XV] AND OTHER HOLLOW STRUCTURES 939 



being forced along. So general is this relation that we may go still 

 further, and presume with great plausibihty in the few exceptional 

 cases (of which the apteryx is the most conspicuous) where the egg 

 is relatively large though not markedly unsymmetrical, that in these 

 cases the oviduct itself is in all probability large (as Giinther had 

 suggested) in proportion to the size of the bird. In the case of the 

 common fowl we can trace a direct relation between the size and 

 shape of the egg, for the first eggs laid by a young pullet are usually 

 smaller, and at the same time are much more nearly spherical than the 

 later ones ; and, moreover, some breeds of fowls lay proportionately 

 smaller eggs than others, and on the whole the former eggs tend to 

 be rounder than the latter*. 



We may now proceed to enquire more particularly how the form 

 of the egg is controlled by the pressures to which it is subjected. 



The egg, just prior to the formation of the shell, is, as we have 

 seen, a fluid body, tending to a spherical shape and enclosed within 

 a membrane. 



Our problem, then, is: Given an incompressible fluid, contained 

 in a deformable capsule, which is either (a) entirely inextensible, or 

 (6) slightly extensible, and which is placed in a long elastic tube the 

 walls of which are radially contractile, to determine the shape under 

 some given distribution of pressure. We may assume, at least to 

 begin with, that the shell-membrane is homogeneous and isotropic — 

 uniform in all parts and in all directions. 



If the capsule be spherical, inextensible, and completely filled 

 with the fluid, absolutely no deformation can take place. The few 

 eggs that are actually or approximately spherical, such as those of 

 the tortoise or the owl, may thus be alternatively explained as cases 

 where little or no deforming pressure has been applied prior to the 

 solidification of the shell, or else as cases where the capsule was so 



* In so far as our explanation involves a shaping or moulding of the egg by 

 the uterus or oviduct (an agency supplemented by the proper tensions of the 

 egg), it is curious to note that this is very much the same as that old view of 

 Telesius regarding the formation of the embryo {De rerum natura, vi, cc. 4 and 10), 

 which he had inherited from Galen, and of which Bacon speaks {Nov. Org. cap. .50; 

 cf. Ellis's note). Bacon expressly remarks that "Telesius should have been able 

 to shew the like formation in the shells of eggs." This old theory of embryonic 

 modelling survives in our usage of the term "matrix" for a "mould." 



