192 THE DEVELOPMENT OF THE CHICK 



and the angle of the body-cavity; it consists of two parts, viz., 

 the primary tubule and the supplementary part. It never pos- 

 sesses a continuous lumen, though there is often a cavity in the 

 supplementary part, which opens into the body-cavity through 

 the nephrostome (Fig. 112 B). 



The pronephros of the chick is a purely vestigial organ, of 

 no apparent functional significance. Its development is accord- 

 ingly highly variable, and it often happens that the right and 

 left sides of the same embryo do not correspond. It is also of 

 very short duration and is usually completely lost on the fourth 

 day. The tubules in the fifth to the tenth somites, moreover, 



Aa 



FIG. 113. Transverse section through the fifteenth somite of the same 

 embryo. 



pr'n. (14), (15), Pronephric tubules of the fourteenth and fifteenth somites, 

 respectively. 



hardly pass the first stage when they appear as thickenings of the 

 somatic layer of the somitic stalk; thus the Wolfnan duct does 

 not extend into this region, and the best developed pronephric 

 tubules are confined to the tenth to the fifteenth somites. 



The pronephric tubules do not form Malpighian corpuscles; 

 but glomeruli develop as cellular buds at the peritoneal orifices 

 of the posterior tubules, projecting into the coelome near the 

 mesentery. Curiously enough these do not form at the time of 

 greatest development of the tubules, but subsequently to this 

 when the tubules themselves are in process of degeneration. 

 Moreover, they are extremely variable as to number, and degree 

 of development. They appear to be best developed on the third 

 and fourth days. They agree in many respects with the so-called 

 external glomeruli of the pronephros of Anamnia, and should be 



