EXCRETORY ORGANS. 



715 



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 1 . 



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 



M.c. 



So- 



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



EMBRYO CHICK OF 45 HOURS. 



M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in 

 contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ; 

 p.p. pleuroperitoneal cavity ; ch. note-chord ; op. boundary of area opaca; v. blood- 

 vessel. 



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 segment 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 (wd\ 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 rudimentary peritoneal funnel. 



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 Sedgwick (No. 549). 



