62 THE NATURE STUDY COURSE. 



Development of the Embryo. — If newly laid eggs be obtained^ 

 removed from the jelly, and placed in water in small saucers 

 or watch-glasses, the initial changes may be observed with a 

 pocket-lens. The older pupils will find these observations 

 extremely interesting. Two meridional grooves 

 and a circumpolar one mark off the little globe 

 into eight areas. These divide again and again ; 

 the globe flattens and lengthens. By and by the 



Egginlststage ° & J J 



of division. neck appears and movement is exhibited. In about 

 two weeks the tadpole, if of a frog, struggles out of the envel- 

 ope and although still mouthless it has a pair of suckers under 

 where the mouth will be by which it attaches itself to a leaf of 

 the water-weed. Watch it closely in this attitude to observe 

 its external gills develop. These are three pairs of thread-like 

 extensions at whose bases are openings that lead into the 

 throat. Water entering by the mouth passes out through 

 these clefts as it does between the gills of a fish. The external 

 gills of the young toad or frog soon disappear and their use is 

 served by internal gills. As this change is taking place, eyes, 

 nostrils and ear patches come into view. It finds the use of 

 its swimming tail ; its mouth opens as a small tough or horny 

 aperture with which it sucks off its food from weeds and stones. 



Heart 



Abdominal Waifs 



Section through the half-hatched embryo (diagrammatic). 



Up to the time it detached itself from the water-weed, where 

 it has been hanging since it was hatched, the food for its 

 growth and all its changes lias been supplied by the yolk of 

 the egg which it carried in its body. Subsequently its 



