1897- NOTES AND COMMENTS. 369 



the posterior region of the gut. In displaying the intestine this 

 bridging vessel has to be cut through, as well as a portion of the 

 mesenteric fold, so as to free the rectal from the duodenal loop, and 

 expose the circular system. These "bridging vessels" are "remark- 

 ably constant in the groups in which they occur, and they seem to 

 present a striking instance of a feature which, apparently, could only 

 have arisen from the * accident ' of contiguous position, and is fixed as 

 a normal part of the structure." The relations of the anterior, 

 median and posterior mesenteric, and of the " short-circuitings " or 

 "bridging-vessels " hold an important place in this paper. 



The circular loop, in its most typical form, is thrown into a 

 number of corrugated folds or secondary loops, suspended at the 

 circumference of the semi-circular expansion of the mesentery after 

 leaving the duodenum. The median mesenteric vein traverses the 

 centre of this fold and runs out to the remnant of the yolk-sac, 

 situated more or less nearly in the centre of this loop. The size and 

 position of this vestige of the yolk-duct, is one of the features of the 

 circular loop, though second in importance to the nature of its 

 convolutions. It seems that some sort of compensation for the loss 

 of caeca is often made by the elongation of the last secondary loop of 

 this circular system. The numerous figures which illustrate this 

 paper show the wonderfully varied forms Avhich the circular loop may 

 present. 



The rectal loop, or last part of the gut, as a rule maintains its 

 primitive straight position, closely attached to the dorsal wall of 

 coelomic cavity by the posterior part of the primitive straight mesentery. 

 It may, however, take the form of an enormous coil swung at the 

 circumference of a semi-circular expansion of the mesentery. Its vein 

 is the posterior mesenteric. 



By his system the author holds that he is able to " display more 

 clearly the relation of the individual cases to each other," and to what 

 he takes to be " the primitive type, and to show the mesentery and 

 the intestinal veins." 



Many, on the strength of less work than is here presented, would 

 have foisted upon us a phylogenetic tree, and added another to the 

 legion of systems already extant or defunct. Not so Mr. Mitchell ; 

 on the contrary, he tells us that he does not " feel justified in attempt- 

 ing to draw any general conclusions as to the relations of the various 

 divergences from the common type " that he has described. Enough 

 however has been done, he thinks, to show that when more facts have 

 been collected, the method of investigation here adopted may furnish 

 " another clue to that riddle of zoology the classification of birds." 

 In this we heartily concur. 



Applying this new method of investigation as far as it will go, 

 the author is led to conclude that, from the point of divergence of 

 type, the Ratitae deserve their place at the bottom of the avian tree, 

 and that Ciconiiformes, Falconiformes, and Charadriiformes seem to 



