EMBRYOLOGY OF TADPOLE AND CHICK 



553 



do not arrange themselves in the manner described until after fertiliza- 

 tion, so that we may say that bilateral symmetry in the frog's egg is 

 potential but not actual until after fertilization and the rearrangement 

 of these three different substances. 



The original cleavage plane lies at right angles to the egg axis, but 

 not at right angles to what is to become the axis of the embryo itselL 

 There is no direct relation between the plane of the first furrow in 

 cleavage and the fertilization meridian. The midline of the developing 

 embryo and the penetration path of the sperm normally correspond to a 

 vertical plane known as a gravitational plane, drawn through the egg 

 after the particles of protoplasm and deutoplasm have rearranged them- 

 selves according to gravity, and all of these correspond to the first cleav- 

 age furrow, though many variations of this occur. 



THE FORMATION OF THE BLASTULA 



One of the chief reasons for studying the embryology of the chick 

 before that of the frog is that the three germ layers of the chick are 

 more readily seen. The frog's egg divides into two portions, then into 

 four, eight, etc., quite like the hen's egg, and by the time there are eight 

 cells present, the four cells in the region of the animal pole are found 





Fig. 321. 



Cleavage of the frog's egg. A Eight-cell stage ; B, beginning of 

 sixteen-cell stage ; C, thirty-two-cell stage ; D, forty-eight-cell stage 

 ( more regular than usual ) ; E, F, G, later stages ; H, I, formation of 

 blastopore. The central light area in / is the yolk-plug while the 

 rinpr which encases the yolk-plug is the margin of the blastopore. 

 (After Morgan.) 



