| OCR Text |
Show 306 DR. H. GADOW ON THE INTESTINAL [May 21, loops. In many instances a spiral is produced by one loop curled upon itself, its apex then forming the centre of the spiral. O n to the apex is attached the diverticulum csecum vitelli; this shows that this spiral is produced by the primitive fold of the embryonic mid-gut. Such is the case in all the Passeres, and since there are only three folds formed by the whole gut, the spiral represents the middle or second fold -, hence this arrangement may be distinguished as mcso-gyrous. The number of turns in such a spiral depends directly upon the length of the intestine ; whilst in the short-gutted Sylviae the spiral is just indicated, there are in the Sparrow (with an intestinal length of 21 centim.) 1 | direct and 1 retrograde turns, and in Pinicola enucleator (which possesses an intestine of 99 centim. in length) there are many direct turns. It is clear that with an original number of only four loops, the conversion of the two middle ones into one spiral will cause such birds as certain Limicolae, Laridae, and Columbae likewise to assume the mesogyrous feature; but the position of the diverticle on the original third loop, and the relations of these birds, like, e. g., Charadrius and Sterna, show that this mesogyrous formation has been brought about in a way different from that of the Passeres. Lastly, the distal portion of any loop originally straight may become coiled up into a spiral, whilst the rest of the loop remains straight. This feature may be termed teloyyrous. With the duodenum this is very rare, it then invariably forms a right-handed spiral, e. y., in Buceros, Ciconia, Milvus ; the duodenum is more irregularlv twisted in certain Pelargi and Accipitres. The ends of the second, third, and fourth loops are never coiled up into a regular spiral, but rather form irregularly coiled up masses, in many Pelargi, Accipitres, and in the Psittaci. W e see, then, that the cycloccelous (meso- or telogyrous) feature by itself cannot be taken as a character which indicates the affinity of the larger groups or orders of birds, unless we take the mode of development of these concentric convolutions into consideration. In fact the cycloccelous formation is the highest mode of stowing away in the smallest compass that portion of the gut which had to be increased in length, the relative length of the mid-gut being dependent upon the nature and composition of the food. In strictly orthoccelous birds the increased length of the gut causes the formation of secondary folds anywhere between the previously existing loops, whereby frequently a very irregular arrangement of all the convolutions is caused. A similar process has produced the plagioccelous feature (fig./, p. 305), which was probably derived from an orthoccelous basis. The highest and perhaps newest mode of stowing away an increased amount of intestinal length is that in which one of the folds already existing is lengthened and, owing to its interstitial growth, turns into a spiral ; in this way the other loops will undergo the least possible disturbance. I do not think it necessary to give here a long and detailed |
