The Development of the Chick 93 



are of almost microscopic size ; hence the 

 mammalian embryo is dependent upon its 

 mother for food during its development, and 

 an arrangement known as ti\.e placenta is pres- 

 ent to permit an interchange of food and gases 

 between the blood of the parent and that of 

 the embryo. The frog's egg, though so much 

 smaller than that of the chick, contains a large 

 amount of food material, as we have already 

 seen, and the embryo frog develops quite inde- 

 pendently of its mother ; but while the chick, 

 at the time of hatching, has practically the 

 adult structure, the young frog, at the time of 

 hatching, is a very different animal from the 

 adult frog, and must obtain food for its further 

 growth from its surroundings. 



The preceding is a description of the egg 

 at the time of its laying. Such an egg has 

 already passed through the earlier stages of its 

 development, and is in a resting condition, 

 simply awaiting suitable conditions of tem- 

 perature, moisture, etc., to proceed with its 

 complete development. The statement that 

 the yolk is a single cell is really true only 

 from the time it leaves the ovary until it is 

 fertilized, or until a short time after fertiliza- 

 tion, when segmentation begins. 



