904 GENERAL METABOLISM [pt. iii 



attractive material for the study of these questions, e.g. the Eleuthero- 

 dactylus of Dominica described by Howes, which lays large trans- 

 parent crystal-clear eggs and has no external tadpole stage. 



It is not without interest that these watery mucilaginous envelopes 

 are found elsewhere in the animal world, e.g. in the eggs of certain 

 insects, mostly Trichoptera (caddis-flies) and midges of the Chironomus 

 class^. The nature of the protective function served by these jellies 

 raises problems of much interest. I have already suggested that the 

 reason why bacteria attack the amphibian egg-jelly so slowly is that 

 it is practically a pure protein, mucin, and that a certain proportion 

 of protein breakdown products is essential for good bacterial growth. 

 There is also much evidence that similar properties may be ascribed 

 to the avian egg-white. There is a large literature on the dietetic 

 aspect of raw and cooked white of tgg which may be found sum- 

 marised in the work of Bateman but without going into it at this 

 point, it may be remarked that raw egg-white is very resistant to 

 digestive enzymes, containing a definite antitrypsin and an antipepsin. 

 In Section 19-3, moreover, we shall see that a natural bacteriolysin 

 is present in raw egg-white (see Sharp & Whitaker) . 



Insect eggs, indeed, provide further light on the evolution of ter- 

 restrial embryos; thus, work on the silkworm [Bombyx mori) brought 

 out the following figures : 



Unincubated eggs 



Finished larvae 



Unused material, membranes, etc. ... 



In the case of this insect, at any rate, 

 more water in it than the original tgg, but the explanation may lie in 

 the fact that at hatching a notable quantity of very dry material is left 

 behind. During metamorphosis also, the organism seems to become 

 richer in water, but here again a mechanism involving the production 

 of dry membranes, cocoon, etc., is perhaps responsible. 



It is possible, however, that the eggs of some insects, like those 

 of reptiles, though apparently self-contained, take in water. Thus 

 Wheeler informs us that ants "salivate" over the eggs in their 

 communities and suggests that this saliva may be absorbed. Simi- 

 larly, Weyrauch reports that the earwig {Forficularia) licks its eggs 

 after laying them; and if this is not done they will not develop. 



1 Also in fishes (e.g. Lepidosiren (Carter & Beadle), where it disappears early in de- 

 velopment and is perhaps functionless) . 



