THE REPRODUCTION OF ANIMALS 215 



occurs in most coelenterate animals is known as metagenesis. (3) Asexual 

 reproduction does not produce so great a diversity and variation among 

 the offspring as is produced by sexual, and especially by bisexual, 

 reproduction. 



There is considerable parallelism between asexual reproduction and 

 the power of organisms to reproduce lost or mutilated parts. When a 

 hydra or a sponge is cut into minute portions, each portion, if it contains 

 cells of all kinds of essential tissues, is able to regenerate a complete new 

 individual. Such an individual will be very small but will be able to grow 

 to full adult size. A flatworm can also be cut into a number of portions, 

 each of which is capable of forming a new, complete individual, provided 

 that it contains all the considerably larger variety of essential tissues. 

 We could, if it were desirable, propagate such animals from cuttings as 

 we do plants. When we turn to higher animals, we find that the power 

 to reproduce new individuals from severed portions of another individual 

 rapidly decreases and is soon lost as we encounter progressively greater 

 complexity of structure. In the lower ranks of the complex Metazoa, there 

 is still, however, a marked ability to regenerate lost legs, tails, and other 

 appendages. This ability, too, diminishes with further increase in struc- 

 tural and physiological complexity (or perhaps, more accurately, with 

 increasing individualization of the organism) until in the higher verte- 

 brates it is limited to the ability to heal a wound, knit a broken bone, and 

 regenerate certain tissues. 



Sexual or Germ-cell Reproduction in the Metazoa 



Unlike the soma cells that are concerned in asexual reproduction, 

 germ cells do not show differentiation into any sort of functional body 

 tissue. They are to be classed neither as epithelial, contractile, sustenta- 

 tive, nor conductive tissue. All organisms that are reproduced by sexual 

 means begin their existence as a single cell, and most of the multiplying 

 descendants of this cell gradually differentiate into the various kinds of 

 cells that make up the tissues needed in the formation of the new in- 

 dividual. However, not all the cells descended from the original germ cell 

 become functional body cells. A few show no differentiation and constitute 

 the germ cells of the new individual. In the higher Metazoa, at least, these 

 germ cells soon migrate into the soma tissues that are destined to form 

 the gonads (ovaries or testes) of the new individual. 



MEIOSIS AND THE MATURATION OF THE GERM CELLS 



When the metazoan approaches sexual maturity, its germ cells begin 

 the characteristic processes by which they are formed into functional 

 eggs or sperms. These processes include two successive, specialized 

 nuclear divisions that together constitute meiosis, and also the cytoplasmic 



