MAMMALIA. 709 



supposed either that the parietes of this cavity, after having secre- 

 ted the requisite quantity of soft material, suddenly assume a new 

 function, and complete the ovum by providing it with the calca- 

 reous covering necessary to enable it to sustain the superincum- 

 bent weight of the mother during incubation ; or that this is 

 effected by a rapid deposition from the cuticular surface of the 

 external passages ; or lastly, according to a more recent but still 

 more improbable supposition, by a calcareous secretion of the ab- 

 dominal glands poured out upon the ovum after its exclusion. 



Fig. 327.* 



But granting that the egg is provided in any of these ways with 

 the necessary external covering, yet, from the evidence afforded by 

 the specimens examined, the ovum is deficient in those parts of its 

 organization which appear to be essential to successful incubation, 

 viz. a voluminous yolk to support the germinal membrane, and the 

 mechanism for bringing the cicatricula into contiguity with the body 

 of the parent. Add to this, that such a mode of developement of 

 the fetus requires that all the necessary nutritive material be accu- 

 mulated in the ovum prior to its exclusion. Now the bony pelvis 

 of the Bird is expressly modified to allow of the escape of an egg, 

 both large from the quantity of its contents, and unyielding from its 

 necessary defensive covering ; but, whatever affinities of structure 



* Owing to an error on the part of the draughtsman, who has neglected to reverse 

 the drawing, the left uterus in the above figure is represented on the right side, and 

 vice versa. 



