THE ANIMAL CLIMAX-THE CHORDATES 



339 



Fig. 13-57. Platypus (Ornifhorhynchus anatinus), an egg- 

 laying mammal. It possesses both reptilian and mam- 

 malian characteristics. 



young are nourished for a time from milk 

 secreted by two rows of glands along the 

 belly side of the mother. There are no teats 

 and the young merely lap up the secreted 

 milk. It is noteworthy to mention again that 

 these archaic animals have survived up to 

 the present time due to their isolation. Had 

 they been forced to compete with modern 

 mammals they would long since have van- 

 ished from the earth. Isolation may be 

 either by actual land or water barriers or 

 by environments in which there is little 

 competition. It is indeed fortimate that such 

 animals have been preserved for us to 

 study, making possible more logical con- 

 jectures as to just how present animals 

 came about. 



The next step on the road toward "true" 

 placental mammals would be the interme- 

 diate types, namely, the marsupials. There 

 are many of these interesting animals, in- 

 cludins the kan2;aroo of Australia and the 

 opossum of North America (Fig. 13-58). 

 These do not lay eggs like the monotremes 

 but retain them in the uterus where a rudi- 

 mentary placenta forms. Since this organ 

 is inadequate to maintain the embryos for 

 any great period of time the young are 

 born in a very immature state ( Fig. 13-56 ) . 

 A 200 pound kangaroo for example may 

 give birth to offspring no longer than 2 

 inches. Following birth, the embryos make 



their way, or are placed, in the marsupium 

 where they grasp and practically swallow 

 the nipples of the mammary glands to 

 which they cling during what is equivalent 

 to later embryonic life of a more advanced 

 mammal (Fig. 13-59). After a time they 

 are able to face the world and they come 

 out of the pouch to feed for themselves, 

 retreating to it only in case of danger. Al- 

 though the marsupials give us a clue to the 

 past history of mammals, their direct line 

 probably diverged from the reptilian mam- 

 mal stock very early in geologic times. 



The placental mammals 



The next step in the evolution of the 

 mammals must have been the formation 

 of a well-developed placenta, one that was 

 adequate to care for the developing off- 

 spring within its mother for the period of 

 time necessary to develop the complex 

 structures essential for success once it was 

 born. This organ increased in size and ef- 

 ficiency until it was capable of receiving 

 sufficient food and oxygen from the blood 

 stream of the mother to allow for advanced 

 development of the offspring (Fig. 13-59). 

 Since, in these animals, so much of the 

 nourishment is obtained from the mother, 

 eggs with large amounts of yolk were un- 

 necessary. Reduction of the yolk is reflected 

 in present-day mammals where the egg is 

 very small (0.5 mm. across), whether it 

 be that of an elephant or a mouse. 



Along with tlie development of a success- 

 ful reproductive system, the placental 

 mammals showed further refinement in an- 

 cestral systems. For example, the brain 

 became an even more conspicuous part of 

 the central nervous system and its functions 

 were much more precise. The coordination 

 and precision of operation of all the parts 

 of the body were brought to an all-time 

 high in these animals. This combination of 

 improvement on systems inherited from 

 their reptilian ancestors has been responsi- 

 ble for the success of mammals. 



