18 • HORMONES AND 

 THE GROWTH OF THE 

 MAMMALIAN EMBRYO 



G. 



rowth of the mammalian embryo follows the same general pattern 

 which we outlined for the frog and chick embryos. What is unique in mam- 

 malian development is the method of obtaining oxygen and substrates and 

 excreting carbon dioxide and nitrogenous wastes. The egg is so small and 

 contains so little in the way of food reserves that it must receive food from 

 the parent blood stream very early in development. This is achieved through 

 the development of embryonic tissues which unite with the tissue of the 

 uterus to form a composite organ, the placenta. In the human, embryonic 

 blood vessels grow into the placenta, where they are bathed with maternal 

 blood, so that the blood of the embryo is in equilibrium with that of the 

 mother. Chemical compounds diffuse between embryo and uterus and the 

 exchange is regulated by the permeability of the cell membranes. There are 

 three membranes separating the maternal blood from that of the embryo 

 (Fig. 168). 



The formation of the placenta and the growth of the embryo in the uterus 

 result from an interaction of the embryo, the pituitary, the ovary, and the 

 placenta. First of all the egg becomes implanted within the wall of the 

 uterus. The egg is liberated at such a time that it reaches the uterine wall 

 when the tissues are receptive to its implantation. After implantation the 

 rhythmic changes of growth and degeneration of the uterine wall must be 



