942 ON THE SHAPES OF EGGS [ch. 



membrane, Tjr + T'jr'. The sum of these curvatures varies from point 

 to point; the internal or hydrodynamical pressure, P, is constant; and 

 therefore the external pressure, jo„, varies from point to point with 

 the curvature, and is a direct function of the shape of the egg. 



Some few eggs, such as the owl's and the kingfisher's, are so nearly 

 spherical that we are apt to speak of them as spheres ; but they are 

 all prolate more or less, and no egg is so nearly circular in meridional 

 section as all eggs are in their circles of latitude. When the egg is 

 all but spherical that shape may be due (as we have seen) to various 

 causes : to a relatively, small size of the egg, allowing it to descend 

 the tube under a minimum of peristaltic pressure; perhaps to an 

 unusually strong shell-membrane, resistant of deformation; in 

 general terms, to a possible diminution of j?„ , or a possible increase 

 of T. But all eggs have approximately spherical ends, and the 

 big anterior end of the large conical eggs of plover or curlew or 

 guillemot is conspicuously so. Here the egg projects into the wide 

 cavity of the uncontracted oviduct, external or peristaltic pressure 

 does not exist, the shell-membrane has to resist internal pressure 

 without further external support, and the resultant spherical cur- 

 vature is an indication of the uniformity, or isotropy, of the mem- 

 brane. The lesser of the two spherical ends, that is to say the 

 posterior end, has by much the greater curvature, and the tension 

 there is correspondingly great. It would seem that the membrane 

 ought to be thicker or stronger at this pointed end than elsewhere, 

 but it is not known to be so. In any case, it is just here, in this 

 presumably weakest part, that we are most apt to find the irregulari- 

 ties and deformities of misshapen eggs. 



Within the egg lies the yolk, and the yolk is invariably spherical 

 or very nearly so, whatever be the form of the entire egg. The 

 reason is simple, and lies in the fact that the fluid yolk is itself 

 enclosed within another membrane, between which and the shell- 

 membrane Hes the fluid albumin, which transmits a uniform hydro- 

 static pressure to the yolk*. The lack of friction between the yolk- 

 membrane and the white of the egg is indicated by the well-known 

 fact that the "germinal spot" on the surface of. the yolk is always 



* In like manner, the cell-nucleus is "usually globular, except in certain 

 specialised tissues, or when it degenerates" (Darlington). Whether it possesses 

 a membrane is matter in dispute, but it at all events possesses a surface, with 

 a phase- difference between it and the surrounding cytoplasm. Cf. above, p. 295. 



