METAZOAN ORGANIZATION 111 



j. The Reproductive System is an organization of glands, ducts, 

 and accessory structures which function in the reproduction of the 

 species. More discussion of this system is found below. 



The body might be thought of as being constructed by relating cells 

 to cells to form tissues, tissues to tissues to form organs, organs to 

 organs to form systems, and sj^stems to systems to form the metazoan 

 organism. These will all be studied in more detail in connection with 

 the study of specific animals. 



Development of Sexual Reproduction 



Keproduction makes great advances among the metazoans. The 

 simple fundamental process of reproduction by cell division or 

 binary fission has been studied already. This is not possible for 

 most metazoan animals, but, in general, this type of animal be- 

 gins life as a single cell resulting from the fusion of two sex cells, 

 one produced by each parent. In some of the colonial Protozoa and 

 also in Sporozoa, as well as possibly in Paramecium, there seems to 

 be the beginning of sexual reproduction. The individuals in a 

 colony by peculiarities in cell division become differentiated into 

 two types: (a) the ordinary, nutritive individuals, whose means of 

 reproduction is fission and (b) reproductive individuals or gametes 

 of two forms : the large, egglike, inactive macrogametes and the 

 smaller, motile microgametes. In reproduction these two types of 

 cells unite to form a single zygote, from which a new colony arises by 

 repeated divisions. In a number of the Sporozoa, both sexual and 

 asexual generations occur. The zygotes, which are formed in the 

 sexual phase or generation, produce a number of spores which de- 

 velop sporozoites (already studied under Plasmodium.). These be- 

 come nutritive trophozoites and are capable of production of another 

 generation of gametes. Conjugation of Paramecium is also looked 

 upon as a forerunner of sexual reproduction. 



In simple Metazoa there are likewise two forms of reproduction : 

 asexual (without sex), including buddijig and fission, and sexual, 

 which involves the union of two germ (sex) cells, one male and one 

 female. In simple forms like sponges and jellyfish the germ cells 

 arise from general formative interstitial cells between the two primi- 

 tive germ layers to form temporary gonads. When the germ cells 

 are mature, they break through the wall to the outside of the body. 

 Again, among the simpler metazoans a single individual produces 

 both male and female germ cells. Such an organism is said to be 



