REPRODUCTION 189 



ectoderm cells, which in one case gives rise to many sperm and in 

 the other to a single egg. The mature sperm are set free from the 

 testis and swim about in the water, but sooner or later one enters 

 the now ruptured covering of the ovary and fuses with the egg. 

 With the conclusion of fertilization the zygote begins to divide 

 and forms an embryo which at an early stage becomes de- 

 tached from the parent. Thus in Hydra there is no complicated 

 apparatus for sexual reproduction; merely now and again the 

 temporary development of the primary sex organs, ovaries and 

 testes. (Fig. 57.) 



The complex bodies of most animals, however, demand more or 

 less permanent gonads, as well as means for transferring the gam- 

 etes directly or indirectly to the exterior. This is brought about 

 by the fact that in coelomate animals the gonads come to lie, not 

 on the outside of the body, but within the coelom. In the Earth- 

 worm, which also is hermaphroditic, the testes and ovaries are 

 permanent organs attached to the partitions between certain seg- 

 ments. The sexual products are set free in the coelom where they 

 are taken up by sperm ducts and oviducts and carried to the 

 outside. Although each Earthworm possesses both male and fe- 

 male reproductive organs, two worms pair and exchange the sperm 

 which are stored in the respective sperm receptacles. Later, when 

 the eggs pass to the exterior, the ' foreign ' sperm are shed on them. 

 Thus cross-fertilization is insured in this hermaphroditic form. In 

 the Crayfish the sexes are represented by separate individuals, 

 males and females, and the appendages of the first and second 

 abdominal segments of the male are modified into copulatory 

 organs for the transfer of the sperm to the body of the female, 

 where they are retained until egg-laying. (Figs. 60, 63.) 



B. Vertebrates 



Throughout all the chief Vertebrate groups the sexes are dis- 

 tinct, although in rare instances abnormal hermaphroditic in- 

 dividuals occur. The definitive primordial germ cells first appear 

 as localized areas of the epithelium lining the coelom, on either 

 side of the vertebral column. As the germ cells develop they be- 

 come associated with connective tissue, blood vessels, and nerves 

 and form the paired gonads. In the most primitive Vertebrates a 

 condition more simple than in the Earthworm is found, for both 

 male and female gametes merely break out of the gonads and 



