656 ON THE SHAPES OF EGGS [ch. 



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 

 probabihty 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 inquire 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 a practically incompressible 

 fluid, contained in a deformable capsule, which is either (a) entirely 

 inextensible, or (b) shghtly extensible, and which is placed in a 

 long elastic tube the walls of which are radially contractile, to 

 determine the shape under pressure. 



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 httle or no deforming pressure has been apphed 

 prior to the solidification of the shell, or else as cases where the 

 capsule was so httle capable of extension and so completely filled 

 as to preclude the possibihty of deformation. 



If the capsule be not spherical, but be inextensible, then 



deformation can take place under the external radial compression, 



* 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 curioxxs 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, co. 4 and 10), 

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

 of. Ellis's note). Bacon expressly remarks thit "Telesius should have been able 

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

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



