MR. OWEN ON THE OVA OF THE ORNITHORHYNCHUS PARADOXUS. 563 



It may, however, be said that the deciduous membrane is here represented by the 

 cortical or outer covering of the ovum : but this membrane, though of a denser 

 structure and without villi, is certainly analogous to the outer tunic of the uterine 

 ovum of the Rabbit and Bitch, which in them is gradually separated from the vitelline 

 membrane by the imbibition of albuminous fluid. Now the relative proportion of 

 the fluid interposed between the cortical and vitelline membranes in the small and 

 large ova of the Ornithorhynchus, shows that the mutual recedence of the two mem- 

 branes is effected in the same way. 



The form, the structure, and the detached condition of the ova of the Ornithorhyn- 

 chus, may still be regarded as compatible with, and perhaps favourable to, the opinion 

 that they are excluded as such, and that the embryo is developed out of the parent's 

 body. But the following objections present themselves to this conclusion ; — the only 

 part of the eff'erent tube of the generative apparatus which can be compared in struc- 

 ture or relative position to the shell-secreting uterus of the Fowl, is the dilated terminal 

 cavity in which, in all the specimens above described, the ova were situated ; and upon 

 the oviparous theory it must be supposed either that the parietes of this cavity, after 

 having secreted the requisite quantity of soft material, suddenly assume a new func- 

 tion, and complete the ovum by providing it with the calcareous covering necessary 

 to enable it to sustain the superincumbent 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 sup- 

 position, by a calcareous secretion of the abdominal glands poured out upon the 

 ovum after its exclusion. 



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

 external covering, yet from the evidence affbrded by the specimens under considera- 

 tion, the ovum is still 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 development of the foetus 

 requires that all the necessary nutritive material be accumulated 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 may exist 

 in other parts of the Ornithorhynchus, it is most important to the question of its 

 generation to bear in mind that it manifests no resemblance to the bird in the 

 disposition of the pubic bones. 



Again, as we have seen that the ova of the Ornithorhynchus have attained a diame- 

 ter of little more than two lines after having traversed the whole of the Fallopian tube, 

 the length of which is six inches, and the internal secreting surface increased by nu- 

 merous folds, it may be reasonably inferred from the analogy of the Rabbit and other 

 Mammalia, that the ovum was of much smaller dimensions when first received into 



