loo DR. P. CHALMERS MITCHELL ON THE 



primitive gut as presenting three definite morphological legions : 

 a proximal and short duodenal region ; the pendant looj), a 

 nearly closed loop, the outgrowth of a very small part of the 

 original straight gut, and divided by the inseition of the 

 umbilical coi'd into pi'oximal and recurrent, or distal, limbs ; and 

 third, the hind-gut, corresponding with a much longer portion 

 of the original straight gut. Next, it possesses a cpecum, or 

 possibly a pair of caeca, homologous with the paired caeca of 

 birds. Unless we accept such a constitution of the primitive 

 or ancestral mammalian gut, we are driven to the much more 

 difficult view that these ver}^ definite subdivisions or parts have 

 arisen independently in many different groups of mammals. 

 I infer, therefore, that where a mammalian gut-pattern presents 

 less specialisation than what I have described as primitive, the 

 condition has come about by secondary reduction. 



In comparing the more differentiated gut-patterns with the 

 primitive pattern, I attach little importance to the secondary 

 connections between proximal and distal regions ; and in this 

 Dr. Beddard appears to agree with me for the most part. The 

 ease with which the more important of these can be established, 

 and the apparent independent establishment of them in different 

 groups, arise from the morphological fact that, as the pendant 

 loop is nearly closed, the colic region and the attachment of the 

 caecum are brought very close to the duodenal region. 



"With regard to the subsidiary loops that may be formed in 

 different portions of the gut, in mammals particularly in the 

 hind-gut, I attach more importance to their morphological 

 positions, and less to whether or no they form what Dr. Beddard 

 calls " fixed " loops. Apparently that author employs two 

 separate criteria in applying the designation. The proximal 

 and distal limbs of his " fixed " loops are held together by 

 a very narrow expanse of mesentery ; this, however, is a 

 qiiestion of degree, and narrow loops ai'e linked by many 

 gradations with what cannot be described as specialised loops 

 at all. Next, " fixed " loops are sometimes bound down by 

 extrinsic ligaments or secondary attachments ; such are obvious 

 adaptations, and appear to come into existence independently in 

 diff'erent groups. 



Nor do I attach much importance to the presence oi- absence 

 ■of a spiral disposition of loops or regions of the gut. Spirals are 

 common growth-forms, and however striking they may appear, 

 there is little reason to suppose that the resemblances they 

 produce are other than convergent. They are far from constant, 

 even in individual life. The intestines of the tadpole, which are 

 long in proportion to the size of the creature, are coiled in a tight 

 spiral ; the spiral has disappeared in the adult frog, in which the 

 intestines are shoi-ter in proportion to the whole length. I have 

 found the intestines of young marsupials coiled in spirals, and 

 comparison of my own obsei-vations with those of othei-s leads me 

 to believe that the chief subsidiary loop of the hind-gut in Lemurs 



