XV] AND OTHER HOLLOW STRUCTURES 937 



Of all the many naturalists of the eighteenth and nineteenth 

 centuries who wrote on the subject of eggs, only two (so far as 

 I am aware) ascribed the form of the egg to direct mechanical 

 causes. Giinther*, in 1772, declared that the more or less rounded 

 or pointed form of the egg is a mechanical consequence of the 

 pressure of the oviduct at a time when the shell is yet unformed 

 or unsolidified ; and that accordingly, to explain the round egg of 

 the owl or the kingfisher, we have only to admit that the oviduct 

 of these birds is somewhat larger than that of most others, or 

 less subject to violent contractions. This statement contains, in 

 essence, the whole story of the mechanical conformation of the egg. 

 A hundred and twenty years after, Dr J. Ryder of Philadelphia 

 gave, as near as may be, the same explanation f. 



Let us consider, very briefly, the conditions to which the egg is 

 subject in its passage down the oviduct. 



(1) The "egg," as it enters the oviduct, consists of the yolk only, 

 enclosed in its vitelHne membrane. As it passes down the first 

 portion of the oviduct the white is gradually superadded, 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 ultimately 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 or until 

 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 peristaltic 



* F, C. Giinther, Sammlung von Nestern und Eyern verschiedener Vogel, Niirnb. 

 1772. Cf. also Raymond Pearl, Morphogenetic activity of the oviduct, J. Exp. Zool. 

 VI, pp. 339-359, 1909. - 



t J. Ryder, The mechanical genesis of the form of the fowl's egg, Proc\ Amer. 

 Philosoph. Soc. Philadelphia, xxxi, pp. 203-209, 1893; cf. A. S. Packard, 

 Inheritance of acquired characters, 'Proc. Amer. Acad. 1894, p. 360. 



