II. Organs. 10. Reproduction and Reproductive Organs. 35 



same animal, or tlie ova and spermatozoa are both formed in a common 

 liermaplirodite gland. 



In many hermaphrodites, ripe ova and spennatozoa are produced at the 

 same time. In others, however, either the ova are fonned first and the sper- 

 matozoa later, so that the animal functions first as a female, then as a male 

 (protogynous hermaphrodites, e.g., Salpa; or they first produce spernnatozoa, 

 later ova (protandrous hermaphrodites, e.g., certain Nematodes), 



Secondary Sexual Characters. — Distinctions between males and females 

 other than those resulting from the difEei-ent nature of the genital organs 

 (primary sexual characters) are of frequent occurrence. The male is provided 

 with special organs, or parts of the body are specially modified for holding the 

 female during copulation ("Water-beetles) ; or they possess special weapons for 

 fighting with other males during the breeding season (Stags); or they are 

 specially ornamented with brilliant colours, peculiar excrescences, etc. (many 

 Birds). On the other hand, the female may be provided with organs of impoi-tance 

 in rearing the young ones (mammary glands of Mammalia), whilst the male 

 is more rarely specialised in this way (Pipe-fish). There is often a perceptible, 

 or even considei-able, difference in the size of the sexes ; sometimes the male is 

 larger than the female (many Mammals, Birds, and Insects), in other 

 cases (Birds of Prey, Round Worms) the female is the larger : the diffe- 

 rence is sometimes extraordinarily great, as in many parasitic Crustacea, 

 where the males only attain a fraction of the size of the female ; or again in 

 an Annelid (Bonellia), where the males are microscopic, and entirely different 

 in appearance from the fairly large female ; indeed, they were at one time 

 regarded as parasites of a distinct species, since they live within the oviducts. 



Fertilisation is effected by the entrance of the spermatozoon 

 into the ovum, and their ultimate fusion. There is evidently a 

 peculiar mutual attraction between ova and spermatozoa; not only 



( ®"M(^«pA( ® J 



Pig. 28. Diagram of fertilisation. fc female pronnoleus, sp male pronuoleua. 

 In 4, union has taken place. See Text. — Orig. 



does the ovum attract the spermatozooUj but at the approach of 

 the latter its protoplasm may rise up to form a small papilla. After 

 entrance, the spermatozoon loses its tail or protoplasm, if it possess 

 any, and its head (nucleus), which is now called the male pronucleus, 

 enlarges, gradually approaches the nucleus of the ovum (female 

 pronucleus), the two come to lie side by side, and finally fuse. 

 This union of the nuclei is evidently the essential part of fertili- 

 sation ; when it has taken place the egg is able to segment and thus 

 to give rise to a new organism. (See Sect. IV. — " Ontogeny.") 



The ovum is fertilised by one spermatozoon only, and as a rule only one enters 

 even though thousands may be swarming round. This is due to the fact that 

 when a spermatozoon has entered, the ovum undergoes oei-tain changes which 



D 2 



