414 THE MAINTENANCE OF SPECIES 



Paedogenesis 



Reproduction by immature individuals is called paedogenesis. 

 As it is rarely encountered in the animal kingdom, only two examples 

 need be mentioned. The first occurs in the trematodes, where imma- 

 ture larval forms, such as sporocytes and rediae, appear to produce 

 the next generation parthenogenetically. These in turn often give 

 rise to another generation through paedogenetic reproduction. In 

 the vertebrates the best known example is the Mexican axolotl, a 

 urodelous amphibian. This interesting animal, while still remaining 

 in its larval form, reproduces its kind sexually without undergoing 

 metamorphosis or losing its external gills. 



Alternation of Generations 



Alternation of a sexual with an asexual generation is called metagen- 

 esis, or simply alternation of generations. Several of the invertebrates, 

 especially the coelenterates, normally exhibit metagenesis. In the 

 hydroid Obelia, for example, the asexual generation is represented 

 by a sessile, colonial hydroid and the sexual generation by the mature, 

 bisexual medusa buds (see page 185). 



Sexual Reproduction and Development in the Vertebrates 



Germ Cells versus Soma Cells 



The early growth and later development of the embryo and its 

 systems, organogeny, are to be considered in some detail. To com- 

 plete the picture it is necessary to envision the continued growth of 

 the organism until it matures, reproduces its kind, and dies. The 

 life of every organism, whether plant or animal, is involved with the 

 mathematical concepts of division, multiplication, addition, and 

 subtraction. In the formation of a new individual by two parents, 

 two germ cells are added together {fertilization). In order that the 

 hereditary genes thus united may not be disastrously doubled in 

 each generation, one half of those present from each contributing 

 parent are subtracted by the elimination of either the maternal or the 

 paternal member of each chromosome pair just prior to maturation. 

 Thus, a constant number of chromosomes with their respective genes 

 is maintained in each body cell of any species. After this preliminary 

 process of subtraction and addition has been accomplished, the newly 

 combined germinal cell, that is, the fertilized egg, or ovule, initiates 



