64 A vwKu. ox p:ggs, 



and never be laid, and it would be quite globular, which an 

 Qgg never is naturally. 



In the case of a pyriform egg, if, as Dr. Thomson says, the 

 small end passes first, this peculiar form may be produced by the 

 restraining vis a fronte acting not merely on the central point of 

 that end of the egg going first, but also more or less on the parts 

 surrounding that centre ; the pressure diminishing gradually as 

 far back as the widest part of the egg, these parts would thus be 

 compressed and tapered back, giving to that end more or less the 

 form of a cone gradually enlarging backwards, and bulging out 

 the hinder part ; at the same time, the propelling vis a tergo act- 

 ing strongly and abruptly against the base or hinder part of the 

 cone, would enlarge and somewhat flatten that part, giving it a 

 blunt rotundity. In this form an egg is well adapted for being 

 gradually insinuated into the closed o^-iduct before it, the apex 

 of the cone acting as a circular wedge, and opening out by degrees 

 the folds and windings of tlie tube as the egg is being diiven on. 



If the large end be the first to pass, the restraining force must 

 be represented by a firm, abrupt, ring-like contraction of the 

 walls of the oviduct in front of the downAvard end of the ag^ 

 which, being forcibly urged from behind against that contraction, 

 would become more or less flattened in front and bulged out 

 laterally ; the rest of the ogg being pressed upon by a graduated 

 force increasing backwards, would assume a conical form in that 

 direction. 



In each case the regular contraction of the circular fibres of 

 the oWduct will preserve the transverse rotundity, and the form 

 thus produced will be stamped upon the egg by the supei-jDosition 

 of the shell membrane in the istlimus. 



In the former of these cases the conical anterior end of the 

 egg is diivcn on like a wedge by pressure exerted on the broad 

 posterior end ; in the latter, the broad end is driven forwai'd by 

 the vis It tergo acting ui)on the greater part of the surface of the 

 egg behind : in tlie former, there is an advantage in form, in the 

 latter, a superiority of power. 



By the application of these muscular forces in variously modified 

 ways and degrees tlic varieties of the form of eggs are produced. 



