THE SPLEEN IX MARSUPIALS. 



lower portion of the spleen. Divisibility is characteristic 

 of its early Mammalian development. In the Platypus it 

 diffuses itself on the right in the meso-duodenum and on 

 the left on the lieno-mesocolic fold. Mesially it is fixed, 

 forming the lower dorsal boundary of the lesser sac. 



With development of the spleen is a correlated great 

 omentum, but no inclusion primarily by the great omen- 

 tum (lesser sac) of distal colon or pancreas. The pan- 

 creas as we ascend in the Mammalian scale becomes more 

 compact. From the point of view of selection it would 

 not be advantageous for it to be included in the lesser sac. 

 It could not then conform to the abdominal law of dorsal 

 fixation of solid bodies by which torsion especially of 

 ducts is prevented in the erect posture. In addition, the 

 dorsal fixation of duodenum, spleen, and left distal colon, 

 of which its own further dorsal fixation is an important 

 factor, would be interfered with; and the significance of 

 the relationship of the pancreas on the left to a fold (left 

 lateral) connecting spleen and distal colon in the produc- 

 tion of that arrangement becomes understood. This fold 

 is completely absent in Koala, and in that Marsupial 

 there is no fixation of distal colon, spleen, or left pancreas. 

 Primary inclusion by the great omentum of the proximal 

 8 cm. of distal colon actually happens in Pseudochirus 

 (Australian Phalanger), and in this Marsupial, though 

 distal colon, spleen, and kidney are proximal, there is a 

 considerable amount of pancreatic tissue free in the lesser 

 sac. Furthermore, a studv of the Mammalia shows that 

 for adaptability to a mixed diet selection requires not 

 atrophy but development of the distal colon, which be- 

 comes ultimately fixed as in erect Man in the left hypo- 

 chondrium. In the Echidna, whose diet is chiefly ants, a 

 failure to develop or atrophy of the distal colon and meso- 

 colon is correlated with the dorsal fixation of that gut 

 and of the pancreas, but not of the spleen. I regard that 

 as the evolutionary tendencv in the Echidna (since I have 



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