Sexual reproduction, i.e., reproduction by means of 

 special germ cells, occurs in some unicellulars, and m most 

 multicellulars. Three methods may be recognized: 



(1) Heterogamy, where the eggs from one parent are 



fertilized by sperms from another parent; 



(2) Autogamy, or hemaphroditism, where the eggs 



are fertilized by sperms from the same parent ; 

 and 



(3) Parthenogenesis, where eggs may develop without 



fertilization, as among Aphids, and in Taraxa- 

 cum, Hieraciu?n, Antennaria, Atchetnilla, etc. 



Sexual reproduction in both plants and animals came 

 into operation after the asexual. In the former, up to the 

 highest forms, asexual and sexual methods have continued 

 side by side, but in the latter sexual reproduction whenever 

 introduced has displaced the asexual, with a few exceptions. 

 Parthenogenetic development is rather rare, as it occurs in 

 a few groups only, while autogamy or hermaphroditism is 

 quite common among the higher plants and many of the 

 animal groups such as the sponges, flat-worms, certain 

 parasitic crustaceans, bryozoans and tunicates. It seems 

 to be associated with parasitism or with a sedentary mode 

 of life. 



The Germ Cells or Gametes 



The female germ cell is called the egg-cell or ovum. It 

 is bounded by a cell wall and contains food substances and a 

 nucleus. The latter, however, is the essential part as it 

 contains the chromatin substance. The size of the egg-cell 

 varies very greatly — usually minute, but large in birds — 

 according to the amount of the food substance or yolk. In 

 the sponges, corals, star-fishes, worms and mammals the 

 yolk is small in amount and is diffused evenly through the 

 egg cell. In the Arthropods the yolk occupies a central 

 position. In the Amphibians the yolk lies at one end, and 

 in fishes, reptiles and birds the yolk occupies nearly the 

 whole egg-cell. The distribution of the yolk determines the 

 method of segmentation after ferilization. (See Fig. 22). 



The male germ cell is called the sperm-cell or sperma- 

 tozoon (in animals). It is very much smaller than the ovum 

 and varies in shape in the different groups of animals. In 

 mammals it consists of a small pointed head, composed 

 almost entirely of nucleus, and a long vibratile tail for pur- 

 poses of movement. 



40 



