168 INTRODUCTION TO EVOLUTION 



struction through drying in a climate characterized by alternating wet and 

 dry seasons, as the latter part of the Paleozoic seems to have been. It is 

 interesting to note in this connection that many modern amphibians man- 

 age to lay their eggs on land, employing a variety of protective devices. 

 But none of these devices are as successful as the ones incorporated into 

 the reptilian egg. 



FIG. 8.23. A Permian vertebrate, Seymouria, combining characteristics of both amphibi- 

 ans and reptiles; length about 20 inches. (Mainly after Case.) 



This important change was made possible by enclosure of each egg 

 within a protective capsule containing enough nourishment to last the em- 

 bryo until it had reached a stage when, as a newly hatched young, it 

 could move about on land and secure its own food. Fig. 8.24 shows the 

 arrangements by which this result is achieved for bird embryos; the rep- 

 tihan egg is identical in its main features. The capsule mentioned is com- 

 posed of a shell and shell membranes; the shell of reptilian eggs is pliable, 

 in contrast to the brittleness of birds' egg shells. The embryo itself is en- 

 closed within a bladderlike membrane called the amnion. This serves as 

 a container for the amniotic fluid in which the embryo floats. The fluid 

 protects the embryo from mechanical injury and from drying. Thus the em- 

 bryos of reptiles and birds resemble those of fishes and amphibians in that 

 all develop while submerged in liquid. In the case of fishes and amphib- 

 ians the liquid is the water of streams and ponds; in the case of reptiles 

 and birds the liquid is the fluid bottled up within the amnion. One may 

 speculate that, since the embryos of ancestors had "formed the habit of" 

 developing in liquid, if descendants were to avoid laying their eggs in wa- 

 ter they would be under necessity of providing a substitute liquid in which 

 embryonic development could occur. Incidentally, we may here appropri- 

 ately recall the many respects in which the embryos of higher vertebrates 

 resemble the embryos of fishes (see Chap. 4). 



The yolk of the reptilian or avian egg constitutes the store of food men- 



