XV] AND OTHER HOLLOW STRUCTURES 943 



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. 



In its passage down the oviduct the egg is not merely thrust but 

 also screwed along; and its spiral course leaves traces on wellnigh 

 all its structure save the shell. When we have broken the shell of 

 a hard-boiled egg the shell-membrane below peels off in spiral strips, 

 and even the white tends to flake off in layers, spirally. In the 

 fresh unboiled egg two knotted cords — the treadles or chalazae — are 

 connected with the yolk, and lie fore-and-aft of it, loose in the 

 albimien. These represent the free ends of a yolk-membrane, which 

 got caught in the constricted oviduct while the yolk between them 

 was being screwed along: very much as we may wrap an apple in 

 a handkerchief, hold the two ends fast, and twirl the apple round. 



These, then, are the general principles involved in, and illustrated 

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

 safely go without actual quantitative determination, in each par- 

 ticular 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 

 is explained the curious form of the egg in Bilharzia (Schistosoma) 

 haematobium, a formidable parasitic worm to which is due a disease 

 wide-spread in Africa and Arabia, and an especial scourge of the 

 Mecca pilgrims. The egg in this worm is provided at one end with 

 a little spine, which is explained as having been moulded within a 

 little funnel-shaped expansion of the uterus, just where it communi- 

 cates with the common duct leading from the ovary and yolk-gland. 

 Owing to some anatomical difference in the uterus, the httle 



* It is a common but unfounded belief among poultry-men that shape and size 

 are related to the sex of the egg, the longer eggs producing mostly male chicks. 

 That there is no such correlation between sex on the one hand and weight, length 

 or shape on the other, has been clearly demonstrated. Cf. M. A. Jull and J. P. 

 Quinn, Journ. Agr. Research, xxix, pp. 195-201, 1924. 



