THE EGGS AND YOUNG TADPOLES 21 



of dissolved oxygen (wliich must be done at the ponds) considerable 

 diificulty was found in drawing up the water into the tubes of the 

 apparatus, because of the blockage due to fragments of jelly. It could 

 certainly not pass through narrow passages such as those that exist in 

 the respiratory systems of the older tadpoles. It is just as this time that 

 the feathery external gills of the tadpoles are at their maximum 

 development. These are really rather puzzling structures. They 

 develop soon after hatching, and not long afterwards disappear when 

 they are overgrown by the development of the operculum, a fold of 

 skin that grows from the anterior portion of the tadpole, and eventually 

 encloses both the external gills and the new set of internal gills that 

 serve the animal during the rest of its life. The external gills are then 

 gradually absorbed. They therefore last for a very short time, and it 

 is surely remarkable that this period should be just the one when the 

 tadpoles are Uving among a glutinous mass of decomposing jelly. 

 They could not use a respiratory device that required this soup to be 

 pumped along channels, but they can use gills that only require to be 

 waved in the water to pick up any oxygen that happens to be there. 

 If now the natural aggregation is examined at this stage, just a litde 

 later than that at which it is first formed, it will be seen that the 

 animals Hve very near to the surface fdm. They are in almost constant 

 motion, vigorous enough for some tadpoles to be lifted almost out of 

 the water by the efforts of those underneath. At other times, they 

 reach the surface by their own efforts, and as soon as the external gills 

 spread out as a feathery fan on the surface of the water, the tadpoles 

 stop swimming and sink down to a lower level. The blocking of 

 respiratory channels is a problem for all animals that hve in water 

 that is thick with sediments of any kind, and it will be shown in 

 Chapter 3 that the full-grown tadpoles have another device for dealing 

 with it, but in the stage now being considered it seems likely that in 

 these feathery gills there is an instance of a special larval organ, 

 developed for a few days to deal with a quite temporary environment. 

 The events that have just been described are not usually to be seen at all 

 in clumps of spawn brought indoors and put into a vessel of water. This 

 is essentially a field phenomenon, and even there it is necessary to find a 

 pond with plenty of spawn in it before its full development can be studied. 

 If it were possible to regard the ingestion of food through the 

 mouth as the only way that the tadpoles could get any nourishment 

 from the envelopes, it could at once be said that the envelopes have no 



