818 DR. HANS GADOW ON OVARIES 



new invention of poueli-life anil nursing caused the development 

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

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

 e. y. a suctorial apparatus with its far-reaching iucident;^! in- 

 fluences upon the future adult structures. 



lu these respects viviparous reptiles. Hypo- and Prototheria 

 euluiiuating in Monotremes, and Metitheria culminating in 

 JMarsupials, represent a continuous progressive series, with a 

 logical terminus characterised by the enormous preponderance 

 of extra- over iutra-uterine development. Compared with these 

 terminal Marsupials the Eutheria seem to be totally diflerent, 

 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 iutra-uterine. And yet the Eutheria have 

 Avith certainty passed through the same IVIetatherian stage as 

 have the Marsupials, and this Metatherian stage compi-ised, 

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

 toic placenta ; marsupium ; diphyodont teeth, the same two 

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

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

 callosum. 



To arrange an}' 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 egg of the truly oviparous Sauropsids 

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

 receives its calcareous shell, so in the Monotremes fliud is added 

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

 that fluid matter is taken into the yolk-s;\c itself by osmosis 

 from the uterine walls, after the keratine shell has already 

 been developed. This process is correlated with an lui- 

 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 panxllel 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 nearly the whole of the albumen, so as to 

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



* Cf. R. Semon ; " Monotremeu u. Mai-supialier," in Zoolog. Fovscluma:. Anstralien, 

 ii. 1891r-97; further, J. T. Wilson and J. P. Hill's papers in Q. J. M. S. 1897, 1898, 

 1900. 



