xin PHYLUM CHORDATA 435 



very uniform in structure, being characterised by its short, rounded 

 hemispheres, large folded cerebellum produced forwards to meet 

 the hemispheres, and laterally placed optic lobes. la the embryo 

 the optic lobes have the normal dorsal position, and the whole brain 

 resembles that of a Reptile. In Apteryx, in correlation with the 

 reduction of the eyes, the optic lobes are very small, and are 

 situated on the under side of the brain. Above the anterior 

 commissure is a small bundle of fibres which has been considered 

 as the homologue of the hippocampal commissure of Mammals, but 

 hippocampi are not developed. 



Apteryx is distinguished by the high development of the 

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

 level of the optic for-amina : the turbinals are large and complex, 

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

 small eye of Apteryx 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, Rheae, 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 ovimi 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. 1074, alh.), which, 

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

 into a twisted cord, the chalaza {ph.). Next, more fluid albumen 

 {alb!) is deposited layer by layer, then a tough, parchment-like 

 shell-Tnertibrane {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 a,nd in Apteryx, branched in the other 

 Eatitse. 



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



