THE DEVELOPMENT OF VERTEBRATA. 245 



and remains rudimentary in the male. The Wolffian duct 

 remains as the duct of the kidney (mesonephros), i.e. 

 ureter in the adult female, urogenital duct in the male. 



The gonads first appear as a pair of ridges (showing no 

 metamerism) just nearer the middle line than the mesone- 

 phric tubules. Like them, they are developed out of the 

 mesoblastic somites, and from the first there is a close 

 connexion between these two structures. The tubules send 

 out branches which enter the gonad: these become the 

 vasa efferentia in the adult male ; and in the female they 

 lose connexion with the tubules and form the cavities in 

 the ovary itself. 



18. The Metamorphosis. We have now described the 

 chief organs that appear in the tadpole in preparation for 

 the adult stage. But when a larva differs so greatly from 

 the adult as the water-breathing, plant-eating, tailed, 

 swimming tadpole does from the air-breathing, insect-eating, 

 tailless leaping frog, there must needs be a great change in 

 the structure of many organs that are needed in both 

 stages. The period during which these organs are changing 

 must be one in which they are not well-adapted to either 

 the old or the new use ; it is therefore a period of helpless- 

 ness, and evidently the more quickly it is got through the 

 better. Accordingly we find in all cases where the differ- 

 ences between larva and adult are very marked, there is 

 some arrangement for a comparatively abrupt change from 

 the one to the other. Such a change is called a meta- 

 morphosis, and it may be much more abrupt than in the 

 case of the frog. 



The tadpole does use its hind legs and its lungs to some 

 extent before the metamorphosis. After a variable time of 

 tadpole life (usually about three months) the metamorphosis 

 begins. The tadpole ceases feeding, but it is supplied with 

 nourishment by the absorption of the tail. Phagocytes 

 (cells resembling the ordinary colourless blood-corpuscles) 

 attack the tissues of the tail just as osteoelasts absorb 

 cartilage and bone, and the digested material is carried 

 away by the blood. Thus the tail gradually shrinks in size. 

 Meanwhile the outer layer of skin is shed, and the horny 



