256 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



with the exception that the posterior vertical takes an outward 

 course, and crosses the horizontal before it dilates into an ampulla. 

 A slightly-curved tube, the cochlea, is connected with the anterior 

 part of the sacculus. The membranous labyrinth contains endo- 

 lymph with otoliths, and is lined by epithelium, patches of which 

 are largely made up of auditory cells, connected with auditory 

 nerve-fibres. 



It has been determined by experiment that the semicircular canals of the 

 ear have to do with the perception of position in space. In this connection 

 the fact that their planes are mutually at right angles deserves notice. 



(5) Visual Organs. The eye agrees in all essential particulars 

 with the description on p. 221, but differs in certain points. 

 The inner half of the eyeball is hemispherical, while its outer 

 part is somewhat conical and terminated by the very convex 

 cornea around which the sclerotic is strengthened by a circlet of 

 small, flat sclerotic plates (Fig. 68). The lens is flattened, and a 

 vascular, pigmented, longitudinally plaited fold, the pecten, pro- 

 jects into the vitreous humour below the entry of the optic 

 nerve. 



The accessory parts connected with the eye are the three eyelids, 

 conjunctiva, two glands, and the eye-muscles. The glands are (1) 

 the Harderian gland in front, and (2) the lachrymal gland behind 

 and above. The eye-muscles are arranged much as in the frog. 



DEVELOPMENT. 



The common fowl has been most studied, and, as birds differ 

 but little in their development, will be here described. 



1. Early Stages. The fertilized ovum, in passing down the 

 oviduct, is covered by several structures secreted by its walls 

 (Fig. 73). These are: (1) The white of the egg, mainly com- 

 posed of semifluid proteid material, and containing a somewhat 

 convoluted cord, the chalaza, at each end, (2) the double-layered 

 shell-membrane, covered by (3) the shell, formed of an organic 

 matrix hardened by salts of lime. The entire egg is elongated, 

 and broader at one end than the other. 



Cleavage (segmentation) (Fig. 74) is unequal and also partial 

 (meroblastic), being confined to the germinal disc. It commences 



