124 ZOOLOGY. 



incapable of motion ; this passive character is an essential 

 feature of the female reproductive cell, and is far more 

 strikingly shown in ova which (like those of birds) are of 

 enormous size and stored with food -material f or the embi yo. 

 On the other hand, the male cell or spermatozoon is an 

 active cell, capable of swimming through a liquid by the 

 lashing of its " tail," which is practically a single, much 

 elongated cilium. It is usually smaller than the ovum even 

 when that is small, and the difference is still more striking 

 when the ova are large. But the nucleus is (at the t .mo 

 when the two cells unite) of equal size and importance in 

 both, and since we know that animals, on the average, 

 show as much resemblance to their fathers as to their 

 mothers, it is natural to suppose that it is the nucleus 

 which is the essential part in transmitting hereditary cha- 

 racters. Thus in respect of the nucleus the two cells 

 are of equal importance ; in respect of other matters there 

 is a kind of division of labour between them the ovum 

 keeping, along with its nucleus, sufficient protoplasm and 

 food-material to start the embryo in its development, while 

 the spermatozoon, unburdened with these, takes on the duty 

 of travelling about to seek the more inert ovum. Sperma- 

 tozoa are also produced in vastly greater numbers than ova, 

 and the vast majority of them fail to reach an ovum and 

 perish. 



5. Male Reproductive Organs, The testes of an 

 adult male rabbit have undergone a far greater shifting of 

 position than the ovaries of the female ; they come to lie in 

 pouches of the body-wall scrotal sacs which project on 

 either side of the urethral aperture. The vas deferens, 

 unlike the oviduct, does not end by an opening into the 

 co3lom it is united to the testis through a tubulated body 

 called the epididymis. From this it runs forwards to get 

 over the base of the ureter and then loops sharply back to 

 open (alongside its fellow) into a median sac called the 

 uterus masculinus a name which does not necessarily 

 imply homology with the paired uterus of the female. 

 This joins with the base of the urinary bladder to form 

 an urethra (corresponding to the female vestibule), which 



