398 



MONOTREMATA. 



nearly the same size (fig. 192). The right 

 uterus in all the specimens had become sym- 

 pathetically affected, being firmer in texture 

 and thicker in its coats. 



The parietes of the impregnated uteri were 

 from three to four lines in thickness; an in- 

 crease which was principally occasioned by 

 the extension of small vascular folds between 

 the fibrous and internal coats, which were so 

 placed at right angles to these tunics as to 

 present an appearance very similar to that of 

 the second cavity of the stomach of the Por- 

 pesse. The fibrous coat was slightly thickened 

 near the cervix, and the serous covering was 

 separated from it by the ramifications of nu- 

 merous large and tortuous uterine vessels. 



There was not the slightest trace of a deci- 

 dual or adventitious membrane in the cavity of 

 the womb ; and especial attention was directed 

 to this circumstance, in consequence of the 

 office assigned to it in a recent work,* as mi- 

 nistering support to the ova in the higher 

 Mammalia, at a period when, like those of the 

 Ornithorhynchus, they have no attachment to 

 the uterine parietes.f 



It may, however, be said that the deciduous 

 membrane is here represented by the cortical 

 or outer covering of the ovum : but this mem- 

 brane, though of a denser structure and with- 

 out 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 pro- 

 portion 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. 



If the form, the structure, and the detached 

 condition of the ova of the Ornithorhynchus, 

 should still be regarded by some 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, 

 the following objections oppose themselves to 

 such an opinion : the only part of the efferent 

 tube of the generative apparatus which can be 

 compared in structure 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 situ- 

 ated; 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 function and complete the ovum by pro- 

 viding it with the calcareous covering neces- 

 sary to enable it to sustain the superincumbent 

 weight of the mother during incubation ; or, 

 that this is effected by a rapid deposition of 

 the same material from the cuticular surface of 

 the external passages ; or lastly, according to 

 a more recent, but still more improbable sup- 



* Breschet, Etudes de 1'GEuf Humain. 

 t In the recent specimens Mr. Bennett noticed 

 besides the ova only a "moisture" in the uterus. 



position, by a calcareous secretion of the ab- 

 dominal 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 co- 

 vering, yet from the evidence afforded by the 

 specimens under consideration, the ovum is 

 still deficient in those parts of its organization 

 which appear to be essential to successful in- 

 cubation, viz. a voluminous yolk to support 

 the germinal membrane, and the mechanism 

 for biinging the cicatricula into contiguity with 

 the body of the parent. Add to this, that such 

 a mode of development of the fetus 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 Ornithorhyn- 

 chus, it is most important to the question of 

 its generation to bear in mind that it manifests 

 no resemblance to the bird in the condition of 

 the pubic bones. 



Again, as we have seen that the ova of the 

 Ornithorhynchus have attained a diameter 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 numerous 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 the oviduct. But the yolk in 

 Birds and Oviparous Reptiles is invariably the 

 product of the ovary, and derives no appre- 

 ciable increase from the secretions of the 

 efferent tube, which supply only the albu- 

 minous part of the egg, or the material for the 

 first formation of the chick. If, therefore, the 

 gestation of the Ornithorliynchus terminates by 

 the exclusion of an egg, as in the Bird or 

 Tortoise, the preparatory steps in the formation 

 of the ovum are widely different, for the parts 

 concerned manifest the essential characters of 

 the Mammiferous type, and the germ itself has 

 a corresponding structure. 



These facts, it is agreeable to find, are in 

 exact accordance with the now ascertained 

 functions of the abdominal glands ; for since 

 the yolk in the Bird, besides its uses in the 

 course of the fcetal development, is intended 

 as an after-substitute for a mammary secretion, 

 remaining, as it does, but little diminished 

 at the close of incubation, it might have been 

 concluded, from a priori physiological deduc- 

 tion, that the Monotremes, in which no such sub- 

 stitute is required, would approximate the other 

 Mammalia in the small size of the ovarian ovum. 



The nature or amount of subsequent devia- 

 tions from a true viviparous generation can be 

 determined only by future examinations of more 

 advanced ova. From the structure of the cor- 



* Geoffrey St. Hilaire, Gazette Medicale, Feb. 

 11, 1833. 



