IXTKSTIXAL TRACT OF MAMMALS. 245 



presents minor loops at its proximal end, which may be grouped 

 so as to form an ansa sinistra. 



In the Bears there is, in addition, a definite ansa clextra. 



In the Prosimiae the rectum is always longer than the distance 

 that it has to traverse, and at its proximal end, close to the distal 

 extremity of the pendant loop, there is frequently a special 

 expansion, forming an ansa sinistra. An ansa dextra, usually 

 large in size, sometimes narrow, sometimes complex and almost 

 doubled, sometimes straight and sometimes spirally twisted, is 

 present in all except a few very small forms. 



In the 8imiae the rectum is always longer than the distance it 

 has to traverse. The proximal region of the hind-gut, composed 

 of the pendant loop distad of the caecum, the colic apex and the 

 proximal portion of the primitively straight rectum are gradually 

 approaching the human condition of nearly straight ascending, 

 transverse, and descending colons, the appearances suggesting 

 that this condition has been reached through a more prosimian 

 stage in which there were definite ansce colt dextrce and sinistrce. 



Systematic Inferences. — In this section I propose to deal only 

 with the facts to which I have myself paid attention. Un- 

 fortunately I am unable to follow, from Dr. Beddard's descriptions 

 and figures, exactly what he means by the " stages of evolution of 

 the intestinal part of the alimentary tract," and so cannot 

 attempt to correlate them with my own results. Stage I. 

 (Beddard, 1908, p. 591, text-fig. 120 A) represents a condition that 

 is at least more primitive than in any known mammal. Two 

 figures are labelled Stage II. (loc cit., text-figs. 1 20 B and 121), and 

 differ in that the second figure shows rotation of the gut ; but in 

 each figure the so-called " cavo-duodenal ligament" is dra.wn and 

 labelled, although in the text its existence is stated to be due to 

 the rotation. In the later figures large portions of the gut are 

 represented as without any mesentery, and much of the mesen- 

 tery that is represented shows relations which I am unable to 

 follow. Zoologists who wish to follow what is known as to the 

 mode in which the rotation of the gut affects the primitive 

 mesentery will find admirable descriptions and figures in the 

 ordinary text-books (as, for instance, Professor D. J. Cunningham's 

 'Text-book of Anatomy,' 1902, pp. 1056, 1057, figs. 711, 712). 

 Klaatsch (1892) is still the best authority on the secondary 

 ligaments and attachments ; but I cannot always follow him in 

 the discrimination between portions of the primitive mesentery 

 and secondary attachments, and suspect that much further 

 investigation is required. 



I am inclined to think, however, that rotation is due largely to 

 simple mechanical causes, and that it is therefore an event that 

 may have occurred repeatedly and independently, the resem- 

 blances caused by it being due not to inheritance from one 

 ancestor in which rotation had occurred, but to a similar efiect 

 producing similar results on similar material. As Meckel's tract 



