THE EGGS AND YOUNG TADPOLES I9 



Hatching 



In a short time after hatcliing, a new phase in the hfe of the tadpole 

 begins, and the nutritional problems of the older tadpoles will be 

 considered in the next chapter, but here it must be considered how the 

 hatching process is effected, and how the colony of tadpoles above the 

 discarded envelopes is formed and maintained in position. 



It is practically certain that the hatching process is produced by the 

 action of an enzyme that dissolves the jelly. This has, apparently, not 

 been found for this species, but Bles (1906) found a hatching enzyme 

 in Xetiopus laevis. Noble (1926) in Alytes ohstetricaus and Cambar (1953) 

 in R. dahnatina, and the process in R. temporaria seems to be similar. 

 Because of the structure of the jelly mass, the tadpoles hatch into the 

 intercapsular chamiels. The problems that now have to be solved are: 

 how they reach the surface when they can hardly swim, and how, 

 when they get there, they manage to get enough oxygen, and how it 

 is that these enormous colonies do not become dispersed throughout 

 the pond sooner than they do. 



This phase in the hfe of the tadpoles does not last long. If the pond 

 is visited during this period, and if there is a large number of clumps 

 of spawn, it will usually be found that the young animals form a dense 

 carpet of black bodies, just beneath the surface of the water and just 

 above the disintegrating mass of envelopes that they have left. Some- 

 times, there are as many as half a milhon of them, and once I estimated 

 that there were about five miUion in one continuous layer. Generally, 

 the animals in these large aggregations are in a state of continuous 

 movement but, if there are only a few clumps, this does not occur. The 

 following experiments showed what was happening (Savage 193 5)- 



Determinations of dissolved oxygen showed amounts of from 3 per 

 cent to 16 per cent of saturation. Some experiments by Helff and 

 Stubblefield (193 1) showed that the tadpoles of R. pipiens camiot hve 

 in water with less than about 18-22 per cent of saturation and fishes 

 have similar requirements. Tadpoles taken from these aggregations 

 and put into the water from which they came died if the vessel was 

 deep, but hved if it was shallow. The habit of aggregation carried 

 with it the risk of death. By putting tadpoles of this age into an 

 experimental vessel (Fig. 4) it was found that, although they were so 

 young that they did not swim when they were stirred round 111 water, 

 and stayed quite still in well-oxygenated water, they were capable of 

 considerable swimming movements, enough to take them to the 



