4 ELEMENTARY BIOLOGY. [CHAP. 



mobility. That which performs the function of the lower 

 eyelid in Man, is a fold of the integument little pigmented 

 and, for the most part, semi-transparent, resembling the 

 nictitating membrane of a bird rather than an ordinary 

 lower lid. If the surface of the cornea be touched, the 

 eyeball is drawn inwards under the upper lid, which de- 

 scends a little, at the same time as the lower lid 

 ascends over the ball, to meet the upper lid and close 

 the eye. 



As is well known, Frogs emit a peculiar croaking sound, 

 their vocal powers being more especially manifested in the 

 breeding season, when they collect together at the surface of 

 ponds, pools and sluggish streams, in great numbers. At 

 this season, which commences in the early spring for the 

 Grass Frog, but much later on in the year for the Edible 

 Frog, the male seeks the female and, clasping her body 

 tightly with his fore-limbs, remains in this position for days 

 or even weeks, until her ova are discharged, when he 

 fecundates them by a simultaneous out-pouring of the 

 seminal fluid. Shortly after the eggs pass into the water, 

 the thin layer of mucus secreted by the oviduct, with 

 which each egg is surrounded, swells up by imbibition and, 

 with that which surrounds the others, it gives rise to a 

 swollen mass, in which the eggs remain imbedded during the 

 early stages of their development. 



The process of fecundation above alluded to, usually 

 results in the fusion between each egg and one of the sper- 

 matozoa contained in the seminal fluid, and it is only 

 when this has been effected that the egg, which is then said 

 to be fertilized, is competent to reproduce the species. It 

 follows that the fertilized ovum is a compound of the egg of 

 the female and a spermatozooid of the male ; and it has been 

 accordingly termed the oosperm, by way of distinction from 



