XIII 



PHYLUM CHORDATA 407 



of the renal portal system. The red blood-corpuscles are always 

 oval and nucleated. 



Nervous System and Sense Organs.- -The brain is also 

 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. In 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. 



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, RheaB, 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 



t/ C7 1' 



are comparatively unimportant. 



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

 the protoplasm forms a small germinal disc at one 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. 1012, 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 

 (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 Avhite or variously coloured by special pigments : it consists of 

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



