402 



MONOTREMATA. 



neum, opposite the middle of the abdomen, 

 along the anterior margin of the suspensory 

 ligament to the liver. It was reduced to a 

 mere filamentary tube, filled with coagulum. 

 From the same cicatrix the remains of the 

 umbilical arteries extending downwards, and 

 near the urinary bladder, were contained within 

 a duplicature of peritoneum, having between 

 them a small flattened oval vesicle, the re- 

 mains of an allantois, which was attached by 

 a contracted pedicle to the fundus of the 

 bladder. 



As both the embryo of the Bird and that of 

 the ovoviviparous Reptile have an allantois 

 and umbilical vessels developed, no certain 

 inference can be drawn from the above appear- 

 ances as to the oviparous or viviparous na- 

 ture of the generation of the Ornithorhynchus. 

 But the structure of the ovary and that of the 

 ovum, both before and after it has quitted the 

 ovisac, afford the strongest analogical proof of 

 the intra-uterine developement of the embryo, 

 and at the same time accord with the ascer- 

 tained fact of the mammary nourishment of the 

 young animal. 



The kidneys were situated remote from the 

 pelvis and high up in the lumbar region. 



The situation of the kidneys with respect to 

 each other varied in the two specimens ; in the 

 larger one, the left was a little higher than the 

 right; in the smaller one it was a little lower; 

 the latter is the ordinary position in the adult. 

 The supra-renal glands did not correspond with 

 this arrangement, but in both instances the 

 right was higher than the left, agreeing with the 

 relative position of the testes in the male, and 

 the ovaries in the female. In Man the large 

 size of the supra-renal glands is noted as a 

 fetal peculiarity, but in the Ornithorhynchus 

 they are of minute size, their greatest diameter 

 not exceeding one-eighth of a line in the smaller 

 specimen here described ; and they increase in 

 size progressively with the growth of the ani- 

 mal, and in a greater proportion than the kid- 

 neys, which increase would appear, therefore, 

 to have relation to the development of the ge- 

 nerative organs. There were no traces of the 

 corpora Wolffiana. 



The testes in the small male specimen were 

 situated a little below the kidneys: they were 

 of an elongated form, pointed at both ends, with 

 the epididymis folded down, as it were, upon 

 their anterior surface. In the female, the ova- 

 ries were freely suspended to the loins in a 

 similar position, the right being at this period 

 as large as the left : it is the persistence of the 

 latter at an early stage of development which 

 occasions the disproportionate size of the two 

 glands in the adult. The still greater inequa- 

 lity of size in the oviducts of the Bird arises, 

 as is well known, from a similar arrest of the 

 development of the one on the right side, but 

 both are equal at an early stage of existence. 

 The uteri were straight linear tubes, scarcely 

 exceeding the size of the ovarian ligaments. 



The lungs were found amply developed in 

 both specimens ; the air-cells remarkably ob- 

 vious, so as to give a reticulate appearance to 

 the surface, and a resemblance to the lungs of 



a turtle. They had evidently been permeated 

 by air in the smaller specimen. 



The heart, in both specimens, was of the 

 adult form, with the apex entire; but the left 

 auricle was proportionately larger than in the 

 adult heart. 



The ductus arteriosus was here very evident, 

 and formed a filamentary chord in the usual 

 situation between the aorta and pulmonary 

 artery, but proportionately longer than in the 

 true viviparous Mammalia. Here also we 

 have the indication of a more prolonged fatal 

 existence than in the marsupial animals, there 

 being no trace of a ductus arteriosus either in 

 the uterine or mammary foetus of the Kangaroo. 



The Ornithorhynchus also deviates from the 

 ordinary Marsupialia in having the thymus 

 gland. This is situated in front of the great 

 vessels of the heart, and consists of two lobes, 

 of which the right is the largest. The traces of 

 foetal structures presented by these young Or- 

 nithorhynchi, and especially the allantoic dila- 

 tation of the urachus, indicate that the Mono- 

 tremata differ from the Marsupialia in a longer 

 continuance of the true fatal or intra-uterine 

 existence. 



Mammary organs. In this section will be 

 adduced the evidence in proof of the essentially 

 Mammalian nature of the Monotremes which 

 the presence and ascertained function of the 

 mammary glands have yielded. 



The most important result of Professor Mec- 

 kel's anatomical investigations of the Oniit/io- 

 rhynchus was his discovery of the two large 

 abdominal subcutaneous glands : these he con- 

 cluded to be the mammary organs, which until 

 that period had been supposed to be absent in 

 the Ornithorhynchus. 



Subjoined is the figure which Meckel has 

 given of one of these glands in its natural 

 relative position, jig. 198. It measured four 

 inches and a half in length, two inches in 

 breadth, and half an inch in thickness. 



From this apparently conclusive evidence of 

 the affinity of the Ornithorhynchus to the Mam- 

 malia, Professor Meckel, however, is far from 

 drawing conclusions as to the identity of their 

 mode of generation. For assuming that the 

 difference between the bringing forth of living 

 young and of eggs is really very small and by 

 no means of an essential nature, and remarking 

 that birds have accidentally hatched the egg 

 within the abdomen and so produced a living 

 fetus, an occurrence which has also been in- 

 duced by direct experiment, and that, lastly, 

 the generation of the Marsupial animals is very 

 similar to the oviparous mode, he deems it 

 very probable that as the Ornithorhynchus ap- 

 proaches still nearer than the Marsupial ani- 

 mals to Birds and Reptiles, its mode of gene- 

 ration may be in a proportionate degree analo- 

 gous. For an animal possessing mammary 

 glands he claims, however, the right to rank 

 with the Mammalia, agreeing with Professor 

 Geoffrey only so far as to consider the Mono- 

 tremata as a distinct order of quadrupeds, 

 which he places, as Cuvier has done, next to 

 the Edentata. 



Against this conclusion, however, Professor 



