PROTECTIVE FEATURES IN YOUNG OF VERTEBRATES. Ill 



their origin, their present purpose and meaning have been 

 involved in much obscurity. It is known that slight mechanical 

 pressure upon the immature young and alterations in the 

 surrounding temperature produce apparently disproportionate 

 effects. Professor Cleland's well-known experiments have 

 furnished ample evidence of these results, usually inimical. To 

 shield the developing organism from hurtful influences, reptiles, 

 birds, and the higher animals are wrapped up in embryonic mem- 

 branes and bathed in abundant fluid. That such structures now 

 serve protective purposes can hardly be questioned. In the chick 

 they are no less than seven in number: — the liquor amnii (i) con- 

 tained in the true amnion (2), outside which is a serous fluid (3) 

 retained within the false amnion (4). External to the amniotic 

 envelopes are the inner shell-membrane (5), the air-chamber (6), 

 and the outer shell-membrane (7), with its calcareous deposit con- 

 stituting the dense outside shell. None of these really form an 

 essential part of the bird, and by the time of hatching most have 

 served their purpose and have degenerated. In the mammalian 

 embryo the amniotic envelopes and fluids are discharged at the 

 close of foetal development. In the lower vertebrates, the fishes 

 and amphibians, no amnion exists; but there are other embryonic 

 structures which serve the same purpose. The frog's egg, for 

 instance, is a small sphere, of which the upper half is tinted 

 black, while the lower half is whitish. Before deposition in water 

 the egg measures barely T \th of an inch in diameter, with a thin 

 outer coat of albumen, like a film of gelatine. The latter on contact 

 with the water swells to more than five times its original size. 

 Massed together these jelly-clothed eggs form the floating spawn 

 so familiar in every wayside pond in spring. A distinguished 

 naturalist has very happily compared the deposited eggs, each a 

 ball of jelly with a black centre, to a number of hen's eggs removed 

 from their shells and placed together. Each yellow yolk corre- 

 sponds to the small black egg of the frog, the white represents the 

 jelly of the spawn. The entire chick is formed from a part of the 

 vitellus, whereas the frog is built up out of the entire tinted vitelline 

 ball. The eggs too, as Professor Miall has pointed out, are kept 

 apart, due aeration is facilitated, and parasitic vegetable growths 

 prevented. The authority named says that the broad-billed duck 

 is one of the few animals which can devour frog-spawn, other 



