326 ORR. ' [\'0L. I. 



article). Some of my sections show, also, that a part of the 

 lumen of the body-cavity is enclosed by the intermediate cell- 

 mass. When the constriction is completed, the lumen at first 

 projecting into the intermediate cell-mass has disappeared. 

 Farther forward the intermediate cell-mass in cross-section ap- 

 pears round, and is completely removed from the protovertebrae, 

 though remaining in contact and fused with the wall of the body- 

 cavity. Still farther forward this fusion disappears, and the 

 intermediate cell-mass is segmented into the Wolffian bodies. 

 The Wolffian bodies are much elongated, and extend in a dorso- 

 lateral direction to near the epiblast, where they unite with the 

 segmental duct. A lumen is visible in the most anterior of 

 the Wolffian bodies, uniting with the lumen of the segmental 

 duct. The gradation of all these changes is apparent in the 

 consecutive sections from behind forwards. 



Haddon^ has recently summarized the facts and reasons for 

 considering the segmental duct as a product of the epiblast. 

 As most of the literature on this subject referred to by 

 him, has been inaccessible to me, I give but a brief account 

 of what I have found in this respect. It is best illustrated in 

 the same embryo of nine somites, in which I have described the 

 development of the Wolffian bodies. Near the region of the 

 neurenteric canal, opposite that part of the unsegmented meso- 

 blast which has not yet divided into a dorsal and a ventral part, 

 there appears a small linear thickening of the epiblast. This 

 thickening is the same on each side, and lies horizontally and a 

 little above the level in which the intermediate cell-mass is to 

 appear. Posteriorly this epiblastic thickening fades away; 

 but in the direction of the head it becomes more marked, and 

 appears in cross-section as a distinct semicircular clump of five 

 to eight cells adhering to the epiblast. (In one embryo in which 

 the epiblast is stained darker than the gnesoblast, the cells in 

 question took the deeper stain.) A little farther forward this 

 thickening of the cells becomes gradually separated from the 

 epiblast, and lies as a solid cord about midway between the epi- 

 blast and the rudiment of the Wolffian body. Still farther for- 

 ward the cord of cells acquires a lumen, and lies in contact with 

 the Wolffian body. It is now easily recognizable as the seg- 



* Haddon. — Suggestion respecting the Epiblastic Origin of the Segmental Duct. — 

 Scientific Proceedings of the Royal Dublin Society (read Feb. l6, 1887). 



