320 



THE RISE OF ANIMAL LIFE 



porates the entire yolk mass into its intes- 

 tine and uses the stored food for growth. 



During this time it is floating in a fluid 

 environment, reminiscent of its amphibian 

 ancestors. A large, fluid-filled sac, called the 



Fig. 13-40. In order for vertebrates to completely divorce 

 themselves from water they needed some means of 

 caring for their young during their early develop- 

 ment. This was accomplished with the evolution of 

 the land egg. 



amnion, develops around the embryo which 

 not only provides a fluid environment, but 

 also protects the developing embryo from 

 injury and desiccation. Shortly after the 

 formation of the amnion, the allantois de- 

 velops from the posterior end of the em- 

 bryo. This enveloping membrane receives 

 discarded material, including carbon diox- 

 ide, from the embryo. It lies very close to 

 the porous, rigid outer shell so that a gase- 

 ous exchange can readily take place. There- 

 fore, in addition to being an organ of ex- 

 cretion, the allantois acts as a temporary 

 respiratory organ during embryonic life. 

 The young reptile need not be immersed in 

 water at any time during its life, and thus 

 the first true land animal has been evolved. 

 This animal can seek out any environment 

 it wishes without regard to water beyond 

 its metabolic needs. This was probably the 

 greatest step forward in conquering the 

 land. 



These changes in the developing embryo 

 were undoubtedly the greatest ones that 



took place in the reptile, although other 

 changes also occurred that made it better 

 suited for a terrestrial existence. Since an 

 aquatic environment was no longer essen- 

 tial, internal fertilization became a neces- 

 sity to protect the delicate reproductive 

 cells. Therefore, efficient copulatory organs 

 developed from the floor of the cloaca of 

 the male, insuring a direct transfer of the 

 sperm into the genital tract of the female. 

 In addition, reptilian legs became longer 

 and were usually more ventrally located, 

 making it possible for them to support the 

 body completely off the ground, a feat 

 which tlie amphibian had not accomplished 

 (Fig. 13-13). The heart also began to form 

 a complete partition in the ventricle, pro- 

 ducing the beginnings of a four-chambered 

 heart and thereby separating the pulmo- 

 nary and systemic blood to make a much 

 more eflFective circulatory system. The 

 greater endurance and strength of reptiles 

 reflects the effectiveness of this change. 



Early reptiles 



This group of animals has had a long and 

 luxuriant history. Living forms, such as the 

 turtles, crocodiles, lizards, and snakes, are 

 relatively insignificant animals on the earth 

 today. However, there was a time during 

 the Mesozoic Era, the so-called Age of 

 Reptiles, when this group dominated all 

 animal life on the earth and reached such 

 peaks that perhaps no other animal, not 

 even mammals, will ever attain. Reptiles 

 may be said to be the most successful ani- 

 mals that have thus far existed, man not 

 excepted. 



Sometime in the distant past there must 

 have been forms stemming from the am- 

 phibians that gradually took on reptilian 

 characteristics. Such animals have been 

 found in fossil remains and are called the 

 Stem Reptiles. Among tliese is Seijmouria, 

 which was probably the first animal that 

 began to show what is now known as rep- 

 tilian characteristics. Setjmouria was found 

 near the small Texas town of Seymour. This 



