THE ANIMAL CLIMAX-THE CHORDATES 



319 



ostium 



posterior vena cava 



odrenol glond 

 kidney. 



Fig. 13-39. Female frog orinogenital system, showing the kidney and ovary enlarged and in cross-section. 



early development of the animal when the 

 sex is not determined, so "to be certain," 

 both organs are produced; later only one 

 becomes functional. 



LAND CONQUERORS: 

 THE REPTILES 



The amphibians were forced to spend 

 their embryonic life in the water and were 

 able to leave water only as adults. The rep- 

 tiles moved one step farther in the long 

 trek to complete terrestrial existence. They 

 spend no part of their life in the water 

 unless they choose to do so. In order to 

 accomplish this feat, radical changes were 

 required in the physical provisions for their 



early development. Means were provided 

 whereby the early embryos could exist in a 

 fluid environment, and this medium was 

 supplied with sufficient nourishment to 

 carry the embryo through the stages equiv- 

 alent to the tadpole stage among amphib- 

 ians. Enough food was stored in the egg so 

 that the oncoming young one might be well 

 along in its development when it emerged 

 on its own, and, as a result, be able to care 

 for itself, eat the adult diet, and move about 

 under its own power on land. For this rea- 

 son, reptilian eggs are large, with great 

 quantities of stored food in the form of yolk 

 and albumin ( Fig. 13-40 ) . The young rep- 

 tile starts its life on the top of a large yolk 

 mass in tlie egg, and eventually it incor- 



