380 THE DEVELOPMENT OF THE CHICK 
Higs. 218 and 219 illustrate the form of these evaginations in 
duck embryos of 40 and 50 somites respectively, as they appear 
in reconstructions of the posterior portion of the mesonephros. 
It will be seen that they gradually 
form sacs opening into the Wolffian 
duct. Subsequently, by elongating, 
these sacs form collecting tubules 
that gather up the secretions of the 
mesonephric tubules proper and con- 
duct them to the Wolffian duet. 
These conducting tubules are stated 
to branch more or less; it is also 
said that they are more highly de- 
veloped in the duck than in the chick. 
Felix proposes to call them meso- 
nephrie ureters. 
In the case of the secondary and 
tertiary tubules, three parts may be 
distinguished: parts one and two (de- 
rived from the nephrogenous tissue) 
are the renal corpuscle and secreting 
tubule respectively; the third part is 
the collecting tubule derived by 
evagination from the Wolffian duct. 
In the case of the primary tubules, 
a conducting part appears to be 
formed secondarily, though in what 
Way is not clear. 
The formation of new tubules 
ceases on the fifth day, all the ne- 
phrogenous tissue being then used 
up. Up to the eighth day at least 
the tubules grow rapidly in length 
and become more differentiated. The 
result is a relatively enormous  pro- 
trusion into the body-cavity on each 
side of the dorsal mesentery.  De- 
generation of the tubules sets in 
about the tenth or eleventh days, 
and the tissue is gradually absorbed; 
