249 



amnion) closely invests the embryo, while the outer layer 

 (false amnion, serous or sub-zonal membrane), which fuses 

 with the vitelline membrane, forms a roof. The space 

 between amnion and embryo becomes full of fluid, serving 

 as a water-cushion. 



From the hind -gut a diverticulum, the allantois, grows 

 out between the amnion and serous membrane ; ultimately 

 it extends round the embryo and yolk-sac, and, fusing with 

 the serous membrane, it becomes closely applied to the 

 shell ; it is the embryonic respiratory organ, it is highly 

 vascular and, absorbing the albumen, it aids in the nutrition 

 of the embryo. 



When the remains of the yolk-sac are withdrawn within 

 the body, and the allantois shrivels, the chick (twenty-first 

 day) ruptures the membranes, and, by means of the knob 

 or caruncle on its beak, breaks open the shell. 



Mention the Principal Divisions of the Class Aves, and give 

 Examples, 



Sub-class ARCHAEOBNITHES. 



Example, Archaeopteryx. Jaws with teeth (in 

 sockets). The three metacarpals and the 

 three digits (with claws) are separate. 

 Pygostyle absent. Each of the (20) tail 

 vertebrae had a pair of feathers. Extinct 

 (Solenhofen Lithographic Stone of Bav- 

 aria). 



Sub-class NEORNITHES. Metacarpals fused. 



Division Ratitae (Running Birds). Sternum without a keel. 

 Scapula and coracoid fused. Clavicles absent or rudi- 

 mentary. Without a pygostyle. 



Examples, Struthio (African Ostrich). Two toes 

 (3rd and 4th) only. A pubic symphysis. 



Rhea (South American Ostrich). Three 

 toes. An ischial symphysis. 



Apteryx (the New Zealand Kiwi). Bill 

 long and .nostrils terminal. Feathers 

 bristle-like. Without distinct tail feathers. 

 Four toes. 



Division Carinatae (Flying Birds). Sternum with a keel. Scapala 

 and ooracoid connected by ligament. Clavicles usually 

 well developed. A pygostyle generally present. 



Example, Coluinba (Pigeon). See pages 42 to 247, 



