440 DE. P. CHALMEES MITCHELL ON THE 



the Monotremes, although, from the large size of the yolk-sac in these Mammals, I 

 had some expectation of finding it. 



The only possible record of its occurrence in a mammal other than Man occurs in 

 the work of that wonderful old naturalist and observer, Pallas. In his monograph on 

 the Glires (20) he describes and figures the presence of a small diverticulum as the 

 cause of Meckel's tract, some inches proximad of the caecum, in a species of Ochotona, 

 and remarks of it that it is peculiar and " ut in avibus quibusdam," as in certain birds. 

 I have myself examined two examples of Ochotona rufescens and did not find the 

 diverticulum in either. 



In Birds, Meckel's tract falls into minor folds or loops, the characters of which 

 present an interesting series of some systematic importance. In Mammals, Meckel's 

 tract is much more uniform ; it may remain relatively short, or increase enormously 

 in length, but in either case it falls into a fairly symmetrical shape, suspended 

 at the circumference of a nearly circular expanse of mesentery. Where it is short, 

 it is thrown into very simple minor loops ; where it is long, these minor loops form 

 a huge convoluted mass, the details of which it is impossible to portray exactly 

 in a diagram. 



The distal portion of Meckel's tract in Mammals, as in Birds, is usually nearly 

 straight, and runs back parallel with the caecum or caeca towards the point where the 

 original pendent loop passed into the rectal portion of the gut. But here there is a 

 considerable difference between the condition in Birds and Mammals. In the series 

 of birds which I examined and described (15) I found that the distal portion of 

 Meckel's tract frequently assumed a certain independence and entered into relations 

 with the duodenum. 



To a parallel, but not identical, disposition developed in some Mammals, I shall call 

 attention later (infra, p. 524). 



In Birds the caeca almost invariably arise at the point where the distal end of 

 Meckel's tract returns to the dorsal line of the primitive straight gut, in fact at the 

 distal root of the pendent loop. In the embryo of Man, and certainly in some other 

 Mammals, the origin of the caecum is some little way from the extreme distal end of 

 the pendent loop. On the other hand, in some Marsupials and a certain number 

 of other forms scattered irregularly through the groups, the caecum arises in the bird- 

 like position. I shall return to the morphology of the mammalian caecum later in this 

 memoir (infra, p. 515), after the systematic description of the modifications of the 

 intestinal canal in the groups. For the present, it is enough to say that it seems to 

 me indubitable that the mammalian caecum is homologous with one of the paired caeca 

 in Birds. 



The third portion of the gut extends from the caeca to the cloaca or anus. To avoid 

 ambiguous terms in their application to the portions of this region, I shall refer to it 



