I.] THE FROG. 5 



the unfertilized egg (ovarian ovum) as it leaves the body 

 of the female. 



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, or segmen- 

 tation which commences within a few hours of impreg- 

 nation, can be readily observed when the eggs are examined 

 as opaque objects under a lower power of the microscope. 

 It is ushered in by the appearance, on the surface of the 

 egg, of a furrow which passes completely round the same, 

 and, gradually deepening, constricts it into two equal 

 halves. Each of these again becomes subdivided, and, the 

 process being repeated, there results a mulberry-like mass 

 of embryo-cells, from which the formative tissues of the 

 body are ultimately derived. Segmentation such as this, 

 in which the first furrow completely cleaves the whole egg 

 into two, each successive one similarly subdividing that cell 

 with which it is related, is termed complete or holoblastic. 



While still within the mucous investment 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. 



After leaving the egg, the larva acquires three pairs of 

 external branchice having the form of branched filaments, at- 

 tached to the sides of the hinder part of the head. Narrow 

 clefts in the skin at the roots of the branchiae lead into the 

 back of the throat or pharynx. Water taken in at the 

 mouth passes out by these branchial clefts. The animal 

 crops the aquatic plants on which it browses, by means of 

 the horny plates with which its jaws are provided. 



In the Tadpole, as the larval Frog is called, the intestine, 

 which is relatively longer than in the adult, is coiled up 

 like a watch-spring in the body cavity. A membranous lip, 



