MAMMALIA 311 



many fishes and amphibians, the act of fertilisation takes place 

 in the water. In the rest, and necessarily in terrestrial forms, 

 it takes place internally and the sperms have to be transferred 

 into the oviduct. Water is thus the medium through which 

 the sperms find their way to the ovum in the former, to which 

 they are chemically attracted. In such the tendency is to the 

 production of large numbers of eggs and sperms, and the death 

 rate especially affects the young stages. In the case of forms 

 which have internally fertilised eggs the rule is that few eggs 

 are fertilised, and the elimination is done, therefore, at the stage 

 of germ cells. Moreover, it is obvious that they have never a 

 free period. 



When fertilisation takes place the protozoon phase gives 

 place to the metazoon in which the cells remain united to form 

 a more or less complex body. In other words, the soma of one 

 generation contains the protozoon phase of the next. 



The protozoon phase of the Metazoa is protected, so usually 

 are the young stages of the metazoon phase. Protection may 

 last, and usually does last, during life in the parasitic, and is 

 given indirectly to the weak and the helpless. But in Metazoa 

 which acquire adequate powers of movement, protection and 

 the need for it are outgrown. This is paralleled by the relation- 

 ship of aquatic life to currents. When young, aquatic animals 

 can only drift with the currents or are denatant. With increase 

 in size and power they become more and more contranatant. 



After the period of multiplication of the germ cells the 

 products are termed respectively spermogones and oogones. 

 A period of rest follows, during which the elements undergo 

 a change into primary spermocytes and oocytes and the 

 metazoon attains maturity. It is during this period also 

 that the metazoon provoked directly by the spermocytes or 

 the oocytes, or through the mediation of the other cells which 

 form the testes or ovaries, exhibits conspicuous growth 

 changes, and may be compelled to migrate. The germ cells 

 themselves undergo a change, the most important evidence 

 of which appears when the cell divides after the rest period is 

 finished in the first of the two maturing divisions. It is then 

 found, and the fact appears to be a universal one, true of plants 

 as well as animals, that the chromosomes are reduced to half 



