AND OVIDUCTS IN THE AMNIOTA. 813 



stronger arterial supply would of course mean mistaking effect 

 for cause *. 



A sufficient cause, however, may be the fact that a full ovi- 

 duct is less liable to disturb the other intestines in the left half 

 of the body-cavity than in the right. The primary intestinal 

 loops ai-e so arranged or packed, that their bases begin on the 

 right whilst their apices extend towards or into the left side of 

 the abdomen. This is especially the case with those loops which 

 starting to the right of the stomach (itself mostly shoved to 

 the left) fill the space between stomach and vent with their 

 distal halves. It needs no further comment that it is the 

 free or apical end, and not the base of a loop where the 

 mesenteric vessels enter, which is displaced easiest and which 

 will easiest resume its original position. But this packing 

 from right to left is not an ada,ptation to, and is not produced 

 by the preponderance of the left oviduct. It can be traced 

 to a much more primitive condition, namely to the fact that 

 the bird's embryo comes to rest with its left side upon the yolk, 

 with its curved back towards the blunt pole. In all probability 

 this is a truly cfenogenetic feature, essentially ontogenetic ; one 

 of those numerous phenomena which, like the gills of tadpoles, 

 the allantois and placenta, are originally incidental to embryonic 

 life, although they may by correlated after-effects profoundly 

 influence even the adult organism. Obvious results of this left- 

 sided position of the embryo are the increasing preponderance of 

 the left vitelline vein ; the yolk-stalk causes the first loop of the 

 midgut ; the stomach itself sinks in, turning the pylorus to the 

 right, upon which side the duodenal loop descends, and further 

 secondary loops of the midgut follow suit. If there are large 

 caeca, they likewise make their way towards the right and back 

 of the stomach. The allantoic bag, containing fluid only, rises 

 and comes to lie upon the embryo, i. e. upon its right side. 



Consequently there is asymmetry introduced at an eax'ly date, 

 which affects the viscera, notably the gut, and introduces a bias 

 in their mutual behaviour within the belly. During the gi-owth 

 of the embryo, by shrinking of the yolk room becomes available 

 for extension of the gut towards the left side. The permanent 

 organs will soon — speaking from the point of evolution — establish 

 an equilibrium, whilst it is clear that any occasional or contingent 

 requirement of space, or disturbance, can be met with easiest in 

 the left half. Such a disturbance is caused by the periodic 

 growth and passage of the eggs which brook no delay. The 

 slightest bias will turn the scales, and now we may apply the 

 censorship of natural selection to its fullest extent. Left eggs 



* The suppression of the right oviduct has had an effect upon the male copulatory 

 organ. Where such is present it is asymmetrical, although unpaired, and stowed 

 away in a left-sided recess of the cloaca. The act invariably takes place from the 

 left side, and the same applies to those birds which are now devoid of such an intro- 

 mittent organ. 



