METAZOAN ORGANIZATION 113 



development by application of chemical, electrical, or mechanical 

 agents. This goes under the name of artificial parthenogenesis. 



Metagenesis is a phenomenon occurring in the life history of a 

 number of scattered species of Metazoa, including the coelenterate, 

 Ohelia; two or three marine worms; and Salpa, the tunicate (a chor- 

 date animal). This process is an alternation of production of sexual 

 individuals in one generation and asexual in the next. The offspring 

 in each case differs from its parents. This is spoken of as alternation 

 of generation. In Ohelia, a coelenterate related to Hydra (to be 

 studied shortly), there is a plantlike, asexual, colonial form, which 

 gives rise to sexual, free-swimming medusae (Fig. 59). The medusae 

 produce eggs and sperms which unite in the water and develop into 

 asexual colonies. Metagenesis really involves two methods of repro- 

 duction in successive generations of the same species. The significance 

 is somewhat uncertain, but possibly it insures better and more com- 

 plete distribution of individuals than could be secured by only the 

 budding colony. Many of the sexually reproducing plants have a 

 similar alternation of sexual and asexual generations. 



Metazoan and Ontogeny- 

 Ontogeny refers to the development and life history of the indi- 

 vidual organism, produced sexually from the union of germ cells 

 or gametes. This process is quite generally similar wherever it 

 occurs, differing only in detail. Embryological development is an 

 expression referring to the processes which occur during the earlier 

 portion of the life of the individual. 



The male and female germ cells or gametes are produced in their 

 respective gonads as previously described. They are in a very im- 

 mature state when they are first differentiated, and are called pri- 

 mordial germ cells. 



The maturation (gametogenesis) or development of the germ cells 

 occurs while they are still within the gonads, except for the latter part 

 of the process in ova which reaches completion after the cells leave 

 the ovary. It consists of a series of mitotic cell divisions which is 

 modified at one point to bring about a fusion and subsequent reduc- 

 tion in the number of chromosomes in the cells. In brief maturation 

 is the preparation of germ cells for fertilization which may follow. 

 The development of the male germ cell is known as spermatogenesis, 

 and the development of the female germ cell is oogenesis. 



