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 \\\& 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 &^'g, 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 ^'g% 

 at the time of its laying. Such an &g^ 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. 



