222 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



layer on the internal side of the retina (ie., the side next the eye- 

 chamber), and are connected through various intermediate layers 

 with elongated sense-cells, the rods and cones, which occupy the 

 outside of the retina. Refracting structures are contained within 

 the chambers of the eye. A clear, watery aqueous humour fills the 

 external chamber, while a gelatinous vitreous humour is contained in 

 the internal chamber. Immediately behind the iris and connected 

 with its outer margin is a firm spheroidal body, the lens. 



A number of accessory parts are connnected with the eye, of 

 which the two eyelids have already been mentioned. Continuous 

 with the lining of these is a delicate transparent membrane, the 

 conjunctiva, which closely covers the cornea. A small Harderian 

 gland, which secretes a fatty substance, is situated in front of and 

 below the eye. 



The eye is moved by a number of small muscles, the most im- 

 portant of which take origin in the walls of the orbit, and are 

 inserted into the eyeball. They are (1) The retractor bulbi, u 

 muscular sheath surrounding the optic nerve ; (2) the four recti 

 muscles, superior, inferior, internal, and external rectus, inserted into 

 the upper, lower, anterior, and posterior sides of the eyeball. All 

 the preceding take origin from the inner wall of the orbit, near 

 its posterior end ; (3) the two oblique muscles, superior and in- 

 ferior, are inserted into the upper and lower sides of the eyeball, 

 taking origin close together in the anterior part of the inner wall 

 of the orbit. 



DEVELOPMENT. 



The frog is a good example of an animal in which there is a 

 free-living embryo or larva, very unlike the adult stage, into 

 which it is converted by a gradual metamorphosis. Frog larvae 

 are familiar to every one as fish-like tadpoles, which hatch out 

 during March and April from the frog " spawn," which consists of 

 fertilized ova surrounded by gelatinous matter. 



1. Early Stages. Cleavage (segmentation) (Fig. 65) is com- 

 plete but unequal. The ovum is pigmented on one side which is 

 termed the upper pole, the unpigmented end being the lower pole. 

 Division may therefore be spoken of as " meridional " or " equa- 

 torial." The two first divisions are meridional, dividing the 

 oosperm into two and four. Then division into eight is effected 



