Chap, in.] SAUROPSWA ; MAMMALIA. 95 



large sac-like structure similar in origin and primitive 

 position to the bladder of the frog; this allantois 

 takes on respiratory functions in the developing reptile 

 or bird, and a nutrient one in the higher Mammalia. 

 From either end of the body there grows out a fold, 

 which passes over the body of the embryo and unites 

 above it with its fellow ; this fold, which is double, 

 forms the amnion; the two layers of the amnion 

 separating from one another give rise to a cavity 

 between them which is more or less occupied by the 

 allantois ; in the Bird the allantois is comparatively 

 larger than it is in the Mammal. 



The differences between the Sauropsida, or reptiles 

 and birds, and the Mammalia are well and sharply 

 marked, and it is almost impossible to suppose that 

 their common ancestor was not more amphibian than 

 amniote in character. Thus, the Sauroids have scales 

 or feathers, the Mammals hairs ; the skull is always 

 articulated to the atlas by a single condyle in the 

 Sauroid, and by two in the Mammal ; the quadrate 

 bone, which is external to the ear in the Sauroid, is 

 enclosed by the otic capsule in the Mammal \ the red 

 blood corpuscles of a Sauroid are, and of a Mammal 

 are not, nucleated ; the connection between the 

 cerebral hemispheres of a Mammal is more intimate 

 than in a Sauroid, and while the eggs of the latter are 

 large, and provided with a quantity of yolk, those of 

 the Mammal are much smaller,* and nutrition is 

 afforded to the young by milk, the secretion of certain 

 modified tegumentary glands. 



The recent investigations of palaeontologists have 



* It has been recently stated that the ova of the lowest 

 Mammals are large, and that they are hatched outside of the body. 

 This observation, coupled with the facts that certain fossil Reptiles 

 (TheriomOrpha) give well-marked indications of mammalian affini- 

 ties, and that some Reptiles (e.g. some of the Amphisbsenidae) 

 have the occipital condyle double, may necessitate a revision of 

 current ideas as to the origin of the Mammalia. 



