658 DR. p. CHALMERS MITCHELL ON THE 



carries the yolk-sac at its extreme point. If, as happens in many 

 birds belonging to widely separated groups, Meckel's tract is 

 twisted in the course of growth, the twist in the mesentery is 

 seen here with the result that in the dissection of the gut as 

 shown in ixiy figures, the mesentery carrying the posterior region 

 of the gut may be tucked or folded under the mesentery carrying 

 Meckel's tract, so that although the two are morphologically 

 continuous, the continuity may not be apparent except where the 

 whole gut is short or very simple. In my diagrams, I have 

 simplified this region, showing the morphological continuity, as 

 the secondary twisting was not a part of my argument. 



The cfeca (or cpecum) are to be looked for a.t this point, which 

 marks the transition from Meckel's tract to the hind gut at the 



Text-fiff. 123. 



Ciiecum of BaJceniceps. 



S. I. Cut small intestine. 



L.I. Cut large intestine. 



B. The lateral wall of the caecum lias been cut away to show at B, the 

 aperture into the small intestine, lying distally of the point 

 where the Intestine appears to pass into the ciecum. 



beginning of the area drained by the posterior branch of the 

 portal vein. In Balceniceps only one is present, as shown in the 

 general diagram (text-fig. 122,0a.). The end of Meckel's tract 

 meets the large intestine almost at a right angle, and the csecum 

 is in continuation of the line of the large intestine. The calibre 

 of the large intestine is rather greater than that of Meckel's tract, 

 the difference being greater than is represented in the general 

 diagram and rather less than in the enlarged figure (text- 

 fig. 123), which was drawn from the specimen after it had been 

 washed out and slightly stretched in the process of opening. The 



