xii PHYLUM CHORDATA 485 



in all has the same main features as in the Pigeon : it has 

 four distinct chambers, two auricles and a ventricle, and 

 there is a single aortic arch, situated on the right side. In the 

 brain the most characteristic points are the short rounded 

 hemispheres, the large folded cerebellum produced forwards 

 to meet the hemispheres, and the laterally placed optic 

 lobes. The internal ear has a large curved cochlea, and 

 the eye has a pecten. The right ovary and oviduct are 

 more or less completely aborted. 



The ovum is always large owing to the great quantity of 

 food-yolk ; the protoplasm forms a small germinal disc at 

 one pole. Impregnation 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. 276, alb\ which, as the egg rotates 

 during its passage, becomes coiled at either end into a 

 twisted cord, the chalaza (cti). Next, more fluid albumen 

 (alb') 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 Carinatae and in Apteryx, branched in the other Ratitae. 



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 be simply 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 



