236 MANUAL OF ANIMAL BIOLOGY 



eventually back to the yolk sac. The embryo, at this stage, has 

 four pairs of gill slits which are similar in structure to those which 

 develop in the aquatic Vertebrates. In the Chick, however, the 

 gills never function and, after a few days, the gill slits disappear. 

 (W. f. 235.) 



The Chick embryo continues to develop, receiving its food from 

 the yolk and carrying on the respiratory interchange through the 

 surface of the shell. The Hen's egg requires about 21 days of incu- 

 bation, and then the Chick breaks through the shell with the aid 

 of a specialized structure which develops on the beak, and comes 

 forth as an active animal able to secure its nourishment, and thus 

 to continue its growth until the adult condition is reached. 



C. The Development of the Mammal 



The Prototheria, such as the Duck-bill and Echidna, constitute 

 the lowest group of the Mammals. It is noteworthy that in this 

 group of Mammals the females are oviparous, that is, they lay 

 eggs which develop outside the body and are similar to the large- 

 yolked eggs of the Birds and Reptiles. In the Prototheria, how- 

 ever, the young animals do not at once begin an independent 

 existence when they hatch from the egg but are nourished for a 

 time by secretions from the mammary glands of the mother. ( W. 

 pp. 191-195 ; 271-273.) 



In the Metatheria and Eutheria the females are viviparous, 

 that is, the fertilized eggs are retained within the body of the 

 female in a specialized portion of the oviducts, known as the 

 uterus, until the embryos have reached a certain stage of devel- 

 opment, at which time they are, as we say, born. The degree of 

 development of the embryo at birth varies a great deal in the 

 different species. In the metatherian mammals, such as the 

 Kangaroo and the Opossum, the young are born in such an imma- 

 ture and helpless condition that it is necessary for the mother to 

 carry them for a time after birth in a special sac, the marsupium, 

 which is present on the ventral abdominal wall of the female. Here 

 the embryos are nourished by milk from the mammary glands 

 until they reach a sufficient degree of maturity to take care of 

 themselves. Even among the eutherian mammals the develop- 

 ment of the embryo at birth shows great variation in the different 

 classes. Thus the new-born embryo is very well-developed among 

 the hoofed Mammals, or Ungulates, while among the Primates 



