AVES. 



379 



head are sometimes, as in Mammalia, the only pneumatic bones : the ver- 

 tebrae, sternum and humerus come next ; whilst in a few birds, e. g. the 

 Toucan, all the bones of the body are stated to be pneumatic. The bones 

 of the fore-arm, of the lower leg, of the hand and foot often retain their 

 medulla. In some instances processes of the air-sacs are prolonged between 

 the muscles and under the skin, e. g. in the Gannet (Sula). The ciliated 

 epithelium of the air passages is replaced by pavement epithelium in the 

 air-sacs and their extensions. 



The kidneys are divided into three lobes which fit into the fossae of 

 the ilia. The glands often meet and even fuse posteriorly, and their ventral 

 surface is often marked by the intestines. The ureter arises from the 

 ventral surface, is dilated at its origin, and opens into the cloaca on a 

 papilla placed to the inner side and a little in front of the genital papilla. 



The sexes differ much externally in colour, development of peculiar 

 feathers, &c., and in Accipitrine birds the females, as is so commonly the case 

 in Arthropoda, are larger than the males. The testes are always retained 

 within the abdomen, and lie anteriorly on the kidneys. They undergo a 

 periodical enlargement at the breeding season and the left is occasionally 

 the larger of the two. The vasa deferentia lie to the outer side of the 

 ureters, and when filled with spermatozoa are disposed in short wavy folds. 

 They are slightly dilated at their cloacal terminations. The right ovary and 

 oviduct are usually atrophied, and when the ovary is persistent, as in some 

 Accipitres (Aetomorphae), its ova do not come to maturity. The oviducal 

 aperture is very wide in correlation with the large size of the ova. The 

 length of the duct and the number of its coils increase with the breeding 

 season. It is divisible into three sections : the first narrow, the second 

 glandular and secreting the albumen, the third muscular and glandular. 

 This last part is often termed ' uterus,' as the egg stays in it for some time 

 whilst the calcareous shell and its colouring matter, if any, are secreted by 

 the glands. There are no accessory glands appended to the generative 

 ducts in either sex. The Ostrich has a solid grooved intromittent organ 

 similar to that of Chelonia and Crocodilia : the Duck and other aquatic 

 birds, the remaining Ratitae and a few others, an eversible grooved organ 

 attached to the front wall of the cloaca. The organ is represented in the 

 female. The egg is impregnated in the upper part of the oviduct or on the 

 ovary. It is incubated by the female, in rare instances by the male as well 

 or exclusively, and is generally laid in a special nest or shelter constructed 

 for the purpose. Development lasts for a time dependent on the size of 

 the bird, e.g. about n days in smaller birds to 7 weeks in the Ostrich. 

 During the growth of the young bird an air space is formed and gradually 

 increases in size at the obtuse end of the egg. Many young birds are pro- 

 vided with a hard knob on the upper surface of the bill for breaking through 

 the shell when ready for hatching. When the food-yolk is large, the young 



