422 ZOOLOGY SECT. 



considered as the homologue of the hippocampal commissure of 

 Mammals. 



Apteryx is also distinguished by the high development of the 

 olfactory chamber, which extends from the tip of the beak to the 

 level of the optic foramina : the turbinals are large and complex, 

 and there is a vestige of the cartilage of Jacobson's organ. The 

 small eye differs from that of all other Birds in the absence of 

 a pecten, although a vestige of that organ occurs in the embryo. 

 The structure of the auditory organ is very uniform throughout 

 the class. 



Urinogenital Organs. In these, also, the general agreement 

 with the Pigeon is very close, the most characteristic feature being 

 the more or less complete atrophy of the right ovary and oviduct. 

 The Megistanes, Ehese, Anseres, and some other Birds have a penis 

 in the form of a thickening of the ventral wall of the cloaca : it 

 has a groove on the dorsal surface acting as a sperm-channel, and 

 its distal end is invaginated, in the position of rest, by an elastic 

 ligament. In the Ostrich there is a solid penis, like that of Chelonia 

 and Crocodiles : it can be retracted into a pouch of the cloaca. 



Development. The process of development in Birds has been 

 most thoroughly worked out in the Common Fowl, but enough is 

 known of the embryology of other Birds to show that the differences 

 are comparatively unimportant. 



The ovum is always large owing to the great quantity of food-yolk ; 

 the protoplasm forms a small germinal disc at the upper pole. Im- 

 pregnation is internal, and as the oosperm passes down the oviduct 

 it is coated by successive secretions from the oviducal glands. It 

 first receives a coat of thick, viscid albumen (Fig. 1085, alb.), which, 

 as the egg rotates during its passage, becomes coiled at either end 

 into a twisted cord, the chalaza (ch.). Next, more fluid albumen 

 (aW) is deposited layer by layer, then a tough, parchment-like 

 shell-membrane (sh. m.), and finally a calcareous shell (sh.). The 

 shell-membrane is double, and, at the broad end of the egg, the 

 two layers are separate and enclose an air-cavity (a.). The shell 

 may be white or variously coloured by special pigments : it consists 

 of three layers, and is traversed by vertical pore-canals, which are 

 unbranched in the Carinatse and in Apteryx, branched in the other 

 Ratitse. 



The eggs may be laid on the bare ground or on the rocks by the 

 sea-shore, as in Penguins and Auks, or on the ledges on inaccessible 

 cliffs, as in the Sooty Albatross (Diomedea fuliginosa) ; but as a rule 

 a nest is constructed for their reception by the parent Birds. This 

 may simply be a hole in the sand, as in the Ostrich ; a mere 

 clearing on the hill-side surrounded by a low wall of earth, as in the 

 Wandering Albatross (Diomedea exulans) ; or a cylinder with 

 excavated top, built of grass, earth, and manure, as in the Molly- 

 mawks (Diomedea melanophrys, &c.). It may take the form of a 



