554 



THE EMBRYOLOGY OF THE FROG 



to be smaller than the 

 four in the region of the 

 vegetal pole. The 

 smaller ones are called 

 micromeres and the 

 larger ones macro- 

 meres. The micro- 

 meres, after the fifth 

 cleavage begins, divide 

 more rapidly than do 

 the macromeres. By a 

 continually more rapid 

 growth of this kind, the 

 smaller animal cells 

 soon almost surround 

 the vegetal or yolk sub- 

 stance. As the yolk be- 

 comes surrounded more 

 and more, there is a 

 somewhat central re- 

 gion where the yolk 

 can still be seen from 

 the exterior. This por- 

 tion of the yolk is 

 called the yolk-plug, 

 while the . margin of 

 darker animal cells im- 

 mediately surrounding 

 the yolk-plug is called 

 the blastopore (Figs. 

 321, 323). 



Fig. 322. 



First two figures, photographs of- Frog Blastulas. A to F, median sections 

 through Blastulas and Gastrulas of A, Cynthia bipartia with 64 cells (this is a 

 member of the tunicates). B, Gastrula of dona intestinalis (also a tunicate). 

 C, Gastrula of Amphioxus; D, Blastula of Axolotl (Mexican salamander which 

 breeds in the larval stage) ; E, Gastrula of an early stage of Turtle (Chdonia 

 caouana), F, Gastrula of a later reptilian stage (in the Gecko). A to F after 

 Rabl and Van Beneden.) 



