XIII.] THE FROG. 155 



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 re- 

 semble 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 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 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 

 observed 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 

 of a minute fish, devoid of limbs and with only rudiments of 

 gills, but provided with two adhesive discs on the ventral 

 side of the head behind the mouth. 



