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 (ig. 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 nght 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, 
SAT 
VWistverar vy Leh cee (15) 
a) 
Fig. 113. — Transverse section through the fifteenth somite of the same 
embryo. 
pr’n. (14), (15), Pronephrie tubules of the fourteenth and fifteenth somites, 
respectively. 
hardly pass the first stage when they appear as thickenings of the 
somatic layer of the somitie stalk; thus the Wolffian duct does 
not extend into this region, and the best developed pronephrie 
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 calome 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 
