48 SEX 



urchins with superficially similar sexes are 

 none the less in a sense very sexual, at the 

 breeding season at least, for they produce 

 enormous quantities of eggs and sperms, and 

 the reproductive organs are huge in proportion 

 to the body. Thus one of the British star- 

 fishes (Luidia ciliaris) produces at least 200 

 millions of eggs. 



FIG. 9. Fertilisation of ovum of starfish after Fol. In 

 I the spermatozoa are seen surrounding the ovum, 

 which is enveloped in a delicate zone of mucus. A 

 minute " receptive cone " is seen on the surface of the 

 ovum. In II a spermatozoon is entering, the locomotor 

 tail is seen protruding, a membrane is being rapidly formed 

 around the ovum. In III the spermatozoon has entered, 

 the membrane is completed, the ovum has become ncn- 

 receptive. Only a small part of the ovum is shown. 



As we ascend the series, the evolution of 

 sex-dimorphism becomes gradually apparent. 

 Following the differentiation of tissues and 

 structures accessory to the reproductive organs 

 the body itself begins as a whole to be sexed. 

 Various structures, quite extra-genital, begin 

 to have alternative expressions, more or less 

 mutually exclusive, and masculine and femi- 

 nine individuals thus at length appear. 

 Animals began to put on secondary sex 



