'34 



NA TURE 



[Dec. ii, i! 



but none could be observed, nor would it be easy to 

 explain how an egg could lie in the vagina to receive its 

 shell, as the urine from the bladder must pass directly 

 over it. Finding they had no resemblance to the oviducts 

 in birds, I was led to compare them with the uteri of 

 those lizards which form an egg that is afterwards de- 

 posited in a cavity corresponding to the uterus of other 

 animals where it is hatched ; which lizards may be there- 

 fore called ovo-viviparous, and I find a very close 

 resemblance between them." 



There has been, however, a certain amount of direct 

 evidence brought forward beyond that which has been 

 quoted from Geoffroy St. Hilaire's paper to prove that 

 Monotremes are really oviparous, notwithstanding the 

 fact that they nourish their young by means of mammary 

 glands. 



Thus a Dr. Nicholson {Phil. Trans., 1S65, p. 683), 

 writing to Owen in 1865, informs him that a Platypus 

 had been captured by workmen on the banks of the 

 River Goulburn in Victoria, and had been placed for a 

 night by the gold-receiver of the district, to whom it had 

 been handed over, in a wooden case, and that whilst in 

 confinement it had laid two eggs — white and soft, with 

 no calcareous covering, and about the size of a crow's 

 egg. Dr. Nicholson does not, however, appear to have 

 taken the trouble to examine the eggs at all carefully, and 

 his evidence is rejected by Owen as certainly insufficient 

 to make him doubt that the Monotremes are ovo-vivi- 

 parous. 



Earlier still in Messrs. Lesson and Garnot's " Voyage 

 de la Coquille" {Zool. Journal, vol. v.), it is stated that 

 the colonists assured the travellers that Ornithorhynchus 

 was oviparous, whilst a Mr. Murdoch, superintendent of 

 the farms on Emu Plains, said that he himself had seen 

 the eggs, that they were two in number, and the size of 

 a hen's egg. 



Again, Dr. Weatherhead {Proc Zool. Soc, 1832, p. 145) 

 read, before the Zoological Society in 1S32, extracts from 

 a letter received by himself from Lieut. Maule in New 

 South Wales, wherein the latter describes in some detail 

 the finding and unearthing of an Ornithorhynchus burrow. 

 The entrance to the latter, he says, is beneath the water, 

 though, after some little distance, the passage rises, and, 

 after a few yards have been traversed, it " branches into 

 two others, which, describing each a circular course to 

 the right and left, unite again in the nest itself, which is a 

 roomy excavation lined with leaves and moss, and situated 

 seldom more than twelve yards from the water or less 

 than two feet below the surface of the earth : several of 

 these nests were with difficulty discovered. No eggs 

 were found in a perfect state, but pieces of a substance 

 resembling egg-shell were picked out of the debris 

 of the nest. In the insides of several Platypi which 

 were shot were found eggs of the size of a musket-ball 

 and downwards, imperfectly formed however, i.e. without 

 the hard outer shell, which prevented their preservation." 



Dr. Bennett also {Proc. Zool. Soc, 1859, p. 213) 

 investigated the structure of the Monotremes and exa- 

 mined the nest of Platypus, but failed to find traces of 

 eggs. 



Lastly, Mr. Patrick Hill, surgeon in the Royal Navy, 

 published in Trans. Linn. Soc, xiii. p. 621, information 

 which had been brought to his notice concerning the 

 Monotremes whilst studying their nature and habits in 

 the district around Sydney. A female specimen was 

 brought to him which had been taken directly from its 

 nest, but which died very soon after being placed in con- 

 finement. On opening the body he discovered in the left 

 ovary a round yellow ovum about the size of a small pea 

 and two of smaller size, whilst no trace of ovisac was to 

 be found in the right ovary ; l but beyond his own investi- 

 gations he brings forward the evidence of a certain 



1 See also Owen, "^Anat. of Vert.," vol. iii. p. 676; also Proc. Zool. S:K., 

 1834, p. 143. 



Cookoogong, chief of the Boorah-Boorah tribes, who, as 

 is quaintly remarked in Geoffroy St. Hilaire's article, " ne 

 manquait ni de lumiere ni de moralite." This native 

 chief stated that it was a fact well known to their tribe 

 that the animal lays two eggs, about the size, shape, and 

 colour of those of a hen, and that the female sits a con- 

 siderable time on her eggs in a nest which is always 

 found among the reeds on the surface of the water ; the 

 native name for the Platypus, he added, was Mullingong. 



The most important part of Mr. Caldwell's communica- 

 tion to the British Association was, however, contained 

 in the two words " ovum meroblastic " ; in other words, 

 the ovum of a Monotreme contains, relatively to the pure 

 protoplasm out of which the tissues of the animal will be 

 formed, so much food-yelk that, when segmentation takes 

 place, it is impossible for the egg to segment as a whole, 

 and therefore the two kinds of protoplasm separate, and 

 we find that the Monotreme embryo possesses a yelk-sac, 

 by the gradual absorption of the contained material of 

 which it is nourished during the early stages of develop- 

 ment. 



The presence of so large an amount of food-yelk thus 

 renders it unnecessary that during this period the tissues 

 of the embryo should enter into such a close relation with 

 the maternal ones as is found to obtain in the rest of the 

 Mammalia, though e\-en amongst the higher members of 

 the latter there are certain signs which point to a former 

 period in their phylogenetic history when they also were 

 possessed of a yelk-sac. 



Figs. 2 and 3 represent respectively examples of holo- 

 blastic and meroblastic ova at very early stages, the one 

 being that of a rabbit, the other that of a Sauropsidan, 

 and it is to the latter that the ovum of Monotremes bears 

 a close resemblance. 



In both cases it is interesting, however, to observe that 

 a structure is eventually formed known in birds and rep- 

 tiles as the yelk-sac and in most mammals as the um- 

 bilical vesicle, the two being really homologous with each 

 other. 



Now that Mr. Caldwell has shown that in the lowest 

 mammals a yelk-sac is present containing food-yelk 

 instead of an umbilical vesicle as in the higher forms, it 

 may be affirmed that the curious stages in the develop- 

 ment of most Mammalia which result in the pinching off 

 of the embryo and the formation of an umbilical vesicle 

 are indications still remaining of the time when these 

 animals were nourished during early stages, not directly by 

 a close union with the maternal tissues, but by means of 

 yelk-sacs : it affords evidence, in fact, that their ancestors 

 were not viviparous, but oviparous, just as are the lowest 

 mammals now known to us. 



During late years various theories have been held 

 concerning the origin of mammals : Balfour formed a 

 hypothetical group— the Pentadactyloidei— in which he 

 supposed the pentadactyle limb characteristic of all the 

 higher vertebrates to have been established : from this he 

 derives two groups— the one including the present exist- 

 ing Amphibia, the other being a hypothetical and some- 

 what generalised group, from which, though along 

 divergent lines, were developed both Mammalia and 

 Sauropsida. Thus, according to him, the two latter were 

 branches from one common stem, but the Sauropsida 

 could not be considered as the ancestor of the Mam- 

 malia. 



Other scientific men have held that mammals were 

 derived from Amphibia-like ancestors : with the present 

 Amphibia they were supposed to agree in the presence of 

 a holoblastic ovum, and in the important fact that in both 

 groups two occipital condyles are present, whilst only one 

 is typically found in the Reptilia. 



It is interesting to notice that Cope has described, 

 amongst the numerous extinct forms of reptiles which he 

 has brought to light during the past few years, one, called 

 by him the Theromorpha {Proc. Am. Phil. Soc, vol. xix. 



