1883.] On the Great Omentum and Transverse Mesocolon. 279 



which are best adapted to their environments. The causes of such 

 variation, not being in any way accounted for by " natural selection, "- 

 must be looked-for either in the influence of the " environment " on 

 the organism, or on some tendency to vary inherent in the organism 

 itself ; and the question which now most occupies the minds of thought- 

 ful Evolutionists, is whether the variations that have conduced to the 

 establishment of the higher types are " aimless," or whether they have 

 followed a definite " plan." 



From a careful consideration of all the circumstances of this case, 

 the author comes to the conclusion not only that such a " plan " can 

 be clearly traced oat in the present case, but that " natural selection " 

 can have had scarcely any share in determining the progressive evolu- 

 tion and relative distribution of the several forms of the OrbUoline 

 type. 



II. " On the Development of the Great Omentum and Trans- 

 verse Mesocolon." By C. B. Lockwood. Communicated 

 by W. S. Savory, F.R.S. Received May 18, 1883. 



(Abstract.) 



The paper begins by quoting the two usually accepted descriptions 

 of the peritoneum as far as the relations of the omentum and trans- 

 verse colon are concerned. The old account, that which makes the 

 transverse colon to be between the two ascending layers of the great 

 omentum, is first given. Afterwards the new account is repeated ; 

 'this says that the colon is not between the layers of the great 

 omentum, but only adherent to them. The development of the colon 

 is next mentioned, and Haller's theory, that the colon and omentum 

 become adherent, is discussed. Reasons are given to show that the 

 old account of the peritoneum is the true one, and that, therefore, 

 Haller's theory is unacceptable. After speaking of the development 

 of the omentum and its relation to the transverse colon, the changes 

 which the author believes to occur are described. Instead of adhesion 

 taking place between the omentum and colon, it is shown that the 

 peritoneal fossa, which at early periods exists between them, 

 gradually disappears, owing to an unfolding or drawing out of the 

 peritoneum at that point. It is further shown that when this has 

 taken place the transverse colon comes to be between the two ascend- 

 ing layers of the great omentum. 



A brief description of the anatomical preparations which accom- 

 pany the paper is also given. 



