THE ONTOGENESIS OF VERTEBRATES 51 



metes respectively of a conjugation, are essentially Protozoa, 

 and thus the first stage in the development of multicellular 

 animals is an historic repetition representing the first and 

 simplest of organisms. The spermatozoon with its motor 

 organ still retains its protozoan character even in the highest 

 of the vertebrates, but the ovum, loaded down with yolk, 

 bears for the most part little resemblance to an active or- 

 ganism. Even here, however, in certain sponges and hydroid 

 polyps, a more primitive form of ovum is still preserved, for 

 it is here amoeboid in form and possesses functional pseudo- 

 podia, being often impossible to distinguish from genuine 

 Amoebae, the simplest of Protozoa. This is a good illustration 

 of Rule V of ontogenesis as given in the previous chapter, since 

 it is to be expected that here, among the lowest and simplest 

 of the Metazoa, the early stages would receive the fullest 

 attention in the ontogenetic recapitulation. 



In size ova vary greatly, but the difference is due mainly 

 to the actual amount of food stuff, or yolk, which is required 

 in each case: this in turn is proportional, not to the size of 

 the adult animal, but to the degree of maturity at which it is 

 most advantageous for the young animal to begin its free 

 existence. Some animals produce a few very large eggs and 

 thus use up their reproductive energy in developing yolk; 

 others produce large quantities of tiny eggs which will develop 

 into innumerable minute larvae. Both extremes and all in- 

 termediate grades are the result of adaptation to the various 

 conditions that surround the different organisms and thus 

 regulate the size of the egg, as well as the size and shape of 

 the parts in the adult. Thus, for example, the ova of jelly- 

 fish, earth-worms, many molluscs, star-fish, and most mam- 

 mals, are very small, almost microscopic; those of insects, 

 crustaceans and fishes are usually of an appreciable size, those 

 of frogs and of certain fish are still larger, while the eggs of 

 reptiles and birds are enormous, those of the latter having 

 reached the extreme limit relative to the size of the parent. 



In the eggs of placental mammals, which are practically 

 yolkless, there is no great difference in actual size between 



