192 ARTHUR E. SHIPLEY. 



crowded with yolk. Scott has described it fully, and ia most 

 respects ray observations confirm his. As he describes, the 

 first structure to appear is the segmental duct which is at first 

 solid. The cells forming this are derived from the mesoblast 

 cells which lie between the already segmented dorsal part of 

 the mesoderm and the ventral unsegmented portion. These 

 cells form a solid cord lying between the mesoblast and the 

 epiblast ; the cord continues to grow backward by a differen- 

 tiation of the cells in situ. A few hours later a lumen appears 

 in the centre of the cord by the separation of the cells ; this 

 soon becomes elliptical in section (fig. 11). It opens into 

 the posterior part of the alimentary canal. 



From this account it will be seen that at first the segmental 

 duct is between the mesoblast and epiblast ; it, however, soon 

 comes to occupy a deeper position by the growth of the sur- 

 rounding tissue. So far we have only considered the duct in 

 that part of its course where the body cavity is not yet deve- 

 loped ; but in the region of the heart, where the body cavity 

 has already appeared, its origin seems to be somewhat difi"erent. 

 The lumen of the segmental duct here becomes continuous 

 with a groove in the parietal peritoneum, lying near the angle 

 where the somatopleure and splanchnopleure diverge. When 

 this groove closes it leaves four or five openings which persist 

 as the openings of the ciliated funnels. This account of the 

 origin of the ciliated funnels agrees with that ofFiirbringer, 

 but differs from Scott^s, who describes the funnels arising 

 as blind projections of the segmental duct which acquire an 

 opening into the body cavity. Each funnel soon acquires 

 cilia, which extend for some distance down its lumen,, and are 

 usually directed downwards towards the tubuli. The funnel 

 is composed of large cubical cells with a large nucleus, at its 

 lip it passes suddenly over into the flat cells of the peritoneal 

 epithelium. At its base it is continuous with a duct which 

 soon becomes elongated and coiled, and ultimately joins the 

 segmental duct. The walls of the tubuli are composed of 

 large clear glandular cells. The posterior end of the seg- 

 mental duct opens into the cloaca. 



