THE GAMETES 39 



as egg capsule, or shell. Vitelline membranes are of very general occur- 

 rence, except in the few naked types of eggs. Chorions are not found quite 

 so often; the insect egg provides a good example of them. The tertiary 

 membranes are particularly well developed in many vertebrates (e.g. the 

 shell and albumen of the bird's egg) but occur also in many other classes 

 (e.g. the egg capsules of molluscs). In some animals, the membranes may 

 be formed before fertilisation, and it is then common to fmd that a special 

 opening (known as the micropyle) is provided, which allows for the 

 passage of the spermatozoon. 



The evolution of suitable egg membranes, and of eggs able to develop 

 inside membranes, was one of the most crucial steps which had to be taken 

 by animals in the colonisation of dry land (cf. Needham, 193 1). All 

 animals provide enough organic food reserves to keep their embryos 

 ahve until they can feed for themselves, but marine invertebrate eggs do 

 not contain enough of either water or salts; these are absorbed by the 

 embryo from the sea. The necessity for such salts has probably been a 

 handicap to invertebrates in the colonisation of freshwater, and explains 

 the relative poverty of freshwater as compared with marine invertebrate 

 faunas. Fish go one better and provide all the salts their embryos will need, 

 but they also do not cater for the water requirements. The same is true of 

 most Amphibia, though in a few enough water is present in the egg to 

 make possible a very much speeded-up development in a damp spot (e.g. 

 the tree-frog Hyla). By this stage in evolution, eggs had become quite large, 

 so as to contain enough organic matter for the development of an embryo 

 which becomes rather compHcated before being able to feed. The reptiles 

 began the next stage, that of enclosing the egg in a shell which could 

 include and retain suffcient water to last till feeding can start, hi many 

 of the turtles, the process is incomplete, and some moisture has to be 

 absorbed from the wet sand in which the eggs are deposited, hi birds the 

 problem has been fully solved; and in mammals it is, of course, circum- 

 vented by the device of uterine development. It is interesting to note that 

 water birds, instead of taking advantage of their environment to relax 

 the effort to retain water in the egg, still provide sufficient stores for the 

 developing embryo, and even evolve a waterproof shell to prevent more 

 water entering. This is an example of a rather general rule of evolution, 

 often known as Dollo's law, which states that evolutionary changes are 

 irreversible. If a later animal returns to a set of circumstances similar to 

 those in which one of its remote ancestors lived, it nearly always meets 

 them, not by an exact return to the ancestral adaptation, but by some new 

 expedient. 



The evolution of an egg-shell which would retain water solved one of 



