814 DR. HANS GADOW ON OVARIES 



and a left duct will be the least liable to set up complications. 

 Two canals may be good, but one improved way is better, and if 

 the traffic goes in one direction only, the other duct falls into 

 abeyance. If goods are still prodviced at the terminus of the 

 obsolete line, they will deteriorate, but this does not matter if 

 the output of the opposite factory is equal to the demand. 



So far so good, and the enquiry need not be carried further 

 back, if it were not for the Monotremes. Although these archaic 

 creatures show no appreciable difference in the size of their 

 paired ovaries and ducts, only those of the left side are functional. 

 According to Semon, Ornithorhynclms invariably produces two 

 eggs, always in the left side ; Echidna lays only one egg, also 

 left-sided. The right ovary forms numerous large eggs which 

 never ripen, and the respective duct and uterus are swollen and 

 much vacuolised during the season. In short, Monotremes behave 

 exactly like certain abnormal birds, e.g. the famous Sparrow-hawk, 

 by the retention of an ancestral feature which is now normally 

 lost. Since the reduction in the Monotremes has made so little 

 pi-ogress, it looks as if it were but of comparatively recent date, 

 but at the same time so ancient as not to have interfered with 

 the inheritance of the full symmetry by the Meta- and Eutheria. 

 The Monotremes are no longer qviite primitive, not even in these 

 organs. Their eggs have lost much of the yolk ; they continue 

 to grow in bulk within the uterus after they have received their 

 keratine shell. Indeed, we cannot well imagine that, compared 

 with oviparous reptiles and birds, the very small egg of the 

 Monotremes, and the imperfect, almost larval condition of 

 the new-born represent truly ancestral conditions, unless — 

 and this is well worth further enquiry — we are prepai-ed to 

 assume that in all Vertebrata the viviparous condition was 

 primary to one in which the foetus is surrounded by a shell and 

 then hatched outside the mother. If this should be the case, we 

 should fui'ther have to distinguish between primordial viviparity 

 (of which recent examples are unlikely) and secondary, pseudo- 

 primitive viviparity, the numerous instances of which have been, 

 and are still being, acquired independently : many Sharks and 

 Teleosts ; many Urodela, even one or two of the Anura, and 

 many scattered cases among the reptiles, as some Chameleons 

 and Lacertidfe, Iguanidfe and Anguidse, all the Scincidfe, all the 

 thoroughly aquatic snakes, the Viperidfe, and hei'e and there 

 some other terrestrial kinds. But to return to the Monotremes. 

 Can their incipient, or perhaps arrested, asymmetry be referred to 

 the same embryonic conditions as those which prevail in Birds ? 

 The bulk of the egg is formed by the yolk, the yolk-stalk might 

 be strong enough to cause a disturbance, the allantois protrudes 

 towards the right, and the left vitelline vein preponderates. 

 How far, and if at all, the viscera are affected by these conditions, 

 remains unknown. For our purpose it is significant that there 

 is incipient asymmetry (functional although scarcely structuial), 

 and that this should be restricted to the only recent Mammals 



