816 DR. HAXS GADOW OX OVAEIES 



Monotremes (cf. loss of the nucleus of red blood corpuscles, 

 structure of the atrio-veutricular valves, the alveolar lungs, the 

 abolishment of the cloaca,, the cranio-dental articulation of 

 the luiderjaw, etc.). 



The Elasmobranchs show various instances of precocious in- 

 ventions, ahead of the times, foredoomed to failure or further 

 improvement, because their owners are after all but low hshes. 

 In some the ovary is single, unpaired, but it lies in the middle. 

 Some Acanthopteri have succeeded in producing a median duct 

 out of their otherwise paired ovarial s:ics. Birds, as we have 

 seen, have suppressed one side ; a clumsy mode of procedure 

 because it has a lopsided result and implies the reduction, by 

 neglect, of one of the precious gonads. It is only the higher- 

 graded organisation of the Mammals which hns succeeded in 

 simplifying the apparatus in the morphologically neatest way, 

 namely, b}^ the partial fusion of the two ducts into one passage, 

 not only unpaired, but median, whilst the upper ends and the 

 ovaries remain intact and functional. 



]\[onotremes, Marsupials, and Placentals foim an unbroken, 

 progressive, thei-efore most probably raonophyletic series. The 

 reduction in question could not be brought about until the 

 reptilian plan of hard-shelled eggs had given way to internal 

 gestation. The differences between oviparous, ovoviviparous, 

 implacentally and placentally vivipaious, are questions of degree 

 only. There is for instance no difference, to be expressed in a 

 few words, between the ovoviviparous fruit of a Yiper and the 

 newborn fa?tus of a Kangaroo, except that the newborn reptile is 

 complete and must shift for itself. The point is that the young 

 bursts its egg, and other membranes, with the act of birth. 

 Whilst no newborn reptile requii'es maternal care, most birds do, 

 and all mammals are absolutely dependent on their mothers for 

 nourishment. 



Within the Class of Birds every stage from almost reptilian 

 to practically mammalian conditions is represented. The 

 typical nidifugous birds are hatched Avith still a considerable 

 amount of yolk slipped into the belly, sufficient for the little ones 

 to hold out for days without food until they are bodily and 

 mentally strong enough to feed themselves ; in many cases they 

 have to be shown by tlie parents how to do it. Next come those 

 which are hatched in a more or less helpless condition and must 

 be fed by the parents with food either in its natural state or 

 already semidigested. Lastly, those which are mused with a 

 milky secretion of the crop. The higher Altrices or ISTidicolaj are 

 born with but small remnants of yolk left, the digestive oi-gans 

 having been hurried on at the expense of the others. The most 

 significant point, however, is that through the crop-secretion 

 the Pigeons have established a parallel with the Mammals in so 

 far as the young are fed actually with parental matter, in which 

 proliferation and fatty degeneration of epithelial cells plays a 

 great part. The analogy can be carried still further, since by 



