2(54 DR. P. CHALMERS MITCHELL ON THE 



therefore exists that a functionless vestige is retained universally in some groups and 

 not in other groups a circumstance to which the attention of those naturalists may be 

 directed who would see a purpose or "selection-value" in every systematic character. 



Meckel's diverticulum presents another condition of great interest. Lonnberg and 

 Jagerskiold (21) drew attention to the fact that in certain cases where the diverticulum 

 is large, it has a patent lumen and a thick wall, and is slightly constricted from the gut 

 (21, cited in Oppel 30, p. 559). These authors, while they suggest that the vestigial 

 organ has been transformed into a gland, do not go further in their microscopical 

 investigation than to point out that the mucous membrane of the diverticulum in these 

 cases is thickened by a series of folds. I have examined the microscopical structure in 

 several Anatiformes, Gruiformes, and Charadriiformes, and find that glands occur 

 frequently in the foldings of the mucous membrane, and that the diverticulum in such 

 cases corresponds, with its folded wall containing glands and lymphoid nodules, very 

 closely to the structure of the caeca in Passerines and Columbae. I am inclined to think, 

 therefore, that in many cases, particularly in the groups that I have mentioned (and 

 possibly in the Falconinae), Meckel's diverticulum has acquired a new function. It is at 

 the least suggestive that where the diverticulum has become glandular, the paired caeca 

 are either rudimentary and functionless, or they are very large, thin-walled, and full of 

 faecal matter. "Where, on the other hand, the paired caeca are chiefly glandular, the 

 diverticulum is either a functionless vestige or has disappeared. This, hoAvever, is not a 

 complete account of the relations of the two organs ; for, in the first place, it is based only 

 on a relatively small number of observations of microscopical structure, and in the second 

 place there are instances, such as the Psittaci, where both diverticulum and paired cseca 

 are absent in the adult. 



THE SUBSIDIARY LOOPS OF MECKEL'S TRACT. 



In the section dealing with systematic description, I have already said all or nearly all 

 that 1 have to say regarding the minor loops into which the Tract so frequently is 

 produced. I have tried to show that these display patterns which persist through 

 systematic groups, the persistency referring to their position with regard to the diverti- 

 culum and to their number. The arrangements of these minor loops, in fact, are 

 instances of what I term uniradial apocentricities. When Meckel's tr.Mct is elongating, 

 in a large bird or in a bird the hahits of which demand a great length of gut, the 

 elongation does not take place at random, but in special regions and in special modes. 

 The combination of position and complexity is of a kind not likely to be repeated 

 independently, but to have common origin, and so to prove of systematic value. The 

 explanation of how these different and complex combinations came into existence I have 

 not attempted. Before that could be done, there is necessary the great labour of 

 following out in every stage of individual development the relations of the growing folds 

 of the gut to the blood-vessels, regions of the coelom, liver, air-sacs, sternum, stomach, 

 and so forth. The beautiful work of Klaatsch and of Mathes, elaborate and prolonged as 

 it was, does little more than to open up the lines of such enquiries. Until something 

 is known in each individual case of the nature of the " growth-forces " in contiguous 



