Ranidae 



that have grown out from the gill plates formed five days ago. 

 If we examine the tadpole carefully with a lens, not only can 

 gills and suckers be seen, but prominences that are to become the 

 eyes (Fig. 210, side view), and an opening just forward of 

 the suckers that is to be the mouth (Fig. 210, front view). 



The low power of the compound microscope used now to 

 examine the outside of the tadpole shows us many wonderful 

 things. The blood is flowing in the gills. The current is easily 

 traced out to the end of each finger-like extension and back again. 

 The movement is in jerks or pulses, which are ryhthmic, corre- 

 sponding to the pumping action of the heart. If we look still 

 more closely, the current is seen to consist of minute circular 

 bodies (blood corpuscles) that chase each other in most irregular 

 fashion in a colourless liquid (blood plasma). 



However, as we look at the blood movement, another motion 

 catches our attention. There is a general wave-like motion of 

 the water over the body of the tadpole in the direction of the tail. 

 Careful examination reveals the presence of a coat of minute hairs 

 (cilia) that are in continual motion, and so create this movement 

 of the water. Cilia appear along the back of the embryo at the 

 time of the formation of the nerve tube. They increase in 

 number as development goes on, until the whole body is covered 

 with them and strong currents of water constantly wash the sur- 

 face of the tadpole from the head backwards. The most rapid 

 streams flow over the back and in the region of the gills. This 

 movement of the cilia, with the resulting circulation of water, 

 aids the young tadpole in the process of respiration before the 

 gills are formed. After the gills are well developed, the cilia are 

 reduced in number so that they do not cover the whole body; 

 they now serve mainly to bring fresh currents of water to the 

 gills. 



On the tenth day all the tadpoles are out of the eggs. They 

 hang quietly from water-weed or other support, or circle about 

 with vigorous wigglings of the tail whenever they are disturbed. 

 The head becomes larger. This is due to backward growing 

 folds on the side of the head. It is easy to see that the folds have 

 progressed quite to the roots of the gills. (Fig. 211, April i8th.) 

 On the eleventh day they have grown still farther back and have 

 united underneath the throat. The tadpole is now 9 mm. long, 

 the increase in length being mainly confined to the tail. On the 



1 80 



