Reproduction 243 



substances of hormone nature pass from the fetal to the maternal 

 blood. By means of this placenta, intervening between mother and 

 young, the nutrition and respiration of the young animal are provided 

 for through the usually long period of intrauterine development. 



Mammals show many variations in the mode of origin and details 

 of structure of the placenta. The marsupial mammals (Meta- 

 theria — the kangaroo and its allies) produce only a weakly developed 

 and briefly temporary placenta or none at all. Accordingly, the devel- 

 opment of the young cannot proceed beyond what is made possible by 

 the initial small yolk supply plus what nutritive material may be ab- 

 sorbed by the embryo and its investing membranes directly from the 

 neighboring uterine tissues and fluids. The young marsupial is, there- 

 fore, necessarily born at an early fetal stage and while very small. The 

 deficiency of the intrauterine arrangements is compensated for by the 

 marsupium, a pouch formed by a fold of abdominal skin. The mam- 

 mary glands are within this pouch. The very immature and quite help- 

 less newborn young (in the great kangaroo, Macropus major, being 

 only about 1 inch long) is transferred to the marsupium by the mother. 

 The young becomes attached to one of the mammary nipples and feeds 

 passively, the milk being pumped in by contraction of muscle about 

 the mammary gland. This "mammary fetus" inhabits the marsupium 

 for a time which is usually much longer than its period of intrauterine 

 development. For example, in the great kangaroo the period of intra- 

 uterine gestation is between five and six weeks, but the young kangaroo 

 is carried in the pouch and nourished by mammary glands for about 

 eight months. 



In placental mammals, as compared to marsupials, the young 

 are born at a relatively advanced stage of development and growth. 

 The mammary organs, however, are in all cases an important postnatal 

 provision for bringing the young animal along to a degree of size and 

 strength favorable to ultimate success. They afford the great advan- 

 tage, too, that the young animal is not thrown upon the world abruptly, 

 but may acquire independence gradually. 



EVOLUTIONARY SIGNIFICANCE 



Surveying the whole group of vertebrates, the great diversity in the 

 conditions and arrangements attending reproduction is most impres- 

 sive. It would be difficult to imagine any practicable reproductive 

 expedient or condition which is not exhibited by some animal. There 

 are microscopic eggs and there are ostrich eggs. The quantity of yolk 

 may be vast or it may be next to nothing. The primary food supply, 

 yolk, may in various ways be supplemented by secondary sources of 

 nutriment — egg albumen, maternal blood, mammary milk, pigeon 



