GENEBAL ACCOUNT OF DEVELOPMENT. 91 



the male frog clasps the female firmly, embracing her with his 

 arms ; and as the eggs are passed out from the cloaca of the 

 female into the water, they are fertilised by spermatozoa dis- 

 charged over them by the male. 



The eggs, which are very numerous, are small spherical 

 bodies about 1'75 mm. in diameter; they are invested by thin 

 coatings of an albuminous substance, which swell up very 

 greatly in the water, and stick together to form the bulky 

 masses we call frog's spawn. Such spawn consists of a trans- 

 parent gelatinous mass, formed by the swollen albuminous 

 matter, in which the eggs are embedded ; these latter appear as 

 small spherical bodies, each presenting a black half and a white 

 half. 



If a number of hen's eggs were broken into a basin, care 

 being taken not to rupture the yolks, a mass would be produced 

 similar to frog's spawn ; the yellow yolks corresponding to the 

 frog's eggs, and the whites or albuminous investments of the 

 yolks to the gelatinous matrix of the spawn. 



The frog's eggs, laid in this way, and fertilised by sperma- 

 tozoa shed over them by the male, begin to develop at once. 

 The rate of development depends very largely on the tempera- 

 ture, and varies within very wide limits, warmth hastening 

 development, and cold retarding it. Freezing of the water in 

 which the eggs are kept merely retards development, and does 

 not injure the eggs, provided the eggs themselves are not 

 actually frozen. The times mentioned in this chapter may be 

 taken as representing the average rate of development in this 

 country. 



Each egg is at first spherical, and remains so during the 

 early stages of development ; at the close of segmentation it 

 becomes slightly ovoid, and then rapidly increases in length. A 

 transverse constriction appears, separating the head from the 

 trunk, and the tail buds out as a small process from the hinder end 

 of the embryo. The embryo soon becomes fish-like in appearance, 

 the tail growing veiy rapidly ; two pairs of branching tufts, the 

 external gills, followed shortly by a third pair, grow out from 

 the sides of the neck, and in about a fortnight from the time of 

 laying of the eggs the young tadpoles, now about 7 mm. in 

 length, wriggle their way out of the gelatinous mass of the 

 spawn, and swim freely in the water (Fig. 44, 3, 4). 



