EXCRETORY ORdAXS. 



In Reptilia (Braun, No. 542), tlie mesonephric tubes develop as seg- 

 men tally-arranged masses on the inner side of the Wolffian duct, and 



M.c. 





<ytt^t. : ,A \%f'.^:<^-'W\Z^wS&ji^ . 







FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN 

 EMBRYO CHICK or 45 HOURS. 



M.c. medullary canal; P.v. mesoblastic somite; !['.</. Wolffian duct which is in 

 contact with the intermediate cell mass ; ,S'o. somatopleure ; S.p. splanchnopleure ; 

 p.p. pleuroperitoneal cavity; cli. notochord; op. boundary of area opaca; r. blood- 

 vessel. 



appear to be at first united with the peritoneal epithelium. Each mass soon 

 becomes an oval vesicle, probably opening for a very short period into the 

 peritoneal cavity by a peritoneal funnel. The vesicles become very early 

 detached from the peritoneal epithelium, and lateral outgrowths from them 

 give rise to the main parts of the segmental tubes, which soon unite with 

 the segmental duct. 



In Birds the development of the segmental tubes is more complicated '. 



The tubules of the Wolffian body are derived from the intermediate 

 cell mass, shewn in fig. 401, between the upper end of the body cavity 

 and the muscle-plate. In the Chick the mode of development of this 

 mass into the segmental tubules is different in the regions in front of and 

 behind about the sixteenth segment. In front of about the sixteenth sec- 



o o 



meiit the intermediate cell mass becomes detached from the peritoneal 

 epithelium at certain points, remaining attached to it at other points, 

 there being several such to each segment. The parts of the intermediate 

 cell mass attached to the peritoneal epithelium become converted into 

 S-shaped cords (fig. 402, st] which soon unite with the segmental duct (></). 

 Into the commencement of each of these cords the lumen of the body 

 cavity is for a short distance prolonged, so that this part constitutes a 



1 Correct figures of the early stages of these structures were first given by 

 Kolliker, but the correct interpretation of them, and the first satisfactory account of 

 the development of the excretory organs of Birds was given by Seclgwick (No. =49). 



