162 ELEMENTARY BIOLOGY. [CHAP. 



The upper eyelid of the Frog is large and covered with 

 ordinary pigmented integument, and it has very little mobility. 

 What performs the function of the lower eyelid in Man, 

 is a fold of the integument of which very little is pigmented 

 and which is, for the most part, semi-transparent, so as to 

 resemble 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 

 descends 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 ot 

 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 fecun- 

 dates them by a simultaneous out-pouring of the seminal 

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

 layer of viscid albumen, 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 gelatinous 

 mass in which the eggs remain imbedded during the early 

 stages of their development. 



The development of the eggs is closely dependent upon 

 temperature, being greatly accelerated by warmth and re- 

 tarded by cold. The process of yelk-division, which com- 

 mences within a few hours of impregnation, is readily ob- 

 served when the eggs are examined as opaque objects under 

 a low power of the microscope. 



While still within the egg the embryo assumes the form 



