INTESTINAL TRACT OF MAMMALS. 185 



is short, thick-walled, and of even calibre, as, for instance, in 

 the case of many carnivorous animals. When the gut is rela- 

 tively long, and when the thickness of its walls and its calibre 

 vary much in different regions, as is frequently the case in omni- 

 vorous and herbivorous creatures, the danger from mechanical 

 strain is greater. The habit of life of the creature also must be 

 taken into consideration. Animals of placid gait, and aquatic 

 animals living in a medium of nearly the specific gravity of their 

 own bodies, subject the contents of their abdominal cavity to the 

 least possible disturbance. Animals that run and leap, and 

 especially climbing animals — as the latter are constantly shifting 

 from a horizontal to an erect posture, — subject the contents 

 of their abdominal cavity to a maximum strain. As adhesions 

 may take place between portions of the gut that, although they 

 belong to difierent regions, are in close contact, it is plain that we 

 may exjject to find them A-ai-ying in correlation with the nature 

 of the food, the structure of the gut, aiid the habits of the animal. 

 We see readily how they may have arisen in many groujDs in- 

 dependently, and that they thus afford no definite indication 

 of affinity. Dr. Beddard, in a communication to this Society 

 (Beddard, 1908, p. 561), has brought together a valuable set of 

 observations, old and new, on such secondary features of the 

 gut, and would appear to agree with me that they cannot, as he 

 phrases it, "yield accurate classificatory I'esults,'"' as he is able to 

 arrange them in a series of ascending stages, and to show that 

 these stages, or some of them, occur independently in different 

 groups. 



The Primitive Mammcdian Giit. 



In text-fig. 1 1 have drawn the primitive type to which 

 the vai-ied patterns displayed by the gut of mammals (when 

 the secondary connections have been severed) can be reduced. 

 The left-hand diagram (A) shows the pattern as it may be seen 

 •in a very young mammalian embryo ; tlie right-hand figure (B) 

 shows it as it appears in some of the simpler adult animals. 

 The whole gut from the stomach (S.) to the distal end of 

 the rectum (R.) is suspended from the dorsal wall by a con- 

 tinuous mesentery (Mes.) containing the blood-vessels. It 

 consists of three definite regions. The proximal region, from 

 the point marked 1 to the point marked 2, is the duodenal 

 region ; in birds this is usually characterised by the outgTowth 

 of a long, narrow, single loop, but in mammals moi'e frequently 

 appears as a bunch of short loops not clearly marked off from the 

 beginning of the next region. The second region, from the point 

 marked 2 to the cfecum (C), I have termed Meckel's tract; 

 it corresponds, according to the position of the caecum, with 

 the whole or the proximal portion of the pendant loop of 

 human embryology, and its apex is fixed in the embryo by the 

 umbilical cord (text-fig. 1 A, M.). As a very rare abnormality 

 in mammals, a diverticulum, known as Meckel's diverticulum. 



