40 PRINCIPLES OF EMBRYOLOGY 



the problems of land life only to raise others. The shell which encloses 

 the water also keeps in the nitrogenous waste products of embryonic meta- 

 bolism. It is a rather general rule that during the earhest stages of embryonic 

 life the main reserve foodstuff utihsed is carbohydrate (glycogen) ; next 

 comes a stage when the protein is consumed, and last of all the fat (but see 

 p. 454). It is the second of these, in particular, which gives rise to large 

 amounts of nitrogenous waste. Animals with shelled eggs cannot avoid 

 using such materials altogether, although they contrive to become very 

 efficient in converting yolk protein into body protein without producing 

 much waste; and they manage also to start rather early to consume fat, 

 which has the added advantage that it produces some extra water as a 

 fmal product of its oxidation. But even so, there is a good deal of nitro- 

 genous waste to get rid of In the simplest marine forms, this is excreted 

 as ammonia, a highly poisonous but rapidly diffusible substance. In fish, 

 more efficient excretory organs are produced, and the waste products are 

 got rid of in the less poisonous, but less diffusible form of urea. It is 

 characteristic of land animals, that most of their nitrogenous waste is 

 excreted as uric acid, which is a very insoluble substance. The reason why 

 this mechanism has been evolved is almost certainly in order to cope 

 with the situation of the embryo within its water-retaining shell; being 

 unable to get rid of its waste products it needs to deposit them in a form 

 which will stay put. Thus the exigencies of ovular life have a lasting effect 

 on the metabolism of the higher vertebrates, though mammals, which 

 escape the closed box of an egg-shell, have returned to urea as their main 

 excretory end-product (a fact which does not fit very well with Dollo's 

 law mentioned above). 



It is worth remarking that in animals which eventually excrete uric 

 acid, we find that there are short early stages which excrete ammonia, 

 like the earliest ancestors, and urea, like the rather more recent ones 

 (Needham 193 1, 1942). This is a biochemical example of the phenomenon 

 of 'recapitulation', which we have discussed as it apphes to morphological 

 events (p. 9). 



4. The morphogenetic structure of the egg 



The nutritive materials in the ovum are more or less 'inert' in that they 

 play little part in determining the structure of the developing embryo. 

 As we have seen, the yolk usually occupies a definite position within the 

 egg-cell, but it can be shifted about, for instance by centrifuging the egg, 

 without making any great difference to the course of development. The 

 underlying basis of the embryonic body is to be found rather in the non- 

 yolky cytoplasm. This is often optically clear and fairly homogeneous, 



