146 EARLY EMBRYOLOGY OF THE CHICK 



nephric ducts of either side are formed first at the level of the 

 pronephric tubules and then extend caudad, eventually reach- 

 ing and opening into the cloaca (See arrows in Fig. 51, A). 



As the pronephric ducts are extended caudal to the level at 

 which pronephric tubules are formed they come in close prox- 

 imity to the developing mesonephric tubules. In their growth 

 the mesonephric tubules extend toward the pronephric ducts 

 and soon open into them (Fig. 51, E). Meanwhile the pro- 

 nephric tubules begin to degenerate. Thus the ducts which 

 originally arose in connection with the pronephros are appro- 

 priated by the developing mesonephros. After the degenera- 

 tion of the pronephric tubules these same ducts are called the 

 mesonephric ducts because of their new associations (Fig. 51, C). 



At a considerably later stage outgrowths develop from 

 the mesonephric ducts near their cloacal ends (Fig. 51, C). 

 These outgrowths form the ducts of the metanephroi. They 

 grow cephalo-laterad and eventually connect with the third 

 group of tubules developed from the intermediate mesoderm, 

 the metanephric tubules (Fig. 51, D). With the establishment 

 of the metanephroi or permanent kidneys the mesonephroi 

 begin to degenerate. The only parts of the mesonephric 

 system to persist, except in vestigial form, are some of the ducts 

 and tubules which in the male are appropriated by the testis 

 as a duct system. 



The Pronephric Tubules of the Chick. The pronephros 

 in the chick is represented by tubules which first appear at about 

 36 hours of incubation. The pronephric tubules arise from the 

 intermediate mesoderm, or nephrotome, lateral to the somites. 

 They are paired, segmen tally arranged structures, a tubule 

 appearing on either side opposite each somite from the fifth to 

 the sixteenth. Transverse sections passing through the loth to 

 I4th somites of an embryo of about 38 hours show the proneph- 

 ric tubules favorably. Each tubule arises as a solid bud of cells 

 organized from the intermediate mesoderm near its junction 

 with the lateral mesoderm (Fig. 52, A). At first the free ends 

 of the buds grow dorsad, passing close to the posterior cardinal 

 veins. Later the end of each tubule is bent caudad coming in 

 contact with the tubule lying posterior to it. In this manner 

 the distal ends of the tubules give rise to a continuous cord of 

 cells, the primordium of the pronephric duct. The pair of cell 



