XV] AND OTHER HOLLOW STRUCTURES 655 



and becomes in turn surrounded by the "shell-membrane." 

 About this latter the shell is secreted, rapidly and at a late period ; 

 the egg having meanwhile passed on into a wider portion of the 

 oviducal tube, called (by loose analogy, as Owen says) the " uterus." 

 Here the egg assumes its permanent form, here it becomes rigid, 

 and it is to this portion of the "oviduct" that our argument 

 principally refers. 



(2) Both the yolk and the entire egg tend to fill completely 

 their respective membranes, and, whether this be due to growth 

 or imbibition on the part of the contents or to contraction on the 

 part of the surrounding membranes, the resulting tendency is for 

 both yolk and egg to be, in the first instance, spherical, unless 

 otherwise distorted by external pressure. 



(3) The egg is subject to pressure within the oviduct, which 

 is an elastic, muscular tube, along the walls of which pass peri- 

 staltic waves of contraction. These muscular contractions may 

 be described as the contraction of successive annuli of muscle, 

 gi\ang annular (or radial) pressure to successive portions of the 

 egg; they drive the egg forward against the frictional resistance 

 of the tube, while tending at the same time to distort its form. 

 While nothing is known, so far as I am aware, of the muscular 

 physiology of the oviduct, it is well known in the case of the 

 intestine that the presence of an obstruction leads to the develop- 

 ment of violent contractions in its rear, which waves of contraction 

 die away, and are scarcely if at all propagated in advance of the 

 obstruction. 



(4) It is known by observation that a hen's egg is always 

 laid blunt end foremost. 



(5) It can be shown, at least as a vpry common rule, that 

 those eggs which are most unsymmetrical, or most tapered off 

 posteriorly, are also eggs of a large size relatively to the parent 

 bird. The guillemot is a notable case in point, and so also are 

 the curlews, sandpipers, phaleropes and terns. We may accord- 

 ingly presume that the more pointed eggs are those that are large 

 relatively to the tube or oviduct through which they have to pass, 

 or, in other words, are those which are subject to the greatest 

 pressure while being forced along. So general is this relation 

 that we may go still further, and presume with great plausibihty 



