I 



ONTOGENY 13 



gonads, which correspond functionally to gametogonidia, divide 

 repeatedly, the products of their multiplication being, with few 

 exceptions, ultimately thrown outside the body as the highly 

 modified germ cells or gametes. In all the Metazoa these germ 

 cells are of two quite unlike forms; the ova or "eggs" are large 

 and ordinarily non-motile, while the spermatozoa or sperm cells, 

 are small and actively motile. These Metazoan germ cells 

 clearly correspond with the ova or macrogametes and the sperms 

 or microgametes of certain of the Protozoa. The gonads 

 forming the ova and Spermatozoa are known as the ovaries 

 and testes respectively. In most of the Metazoa germ cells of 

 only a single kind are formed by a single organism, a condition 

 which leads to the primary distinction of sex; individuals 

 forming ova are called females, those forming spermatozoa, 

 males. * 



& In comparatively few kinds of animals does a single individual 

 normally possess gonads of both types, and thus become capable 

 of forming both kinds of germ cells, either simultaneously or 

 successively. Such a condition is known as hermaphroditism; 

 it occurs chiefly among the lower Metazoa,/ such as the Platy- 

 helminthes, Nemertea, some Annulates and Tunicates, and 

 less frequently among the Molluscs, Echinoderms, Bryozoa, 

 Brachiopoda, and Crustacea. Among the Chordata normal 

 hermaphroditism is found only rarely (some Teleosts) , though it 

 may occur as an abnormality in any group. In a few animals 

 a special form of hermaphroditism occurs, where a single gonad 

 may produce first sperm and later ova, a condition known as 

 protandry and found in some Nematode worms and in the 

 Cyclostome, Myxine, for example. The process of forming 

 first ova and later spermatozoa, known as protogony, is very 

 rare among animals. 



In the reproduction of all except a very few of the Metazoa, 

 the initial phase is the union of an egg cell and a sperm cell; 

 this process is known as fertilization or syngamyj and the double 

 cell thus formed, which is called the'^gofe or odsperm, then 

 gives rise directly to the new individual. Not all of the germ 

 cells formed by an organism actually happen to give origin to 



