174 DE. P. CHALMERS MITCHELL ON THE 



the " median loop," exteuding from the duodenum to the unpaired caecum ; the " colic 

 loop," extending from the median loop to the insertion of the paired caeca ; and the 

 " rectum." Following Meckel, he identified the " unpaired caecum" as the rudiment of 

 the yolk-sac. Cuvier, however, had no great range of facts before him, and refrained 

 from any general conclusions. Owen, in Todd's ' Cyclopaedia of Anatomy and Physiology ' 

 (31), added little to the investigations of Home and Cuvier; and Macgillivray (22) did 

 little more than to point out that the subject might yet afford useful facts for taxonomy. 

 Thereafter the svibject was apparently completely neglected until Dr. Gadow (10, 11, 12) 

 began his extensive and extremely interesting investigations. Gadow studied and 

 figured the Ioojds and blood-vessels in a large number of forms, distinguished the loops 

 of the intestine as being " open" or " closed " according to the width of the mesentery 

 between the limbs of the fold, named the folds '" right-handed " or " left-handed " 

 according to the position of the descending limb ; but^. above all, described at length 

 and based taxonomic distinctions on the mode in which the loops were folded within the 

 coelom. 



In 1894, working in the Prosectorium of the Zoological Society, I examined the 

 intestines of a number of birds in the method from which Gadow obtained his notable 

 conclusions ; but I found not infrequently that there were individual variations in the 

 disposition of the loops, particulai'ly as regarded their " right "- and " left-handedness," 

 no doubt in connection with the writhing movements of the gut during life, and 

 came to the conclusion that there was more to be leai'ued from investigation of the 

 relation of the loo2)s to the unfolded mesentery than from consideration of the mode in 

 which the folds were packed. The method of examination, which I have since found 

 to be an extension of Cuvier's method, I described in 1895 (25), and, at greater length, 

 in 1896 (26). It depends on the morphological nature of the intestinal tract of Verte- 

 brates, which is a tube suspended in the coelom by a fold of mesentery attached to the 

 dorsal wall. The tube, in its course, describes an arc between two fixed points, the 

 pyloric end of the stomach anteriorly, and the insertion of the rectum to the cloaca 

 posteriorly. A third fixed point is given in the embryonic stage by the attachment of 

 the yolk-sac nearly in the middle of the ventral edge of the arc, and this point is often 

 marked in the adult by Meckel's diverticulum (see infra, p. 175). The intestinal tract 

 increases considerably in length between the fixed points, and along with its mesentery, 

 which similarly increases, it is thrown into a series of loops which are folded on one 

 another in the various modes described by Gadow. When the intestines have been 

 removed from the bird by section at the pylorus and cloaca and by cutting tlie mesentery 

 along its dorsal attachment, the cut end of the duodenum and of the rectum may be 

 pinned on a board to the operator's right-hand, these two points and the cut edge of the 

 mesentery stretching between them being placed in their natural relative jjositions. Then, 

 with some trouble in the more complicated cases, the various loops may be unfolded 

 to the left and pinned out ; whereuj)on the mesentery appears as a flat sheet, in shape 

 roughly the segment of a circle, the cut dorsal edge of the mesentery being the sector, 

 and the arc, which carries the intestinal tract, being irregularly distorted. The sheet of 

 mesentery is of course double, and tin; Iduod-vessels and autonomic nervous system run 

 between the two sheets, being situated outside the crelom. The figures which illustrate 



