882 LIFE : OUTLINES OF GENERAL BIOLOGY 



the true ivory tooth that projects forwards horizontally from the 

 tip of the upper jaw in some lizards and snakes, such as gecko and 

 adder. When there is a true bony "egg- tooth" there is never a horny 

 "shell-breaker", and vice versa. 



As to embryonic resemblances between birds and reptiles, they are 

 manifold. The egg of a goose is externally like that of a crocodile, 

 and the egg of a swan is like one of the smaller Dinosaur eggs which 

 American explorers found in considerable numbers in the Desert of 

 Gobi in Mongoha. Internally also the structure of the egg is the same, 

 with shell-membrane, albumen, and central yolk, and the minute 

 drop of living matter lying like an inverted watch-glass on the top 

 of the yolk — the drop out of which the whole embryo is built up. 

 The mode of ovum-cleavage or segmentation is the same (partial 

 and discoidal), and all the important parts develop in the same 

 fashion. The ante-natal membranes — the protective amnion and 

 the respiratory-nutritive-excretory allantois — are almost exactly 

 the same in reptile and bird. For the first few days the embryo bird 

 and the embryo reptile proceed side by side on the great highway of 

 Amniote development ; gradually the paths begin to diverge — when, 

 for instance, the embryo bird shows the first traces of feathers. 



In many ways the developing bird within the egg exhibits struc- 

 tures and phases which are recapitulations of what may be found 

 in developing reptiles to-day, or in adult extinct forms in distant 

 ages. The vertebrae may pass through a biconcave stage (recalling 

 those of fishes) ; the embryo shows, to begin with, only two sacral 

 vertebrae ; the ploughshare bone at the tip of the tail is represented 

 by six distinct vertebrae ; the breastbone is at first a paired structure ; 

 there are seven separate elements in the wrist and two rows at the 

 ankle ; the metatarsals are separate in the embryo; there is sometimes 

 an embryonic dental ridge on each jaw, though there are never 

 distinct tooth-germs; Jacobson's organ, a sensory structure on the 

 roof of the mouth below the anterior part of the nasal passage in 

 many reptiles, has been found in some bird embryos, though never 

 in an adult bird. These are but a few instances of the detailed way 

 in which the bird's individual development (ontogeny) condensedly 

 recapitulates the racial evolution (phylogeny), though its advance of 

 a day may represent the slow progress of many thousand years. 



As to annectant types, several extinct toothed birds are known, 

 such as Hesperornis and Ichthyornis, and many bipedal extinct 

 reptiles. But Hesperornis does not take us very far back from the 

 familiar bird type; thus D'Arcy Thompson points out no fewer than 

 twenty-five skeletal resemblances between it and the Great Northern 

 Diver (Colymbus) . In Ichthyornis there are no doubt more lingering 

 traces of the reptile: thus the halves of the lower jaw are quite 

 separate ; the enamel- tipped teeth are in sockets, and show successors 

 vertically below them, just as in crocodiles; the vertebrae are bicon- 



