408 ZOOLOGY 



SECT. 



(/. od.) is long and convoluted; its anterior end is enlarged to form 

 a wide, membranous, coelomic funnel (/. od.") into which the ripe ova 

 pass on their liberation from the ovisacs ; the rest of the tube has 

 thick muscular walls, lined with glandular epithelium, and opens 

 into the urodseum. A fair-sized vestige of the right oviduct (r. od.) 

 is found in connection with the right side of the cloaca, and a 

 more or less extensive vestige of the right ovary is frequently 

 present. 



Internal impregnation takes place. As the ova or "yolks" 

 pass down the oviduct they are invested with the secretions of its 

 various glands ; first with layers of albumen or " white," next with 

 a parchment-like, double shell-membrane, and lastly with a white 

 calcareous shell. They are laid, two at a time, in a rough nest, 

 and are incubated or sat upon by the parents for fourteen days, the 

 temperature being in this way kept at about 38 to 40 C. (100 

 to 101 F.). At the end of incubation the young Bird is sufficiently 

 developed to break the shell and begin free life. It is at first 

 covered with fine down, and is fed by the parents with a secretion 

 from the crop, the so-called "Pigeon's milk." 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



Aves are Craniate in which the epidermal exoskeleton takes the 

 form of feathers over the greater part of the body, of a rhampho- 

 theca or horny sheath to the beak, and of claws on the digits of the 

 foot and sometimes of the hand. In the standing position the 

 body is entirely supported on the hind-limbs, the articulations of 

 which are thrown forward. The fore-limbs are modified to form 

 wings, usually provided with large feathers for the support of the 

 body during flight. The cervical and free thoracic vertebrae are 

 usually heteroccelous, but may be proccelous or amphicoelous. The 

 sacral vertebrae are fused with the lumbar and with more or fewer 

 of the posterior thoracic and anterior caudal to form a synsacrum 

 for the support of the ilia. The posterior caudal vertebrae are 

 usually fused to form a pygostyle around which the tail-quills are 

 arranged in a semicircle. The bones of the skull undergo early 

 ankylosis. There is a single, rounded, occipital condyle ; the united 

 premaxillae form nearly the whole of the upper jaw ; and the lower 

 jaw is composed originally of five or six bones in each ramus, and 

 is supported by a freely articulated quadrate. The vertebral ribs 

 are double-headed, provided with bony uncinates, and articulate 

 with the bony sternal ribs by synovial joints. The sternum is 

 broad, and is typically produced into a longitudinal ventral keel, 

 having a separate centre of ossification. The coracoid is usually 

 more or less pillar-like, the scapula is sabre-shaped, and the clavicles 

 and interclavicle unite to form a furcula. Except in one extinct 



