EMBRYO LIFE 
23 
phant. As the embryo enlarges, the uterus grows to accom¬ 
modate it, and the muscles in the walls of the sack increase in 
preparation for the work they have to do at the birth of the 
child. For it is by the contraction of the uterine muscles 
that the young is finally expelled from its first warm nest into 
the cold world. 
After the child is born and begins to get its oxygen by 
breathing, the cord is cut. The piece remaining attached 
to the child shrivels and drops off after a few days, leaving the 
navel, or umbilicus. While the foetus is developing, the 
mother’s milk glands, mammae , are also growing, that the 
food may be ready when it is needed. A few hours or some¬ 
times a day or two after the birth of the young, milk forms 
in the mammae and the instinct of sucking stimulates the 
young to seek its food. Henceforth the care of the young is 
not simply the working of the unseen bodily functions; it 
involves the conscious actions of nursing and protection, con¬ 
trolled by the faithful parental instincts. 
1. How large are mammal eggs? 
2. How do mammals give their eggs more secure protection and 
certain warmth than do the lower animals ? 
3. From what source does the foetus get its food and oxygen? 
4. How does it get rid of carbon dioxid and nitrogenous waste? 
5. What is the placenta? 
6. What is the umbilical cord ? 
7. What is the use of the muscles in the walls of the uterus? 
8. How are mammalian young nourished ? 
