660 ON THE SHAPES OF EGGS [ch. 



even to burst, and it is here that we most commonly find irregu- 

 larities of shape in abnormal eggs. 



If one portion of the envelope were to become practically stifi 

 before jp ceases to vary, that would be tantamount to a sudden 

 variation of T, and would introduce asymmetry by the imposition 

 of a boundary condition in addition to the above equation. 



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 yolk is itself enclosed 

 in another membrane, between which and the outer membrane 

 lies a fluid the presence of which makes jo„ for the inner membrane 

 practically constant. The smallness of friction is indicated by 

 the well-known fact that the "germinal spot" on the surface of 

 the yolk is always found uppermost, however we may place and 

 wherever we may open the egg; that is to say, the yolk easily 

 rotates within the egg, bringing its lighter pole uppermost. So, 

 owing to this lack of friction in the outer fluid, or white, whatever 

 shear is produced within the egg will not be easily transmitted 

 to the yolk, and, moreover, owing to the same fluidity, the yolk 

 will easily recover its normal sphericity after the egg-shell is 

 formed and the unequal pressure relieved. 



These, then, are the general principles involved in, and illus- 

 trated by, the configuration of an egg; and they take us as far 

 as we can safely go without actual quantitative determinations, 

 in each particular case, of the forces concerned. 



In certain cases among the invertebrates, we again find 

 instances of hard-shelled eggs which have obviously been 

 moulded by the oviduct, or so-called "ootype," in which they 

 have lain : and not merely in such a way as to shew the effects 

 of peristaltic pressure upon a uniform elastic envelope, but so 

 as to impress upon the egg the more or less irregular form 

 of the cavity, within which it had been for a time contained 

 and compressed. After this fashion Dr Looss* of Cairo has 



* Journal of Tropical Medicine, 15th June, 1911. I leave this paragraph as it 

 was written, though it is now once more asserted that the terminal and lateral- 

 spined eggs belong to separate and distinct species of Bilharzia (Leiper, Brit. Med. 

 Journ., 18th March, 1916, p. 411). 



