REPRODUCTION AND HEREDITY 285 



Two fundamentally distinct methods may be discerned in 

 their nutrition : either the food-cells themselves serve as food and 

 are simply eaten up by the growing sex-cells, or they draw a 

 continual stream of food-material from the maternal body and 

 pass it on to the ova and spermatozoa. They act therefore 

 as intermediaries. 



That the food-cells, as a rule, are nothing but degenerate 

 germ-cells we have already heard on a previous occasion, where 

 I also referred to the silent but fierce struggle which takes place 

 in the sexual glands, as the result of which competition only the 

 best and strongest germ-cells are able to develop. But even the 

 complete germ-cells are sometimes subjected to a severe test of 

 their fitness. 



To give only one instance, the spermatozoa of the higher 

 animals possess the peculiar habit of taking up their position in 

 the fluid with the head opposite to the direction of the current. 

 If we now gradually increase the strength of the current we see 

 under the microscope that numerous spermatozoa become ex- 

 hausted and are carried away by the stream, but that the 

 strongest pluckily work their way up stream. Though this 

 seems insignificant, it has a very important meaning. In the 

 female uterus and oviduct there is a slight current to the outside, 

 which indicates the direction to the invading spermatozoa and 

 must be overcome by them before they can penetrate to the 

 ovum and complete the union. In this simple manner once 

 more selection takes place, by which weakly elements are generally 

 eliminated. 



In order that the germ-cells may fulfil the purpose of their 

 existence it is above everything necessary that there should be 

 suitable preparations for bringing ova and spermatozoa together. 

 The first condition is a permanent, or at any rate periodical, 

 coming .together of the two sexes. Most favourably situated 

 appear to be in this regard the hermaphrodites, those animals 

 in which both kinds of germ-cells originate in one and the same 

 individual. Especially in the lowest classes of multicellular 

 animals, hermaphroditism is still fairly general, and in the 

 numerous species which are either fixed or slow-moving it 

 becomes the general rule. We know hermaphrodites among 

 sponges, jelly-fishes, vermes, echinoderms, molluscs, crustaceans, 



