1 86 BR. r. CHALMERS MITCHELL ON THE 



the equivalent of the yolk-sac diverticulum which persists 

 throughout life in a very large number of the families of 

 birds, is to be found at the point marked in the embrj'o by 

 the attachment of the umbilical cord to the apex of Meckel's 

 tract. Meckel's tract forms the small intestine, and however 

 the gut may be lengthened it remains a nearly closed loop, the 

 point (3) where the post-caecal gut passes into the hind-gut 

 remaining extremely close to the point (2) where the duodenal 

 region passes into Meckel's tract. It is also of some importance 

 to notice that the lengthening of Meckel's tract to form the coils 

 of the small intestine takes place chiefly on the proximal limb of 

 the primitive tract ; in the vast majority of mammals, however 

 long the small intestine may be, the distal limb of Meckel's tract 

 remains as an almost straight tube running up until it nearly 

 meets the distal extremity of the duodenal loop. Meckel's tract 

 in mammals difiers notably from the similar region in birds. In 

 birds the tract tends to be drawn out into definite minor loops, 

 the disposition of which forms characteristic patterns in different 

 groups, and the distal region of the tract, immediately proximal 

 to the cfeca, tends to form a specialised loop, folded over, and 

 secondarily attached to the duodenal loop. The third region 

 of the gut is what I term the large intestiire or hind-gut ; it 

 stretches from the csecum to the anus, and occupies a greater 

 portion of the antei^o-posterior axis of the body than the 

 duodenal region and Meckel's tract together. 



The caecum of mammals occupies nearly the same morphological 

 position as the cpeca of birds. In birds like the Osti'ich, and in 

 mammals like the Kangaroo or Elephant, where the hind-gut is 

 relatively long, but little diffeientiated, the cfeca throughout life 

 occupy almost exactly the position indicated in text-fig. 1 B. 

 In birds where the rectum is very short, and especially when 

 the distal poi-tion of Meckel's tract is prolonged into a loop, 

 and in mammals such as Carnivores in which the hind-gut is 

 very short and straight, the ceeca appear to lie more close to 

 the rectum. In mammals in which the hind-gut is highly 

 differentiated, the c?eca occur on the straight portion of the 

 recurrent limb of the pendant loop at a varying distance from 

 the point marked 3 in text-fig. 1. Thus, when the csecum is 

 situated distally, the recurient limb of the pendant loop gives 

 rise to the distal portion of Meckel's tract. When, as is more 

 usual in mammals, the cpecum is attached proximad of the distal 

 end of the pendant loop, the recurrent limb of the latter gives 

 rise partly to the distal and usuallj' straight portion of Meckel's 

 tract, and partly to the proximal portion of the hind -gut. In 

 birds the cteca are almost invariably paired, but as an individual 

 abnormality a single csecum has been recorded in several cases 

 (Ploius, Falconiformes, Columbje), and in Herons and Balceniceps 

 a single caecum is the normal condition. In mammals a single 

 cajcum is the normal condition ; but there are many anatomical 

 facts most easily explained as vestiges of a paired condition 



