REPRODUCTION 183 



medusae are free and swim about by opening and closing 

 the umbrella, and nourish themselves like the jelly-fishes. 

 After a certain duration of vegetative life, gonads, or 

 reproductive organs, make their appearance. These are, 

 however, different in the different medusae, some pro- 

 ducing ova, others spermatozoa, so that these little 

 animals are cfesms, or of tw^ sexes. The eggs, being 

 discharged, are fertilized by conjugation with spermat- 

 ozoa in the water. From these eggs a larva is developed 

 which swims to the bottom of the water, attaches itself 

 to some object, and develops into a hydroid polyp. 

 Thus, in the life history of this little animal we find two 

 stages which alternate, one of fixed hydroid and one of 

 free swimming medusa form. It is thus said to exhibit 

 metagenesis or alternation of generation. 



Among the sponges the sexes seem to be clearly sepa- 

 rated so that the animals are all dicecius. The cells 

 which take on the reproductive function are known as 

 gonocytes and are amoeboid. The spermatic cells 

 break up into a considerable number of spermatozoa 

 which are liberated into the water. The ova which 

 form in some other sponge are also amoeboid and force 

 their way through the entodermal cells until they pro- 

 trude into one of the inhalant canals where they await 

 the arrival of a spermatozoon in the water. When 

 conjugation has taken place and fertilization been 

 effected, the ovum again draws back into the body of 

 the sponge and shortly undergoes segmentation by the 

 formation of a considerable number of equal-sized divi- 

 sions resulting in a blastula or hollow sphere composed 

 of one layer of cells. The cells of one hemisphere next 

 increase in number above those of the other, and acquire 

 flagella, after which the embryo leaves the body of its 

 parent and swims away. Soon the non-flagellated cells, 

 which appear granular, begin to grow and invaginate 

 the flagellated cells so that the embryo, being no longer 

 able to swim, settles down upon some object to which 

 it later becomes attached. An osculum opens at the 



