402 THE EVOLUTION OF MAN. 



Though we must recognize the formation of the two 

 kinds of sexual cells, and in their union at fertilization as 

 the one essential act of sexual reproduction, yet, in the great 

 majority of animals, other organs exist which also take 

 part in the act of fertilization. The most important of 

 these secondary sexual organs are the exit-ducts which 

 serve to conduct the mature sexual cells out of the body, 

 and, next to these, the copulative organs, which transmit 

 the fertilizing sperm from the male person to the female, 

 in which the eggs are situated. These latter organs exist 

 only in the higher animals of various tribes, and are far less 

 widely distributed than the exit-ducts. Even these latter, 

 however, are only of secondary formation, and are wanting 

 in many animals of the lower groups. In these, as a rule, 

 the mature sexual cells are simply ejected from the body. 

 In some cases they pass out directly through the outer 

 skin-covering (as in the Hydra and many of the Hy- 

 droidea) ; in other cases, they enter the stomach-cavity, 

 and are ejected through the mouth-opening (in Gastrseads, 

 Sponges, and other Hydroid Polypes and Coral Animals) ; 

 in yet other cases, they enter the body-cavity and 

 pass out through a special aperture in the ventral wall 

 (porus genitalis). The latter is the case in many Worms 

 and even in a few lower Vertebrates (Cyclostoma and 

 a few Fishes). These indicate the earliest condition of 

 this matter as it was in our ancestors. On the other 

 hand, in all higher, and most lower Yertebrates (as also 

 in most higher Invertebrates) special tube-shaped exit- 

 ducts from the sexual cells, or sexual ducts (gonophori), 

 are present in both sexes. In the female these convey the 

 egg-ceUs out from the ovaries, and hence they have been 



