506 EMBRYOLOGY OF THE CHICK 



degenerate, so that now the ducts which were originally called pronephric 

 become the mesonephric ducts. 



In the real kidney, or metanephros, the tubules do not grow toward 

 the mesonephric ducts, but from these ducts. They grow headward and 

 laterad and ultimately connect with the tubules of the mesonephros, 

 after which the mesonephros itself degenerates with the exception of 

 the Wolffian or mesonephric ducts, which in the male become the tubules 

 through which the sperm pass. 



With this in mind, the excretory system of the chick can be studied 

 with some understanding. 



At about thirty-six hours, it will be remembered, the pronephric 

 tubules were seen to arise from the nephrotome, one pair lateral to each 

 somite from the fifth to the sixteenth. Each tubule arises as a solid bud 

 of cells with the free ends growing dorsad, close to the posterior cardinal 

 veins. The distal end of each tubule is bent caudad later, until it reaches 

 the tubule directly posterior to it. Thus is formed a continuous cord of 

 cells which is to become the pronephric duct. These ducts continue to 

 extend caudad beyond the region where the tubules were formed, and 

 soon develop a lumen. The ducts ultimately reach the cloaca, extending 

 ventrally and opening into it. 



The best way to study a series of cross sections is to begin caudad 

 and observe them serially toward the head, because the posterior por- 

 tions are not so well developed as are the anterior. 



The pronephros (Figs. 285 and 299, D) varies in its development, 

 although it usually can be noted in from the fifth to the fifteenth or six- 

 teenth somite. Typically it develops from the tenth to fifteenth, inclu- 

 sive. No duct is formed anterior to the tenth somite, the pronephric 

 buds in that region disappearing by the end of the second day. 



. Mesonephric tubules (Fig. 299, A, B, C), develop in all segments 

 from the thirteenth or fourteenth to the thirtieth, so that the most an- 

 terior mesonephric tubules develop in the same segments where the 

 pronephric tubules also developed, although it is only posterior to the 

 twentieth segment that the mesonephros develops typically. 



The mesonephric tubules, which are to connect with the ducts, are 

 developed from radially arranged cell masses lying ventral and medial 

 to the ducts. The most anterior of these tubules acquire a lumen by 

 the time the ducts have developed their lumen. These tubules grow 

 toward and connect with the duct. Later they remain as isolated vesi- 

 cles. The grouping of the mesonephric tubules constitutes the 

 mesonephros or Wolffian. body. Some of the more cephalad mesonephric 

 tubules seem to develop nephrostomes opening into the coelom. 



The tubules themselves, having formed separately from the ducts 

 and then grown outward and connected with them, have had their out- 

 ward ends develop a cluster of closely packed cells which lies in close 

 relationship to the dorsal aorta. This cluster becomes the glomeruli. 



