteristic of the functional pronephros in primitive 

 vertebrates. 



The Mesonephric Tubule s.^The mesonephric tubules de- 

 velop from the intermediate mesoderm caudal to the pronephros. 

 The early steps in their formation are well shown in transverse 

 sections of chicks of 29 to 30 somites (about 55 hours). In 

 the posterior somites conditions are less advanced than they 

 are more anteriorly. Consequently by studying the posterior 

 sections of a transverse series first and then progressing cephalad 

 a graded series of developmental stages may be obtained. 



The mesonephric tubules appear first as cell clusters formed 

 in the intermediate mesoderm. They lie ventro-mesial to the 

 cord of cells which is the primordium of the pronephric duct. 

 The cells of the developing tubules acquire a more or less radial 

 arrangement, and at the same time become more distinctly 

 isolated from the surrounding mesoderm cells. By 55 hours of 

 incubation the primordial cell cord representing the pronephric 

 duct has become hollowed out to estabhsh a definite lumen. 

 The most anterior of the mesonephric tubules also have 

 acquired a lumen. The growth of the^ tubules brings them in 

 close association with the duct. In some of the more differen- 

 tiated tubules indications can be made out of their opening into 

 the duct which is soon to be definitely established. The more 

 posterior mesonephric tubules do not become associated 

 with the duct until somewhat later, but remain as a series of 

 isolated vesicles. 



Figure 52, Z>, shows the scheme of organization of a functional 

 mesonephric tubule of primitive type. As is the case with the 

 pronephric tubule, its ciliated nephrostome draws in fluid 

 from the coelom. The mesonephric tubule differs from the 

 pronephric chiefly in its relation to the blood vessels associated 

 with it. It develops a cup-like outgrowth into which a knot 

 of capillaries is pushed. The cup-shaped outgrowth from the 

 tubule is called the capsule (of Bowman) and the tuft of capil- 

 laries, a glomerulus. Waste-laden fluid is extracted from 

 the capillaries of the glomerulus, mingles with the fluid coming 

 in by way of the nephrostome, and is eventually discharged into 

 the nephric duct. In mesonephric tubules of a more highly 

 differentiated type the nephrostome becomes closed and all the 



