i8o ESSENTIALS OF BIOLOGY 



division of a pre-existing gonidial cell in the male gonads or 

 testis, or essential male sex-gland. The female germ-cell, or 

 gamete is usually large and inactive and it is called the ovum 

 in animal organisms. It is formed in the ovary, or essential 

 female sex-gland, by the division of a pre-existing gonidial cell. 



The conditions in the higher plants are essentially similar — 

 when we allow for the alternation of sexual and asexual genera- 

 tions. The male gamete (corresponding to the spermatozoon in 

 animals) is the antherozoid, pollen-micleus, etc. The female gamete 

 (that corresponds to the animal ovum) is the oosphere, egg-cell, etc. 



Thus two kinds of germ-cells, the gametes, are reproduced 

 whereas, in the typical unicellular organism, there is only one 

 kind of germ-cell and this is also the unicellular organism itself. 

 In typically sexual animals there are clear indications that either 

 the male or female gametes may, of themselves, develop into 

 organisms. The ovum may do so in parthenogenetic animals 

 (see Section 68) and in irregular fertilizations it may happen that 

 the spermatozoon may, by itself, proceed to develop. 



But what usually happens in sexual reproduction is that the 

 ovum (or oosphere, in plants) is fertilized by the spermatozoon 

 or antherozoid, or pollen-nucleus, in plants). There is conjuga- 

 tion of the gametes, just as there may be conjugation of unicellular 

 organisms. 



66a. Secondary Sexual Characters. In general animals 

 are differentiated bodily into males and females. The essentials 

 of maleness and femaleness are the existence of the male sex- 

 glands, or testes, and of the female sex-glands, or ovaries, respect- 

 ively. But there are also bodily differences. 



External sexual organs. Typically there is copulation of male 

 and female animals. This is analogous to the partial coalescence 

 of Paramoecia (Section 636). In copulation the gametes (ova and 

 spermatozoa) are brought together and so there are generally 

 receptive cavities (vagina, etc.) in the females while there are 

 intromittent organs (the penis, etc.) in the males. In copulation 

 the spermatozoa are placed in the receptive cavity, where they 

 come into proximity with the ova. 



But in some animals (some fishes) there may be no external 

 genitalia whatever. Ova and spermatozoa are simply extruded 

 from the bodies, or spawned, and they come into proximity in the 

 water in which the animals live. 



