AND OVIBUCTS IN THE AMNIOTA. 815 



which still possess comparatively large-yolked eggs. However, 

 the Monotreme embryo does not turn upon its left side, it merely 

 sinks into the cavity of the emptied yolk-sac, forming a 

 proamnion ; the bird, owing to its enormous yolk, turns over 

 and ultimately comes to lie on its curved back ; the reptilian 

 embryo must also sink in, but it does not turn. This turning 

 over, so marked a feature in the bird, may be correlated with 

 the conditions of incubation. Everybody knows that the 

 chalazse keep the blastoderm " on the top," i. e. nearest the 

 source of warmth, against the hen's body *. This does not apply 

 to reptiles which deposit their eggs in the ground, nor to the one 

 or two eggs in the moist pouch of the Monotremes ; lastly, to the 

 embryo of viviparous and ovoviviparous creatures " orientation " 

 towards the source of heat is not only unnecessary, it would 

 also be impossible because the mother changes so much her own 

 position by moving about. 



Since no trace of functional asymmetry of ovaries and ducts 

 appears in Meta- and Eutheria, and as that of the Monotremes 

 cannot well be a reptilian inheritance (because their asymmetry 

 is usually marked by an enlargement of the right gonads and 

 ducts, e. g. Snakes and Crocodiles, both sides however being 

 functional), the asymmetry of the Monotremes must be due to a 

 departure within the Prototheria, but so slight as not to have 

 caused any irreparable moi-phological reduction of either ovaries 

 or ducts by the time that the Prototheria entered upon the next 

 higher or Metatherian stage, excepting of course the Monotremes. 

 If Prototheria ever laid eggs much larger than they are now, the 

 asymmetry may have been greater and be referable to the same 

 primary causes as those suggested for bii-ds, but since the recent 

 Monotremes seem to be actually in the process of reducing them 

 now, and moreover to the last possible number, the left-sidedness 

 seems to be a case of mere coincidence with birds. 



Some simplification of the completely double female apparatus 

 of the Vertebrates was bound to come ; it was a matter of time, 

 and success depended upon the grade or height of the general 

 organisation of those who attempted it. Any agreement 

 between birds and mammals versus reptiles cannot be anything 

 more than convergent resemblances, at best cases of Isotely, 

 The classes of both Birds and Mammals have gone beyond the level 

 of the Reptilian organisation and they represent highest termini ; 

 but, although the class of Birds is by far the most specialised and 

 in vai'ious respects has reached seemingly unsurpassable perfec- 

 tion, the class of Mammals is morphologically the highest, in 

 spite of its still comprising such lowly, undecided types as the 



* The usual statement that the hen turns the eggs over from time to time in 

 order to ensure the equal w arming of the whole egg, now the upper and then the 

 lower half, implies nonsense. What the sitting bird does, is to rearrange the 

 position of the eggs with reference to each other, to give those now lying peripherally 

 an equal chance of best position near the centre. With a small clutch this is not 

 necessary, but with a dozen eggs the frequent rearrangement is \ery noticeable, at 

 least with thoughtful sitters. 



