818 DR. HANS GADOW ON OVARIES 



new invention of pouch-life and nursing caused the development 

 of an entirely new category of features — of features which were 

 not required either by the foetus or the adult, therefore larval, 

 e. y. a suctorial apparatus with its far-reaching incidental in- 

 fluences upon the future adult structures. 



In these respects viviparous reptiles, Hypo- and Prototheria 

 culminating in Monotremes, and Metatheria culminating in 

 Marsupials, represent a continuous progressive series, with a 

 logical terminus characterised by the enormous preponderance 

 of extra- over intra-utei'ine development. Compared with these 

 terminal Marsupials the Eutheria seem to be totally different, 

 provided we take as their type those which are born complete, 

 in this respect like the hypothetical Sauro-mammals, the whole 

 of the " 50 days" being intra-uterine. And yet the.Eutheria have 

 with certainty passed through the same Metatherian stage as 

 have the Marsiipials, and this Metatherian stage comprised, 

 besides others, the following features * : Truly viviparous ; allan- 

 toic placenta; marsupium ; diphyodont teeth, the .same two 

 naiddle series of a total of prelacteal, lacteal, permanent and post- 

 permanent sets ; nipples ; semi-cloaca ; absence of a corpus 

 callosum. 



To arrange any one of these features into successive morpho- 

 logical stages is comparatively easy, but it does not follow that 

 these represent exactly the phylogeny of the groups, because of 

 the complicated correlations with other organs which by no 

 means keep step with each other, neither in the same species nor 

 in the greater groups. Some are precocious, even hypertelic, 

 while others lag behind. 



Just as to the large eg^ of the truly oviparous Sauropsids 

 albumen (more watery but less fatty yolk) is added, before it 

 receives its calcai-eous shell, so in the Monotremes fluid is added 

 to the contents of the egg, but with the remarkable difference 

 that fluid matter is taken into the yolk-sac itself by osmosis 

 from the uterine walls, after the keratine shell has already 

 been developed. This process is correlated with an un- 

 doubted previously acquired reduction of the amount of ovarial 

 yolk, and is as much a secondary process as the loss of calcareous 

 matter in the parchment-like "keratine" shell. 



As Semon has shown, the whole shell-enclosed egg multiplies 

 its size during its passage through the oviduct. This mode of 

 growth finds a curious parallel analogy in various Lacertilia, the 

 parchment-shelled eggs of which grow considerably after they 

 have been deposited. 



Whilst the Sauropsidan allantois comes to surround the whole 

 yolk-sac and also neai'ly the whole of the albumen, so as to 

 spread over most of the inner surface of the egg, the Monotreme 



* Cf. R. Semon ; " Monotremen u. Marsupialier," in Zoolog. Forschung, Australien, 

 ii. 189i-97 ; fuvther, J. T. Wilson and J. P. Hill's papers in Q. J. M. S. 1897, 1898, 

 1900. 



