xii PHYLUM CHORDATA 489 



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. 



Fig. 296. — Gallus bankiva (domestic fowl). Semi-diagrammatic view of the egg 

 at the time of hatching, a, air-space; alb, dense layer of albumen; alb' , more 

 fluid albumen; bl, blastoderm; ch, chalaza; sh, shell; sh. m, shell-membrane; 

 sh. I, sh. 2, its two layers separated to enclose air-cavity. (From Marshall's 

 Embryology, slightly altered.) 



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 incipient egg or 

 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. 296, alb), which, as the egg 



