INTESTINAL TliACT OF MAMMALS, 241 



and that the rectal portion is usually rather longer than the 

 length that it has to traverse and is thus thrown into occasional 

 minor folds. The colon presents no definite expansions that can 

 be compared with the ansa coll dextra et sinistra., but the width 

 of the transverse region suggests derivation from a condition in 

 which both these loops were present. It is not difficult to see 

 in the pattern of the Simiie a condition that might have been 

 reached by reduction from the Prosimian pattern. 



. General Conclusions. 



In my earlier memoir (Mitchell, 1905) I dealt at some length 

 with the inferences that seemed to follow from my observations, 

 and 1 propose now to deal only with matters that call for addition 

 or modification. It may be useful to say, in the first place, that 

 the figures I gave formerly and those in this communication are, 

 in the strictest sense, diagrams. That is to say, they are inter- 

 pretations, not exact reproductions of the precise details of the 

 individual specimens. fio far as I know, they give a fair 

 presentment of the sigiiificnnt features of the difleient patterns. 

 Tliey form, I hope, a. good basis for intensive study of the 

 details. It must be remembered, however, that I have attempted 

 to represent the primitive continuous mesentery of the gut, and 

 that, m actual fact, especially in the more elaborate types of 

 intestinal tract, portions of this mesentery have disappeared. 

 Notwithstanding the work of Klaatsch (18t»2) and others, much 

 intensive study of individual types is still required to trace the 

 precise portions that have been lost or retained. Moreovei', I a,m 

 certain that detailed study of the blood-vessels, after cai-eful 

 injection of fresh material, would yield useful results. It is 

 well known that the mesenterial arteries and veins vary con- 

 siderably in man, and doubtless this also is the case in other 

 mammals. Kone the less, the general ariangement of the blood- 

 vessels appears to me, on such slight study as I have been able to 

 give, to follow the main morphological features of the gut-pattern, 

 and in a number of cases whei'e one region of the gut is difficult 

 to distinguish from another, as, for instance, in the Bears, where 

 there is no c?ecum to mark the boundary between the ileum and 

 the ansa coli dextra, the arrangement of the vessels in two 

 groups clearly delimits the regions. I hope that in my diagrams 

 the main features of the grouping of the blood-vessels are given, 

 but very much more work than I was able to give is required. 



The Ccecum. — Further work has confirmed me in the opinion 

 that the csecum of Mammals is one member of a primitive pair, 

 homologous with the paired casca of Birds. I have already 

 sufficiently stated the facts that lead to this conclusion (Mitchell, 

 1905, p. 515), but I may refer to a curious side-light on the 

 subject. Balcenice-ps is one of the few birds in which the normal 



Pkoc. Zool. See— 1916, No. XYI. 16 



