192 ANIMAL BIOLOGY 



body which supplies it with food and oxygen, and removes carbon 

 dioxide, urea, and other metabolic wastes. This, of course, imposes 

 a considerable amount of extra work on the various maternal 

 organs, especially the lungs, kidneys, and digestive system, and 

 therefore special provisions must be made for this intimate inter- 

 change of materials between the blood vascular systems of mother 

 and offspring. (Fig. 134A.) 



The embryo at first is nourished by materials absorbed from 

 the rich blood supply of the uterine wall, but soon this proves in- 

 adequate for the rapidly increasing demands of the growing em- 

 bryo, and a new structure, the placenta, is formed jointly by the 

 tissues of the uterus and embryo to meet the need. The connection 

 of the embryonic body with the placenta is by the umbilical cord. 

 The blood of the embryo passes by its arteries through the umbil- 

 ical cord to capillaries in the placenta, and after interchanging 

 wastes for food and oxygen by diffusion with the maternal blood, 

 returns by veins to the embryo. Accordingly there is no direct 

 intermixture of maternal and embryonic blood: the embryo from 

 the beginning is a separate organism whose blood supply inter- 

 changes materials with that of the mother through the placenta. 

 This temporary dependence is terminated when the child is ex- 

 pelled from the uterus, or born. 



2. Hormones 



Thus, except in the simplest animals, there is a special repro- 

 ductive system : a series of organs connected with the reproductive 

 function. But it must be emphasized that the essential organs are 

 the gonads themselves and all the rest are accessory. Furthermore, 

 in relation to the sexual differentiation of male and female in- 

 dividuals, many so-called secondary sexual characters arise 

 which are not directly connected with the reproductive organs, 

 but nevertheless depend very largely for their development upon 

 hormones liberated by the gonads. For example, early castration 

 of the Stag inhibits the growth of a distinctive male secondary 

 sexual character, the antlers; while if performed later when the 

 antlers are full grown, they are shed and abnormal ones take their 

 place. 



Indeed, the sexual life of the Vertebrates, including Man, is 

 largely controlled by hormones. Thus after the release of the egg 

 from the human ovary, its former location is filled by the corpus 



